Tobiasz:
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. what point exactly? that you should overlook the image circle a telescope has because only aperture matters? what I have said and still do is that a larger sensor will collect more light as an smaller sensor and that is as universally true as the fact that you get more photons per arc second of sky if your aperture is bigger. you are aware that countless lenses with an f ratio of f1.4 and even some <f1 exist? cs Andi
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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. Epsilons can easily cover a full frame sensor at f/3.3, and far cheaper than equivalent APOs. In fact there is an entire thread about it here on Astrobin. https://www.astrobin.com/forum/c/equipment-forums/takahashi-epsilon-160ed/setting-up-the-e160-ed-for-full-frame-imaging/?page=1 |
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Tobiasz:
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. @ I would love to see this calculation done with everything taken into account. It's not difficult to estimate all of the other obstructions in the system, reflectivity is also known if you know what type of coating was used. There's a huge range there from 89 to 95 percent reflectivity. When I run the numbers for a popular F/4 Newtonian astrograph that takes everything into account I get a total efficiency of 74%. A typical APO refractor runs about 94%. All my original post says is that a smaller aperture refractor will gather as much light as a larger aperture Newtonian. How much smaller depends on how you specify the Newtonian. When you quote the F Ratio of any telescope by using just the geometrical f ratio, without taking light losses into account you really are not painting an accurate picture. Imagine Canon advertising a 200mm F/4 lens that actually turns out to be F/5.6 no one would stand for it yet that is exactly what we accept in the commercial astronomy world. Newts are great scopes and give you a LOT of bang for your buck, they are just not as efficient as a similar sized refractor. I don't think anyone would argue that point. It's just a matter of how much.
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Where is the 94% efficiency for apo comming from? Is flattener included? Is that dublet/triplet or more lenses? I think it's obvious that newton design is less effective then refractor at same aperture, but the aperture gains of 150 newton against 102 apo overhelm those loses. Not talking about higher resolution.APO is more plug&play thingie….
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I based that on a triplet design, 6 air to glass surfaces with a 1% loss at each surface. Even if you add a flattener, you'll still be above 90%.
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Ashraf AbuSara:
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.
Epsilons can easily cover a full frame sensor at f/3.3, and far cheaper than equivalent APOs. In fact there is an entire thread about it here on Astrobin.
https://www.astrobin.com/forum/c/equipment-forums/takahashi-epsilon-160ed/setting-up-the-e160-ed-for-full-frame-imaging/?page=1 They most definitely can. At this point I'm not really sure what the point of this thread is. John Hayes' presentation is fantastic and covers this topic rather well.
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Dark Matters Astrophotography:
Ashraf AbuSara:
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.
Epsilons can easily cover a full frame sensor at f/3.3, and far cheaper than equivalent APOs. In fact there is an entire thread about it here on Astrobin.
https://www.astrobin.com/forum/c/equipment-forums/takahashi-epsilon-160ed/setting-up-the-e160-ed-for-full-frame-imaging/?page=1
They most definitely can. At this point I'm not really sure what the point of this thread is. John Hayes' presentation is fantastic and covers this topic rather well. The point? It's idiot astro-tennis.
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Dark Matters Astrophotography:
Ashraf AbuSara:
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.
Epsilons can easily cover a full frame sensor at f/3.3, and far cheaper than equivalent APOs. In fact there is an entire thread about it here on Astrobin.
https://www.astrobin.com/forum/c/equipment-forums/takahashi-epsilon-160ed/setting-up-the-e160-ed-for-full-frame-imaging/?page=1
They most definitely can. At this point I'm not really sure what the point of this thread is. John Hayes' presentation is fantastic and covers this topic rather well. One quick google search says that, as an example, the epsilon 180 has an light fall off from around 40% at the edges of its full frame circle. A 10 inch lacerta newton paired with a gpu corrector will maybe deliver 100% illumination for half the size of an apsc chip. A toa 130 or 150 has an 100% illumination circle of 70mm.
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Anderl:
Dark Matters Astrophotography:
Ashraf AbuSara:
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.
Epsilons can easily cover a full frame sensor at f/3.3, and far cheaper than equivalent APOs. In fact there is an entire thread about it here on Astrobin.
https://www.astrobin.com/forum/c/equipment-forums/takahashi-epsilon-160ed/setting-up-the-e160-ed-for-full-frame-imaging/?page=1
They most definitely can. At this point I'm not really sure what the point of this thread is. John Hayes' presentation is fantastic and covers this topic rather well.
One quick google search says that, as an example, the epsilon 180 has an light fall off from around 40% at the edges of its full frame circle. A 10 inch lacerta newton paired with a gpu corrector will maybe deliver 100% illumination for half the size of an apsc chip.
A toa 130 or 150 has an 100% illumination circle of 70mm. The Epsilon referred to by the previous poster is the redesigned E160ED. You guys are welcome to go back and forth about this as you'd like. I'm not particularly interested, I do think John's presentation is great though.
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Dark Matters Astrophotography:
Anderl:
Dark Matters Astrophotography:
Ashraf AbuSara:
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.
Epsilons can easily cover a full frame sensor at f/3.3, and far cheaper than equivalent APOs. In fact there is an entire thread about it here on Astrobin.
https://www.astrobin.com/forum/c/equipment-forums/takahashi-epsilon-160ed/setting-up-the-e160-ed-for-full-frame-imaging/?page=1
They most definitely can. At this point I'm not really sure what the point of this thread is. John Hayes' presentation is fantastic and covers this topic rather well.
One quick google search says that, as an example, the epsilon 180 has an light fall off from around 40% at the edges of its full frame circle. A 10 inch lacerta newton paired with a gpu corrector will maybe deliver 100% illumination for half the size of an apsc chip.
A toa 130 or 150 has an 100% illumination circle of 70mm.
The Epsilon referred to by the previous poster is the redesigned E160ED.
You guys are welcome to go back and forth about this as you'd like. I'm not particularly interested, I do think John's presentation is great though.
You are right and to be honest i don’t know why this topic bothers me enough to comment but there is just something about this whole aperture is the only thing that matter’s attitude that triggers me. therefore…  the e160ed has an light fall off about the same size of the before mentioned epsilon 180. like any newton i heard of it can only deliver f3.3 over an way smaller area than its corrected image circle. The f3.3 anyways is more like f3.7 if you calculate the secondary (and other things) in. i guess an super slow toa 130 paired with the 150mp qhy411 would actually deliver more photons than the e160ed paired with a full frame camera. cs andi
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Anderl:
Dark Matters Astrophotography:
Ashraf AbuSara:
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.
Epsilons can easily cover a full frame sensor at f/3.3, and far cheaper than equivalent APOs. In fact there is an entire thread about it here on Astrobin.
https://www.astrobin.com/forum/c/equipment-forums/takahashi-epsilon-160ed/setting-up-the-e160-ed-for-full-frame-imaging/?page=1
They most definitely can. At this point I'm not really sure what the point of this thread is. John Hayes' presentation is fantastic and covers this topic rather well.
A toa 130 or 150 has an 100% illumination circle of 70mm. At f/7.7...
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It looks to me like you guys are more interested in arguing than in learning something. It's clear from many of the statements made here that neither of you guys bothered to look at the presentation that I pointed out to you earlier in this thread. You are arguing about beginner level radiometry, which is not at all controversial. Go look at the presentation and if you still don't understand it, then I didn't do a very good job of explaining it–and it's on me.
John
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Aperture is king if you understand what it allows you to do. It allows you to use a camera with larger pixels (or to bin, but please lord lets not start a debate on binning) while still having a respectable image scale. That is how a larger, but slower telescope is able to beat a smaller but faster telescope in gathered signal when it comes to the same image scale.
At face value, a RedCat51 beats a CDK in signal gathering using the same camera - until you try to get the RedCat51 to the same image scale as the CDK.
This fact does not take FoV into account, or other factors that might be important to the end user. So everyone's job is to use this knowledge and the calculators available to them to select an imaging system that works for them.
In essence, just watch the John Hayes video, lol.
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@ I would love to see this calculation done with everything taken into account. It's not difficult to estimate all of the other obstructions in the system, reflectivity is also known if you know what type of coating was used. There's a huge range there from 89 to 95 percent reflectivity. When I run the numbers for a popular F/4 Newtonian astrograph that takes everything into account I get a total efficiency of 74%. A typical APO refractor runs about 94%. All my original post says is that a smaller aperture refractor will gather as much light as a larger aperture Newtonian. How much smaller depends on how you specify the Newtonian. When you quote the F Ratio of any telescope by using just the geometrical f ratio, without taking light losses into account you really are not painting an accurate picture. Imagine Canon advertising a 200mm F/4 lens that actually turns out to be F/5.6 no one would stand for it yet that is exactly what we accept in the commercial astronomy world. Newts are great scopes and give you a LOT of bang for your buck, they are just not as efficient as a similar sized refractor. I don't think anyone would argue that point. It's just a matter of how much. Yes...you'll see that computation done in the presentation that I pointed out earlier in this thread! All you have to do is go watch it... John
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I agree with John. I usually have a notepad or Word document open to take notes with when I watch his presentations. He's a professor with a PhD in Optical Sciences (and a very nice guy) and does a great job explaining things and translating them to the Astrophotography hobby so you can apply what he's explaining to a domain you already have familiarity with.
Seriously, go watch the presentation and I'll bet you'll take a lot away from it.
Bill
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Dark Matters Astrophotography:
I agree with John. I usually have a notepad or Word document open to take notes with when I watch his presentations. He's a professor with a PhD in Optical Sciences (and a very nice guy) and does a great job explaining things and translating them to the Astrophotography hobby so you can apply what he's explaining to a domain you already have familiarity with.
Seriously, go watch the presentation and I'll bet you'll take a lot away from it.
Bill True. This discussion ended the moment John dropped his presentation link.
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Ashraf AbuSara:
Dark Matters Astrophotography:
I agree with John. I usually have a notepad or Word document open to take notes with when I watch his presentations. He's a professor with a PhD in Optical Sciences (and a very nice guy) and does a great job explaining things and translating them to the Astrophotography hobby so you can apply what he's explaining to a domain you already have familiarity with.
Seriously, go watch the presentation and I'll bet you'll take a lot away from it.
Bill
True. This discussion ended the moment John dropped his presentation link. John Hayes:
@ I would love to see this calculation done with everything taken into account. It's not difficult to estimate all of the other obstructions in the system, reflectivity is also known if you know what type of coating was used. There's a huge range there from 89 to 95 percent reflectivity. When I run the numbers for a popular F/4 Newtonian astrograph that takes everything into account I get a total efficiency of 74%. A typical APO refractor runs about 94%. All my original post says is that a smaller aperture refractor will gather as much light as a larger aperture Newtonian. How much smaller depends on how you specify the Newtonian. When you quote the F Ratio of any telescope by using just the geometrical f ratio, without taking light losses into account you really are not painting an accurate picture. Imagine Canon advertising a 200mm F/4 lens that actually turns out to be F/5.6 no one would stand for it yet that is exactly what we accept in the commercial astronomy world. Newts are great scopes and give you a LOT of bang for your buck, they are just not as efficient as a similar sized refractor. I don't think anyone would argue that point. It's just a matter of how much.
Yes...you'll see that computation done in the presentation that I pointed out earlier in this thread! All you have to do is go watch it...
John, I did watch it and found it very complete. |
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As I am traveler imager, and interested in wide field, now I use two refractors to one target.
Askar 151PHQx0.7 + 6200MM & Askar 130PHQx0.7 + 6200MC
= 735mm f/5 + 700mm f/5.4 (Full Frame)
≒ 735mm f/3.6 full frame
≒ 525mm f/2.6 APS/C drizzle
< bit less than my EdgeHD11 HyperStar but easy setup, no tweaking at dark, less waiting time, clean round stars.
also it can be 151PHQ+6200MM APS/C & 130PHQ+6200MC APS/C for smaller target like M81+82
≒ my TS 200mm f/6 Newtonian for light gathering power, but better wind tolerance, refractors have better resolution.
So far I'm satisfied these wide field configurations, maybe will not go tak e160ED or SharpStar 130HNT.
around 500mm : Vixen VSD90SS + Askar 80PHQx0.76 = about f/3.9
around 300mm : Askar FRA300 + 65PHQx0.7 -> will be SQA55 later this month = about f/3.5
The weak point of "twin refractor" is comet processing, it is hell confusing.
============================================
For DSO imaging, "photons you can get from specific target area (for example 1deg x 1deg)" is calculated only from aperture, so simply bigger is better
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Not to beat the dead horse that this topic has become but I want to pose the question: How do quantify the optical efficacy of a telescope. It's clear that all is not equal so what is accepted and correct way to do it?
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Just answered my own question here. The way to express it is in T Stops. This comes from the film industry and is a measured value. T stop is calculated by dividing the F number by the square root of the percentage efficiency of the system. For example, an F/2 system with an efficiency of 75% gives a T Stop value of 2.3 The T stop value is always larger than the F stop because no system gives 100% efficiency.
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