What is your F/ratio [Deep Sky] Acquisition techniques · Tony Gondola · ... · 44 · 1184 · 0

Seemingly a simple question but not really. Would anyone be surprised to know that a  6"  F/4 Newtonian astrograph actually delivers an image brightness to the sensor that's about f/6.3? Your actual light gathering power would be roughly equivalent to a 102mm refractor. Interesting to think about…

Central obstruction, vanes, clips, reflectivity of the coatings, losses at every air to glass surface, so many things…

I am not talking about geometric focal ratio, as you point out, that is just the product of aperture diameter divided by focal length and does not change. What I am talking about is the brightness of the image delivered to the focal plane. Depending on the efficiency of the system, the flux at the focal plane will never match that expected if the system was 100% efficient. You can quibble about the actual losses but losses do exist and will be different for every system. Heck, even aluminum coated mirrors will very in reflectivity between 89% and 97% and we are not even talking about wavelength. There are many variables and within them are variations. Your mileage may vary but the basic idea is sound.

That's my point, it's not. Aperture will not always win. How much is negligible? I came to this because I wanted to see if getting a 6" f/4 Newtonian astrograph would be a considerable gain in light grasp over a 102mm refractor. Turns out, it really isn't. I would encourage anyone to do the math to see for themselves. You might be surprised, I certainly was. The easiest way to do it is to start with the full aperture, then subtract out the area of all obstructions, secondary, spider vanes, mirror clips, focuser tube, everything.  Then you have to account for the reflectivity of the mirrors. A super clean aluminum  coating gives you anything from 89% to 96%, from each surface. Lastly, the astrograph has a coma corrector. This will add at least 2 air to glass surfaces. This can vary a lot depending on the type of AR coating, as an estimate, assume you'll loose about 1% from each interface.

Comparing a 6" Newtonian with a 102mm refractor at the same focal length (around 600mm): Click

Sensor with 3,7 micron pixels and 80% quantum efficiency. Newton with 30% Obstruction and APO with around 96% transmission.

# Telescope 1 f/ 4.00 fl= 600mm D=150mm O=30% res=1.27"/p FOV=21.2'x21.2'= 1.04x eoi= 2.25x poi= 4.43x e= 2.05x pe= 2.05x ps= 2.13x os= 2.05x
# Telescope 2 f/ 6.00 fl= 612mm D=102mm O= 0% res=1.25"/p FOV=20.8'x20.8'= 0.96x eoi= 0.44x poi= 0.23x e= 0.49x pe= 0.49x ps= 0.47x os= 0.49x

Every pixel gets over double the signal on the newt, even if the target fits in the FOV of both scopes. The newt will still be much "faster".

Regards

Seemingly a simple question but not really. Would anyone be surprised to know that a  6"  F/4 Newtonian astrograph actually delivers an image brightness to the sensor that's about f/6.3? Your actual light gathering power would be roughly equivalent to a 102mm refractor. Interesting to think about...

*** 
It is not. More like a 120mm triplet apo. And then you'll be hard-pushed to squeeze 480mm of said refractor and still you wouldn't get the same image scale. Pointless exercise...

This is interesting but I'm not sure what I'm looking at here, can you go through it?

Also, I think 96% transmission is about right on the APO, what transmission percentage did you use for the Astrograph? The obstruction should be closer to 40% and I don't see anything else being taken into account (other obstructions, reflectivity, etc.)

You can take other obstructions and reflectivity into account with the total transmittance factor.

If I use the obstruction of a Quattro 150P (42%) and and the transmission of 90% (we dont know the mirrors, spider vanes, etc. in the light path) the newt is still much faster at same focal length. 1.74x times faster than the 102mm APO.

# Telescope 1 f/ 4.00 fl= 600mm D=150mm O=42% res=1.27"/p FOV=21.2'x21.2'= 1.04x eoi= 2.25x poi= 4.01x e= 1.85x pe= 1.85x ps= 1.74x os= 1.67x
# Telescope 2 f/ 6.00 fl= 612mm D=102mm O= 0% res=1.25"/p FOV=20.8'x20.8'= 0.96x eoi= 0.44x poi= 0.25x e= 0.54x pe= 0.54x ps= 0.58x os= 0.60x

Let's see which size of APO reaches the same signal gathering power as the entry level newton.

# Telescope 1 f/ 4.00 fl= 600mm D=150mm O=42% res=1.27"/p FOV=21.2'x21.2'= 1.00x eoi= 1.33x poi= 1.46x e= 1.10x pe= 1.10x ps= 1.03x os= 1.03x
# Telescope 2 f/ 4.62 fl= 600mm D=130mm O= 0% res=1.27"/p FOV=21.2'x21.2'= 1.00x eoi= 0.75x poi= 0.69x e= 0.91x pe= 0.91x ps= 0.97x os= 0.97x

A hefty 130mm one, which has to use a reducer to reach the same FOV as the newton. And then it's only as fast as the newt. Price per mm of aperture will be better on the newt.

APOs are convenient and easy to use, but they are no light buckets and never will be. People still love them, which is fine.

--

I would definetly recommend that you watch John Hayes presentation on the Astro Imaging youtube channel. It's the link he posted earlier. A very interesting and informative video about why Aperture is king.

Tobiasz:

Anderl:
The right way to calculate your t-ratio should be 
(Primary)75x75x3.14 - (secondary)35x35x3.14 = 13.800 = effective light collecting area. 
effective aperture = 13.800/3.14 = 4394.
now calculate the square root of 4394 = 66.3 and multiply it by 2 to get the aperture —> 132.6mm

To get the t-ratio you now need to divide your focal length (600mm) by 132.6 and you get something around t4.5 as an result. 

The real reason refractors are waaay better is because they just look better ;) newtonians are just highly effective astronomically instruments looking like trash cans. 
oh and don’t forget that most apos will be able to illuminate a full frame sensor while even expensive newtonians will already show significant vignetting on apsc. If you look at it that way a f7 apo and an f4 newtonian have similar light collecting abilities. 

cs
andi

I don't know if you're trolling, but we can do the calculations for your "f/7 apo is as fast as the f/4 newton" comparison. I already posted the results on gondolas comparison, so yes a reduced 130mm apo is needed to reach the SAME light gathering power as an entry level newton on the SAME FOV.

Your statement on "significant vignetting on apsc even on expensive newtons" is simply not true and I will ignore that. Newtons can be build like that to fully support an full frame image circle, e.g. TS ONTC.

Let's take your 132mm f/7 apo and compare it to a newton with the same focal length. The most common one would be a 8" F/4 Newton.

# Telescope 1 f/ 4.00 fl= 800mm D=200mm O=42% res=0.95"/p FOV=15.9'x15.9'= 0.85x eoi= 1.96x poi= 3.71x e= 1.61x pe= 1.61x ps= 1.51x os= 1.77x
# Telescope 2 f/ 5.60 fl= 739mm D=132mm O= 0% res=1.03"/p FOV=17.2'x17.2'= 1.17x eoi= 0.51x poi= 0.27x e= 0.62x pe= 0.62x ps= 0.66x os= 0.56x

I had to reduce your APO by 0.8 to get rougly in the ballpark with the focal length. Should be an advantage for the APO here. Same obstruction and transmission as in gondolas comparison. Sensor with 3,7 micron pixels and 80% QE. Who would have thought, the toys get bigger but nothing changes on the light gathering power. The newton is still much faster.

Same FOV, for the same target the newt would acquire 1,77x times more signal on the object than your reduced 132mm apo.

With those few clear nights in central europe, I'd rather have the photon trash can over the slower APO.

Regards

woa woh wa. chill your nuggets ;)
i have asked a few people here and on other astro communities to give feedback about those "full frame capable newton telescopes" and there is almost no scope that can do it. 
the ts options seem to produce vignetting and bad stars, lacerta newton is only good up to apsc (and already vignettes), epsilon works but needs a lot of work, I couldn't find out how good or bad the corner performance of discontinued scopes like asa n8 n10 are but as I can't get one It doesn't matter. 
there are surely newtonians out there that have great edge to edge performance over an full frame area but I think it is true to state that it is way easier to get that with an apo. 
and as the area of an full frame chip is around 2.5 times the area of an apsc sized chip you are also getting around 2.5 times the light, equaling the faster f-ratio of the newton again. 
note that I don't look at same target same fov here. I just look at the light that lands on the sensor area. 

and to state it again: the most important reason to get an apo refractor is because it is looking better. this really should tell you how serious you should take my comment ;)

cs
Andi

Tobiasz:

Anderl:

Tobiasz:

Anderl:
The right way to calculate your t-ratio should be 
(Primary)75x75x3.14 - (secondary)35x35x3.14 = 13.800 = effective light collecting area. 
effective aperture = 13.800/3.14 = 4394.
now calculate the square root of 4394 = 66.3 and multiply it by 2 to get the aperture —> 132.6mm

To get the t-ratio you now need to divide your focal length (600mm) by 132.6 and you get something around t4.5 as an result. 

The real reason refractors are waaay better is because they just look better ;) newtonians are just highly effective astronomically instruments looking like trash cans. 
oh and don’t forget that most apos will be able to illuminate a full frame sensor while even expensive newtonians will already show significant vignetting on apsc. If you look at it that way a f7 apo and an f4 newtonian have similar light collecting abilities. 

cs
andi

I don't know if you're trolling, but we can do the calculations for your "f/7 apo is as fast as the f/4 newton" comparison. I already posted the results on gondolas comparison, so yes a reduced 130mm apo is needed to reach the SAME light gathering power as an entry level newton on the SAME FOV.

Your statement on "significant vignetting on apsc even on expensive newtons" is simply not true and I will ignore that. Newtons can be build like that to fully support an full frame image circle, e.g. TS ONTC.

Let's take your 132mm f/7 apo and compare it to a newton with the same focal length. The most common one would be a 8" F/4 Newton.

# Telescope 1 f/ 4.00 fl= 800mm D=200mm O=42% res=0.95"/p FOV=15.9'x15.9'= 0.85x eoi= 1.96x poi= 3.71x e= 1.61x pe= 1.61x ps= 1.51x os= 1.77x
# Telescope 2 f/ 5.60 fl= 739mm D=132mm O= 0% res=1.03"/p FOV=17.2'x17.2'= 1.17x eoi= 0.51x poi= 0.27x e= 0.62x pe= 0.62x ps= 0.66x os= 0.56x

I had to reduce your APO by 0.8 to get rougly in the ballpark with the focal length. Should be an advantage for the APO here. Same obstruction and transmission as in gondolas comparison. Sensor with 3,7 micron pixels and 80% QE. Who would have thought, the toys get bigger but nothing changes on the light gathering power. The newton is still much faster.

Same FOV, for the same target the newt would acquire 1,77x times more signal on the object than your reduced 132mm apo.

With those few clear nights in central europe, I'd rather have the photon trash can over the slower APO.

Regards

woa woh wa. chill your nuggets ;)
i have asked a few people here and on other astro communities to give feedback about those "full frame capable newton telescopes" and there is almost no scope that can do it. 
the ts options seem to produce vignetting and bad stars, lacerta newton is only good up to apsc (and already vignettes), epsilon works but needs a lot of work, I couldn't find out how good or bad the corner performance of discontinued scopes like asa n8 n10 are but as I can't get one It doesn't matter. 
there are surely newtonians out there that have great edge to edge performance over an full frame area but I think it is true to state that it is way easier to get that with an apo. 
and as the area of an full frame chip is around 2.5 times the area of an apsc sized chip you are also getting around 2.5 times the light, equaling the faster f-ratio of the newton again. 
note that I don't look at same target same fov here. I just look at the light that lands on the sensor area. 

and to state it again: the most important reason to get an apo refractor is because it is looking better. this really should tell you how serious you should take my comment ;)

cs
Andi

"and as the area of an full frame chip is around 2.5 times the area of an apsc sized chip you are also getting around 2.5 times the light, equaling the faster f-ratio of the newton again."

No, this is just wrong. You are not getting more light, you just capture more of the surrounding background, which is low on signal. Per pixel signal does not increase with bigger sensor size, only the FOV.

Your 132mm f/7 APO 0.8 reducer + 6200mm FF vs Sharpstar 150mm f/2.8 astrograph + 2600mm APS-C. Both have roughly the same FOV and the newt should be capable of illuminating the APS-C sensor.

# Telescope 1 f/ 2.80 fl= 420mm D=150mm O=40% res=1.82"/p FOV=189.2'x126.5'= 1.32x eoi= 4.00x poi= 4.34x e= 1.43x pe= 3.36x ps= 3.15x os= 1.02x
# Telescope 2 f/ 5.60 fl= 739mm D=132mm O= 0% res=1.03"/p FOV=164.8'x109.9'= 0.76x eoi= 0.25x poi= 0.23x e= 0.70x pe= 0.30x ps= 0.32x os= 0.98x

Your 2.5 times more light statement is almost correct, for the signal gathering per pixel of the APS-C sensor at least :-D Newton is not faster even, it blows the APO out of the water.

The resolution is a bit worse, but you can drizzle the APS-C subs, because the Newton delivers the appropriate aperture for it. Noise will be less of a concern with 3 times more signal per sub than the APO.

"note that I don't look at same target same fov here. I just look at the light that lands on the sensor area."

Ok, so you want to compare supported image circles, which is a different topic. Still, it does not change the physical characteristics and performance of the telescopes.

you can try to capture the same fov of an 800mm f7 apo (full frame) with an f4 800mm newton (apsc mosaic). Both will more or less require the same time. 
The supported image circle is an often overlooked thing within our hobby. If you use an 1inch sensor on your newton instead of an apsc sized chip you are just wasting light. 
An f7 toa 150 becomes equally fast as an epsilon astrograph once you use his full image circle (100% illumination @70mm) + it will provide better resolution, look better ;) and will be easier 2use.

cs
andi


cs
andi

Hi all,

Regarding the senzor size, bigger will collect more light, but from bigger area. I guess (logic tells me) that important thing here is the sampling - sky are per pixel. So doesn't matter how big the senzor is, but sampling rate.

Jan

Tobiasz:

Anderl:

Tobiasz:

Anderl:

Tobiasz:

Anderl:
The right way to calculate your t-ratio should be 
(Primary)75x75x3.14 - (secondary)35x35x3.14 = 13.800 = effective light collecting area. 
effective aperture = 13.800/3.14 = 4394.
now calculate the square root of 4394 = 66.3 and multiply it by 2 to get the aperture —> 132.6mm

To get the t-ratio you now need to divide your focal length (600mm) by 132.6 and you get something around t4.5 as an result. 

The real reason refractors are waaay better is because they just look better ;) newtonians are just highly effective astronomically instruments looking like trash cans. 
oh and don’t forget that most apos will be able to illuminate a full frame sensor while even expensive newtonians will already show significant vignetting on apsc. If you look at it that way a f7 apo and an f4 newtonian have similar light collecting abilities. 

cs
andi

I don't know if you're trolling, but we can do the calculations for your "f/7 apo is as fast as the f/4 newton" comparison. I already posted the results on gondolas comparison, so yes a reduced 130mm apo is needed to reach the SAME light gathering power as an entry level newton on the SAME FOV.

Your statement on "significant vignetting on apsc even on expensive newtons" is simply not true and I will ignore that. Newtons can be build like that to fully support an full frame image circle, e.g. TS ONTC.

Let's take your 132mm f/7 apo and compare it to a newton with the same focal length. The most common one would be a 8" F/4 Newton.

# Telescope 1 f/ 4.00 fl= 800mm D=200mm O=42% res=0.95"/p FOV=15.9'x15.9'= 0.85x eoi= 1.96x poi= 3.71x e= 1.61x pe= 1.61x ps= 1.51x os= 1.77x
# Telescope 2 f/ 5.60 fl= 739mm D=132mm O= 0% res=1.03"/p FOV=17.2'x17.2'= 1.17x eoi= 0.51x poi= 0.27x e= 0.62x pe= 0.62x ps= 0.66x os= 0.56x

I had to reduce your APO by 0.8 to get rougly in the ballpark with the focal length. Should be an advantage for the APO here. Same obstruction and transmission as in gondolas comparison. Sensor with 3,7 micron pixels and 80% QE. Who would have thought, the toys get bigger but nothing changes on the light gathering power. The newton is still much faster.

Same FOV, for the same target the newt would acquire 1,77x times more signal on the object than your reduced 132mm apo.

With those few clear nights in central europe, I'd rather have the photon trash can over the slower APO.

Regards

woa woh wa. chill your nuggets ;)
i have asked a few people here and on other astro communities to give feedback about those "full frame capable newton telescopes" and there is almost no scope that can do it. 
the ts options seem to produce vignetting and bad stars, lacerta newton is only good up to apsc (and already vignettes), epsilon works but needs a lot of work, I couldn't find out how good or bad the corner performance of discontinued scopes like asa n8 n10 are but as I can't get one It doesn't matter. 
there are surely newtonians out there that have great edge to edge performance over an full frame area but I think it is true to state that it is way easier to get that with an apo. 
and as the area of an full frame chip is around 2.5 times the area of an apsc sized chip you are also getting around 2.5 times the light, equaling the faster f-ratio of the newton again. 
note that I don't look at same target same fov here. I just look at the light that lands on the sensor area. 

and to state it again: the most important reason to get an apo refractor is because it is looking better. this really should tell you how serious you should take my comment ;)

cs
Andi

"and as the area of an full frame chip is around 2.5 times the area of an apsc sized chip you are also getting around 2.5 times the light, equaling the faster f-ratio of the newton again."

No, this is just wrong. You are not getting more light, you just capture more of the surrounding background, which is low on signal. Per pixel signal does not increase with bigger sensor size, only the FOV.

Your 132mm f/7 APO 0.8 reducer + 6200mm FF vs Sharpstar 150mm f/2.8 astrograph + 2600mm APS-C. Both have roughly the same FOV and the newt should be capable of illuminating the APS-C sensor.

# Telescope 1 f/ 2.80 fl= 420mm D=150mm O=40% res=1.82"/p FOV=189.2'x126.5'= 1.32x eoi= 4.00x poi= 4.34x e= 1.43x pe= 3.36x ps= 3.15x os= 1.02x
# Telescope 2 f/ 5.60 fl= 739mm D=132mm O= 0% res=1.03"/p FOV=164.8'x109.9'= 0.76x eoi= 0.25x poi= 0.23x e= 0.70x pe= 0.30x ps= 0.32x os= 0.98x

Your 2.5 times more light statement is almost correct, for the signal gathering per pixel of the APS-C sensor at least :-D Newton is not faster even, it blows the APO out of the water.

The resolution is a bit worse, but you can drizzle the APS-C subs, because the Newton delivers the appropriate aperture for it. Noise will be less of a concern with 3 times more signal per sub than the APO.

"note that I don't look at same target same fov here. I just look at the light that lands on the sensor area."

Ok, so you want to compare supported image circles, which is a different topic. Still, it does not change the physical characteristics and performance of the telescopes.

you can try to capture the same fov of an 800mm f7 apo (full frame) with an f4 800mm newton (apsc mosaic). Both will more or less require the same time. 
The supported image circle is an often overlooked thing within our hobby. If you use an 1inch sensor on your newton instead of an apsc sized chip you are just wasting light. 
An f7 toa 150 becomes equally fast as an epsilon astrograph once you use his full image circle (100% illumination @70mm) + it will provide better resolution, look better ;) and will be easier 2use.

cs
andi


cs
andi

Sorry, but throwing in random telescopes and now including mosaics does not make it right that a bigger sensor makes the telescope faster. You don't want to compare FOVs but you are talking about FOVs the whole time.

My explanation still stands, in the current market and at equal FOV a Newton will always have more effective aperture available than an APO and therefore will be faster. If a picture will look "better" depends on so much more than the telescope, e.g. post-processing, collimation, pinched optics or seeing.

Lets look what ai has to say and yes i am very aware of the limitations of such.


To perform a detailed analysis of the total exposure time required to achieve the same picture, SNR, and FOV with the two setups—an f/7 800mm APO refractor with a full frame sensor and an f/4 800mm Newtonian with an APS-C sensor—we need to consider several key factors:

1. **Focal Length and Focal Ratio:**
   - Both telescopes have the same focal length (800mm), but different focal ratios (f/7 and f/4).
   - The focal ratio affects the amount of light per unit area reaching the sensor, with the f/4 system being faster.

2. **Aperture Size:**
   - APO refractor: \( \text{Aperture} = \frac{800}{7} \approx 114.3 \text{mm} \)
   - Newtonian: \( \text{Aperture} = \frac{800}{4} = 200 \text{mm} \)
   - Newtonian with secondary obstruction: Effective aperture \( = \sqrt{200^2 - 70^2} \approx 187.3 \text{mm} \)

3. **Light-Gathering Power:**
   - Light-gathering power is proportional to the area of the aperture.
   - APO refractor: \( \text{Area} \approx \pi \times \left(\frac{114.3}{2}\right)^2 \approx 10,263 \text{mm}^2 \)
   - Newtonian: \( \text{Effective area} \approx \pi \times \left(\frac{187.3}{2}\right)^2 \approx 27,573 \text{mm}^2 \)

4. **Field of View (FOV):**
   - Full Frame Sensor: \( 36 \text{mm} \times 24 \text{mm} \)
   - APS-C Sensor: \( 22 \text{mm} \times 15 \text{mm} \)
   - FOV is given by \( \text{FOV} = \frac{206.265 \times \text{sensor size}}{\text{focal length}} \)

### FOV Calculation:
#### APO Refractor (Full Frame Sensor):
- FOV (width): \( \frac{206.265 \times 36}{800} \approx 9.28^\circ \)
- FOV (height): \( \frac{206.265 \times 24}{800} \approx 6.19^\circ \)

#### Newtonian (APS-C Sensor):
- FOV (width): \( \frac{206.265 \times 22}{800} \approx 5.68^\circ \)
- FOV (height): \( \frac{206.265 \times 15}{800} \approx 3.87^\circ \)

### Mosaic Calculation:
To cover the same FOV with the Newtonian setup, we need to calculate how many tiles are required.

#### Area Covered by Each Configuration:
- APO Refractor: \( 9.28^\circ \times 6.19^\circ \approx 57.43 \text{ square degrees} \)
- Newtonian: \( 5.68^\circ \times 3.87^\circ \approx 21.99 \text{ square degrees} \)

#### Number of Tiles for Mosaic:
\[ \text{Number of tiles} = \frac{57.43}{21.99} \approx 2.61 \]

Rounding up, the Newtonian requires 3 tiles to cover the same area as the APO.

### Exposure Time Comparison:
To compare the total exposure time, we must consider the light-gathering power and the number of tiles required.

#### Light-Gathering Efficiency:
- APO Refractor: \( \text{Light gathering power} = 10,263 \text{mm}^2 \)
- Newtonian: \( \text{Light gathering power} = 27,573 \text{mm}^2 \)

The Newtonian gathers approximately \( \frac{27,573}{10,263} \approx 2.69 \) times more light than the APO.

#### Focal Ratio Impact:
- Faster focal ratio (f/4) means more light per unit area.
- Exposure time \( t \) is inversely proportional to the square of the focal ratio:
  - \( \text{Refractor exposure time} \propto (7)^2 = 49 \)
  - \( \text{Newtonian exposure time} \propto (4)^2 = 16 \)

#### Individual Exposure Time Comparison:
\[ \frac{t_{\text{Newtonian}}}{t_{\text{Refractor}}} = \frac{16}{49} \approx 0.33 \]

Since the Newtonian requires 3 tiles, the total exposure time for the Newtonian setup is:
\[ \text{Total exposure time for Newtonian} = 3 \times 0.33 \times t_{\text{Refractor}} = 0.99 \times t_{\text{Refractor}} \]

### Conclusion:
The total exposure time for the finished and stacked image using the 200mm f/4 Newtonian with an APS-C sensor will be approximately the same as the exposure time required for the 114.3mm f/7 APO refractor with a full frame sensor. Despite the need to create a mosaic with the Newtonian, its faster focal ratio and larger effective aperture compensate for the additional images, resulting in nearly equivalent total exposure times for the final image.

Anderl:

Tobiasz:

Anderl:

Tobiasz:

Anderl:

Tobiasz:

Anderl:
The right way to calculate your t-ratio should be 
(Primary)75x75x3.14 - (secondary)35x35x3.14 = 13.800 = effective light collecting area. 
effective aperture = 13.800/3.14 = 4394.
now calculate the square root of 4394 = 66.3 and multiply it by 2 to get the aperture —> 132.6mm

To get the t-ratio you now need to divide your focal length (600mm) by 132.6 and you get something around t4.5 as an result. 

The real reason refractors are waaay better is because they just look better ;) newtonians are just highly effective astronomically instruments looking like trash cans. 
oh and don’t forget that most apos will be able to illuminate a full frame sensor while even expensive newtonians will already show significant vignetting on apsc. If you look at it that way a f7 apo and an f4 newtonian have similar light collecting abilities. 

cs
andi

I don't know if you're trolling, but we can do the calculations for your "f/7 apo is as fast as the f/4 newton" comparison. I already posted the results on gondolas comparison, so yes a reduced 130mm apo is needed to reach the SAME light gathering power as an entry level newton on the SAME FOV.

Your statement on "significant vignetting on apsc even on expensive newtons" is simply not true and I will ignore that. Newtons can be build like that to fully support an full frame image circle, e.g. TS ONTC.

Let's take your 132mm f/7 apo and compare it to a newton with the same focal length. The most common one would be a 8" F/4 Newton.

# Telescope 1 f/ 4.00 fl= 800mm D=200mm O=42% res=0.95"/p FOV=15.9'x15.9'= 0.85x eoi= 1.96x poi= 3.71x e= 1.61x pe= 1.61x ps= 1.51x os= 1.77x
# Telescope 2 f/ 5.60 fl= 739mm D=132mm O= 0% res=1.03"/p FOV=17.2'x17.2'= 1.17x eoi= 0.51x poi= 0.27x e= 0.62x pe= 0.62x ps= 0.66x os= 0.56x

I had to reduce your APO by 0.8 to get rougly in the ballpark with the focal length. Should be an advantage for the APO here. Same obstruction and transmission as in gondolas comparison. Sensor with 3,7 micron pixels and 80% QE. Who would have thought, the toys get bigger but nothing changes on the light gathering power. The newton is still much faster.

Same FOV, for the same target the newt would acquire 1,77x times more signal on the object than your reduced 132mm apo.

With those few clear nights in central europe, I'd rather have the photon trash can over the slower APO.

Regards

woa woh wa. chill your nuggets ;)
i have asked a few people here and on other astro communities to give feedback about those "full frame capable newton telescopes" and there is almost no scope that can do it. 
the ts options seem to produce vignetting and bad stars, lacerta newton is only good up to apsc (and already vignettes), epsilon works but needs a lot of work, I couldn't find out how good or bad the corner performance of discontinued scopes like asa n8 n10 are but as I can't get one It doesn't matter. 
there are surely newtonians out there that have great edge to edge performance over an full frame area but I think it is true to state that it is way easier to get that with an apo. 
and as the area of an full frame chip is around 2.5 times the area of an apsc sized chip you are also getting around 2.5 times the light, equaling the faster f-ratio of the newton again. 
note that I don't look at same target same fov here. I just look at the light that lands on the sensor area. 

and to state it again: the most important reason to get an apo refractor is because it is looking better. this really should tell you how serious you should take my comment ;)

cs
Andi

"and as the area of an full frame chip is around 2.5 times the area of an apsc sized chip you are also getting around 2.5 times the light, equaling the faster f-ratio of the newton again."

No, this is just wrong. You are not getting more light, you just capture more of the surrounding background, which is low on signal. Per pixel signal does not increase with bigger sensor size, only the FOV.

Your 132mm f/7 APO 0.8 reducer + 6200mm FF vs Sharpstar 150mm f/2.8 astrograph + 2600mm APS-C. Both have roughly the same FOV and the newt should be capable of illuminating the APS-C sensor.

# Telescope 1 f/ 2.80 fl= 420mm D=150mm O=40% res=1.82"/p FOV=189.2'x126.5'= 1.32x eoi= 4.00x poi= 4.34x e= 1.43x pe= 3.36x ps= 3.15x os= 1.02x
# Telescope 2 f/ 5.60 fl= 739mm D=132mm O= 0% res=1.03"/p FOV=164.8'x109.9'= 0.76x eoi= 0.25x poi= 0.23x e= 0.70x pe= 0.30x ps= 0.32x os= 0.98x

Your 2.5 times more light statement is almost correct, for the signal gathering per pixel of the APS-C sensor at least :-D Newton is not faster even, it blows the APO out of the water.

The resolution is a bit worse, but you can drizzle the APS-C subs, because the Newton delivers the appropriate aperture for it. Noise will be less of a concern with 3 times more signal per sub than the APO.

"note that I don't look at same target same fov here. I just look at the light that lands on the sensor area."

Ok, so you want to compare supported image circles, which is a different topic. Still, it does not change the physical characteristics and performance of the telescopes.

you can try to capture the same fov of an 800mm f7 apo (full frame) with an f4 800mm newton (apsc mosaic). Both will more or less require the same time. 
The supported image circle is an often overlooked thing within our hobby. If you use an 1inch sensor on your newton instead of an apsc sized chip you are just wasting light. 
An f7 toa 150 becomes equally fast as an epsilon astrograph once you use his full image circle (100% illumination @70mm) + it will provide better resolution, look better ;) and will be easier 2use.

cs
andi


cs
andi

Sorry, but throwing in random telescopes and now including mosaics does not make it right that a bigger sensor makes the telescope faster. You don't want to compare FOVs but you are talking about FOVs the whole time.

My explanation still stands, in the current market and at equal FOV a Newton will always have more effective aperture available than an APO and therefore will be faster. If a picture will look "better" depends on so much more than the telescope, e.g. post-processing, collimation, pinched optics or seeing.

Lets look what ai has to say and yes i am very aware of the limitations of such.


To perform a detailed analysis of the total exposure time required to achieve the same picture, SNR, and FOV with the two setups—an f/7 800mm APO refractor with a full frame sensor and an f/4 800mm Newtonian with an APS-C sensor—we need to consider several key factors:

1. **Focal Length and Focal Ratio:**
   - Both telescopes have the same focal length (800mm), but different focal ratios (f/7 and f/4).
   - The focal ratio affects the amount of light per unit area reaching the sensor, with the f/4 system being faster.

2. **Aperture Size:**
   - APO refractor: \( \text{Aperture} = \frac{800}{7} \approx 114.3 \text{mm} \)
   - Newtonian: \( \text{Aperture} = \frac{800}{4} = 200 \text{mm} \)
   - Newtonian with secondary obstruction: Effective aperture \( = \sqrt{200^2 - 70^2} \approx 187.3 \text{mm} \)

3. **Light-Gathering Power:**
   - Light-gathering power is proportional to the area of the aperture.
   - APO refractor: \( \text{Area} \approx \pi \times \left(\frac{114.3}{2}\right)^2 \approx 10,263 \text{mm}^2 \)
   - Newtonian: \( \text{Effective area} \approx \pi \times \left(\frac{187.3}{2}\right)^2 \approx 27,573 \text{mm}^2 \)

4. **Field of View (FOV):**
   - Full Frame Sensor: \( 36 \text{mm} \times 24 \text{mm} \)
   - APS-C Sensor: \( 22 \text{mm} \times 15 \text{mm} \)
   - FOV is given by \( \text{FOV} = \frac{206.265 \times \text{sensor size}}{\text{focal length}} \)

### FOV Calculation:
#### APO Refractor (Full Frame Sensor):
- FOV (width): \( \frac{206.265 \times 36}{800} \approx 9.28^\circ \)
- FOV (height): \( \frac{206.265 \times 24}{800} \approx 6.19^\circ \)

#### Newtonian (APS-C Sensor):
- FOV (width): \( \frac{206.265 \times 22}{800} \approx 5.68^\circ \)
- FOV (height): \( \frac{206.265 \times 15}{800} \approx 3.87^\circ \)

### Mosaic Calculation:
To cover the same FOV with the Newtonian setup, we need to calculate how many tiles are required.

#### Area Covered by Each Configuration:
- APO Refractor: \( 9.28^\circ \times 6.19^\circ \approx 57.43 \text{ square degrees} \)
- Newtonian: \( 5.68^\circ \times 3.87^\circ \approx 21.99 \text{ square degrees} \)

#### Number of Tiles for Mosaic:
\[ \text{Number of tiles} = \frac{57.43}{21.99} \approx 2.61 \]

Rounding up, the Newtonian requires 3 tiles to cover the same area as the APO.

### Exposure Time Comparison:
To compare the total exposure time, we must consider the light-gathering power and the number of tiles required.

#### Light-Gathering Efficiency:
- APO Refractor: \( \text{Light gathering power} = 10,263 \text{mm}^2 \)
- Newtonian: \( \text{Light gathering power} = 27,573 \text{mm}^2 \)

The Newtonian gathers approximately \( \frac{27,573}{10,263} \approx 2.69 \) times more light than the APO.

#### Focal Ratio Impact:
- Faster focal ratio (f/4) means more light per unit area.
- Exposure time \( t \) is inversely proportional to the square of the focal ratio:
  - \( \text{Refractor exposure time} \propto (7)^2 = 49 \)
  - \( \text{Newtonian exposure time} \propto (4)^2 = 16 \)

#### Individual Exposure Time Comparison:
\[ \frac{t_{\text{Newtonian}}}{t_{\text{Refractor}}} = \frac{16}{49} \approx 0.33 \]

Since the Newtonian requires 3 tiles, the total exposure time for the Newtonian setup is:
\[ \text{Total exposure time for Newtonian} = 3 \times 0.33 \times t_{\text{Refractor}} = 0.99 \times t_{\text{Refractor}} \]

### Conclusion:
The total exposure time for the finished and stacked image using the 200mm f/4 Newtonian with an APS-C sensor will be approximately the same as the exposure time required for the 114.3mm f/7 APO refractor with a full frame sensor. Despite the need to create a mosaic with the Newtonian, its faster focal ratio and larger effective aperture compensate for the additional images, resulting in nearly equivalent total exposure times for the final image.

Why should one choose a newton with 800mm fl and APS-C, when his/her target preference would require mosaics? Just choose a high resolution telescope with lower FL, like a newton or RASA astrograph and do it in one shot. Bigger RASAs even support full frame which increases the FOV even further and we are talking about f/2 here. Need a better sampling rate? Drizzle. The aperture supports it.

Don't want the hassle with those fast astrographs? Choose an APO, enjoy the convenience and take the light gathering performance hit. It will be fine.

Even the AI says the same:
"Despite the need to create a mosaic with the Newtonian, its faster focal ratio and larger effective aperture compensate for the additional images"

Newton is faster, because of its bigger effective aperture. Aperture is king, simple as.

Anderl:

Tobiasz:

Anderl:

Tobiasz:

Anderl:

Tobiasz:

Anderl:

Tobiasz:

Anderl:
The right way to calculate your t-ratio should be 
(Primary)75x75x3.14 - (secondary)35x35x3.14 = 13.800 = effective light collecting area. 
effective aperture = 13.800/3.14 = 4394.
now calculate the square root of 4394 = 66.3 and multiply it by 2 to get the aperture —> 132.6mm

To get the t-ratio you now need to divide your focal length (600mm) by 132.6 and you get something around t4.5 as an result. 

The real reason refractors are waaay better is because they just look better ;) newtonians are just highly effective astronomically instruments looking like trash cans. 
oh and don’t forget that most apos will be able to illuminate a full frame sensor while even expensive newtonians will already show significant vignetting on apsc. If you look at it that way a f7 apo and an f4 newtonian have similar light collecting abilities. 

cs
andi

I don't know if you're trolling, but we can do the calculations for your "f/7 apo is as fast as the f/4 newton" comparison. I already posted the results on gondolas comparison, so yes a reduced 130mm apo is needed to reach the SAME light gathering power as an entry level newton on the SAME FOV.

Your statement on "significant vignetting on apsc even on expensive newtons" is simply not true and I will ignore that. Newtons can be build like that to fully support an full frame image circle, e.g. TS ONTC.

Let's take your 132mm f/7 apo and compare it to a newton with the same focal length. The most common one would be a 8" F/4 Newton.

# Telescope 1 f/ 4.00 fl= 800mm D=200mm O=42% res=0.95"/p FOV=15.9'x15.9'= 0.85x eoi= 1.96x poi= 3.71x e= 1.61x pe= 1.61x ps= 1.51x os= 1.77x
# Telescope 2 f/ 5.60 fl= 739mm D=132mm O= 0% res=1.03"/p FOV=17.2'x17.2'= 1.17x eoi= 0.51x poi= 0.27x e= 0.62x pe= 0.62x ps= 0.66x os= 0.56x

I had to reduce your APO by 0.8 to get rougly in the ballpark with the focal length. Should be an advantage for the APO here. Same obstruction and transmission as in gondolas comparison. Sensor with 3,7 micron pixels and 80% QE. Who would have thought, the toys get bigger but nothing changes on the light gathering power. The newton is still much faster.

Same FOV, for the same target the newt would acquire 1,77x times more signal on the object than your reduced 132mm apo.

With those few clear nights in central europe, I'd rather have the photon trash can over the slower APO.

Regards

woa woh wa. chill your nuggets ;)
i have asked a few people here and on other astro communities to give feedback about those "full frame capable newton telescopes" and there is almost no scope that can do it. 
the ts options seem to produce vignetting and bad stars, lacerta newton is only good up to apsc (and already vignettes), epsilon works but needs a lot of work, I couldn't find out how good or bad the corner performance of discontinued scopes like asa n8 n10 are but as I can't get one It doesn't matter. 
there are surely newtonians out there that have great edge to edge performance over an full frame area but I think it is true to state that it is way easier to get that with an apo. 
and as the area of an full frame chip is around 2.5 times the area of an apsc sized chip you are also getting around 2.5 times the light, equaling the faster f-ratio of the newton again. 
note that I don't look at same target same fov here. I just look at the light that lands on the sensor area. 

and to state it again: the most important reason to get an apo refractor is because it is looking better. this really should tell you how serious you should take my comment ;)

cs
Andi

"and as the area of an full frame chip is around 2.5 times the area of an apsc sized chip you are also getting around 2.5 times the light, equaling the faster f-ratio of the newton again."

No, this is just wrong. You are not getting more light, you just capture more of the surrounding background, which is low on signal. Per pixel signal does not increase with bigger sensor size, only the FOV.

Your 132mm f/7 APO 0.8 reducer + 6200mm FF vs Sharpstar 150mm f/2.8 astrograph + 2600mm APS-C. Both have roughly the same FOV and the newt should be capable of illuminating the APS-C sensor.

# Telescope 1 f/ 2.80 fl= 420mm D=150mm O=40% res=1.82"/p FOV=189.2'x126.5'= 1.32x eoi= 4.00x poi= 4.34x e= 1.43x pe= 3.36x ps= 3.15x os= 1.02x
# Telescope 2 f/ 5.60 fl= 739mm D=132mm O= 0% res=1.03"/p FOV=164.8'x109.9'= 0.76x eoi= 0.25x poi= 0.23x e= 0.70x pe= 0.30x ps= 0.32x os= 0.98x

Your 2.5 times more light statement is almost correct, for the signal gathering per pixel of the APS-C sensor at least :-D Newton is not faster even, it blows the APO out of the water.

The resolution is a bit worse, but you can drizzle the APS-C subs, because the Newton delivers the appropriate aperture for it. Noise will be less of a concern with 3 times more signal per sub than the APO.

"note that I don't look at same target same fov here. I just look at the light that lands on the sensor area."

Ok, so you want to compare supported image circles, which is a different topic. Still, it does not change the physical characteristics and performance of the telescopes.

you can try to capture the same fov of an 800mm f7 apo (full frame) with an f4 800mm newton (apsc mosaic). Both will more or less require the same time. 
The supported image circle is an often overlooked thing within our hobby. If you use an 1inch sensor on your newton instead of an apsc sized chip you are just wasting light. 
An f7 toa 150 becomes equally fast as an epsilon astrograph once you use his full image circle (100% illumination @70mm) + it will provide better resolution, look better ;) and will be easier 2use.

cs
andi


cs
andi

Sorry, but throwing in random telescopes and now including mosaics does not make it right that a bigger sensor makes the telescope faster. You don't want to compare FOVs but you are talking about FOVs the whole time.

My explanation still stands, in the current market and at equal FOV a Newton will always have more effective aperture available than an APO and therefore will be faster. If a picture will look "better" depends on so much more than the telescope, e.g. post-processing, collimation, pinched optics or seeing.

Lets look what ai has to say and yes i am very aware of the limitations of such.


To perform a detailed analysis of the total exposure time required to achieve the same picture, SNR, and FOV with the two setups—an f/7 800mm APO refractor with a full frame sensor and an f/4 800mm Newtonian with an APS-C sensor—we need to consider several key factors:

1. **Focal Length and Focal Ratio:**
   - Both telescopes have the same focal length (800mm), but different focal ratios (f/7 and f/4).
   - The focal ratio affects the amount of light per unit area reaching the sensor, with the f/4 system being faster.

2. **Aperture Size:**
   - APO refractor: \( \text{Aperture} = \frac{800}{7} \approx 114.3 \text{mm} \)
   - Newtonian: \( \text{Aperture} = \frac{800}{4} = 200 \text{mm} \)
   - Newtonian with secondary obstruction: Effective aperture \( = \sqrt{200^2 - 70^2} \approx 187.3 \text{mm} \)

3. **Light-Gathering Power:**
   - Light-gathering power is proportional to the area of the aperture.
   - APO refractor: \( \text{Area} \approx \pi \times \left(\frac{114.3}{2}\right)^2 \approx 10,263 \text{mm}^2 \)
   - Newtonian: \( \text{Effective area} \approx \pi \times \left(\frac{187.3}{2}\right)^2 \approx 27,573 \text{mm}^2 \)

4. **Field of View (FOV):**
   - Full Frame Sensor: \( 36 \text{mm} \times 24 \text{mm} \)
   - APS-C Sensor: \( 22 \text{mm} \times 15 \text{mm} \)
   - FOV is given by \( \text{FOV} = \frac{206.265 \times \text{sensor size}}{\text{focal length}} \)

### FOV Calculation:
#### APO Refractor (Full Frame Sensor):
- FOV (width): \( \frac{206.265 \times 36}{800} \approx 9.28^\circ \)
- FOV (height): \( \frac{206.265 \times 24}{800} \approx 6.19^\circ \)

#### Newtonian (APS-C Sensor):
- FOV (width): \( \frac{206.265 \times 22}{800} \approx 5.68^\circ \)
- FOV (height): \( \frac{206.265 \times 15}{800} \approx 3.87^\circ \)

### Mosaic Calculation:
To cover the same FOV with the Newtonian setup, we need to calculate how many tiles are required.

#### Area Covered by Each Configuration:
- APO Refractor: \( 9.28^\circ \times 6.19^\circ \approx 57.43 \text{ square degrees} \)
- Newtonian: \( 5.68^\circ \times 3.87^\circ \approx 21.99 \text{ square degrees} \)

#### Number of Tiles for Mosaic:
\[ \text{Number of tiles} = \frac{57.43}{21.99} \approx 2.61 \]

Rounding up, the Newtonian requires 3 tiles to cover the same area as the APO.

### Exposure Time Comparison:
To compare the total exposure time, we must consider the light-gathering power and the number of tiles required.

#### Light-Gathering Efficiency:
- APO Refractor: \( \text{Light gathering power} = 10,263 \text{mm}^2 \)
- Newtonian: \( \text{Light gathering power} = 27,573 \text{mm}^2 \)

The Newtonian gathers approximately \( \frac{27,573}{10,263} \approx 2.69 \) times more light than the APO.

#### Focal Ratio Impact:
- Faster focal ratio (f/4) means more light per unit area.
- Exposure time \( t \) is inversely proportional to the square of the focal ratio:
  - \( \text{Refractor exposure time} \propto (7)^2 = 49 \)
  - \( \text{Newtonian exposure time} \propto (4)^2 = 16 \)

#### Individual Exposure Time Comparison:
\[ \frac{t_{\text{Newtonian}}}{t_{\text{Refractor}}} = \frac{16}{49} \approx 0.33 \]

Since the Newtonian requires 3 tiles, the total exposure time for the Newtonian setup is:
\[ \text{Total exposure time for Newtonian} = 3 \times 0.33 \times t_{\text{Refractor}} = 0.99 \times t_{\text{Refractor}} \]

### Conclusion:
The total exposure time for the finished and stacked image using the 200mm f/4 Newtonian with an APS-C sensor will be approximately the same as the exposure time required for the 114.3mm f/7 APO refractor with a full frame sensor. Despite the need to create a mosaic with the Newtonian, its faster focal ratio and larger effective aperture compensate for the additional images, resulting in nearly equivalent total exposure times for the final image.

Why should one choose a newton with 800mm fl and APS-C, when his/her target preference would require mosaics? Just choose a high resolution telescope with lower FL, like a newton or RASA astrograph and do it in one shot. Bigger RASAs even support full frame which increases the FOV even further and we are talking about f/2 here. Need a better sampling rate? Drizzle. The aperture supports it.

Don't want the hassle with those fast astrographs? Choose an APO, enjoy the convenience and take the light gathering performance hit. It will be fine.

Even the AI says the same:
"Despite the need to create a mosaic with the Newtonian, its faster focal ratio and larger effective aperture compensate for the additional images"

Newton is faster, because of its bigger effective aperture. Aperture is king, simple as.

so you disagree that the 2 telescopes in that scenario need around equally long to get the desired image?

cs
Andi

???

No, never did. You are not getting the point.

I think I explained myself enough and will stop here. I gave you enough examples why reflectors are faster and always will be.