What's point of more than 1300mm - 1500mm focal length for Galaxies and PN?

2.11 John Stone ... · view views · 1 like · Share link
So, I want to buy a longer focal length telescope to image Galaxies and PN and other smaller things. I've been running some numbers and with the IMX571, IMX533 sensors at 3.76u 1300mm gets you 0.6"/px and 1550 gets you 0.5"/px. Using the rule of thumb to sample your seeing at about 3.33x that gives a sky of 2" @ 1300mm and 1.67" @ 1550mm which is about as low as things go where I'm from. Also using the rule of thumb that you need to guide at minimum 1/2 to 1/3 your sampling we need a mount that can guide at 0.3" rms - 0.2" rms for 0.6"/px and 0.25" rms - 0.15" rms for 0.5"/px … And what kind of mounts can do that? 10Micron? AP? Paramount? … all $12K and up, right? Do I really need to buy a $12K mount to sample at my seeing? So what's the point of anyone using an SCT, RC, CC, or any 8"+ Cassegrain on anything but planetary? (I'm assuming they don't have a $12k mount). My reduced 8" SCT is 1450mm @ 0.53"/px and my poor EQ6-R only guides at the absolute best 0.3" rms (typically >=0.42" rms but only in the very best seeing; it goes up to 0.5 rms - 0.7 rms in poor seeing) so I feel like there's no hope. I want to stay away from fast optics since the normal 3 nm filters really don't work below F5/F4 … I suppose a 10" F4 Newt could work with a Paracorr at 1160mm - 0.67"/px, but that' a big beast. Is there a more moderately priced mount that the $12K options above that can guide it at 0.3" rms? Could a Skywatcher CQ350Pro @ $3K or a iOptron CEM70 also at $3K guide at 0.3 rms and handle a galaxy scope? Like

2.11

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So, I want to buy a longer focal length telescope to image Galaxies and PN and other smaller things.

I've been running some numbers and with the IMX571, IMX533 sensors at 3.76u 1300mm gets you 0.6"/px and 1550 gets you 0.5"/px.

Using the rule of thumb to sample your seeing at about 3.33x that gives a sky of 2" @ 1300mm and 1.67" @ 1550mm which is about as low as things go where I'm from.

Also using the rule of thumb that you need to guide at minimum 1/2 to 1/3 your sampling we need a mount that can guide at 0.3" rms - 0.2" rms for 0.6"/px and 0.25" rms - 0.15" rms for 0.5"/px … And what kind of mounts can do that? 10Micron? AP? Paramount? … all $12K and up, right?

Do I really need to buy a $12K mount to sample at my seeing?

So what's the point of anyone using an SCT, RC, CC, or any 8"+ Cassegrain on anything but planetary? (I'm assuming they don't have a $12k mount).

My reduced 8" SCT is 1450mm @ 0.53"/px and my poor EQ6-R only guides at the absolute best 0.3" rms (typically >=0.42" rms but only in the very best seeing; it goes up to 0.5 rms - 0.7 rms in poor seeing) so I feel like there's no hope.

I want to stay away from fast optics since the normal 3 nm filters really don't work below F5/F4 …

I suppose a 10" F4 Newt could work with a Paracorr at 1160mm - 0.67"/px, but that' a big beast. Is there a more moderately priced mount that the $12K options above that can guide it at 0.3" rms?

Could a Skywatcher CQ350Pro @ $3K or a iOptron CEM70 also at $3K guide at 0.3 rms and handle a galaxy scope?

7.30 Jay Hovnanian ... · view views · 1 like · Share link
I could be wrong, John ... but atmospheric conditions are always going to influence guiding accuracy. Here in northern New England, on the best of nights, CEM70 guides at .20-.30" rms. Unfortunately, that is extremely rare. On nights of "above average" atmospheric Transparency and Seeing, the rms is .40-.60". Those numbers, however, only mean that the mount is correcting for some smaller deviation in both axes. Even on "below average" nights, I still get round stars, and I'm sure that detail is compromised to some degree, but then there is BLX, which does a mighty fine job at eliminating imperfections. BTW, my CEM70 handles every bit of weight I load onto it. I suppose the best formula ~ if one has the budget ~ is to place a set-up at some remote site whose atmospheric conditions aren't as compromised as most .... Jay Like

7.30

Jay Hovnanian

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I could be wrong, John ... but atmospheric conditions are always going to influence guiding accuracy. Here in northern New England, on the best of nights, CEM70 guides at .20-.30" rms. Unfortunately, that is extremely rare. On nights of "above average" atmospheric Transparency and Seeing, the rms is .40-.60". Those numbers, however, only mean that the mount is correcting for some smaller deviation in both axes. Even on "below average" nights, I still get round stars, and I'm sure that detail is compromised to some degree, but then there is BLX, which does a mighty fine job at eliminating imperfections. BTW, my CEM70 handles every bit of weight I load onto it.

I suppose the best formula ~ if one has the budget ~ is to place a set-up at some remote site whose atmospheric conditions aren't as compromised as most .... Jay

9.89 andrea tasselli ... · view views · 3 likes · Share link
You won't need those kind of expenditures to have a mean tracking error of around 0.3" (and importantly, AT ALL DECLINATIONS) but you will still need to spend more than your average SW CQ350 or CEM70 would require. Around here only short exposures will bring you below 2" fwhm and in rare nights too, so it won't really be a point. Even in far away places with top of the line mounts and 1/2 meter scopes I can't get better than 1.2"-1.3" fwhm on long (5 min) integrations. While there isn't much I can do about the seeing I can do a lot more about SNR so fast large newts are my answer. Like

9.89

andrea tasselli

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You won't need those kind of expenditures to have a mean tracking error of around 0.3" (and importantly, AT ALL DECLINATIONS) but you will still need to spend more than your average SW CQ350 or CEM70 would require. Around here only short exposures will bring you below 2" fwhm and in rare nights too, so it won't really be a point. Even in far away places with top of the line mounts and 1/2 meter scopes I can't get better than 1.2"-1.3" fwhm on long (5 min) integrations. While there isn't much I can do about the seeing I can do a lot more about SNR so fast large newts are my answer.

4.52 ScottF ... · view views · 1 like · Share link
Something to consider is Active Optic units. Starlight Express https://www.sxccd.com/active-optics/ makes them. It will iron out most bad mount behaviours and provide better overall results. Like

2.11 Tobiasz ... · view views · 1 like · Share link
Something to consider is Active Optic units. Starlight Express https://www.sxccd.com/active-optics/ makes them. It will iron out most bad mount behaviours and provide better overall results. It is a nice piece of kit, but expensive. I use something that goes in the same directon with rapid and small guide corrections but it's software-based and free, "MetaGuide". Improved my HFR and eccentricity of my subs a lot (Mobile setup, 2000mm, 0.76"/pix) Fast and aggressive guiding suits middle-priced GEMs and HD mounts very well. CS Like

2.11

Tobiasz

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Something to consider is Active Optic units. Starlight Express https://www.sxccd.com/active-optics/
makes them. It will iron out most bad mount behaviours and provide better overall results.

It is a nice piece of kit, but expensive. I use something that goes in the same directon with rapid and small guide corrections but it's software-based and free, "MetaGuide". Improved my HFR and eccentricity of my subs a lot (Mobile setup, 2000mm, 0.76"/pix)

Fast and aggressive guiding suits middle-priced GEMs and HD mounts very well.

CS

6.67 GalacticRAVE ... · view views · 7 likes · Share link
couple of reasons, @John Hayes had an excellent talk on this very issue a couple of months ago on the astro imaging channel https://www.youtube.com/watch?v=HiJoqQp1qFI&t=2304s main points: a) seeing/turbulence is a complex beast, which cannot fully be crasped with one number, i.e. at the same seeing a bigger scope still has a higher resolution than a smaller one - but it does not go as fast as 1/D which you would expect from the optics. b) similar the Nyquist limit and the canonical factor 3 for oversampling: it is a guide line, not a rigid limit. In fact, the various deconvolution techniques around seem to benefit considerably from a moderate oversampled signal. c) there is a light gathering factor: a bigger telescope gives you more signal, and photon starved as we are this is always a good thing! at the end, there is a reason why in professional astronomy we are building 10m telescopes (and soon a 40m telescope) ... CS Matthias Edited ... Like

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GalacticRAVE

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couple of reasons, @John Hayes had an excellent talk on this very issue a couple of months ago on the astro imaging channel https://www.youtube.com/watch?v=HiJoqQp1qFI&t=2304s

main points:

a) seeing/turbulence is a complex beast, which cannot fully be crasped with one number, i.e. at the same seeing a bigger scope still has a higher resolution than a smaller one - but it does not go as fast as 1/D which you would expect from the optics.

b) similar the Nyquist limit and the canonical factor 3 for oversampling: it is a guide line, not a rigid limit. In fact, the various deconvolution techniques around seem to benefit considerably from a moderate oversampled signal.

c) there is a light gathering factor: a bigger telescope gives you more signal, and photon starved as we are this is always a good thing!

at the end, there is a reason why in professional astronomy we are building 10m telescopes (and soon a 40m telescope) ...

CS Matthias

Edited ...

7.63 Scott Badger ... · view views · 5 likes · Share link
Jay Hovnanian: I could be wrong, John ... but atmospheric conditions are always going to influence guiding accuracy. Here in northern New England, on the best of nights, CEM70 guides at .20-.30" rms. Unfortunately, that is extremely rare. On nights of "above average" atmospheric Transparency and Seeing, the rms is .40-.60". Those numbers, however, only mean that the mount is correcting for some smaller deviation in both axes. Even on "below average" nights, I still get round stars, and I'm sure that detail is compromised to some degree, but then there is BLX, which does a mighty fine job at eliminating imperfections. BTW, my CEM70 handles every bit of weight I load onto it. I suppose the best formula ~ if one has the budget ~ is to place a set-up at some remote site whose atmospheric conditions aren't as compromised as most .... Jay I'm also in poor seeing central (northern new england) and it's the same for me -- seeing and guiding RMS go hand-in-hand. But.....though the RMS number may look terrible. Stars are still round and fwhm's at 5 min aren't any worse than 4 sec exposures. When the seeing is best (2.5" - 3"), I shoot Lums to maximize detail resolution in the final image, and when it's poor, I shoot RGB since poor resolution isn't as much of an issue if I'm using a higher resolution Lum. There are nights where fwhm's are closer to 2", but that only occurs during a blue moon...... Though I'd love to have better data overall, I enjoy acquisition (outside under the stars) too much to give it up for a remote site. Cheers, Scott Like

7.63

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Jay Hovnanian:
I could be wrong, John ... but atmospheric conditions are always going to influence guiding accuracy. Here in northern New England, on the best of nights, CEM70 guides at .20-.30" rms. Unfortunately, that is extremely rare. On nights of "above average" atmospheric Transparency and Seeing, the rms is .40-.60". Those numbers, however, only mean that the mount is correcting for some smaller deviation in both axes. Even on "below average" nights, I still get round stars, and I'm sure that detail is compromised to some degree, but then there is BLX, which does a mighty fine job at eliminating imperfections. BTW, my CEM70 handles every bit of weight I load onto it.

I suppose the best formula ~ if one has the budget ~ is to place a set-up at some remote site whose atmospheric conditions aren't as compromised as most .... Jay

I'm also in poor seeing central (northern new england) and it's the same for me -- seeing and guiding RMS go hand-in-hand. But.....though the RMS number may look terrible. Stars are still round and fwhm's at 5 min aren't any worse than 4 sec exposures. When the seeing is best (2.5" - 3"), I shoot Lums to maximize detail resolution in the final image, and when it's poor, I shoot RGB since poor resolution isn't as much of an issue if I'm using a higher resolution Lum. There are nights where fwhm's are closer to 2", but that only occurs during a blue moon......

Though I'd love to have better data overall, I enjoy acquisition (outside under the stars) too much to give it up for a remote site.

Cheers,
Scott

8.11 Tony Gondola ... · view views · 2 likes · Share link
The techniques used in lucky imaging can help you here. While it's the standard technique used in lunar/planetary imaging, because of camera and telescope limitations it can't be used completely by deep sky imagers (yet) but you can take advantage of some of it's principles to gain resolution. On the hardware side you want a large aperture with a low f/ratio that still gives you the focal length you need. The idea is you want as much light per-pixel as you can get. The camera should be in the 585 class in terms of read noise because you'll be taking a lot of subs. On the data gathering side of the house you will want to target the nights with the best seeing and use the others for wide field work. Limit your sessions to high object elevations, avoid shooting below 50 degree if you can. Subs should be as short as possible. I can go as short as ten seconds at F/6 without any issues and could likely go shorter. Yes you'll need hardware and storage that can handle a lot of subs for stacking. Siril does that really well, not sure about PI but maybe it's faster now? Depending on your mount, at very short integrations, guiding might do more harm then good so you can probably do without it, dither on-mount and have a re-center trigger in your capture sequence (NINA). The last and most important part is aggressively cull your data. You'll want to only use the very best. It will depend on your targets but I'm talking about culling down to 30% or less. It won't be easy and you'll need a lot of imaging time to get the quality you're after but it's an approach that's worth incorporating, at least in part. Good luck! Like

8.11

Tony Gondola

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The techniques used in lucky imaging can help you here. While it's the standard technique used in lunar/planetary imaging, because of camera and telescope limitations it can't be used completely by deep sky imagers (yet) but you can take advantage of some of it's principles to gain resolution. On the hardware side you want a large aperture with a low f/ratio that still gives you the focal length you need. The idea is you want as much light per-pixel as you can get. The camera should be in the 585 class in terms of read noise because you'll be taking a lot of subs.

On the data gathering side of the house you will want to target the nights with the best seeing and use the others for wide field work. Limit your sessions to high object elevations, avoid shooting below 50 degree if you can. Subs should be as short as possible. I can go as short as ten seconds at F/6 without any issues and could likely go shorter. Yes you'll need hardware and storage that can handle a lot of subs for stacking. Siril does that really well, not sure about PI but maybe it's faster now? Depending on your mount, at very short integrations, guiding might do more harm then good so you can probably do without it, dither on-mount and have a re-center trigger in your capture sequence (NINA). The last and most important part is aggressively cull your data. You'll want to only use the very best. It will depend on your targets but I'm talking about culling down to 30% or less. It won't be easy and you'll need a lot of imaging time to get the quality you're after but it's an approach that's worth incorporating, at least in part.

Good luck!

1.81 Michael Maas ... · view views · 6 likes · Share link
John I would like to add to the conversation, focusing your decision on just seeing conditions provides only part of the answer. Overall image resolution is determined by the seeing conditions, the optical performance of the telescope (Airy Pattern) and the size of the pixels in the camera. The size of the Airy Pattern is a function of the optical diameter, so a larger telescope will produce smaller Airy Disk and thus higher resolution. The observed performance a telescope is determined by the convolution of the scopes resolving power with the seeing conditions. When the seeing conditions are such that the effects of seeing are larger than the Airy Pattern then the resolving power of the telescope is limited by the seeing conditions and as a result the telescope cannot achieve the resolution limit it is capable of. However, when the seeing conditions are such that the effects are smaller than the Airy Disk pattern, it is then the telescope that becomes the limiting factor determining the ultimate resolution. So if you size your scope choice by just average or poor seeing then you are limiting the resolution potential that can be achieved under good or excellent seeing conditions. Complicating this choice is the impact of pixel size which drives the image scale of the optical setup. Pixel sizes smaller than the Airy Pattern result in oversampling which impacts the signal strength impingent upon each pixel. Ideally, the choice of pixel size should be such that the pixel is a bit larger than the Airy Pattern, this results in the maximum energy landing on the pixel. A larger pixel is bad because it collects energy not within the Airy Pattern and this decreases the contrast. A pixel smaller than the Airy Pattern loses energy to adjacent pixels again reduces the energy/signal that should be falling on that pixel. I have four telescopes with the following image scales and analysis at 2 arc seconds of seeing: 80mm Meade Refractor with a focal reducer/field flattener with an image scale of 3.2 arc seconds (384mm FL at f/4.8) which should use 2x or 3x drizzle with 6um pixels to achieve optimum resolution. I used this for extended objects of 100 to 200 arc minutes in size. This configuration is slightly under sampled. 115mm Stellarvue Refractor with a field flattener with an image scale of 1.5 arc seconds (805mm FL at f/7.0) which should use 2x drizzle with 6um pixels to achieve optimum resolution. I used this for extended objects of 50 to 100 arc minutes in size. This configuration is slightly under sampled. 8" Celestron SCT with a 0.7x focal reducer with an image scale of 0.87 arc seconds (1,422 mm FL at f/7), no binning or drizzle is required to achieve maximum detail with this configuration which has some oversampling. I used this for extended objects of 25 to 50 arc minutes in size. 14" Celestron SCT with a 0.7x focal reducer with an image scale of 0.452 arc seconds (2,737 mm FL at f/7.7), I normally run this without any binning but could run with 2 x 2 binning with almost no loss in detail and achieve higher signal levels with just a bit of oversampling. I used this for extended objects of 10 to 20 arc minutes in size. If you really want to image small objects, bigger is better unless seeing is bad. If you want to see how the Modulation Transfer Function of potential configuration is effected by all these parameters checkout RC Astros MFT Analyzer ( https://www.rc-astro.com/mtf-analyzer/ ) Hope this helps Michael Like

1.81

Michael Maas

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John

I would like to add to the conversation, focusing your decision on just seeing conditions provides only part of the answer. Overall image resolution is determined by the seeing conditions, the optical performance of the telescope (Airy Pattern) and the size of the pixels in the camera. The size of the Airy Pattern is a function of the optical diameter, so a larger telescope will produce smaller Airy Disk and thus higher resolution. The observed performance a telescope is determined by the convolution of the scopes resolving power with the seeing conditions. When the seeing conditions are such that the effects of seeing are larger than the Airy Pattern then the resolving power of the telescope is limited by the seeing conditions and as a result the telescope cannot achieve the resolution limit it is capable of. However, when the seeing conditions are such that the effects are smaller than the Airy Disk pattern, it is then the telescope that becomes the limiting factor determining the ultimate resolution. So if you size your scope choice by just average or poor seeing then you are limiting the resolution potential that can be achieved under good or excellent seeing conditions.

Complicating this choice is the impact of pixel size which drives the image scale of the optical setup. Pixel sizes smaller than the Airy Pattern result in oversampling which impacts the signal strength impingent upon each pixel. Ideally, the choice of pixel size should be such that the pixel is a bit larger than the Airy Pattern, this results in the maximum energy landing on the pixel. A larger pixel is bad because it collects energy not within the Airy Pattern and this decreases the contrast. A pixel smaller than the Airy Pattern loses energy to adjacent pixels again reduces the energy/signal that should be falling on that pixel.

I have four telescopes with the following image scales and analysis at 2 arc seconds of seeing:

80mm Meade Refractor with a focal reducer/field flattener with an image scale of 3.2 arc seconds (384mm FL at f/4.8) which should use 2x or 3x drizzle with 6um pixels to achieve optimum resolution. I used this for extended objects of 100 to 200 arc minutes in size. This configuration is slightly under sampled.

115mm Stellarvue Refractor with a field flattener with an image scale of 1.5 arc seconds (805mm FL at f/7.0) which should use 2x drizzle with 6um pixels to achieve optimum resolution. I used this for extended objects of 50 to 100 arc minutes in size. This configuration is slightly under sampled.

8" Celestron SCT with a 0.7x focal reducer with an image scale of 0.87 arc seconds (1,422 mm FL at f/7), no binning or drizzle is required to achieve maximum detail with this configuration which has some oversampling. I used this for extended objects of 25 to 50 arc minutes in size.

14" Celestron SCT with a 0.7x focal reducer with an image scale of 0.452 arc seconds (2,737 mm FL at f/7.7), I normally run this without any binning but could run with 2 x 2 binning with almost no loss in detail and achieve higher signal levels with just a bit of oversampling. I used this for extended objects of 10 to 20 arc minutes in size.

If you really want to image small objects, bigger is better unless seeing is bad. If you want to see how the Modulation Transfer Function of potential configuration is effected by all these parameters checkout RC Astros MFT Analyzer ( https://www.rc-astro.com/mtf-analyzer/ )

Hope this helps

Michael

26.84 John Hayes ... · view views · Share link
Watch the video that Matthias recommended. It goes through a lot of the things you are asking about. - John Like

2.11 John Stone Topic starter ... · view views · Share link
Something to consider is Active Optic units. Starlight Express https://www.sxccd.com/active-optics/ makes them. It will iron out most bad mount behaviours and provide better overall results. ScottF, Wow! Does this device really work? Do you think it could take my 0.5 rms - 0.4 rms performing mount down to about 0.3 rms - 0.2 rms? Does anyone have personal experience with this device? J. Like

8.11 Tony Gondola ... · view views · Share link
It's pricey but it makes sense. Why try and make fine adjustments to the entire mass carried by the mount when you can just tilt a bit of glass. That said, the example image is a little underwhelming. That makes sense since it can only correct for image displacement but not blurring I suspect you could get to a very similar result with short exposures and aggressive culling. Like

5.76 Dave Rust ... · view views · Share link
I’m in Bortle 5 and my 2350mm scope easily reveals more detail than my 540mm scope. High power reflectors do indeed produce images that lack the “pop” of lesser power refractors. Nevertheless, they can show more features. This despite guiding at 0.5 with standard mid-level equipment. In fact, our club’s observatory is equipped with a 9300mm F10 reflector that produces pin point stars. And it’s 80 years old! It’s visual only and there are plans to add a camera. Of course, to make stackable subs, adaptive optics will likely be required since the scope is too massive to be moved in response to rapid atmospheric fluctuations. I’ve been told there are significant challenges for amateur rigs with getting computer controllers that are fast enough to analyze and send meaningful corrections so many times per second, but the idea is certainly promising. Like

5.76

Dave Rust

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I’m in Bortle 5 and my 2350mm scope easily reveals more detail than my 540mm scope. High power reflectors do indeed produce images that lack the “pop” of lesser power refractors. Nevertheless, they can show more features. This despite guiding at 0.5 with standard mid-level equipment.

In fact, our club’s observatory is equipped with a 9300mm F10 reflector that produces pin point stars. And it’s 80 years old! It’s visual only and there are plans to add a camera. Of course, to make stackable subs, adaptive optics will likely be required since the scope is too massive to be moved in response to rapid atmospheric fluctuations.

I’ve been told there are significant challenges for amateur rigs with getting computer controllers that are fast enough to analyze and send meaningful corrections so many times per second, but the idea is certainly promising.

4.52 ScottF ... · view views · Share link
John Stone: Something to consider is Active Optic units. Starlight Express https://www.sxccd.com/active-optics/ makes them. It will iron out most bad mount behaviours and provide better overall results. ScottF, Wow! Does this device really work? Do you think it could take my 0.5 rms - 0.4 rms performing mount down to about 0.3 rms - 0.2 rms? Does anyone have personal experience with this device? J. Yes, it does work. I have that unit and it makes a big difference. I haven't used it lately because it eats up backfocus distance that I couldn't spare. The included adapters were not optimal, but I have since had a custom adapter made that will allow me to use it on my longer focal length scopes. It has to be used with a computer versus ASIAIRs to work. Like

4.52

ScottF

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John Stone:
Something to consider is Active Optic units. Starlight Express https://www.sxccd.com/active-optics/
makes them. It will iron out most bad mount behaviours and provide better overall results.

ScottF,

Wow! Does this device really work? Do you think it could take my 0.5 rms - 0.4 rms performing mount down to about 0.3 rms - 0.2 rms?

Does anyone have personal experience with this device?

J.

Yes, it does work. I have that unit and it makes a big difference. I haven't used it lately because it eats up backfocus distance that I couldn't spare. The included adapters were not optimal, but I have since had a custom adapter made that will allow me to use it on my longer focal length scopes. It has to be used with a computer versus ASIAIRs to work.

2.11 John Stone Topic starter ... · view views · Share link
but I have since had a custom adapter made Scott, What can you tell me about the custom adapter? Where did you get it? What is it like? I'm trying to squeeze something into the 90.3mm +/- 2mm backfocus the SCTCORR-4 gives me. I need a custom M42 x 0.72 Female -> M72.2 x 1.0 Male telescope side and then something to connect the Camera side to a Pegasu OAG (leaving room for the guide camera). None of their diagrams show just the AO device without their OAG attached. Like

2.11

John Stone

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but I have since had a custom adapter made

Scott,

What can you tell me about the custom adapter? Where did you get it? What is it like?

I'm trying to squeeze something into the 90.3mm +/- 2mm backfocus the SCTCORR-4 gives me.

I need a custom M42 x 0.72 Female -> M72.2 x 1.0 Male telescope side and then something to connect the Camera side to a Pegasu OAG (leaving room for the guide camera).

None of their diagrams show just the AO device without their OAG attached.

1.20 MaksPower ... · view views · 1 like · Share link
John Stone: Could a Skywatcher CQ350Pro @ $3K or a iOptron CEM70 also at $3K guide at 0.3 rms and handle a galaxy scope? John, It will work but whether you like the results is another matter. The average FWHM of stars is a function of 3 things - the optical qualities of the scope, the guiding and the seeing, so you're going to be limited by whichever is the worst of those three parameters. Hence guiding better than 0.5 arcsec RMS is not going to matter if your seeing is 1.5 anyway. FWIW I use a CQ350 carrying a 10” f/12 mak (3000mm), controlled by ASIAir, the main and guide camera are an ASI 2600 DUO - and no reducer. Image scale is 0.25 arcsec/pixel. While it may seem slightly crazy to image at 0.25 arsec/pixel, this scope is capable of turning in images with stars 1...1.5 arcsec quote often, and 0.4 arcsec stars is possible in very short exposures for things like double stars and small bright nebulaea - and I don't need to dither or drizzle. Yes its slow but patience can deliver some nice results. https://www.astrobin.com/f74g01/ https://www.astrobin.com/y8n6ro/ Edited ... Like

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MaksPower

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John Stone:
Could a Skywatcher CQ350Pro @ $3K or a iOptron CEM70 also at $3K guide at 0.3 rms and handle a galaxy scope?

John,

It will work but whether you like the results is another matter. The average FWHM of stars is a function of 3 things - the optical qualities of the scope, the guiding and the seeing, so you're going to be limited by whichever is the worst of those three parameters. Hence guiding better than 0.5 arcsec RMS is not going to matter if your seeing is 1.5 anyway.

FWIW I use a CQ350 carrying a 10” f/12 mak (3000mm), controlled by ASIAir, the main and guide camera are an ASI 2600 DUO - and no reducer.
Image scale is 0.25 arcsec/pixel. While it may seem slightly crazy to image at 0.25 arsec/pixel, this scope is capable of turning in images with stars 1...1.5 arcsec quote often, and 0.4 arcsec stars is possible in very short exposures for things like double stars and small bright nebulaea - and I don't need to dither or drizzle.

Yes its slow but patience can deliver some nice results.

https://www.astrobin.com/f74g01/
https://www.astrobin.com/y8n6ro/

Edited ...

5.26 Lorenzo Siciliano ... · view views · Share link
John Stone: (...) Could a Skywatcher CQ350Pro @ $3K or a iOptron CEM70 also at $3K guide at 0.3 rms and handle a galaxy scope? * Oh, yes, it can. I used a Cem70 with a C11 @ 0.45 arcsec/pix without an hassle... Edited ... Like

5.76 Dave Rust ... · view views · Share link
I use the CEM40 with a 9.25 inch Celestron with similar .45 average performance. Scope/camera resolution is theoretically .335… so very close. The atmosphere is likely to make achieving that spec unrealistic anyway. adaptive optics would improve results, but I’m portable, on WiFi, and don’t want to wire in a dedicated computer for the mechanism. Edited ... Like

2.15 Pistachio_Enjoyer ... · view views · Share link
I'll drop my two cents here. I image with an RC 10" and an IMX 571 camera, giving me an image scale of .39"/px. For my mount, I use an old EQ8 and I guide off-axis, so I don't have to worry about guiding scale. When I look at my numbers in PHD2, as long as the guiding error is below .5" rms, my images have an acceptable FWHM that I can later use for processing (I don't process anything with a FWHM > 3.2"). I am limited by atmospheric conditions far more than any mechanical errors in my mount, so I would say having an expensive mount is not necessary, unless you're doing remote imaging, where reliability becomes an extremely important factor as you're not always there to make sure everything runs smoothly. I could probably swap out for a CEM70/CQ350 and be just fine. I suggest you get an off-axis guider if you want to image at very long focal lengths. Like

2.15

Pistachio_Enjoyer

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I'll drop my two cents here. I image with an RC 10" and an IMX 571 camera, giving me an image scale of .39"/px. For my mount, I use an old EQ8 and I guide off-axis, so I don't have to worry about guiding scale. When I look at my numbers in PHD2, as long as the guiding error is below .5" rms, my images have an acceptable FWHM that I can later use for processing (I don't process anything with a FWHM > 3.2"). I am limited by atmospheric conditions far more than any mechanical errors in my mount, so I would say having an expensive mount is not necessary, unless you're doing remote imaging, where reliability becomes an extremely important factor as you're not always there to make sure everything runs smoothly. I could probably swap out for a CEM70/CQ350 and be just fine. I suggest you get an off-axis guider if you want to image at very long focal lengths.

4.52 ScottF ... · view views · 1 like · Share link
John Stone: but I have since had a custom adapter made Scott, What can you tell me about the custom adapter? Where did you get it? What is it like? I'm trying to squeeze something into the 90.3mm +/- 2mm backfocus the SCTCORR-4 gives me. I need a custom M42 x 0.72 Female -> M72.2 x 1.0 Male telescope side and then something to connect the Camera side to a Pegasu OAG (leaving room for the guide camera). None of their diagrams show just the AO device without their OAG attached. I’ll have to see if I can find the info. I know places like precise parts can make custom adapters, but be darn sure you have all your measurements correct before ordering from them. Like

4.52

ScottF

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John Stone:
but I have since had a custom adapter made

Scott,

What can you tell me about the custom adapter? Where did you get it? What is it like?

I'm trying to squeeze something into the 90.3mm +/- 2mm backfocus the SCTCORR-4 gives me.

I need a custom M42 x 0.72 Female -> M72.2 x 1.0 Male telescope side and then something to connect the Camera side to a Pegasu OAG (leaving room for the guide camera).

None of their diagrams show just the AO device without their OAG attached.

I’ll have to see if I can find the info. I know places like precise parts can make custom adapters, but be darn sure you have all your measurements correct before ordering from them.

7.63 Scott Badger ... · view views · Share link
MaksPower: Hence guiding better than 0.5 arcsec RMS is not going to matter if your seeing is 1.5 anyway. That lines up with what I see. My average seeing is about 3” (in the summer, worse in the winter) and though I’m usually guiding at about 0.6 - 0.7” RMS in those conditions (2350mm FL), it has to go past 1” before there’s much direct effect on the subs. A bigger issue, that’s plaguing me right now, is getting disparate DEC and RA errors. Even if total RMS is rockstar, it won’t be round-star…. Regarding aggressive culling, to a point, I find more is better than best. Increased signal helps throughout processing, and particularly with BlurX, so any sacrifice in resolution can be regained. Cheers, Scott Like

7.63

Scott Badger

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MaksPower:
Hence guiding better than 0.5 arcsec RMS is not going to matter if your seeing is 1.5 anyway.

That lines up with what I see. My average seeing is about 3” (in the summer, worse in the winter) and though I’m usually guiding at about 0.6 - 0.7” RMS in those conditions (2350mm FL), it has to go past 1” before there’s much direct effect on the subs. A bigger issue, that’s plaguing me right now, is getting disparate DEC and RA errors. Even if total RMS is rockstar, it won’t be round-star….

Regarding aggressive culling, to a point, I find more is better than best. Increased signal helps throughout processing, and particularly with BlurX, so any sacrifice in resolution can be regained.

Cheers,
Scott

7.63 Scott Badger ... · view views · Share link
=16pxI’ll have to see if I can find the info. I know places like precise parts can make custom adapters, but be darn sure you have all your measurements correct before ordering from them. Precise parts is a great service, and it helps that they’re familiar with astro imaging, but if there’s much complexity to the part, using a local machine shop that you can talk with directly reduces the risk of it not being made as you expect. Cheers, Scott Like

7.63

Scott Badger

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=16pxI’ll have to see if I can find the info. I know places like precise parts can make custom adapters, but be darn sure you have all your measurements correct before ordering from them.

Precise parts is a great service, and it helps that they’re familiar with astro imaging, but if there’s much complexity to the part, using a local machine shop that you can talk with directly reduces the risk of it not being made as you expect.

Cheers,
Scott

8.11 Tony Gondola ... · view views · Share link
Scott Badger: MaksPower: Hence guiding better than 0.5 arcsec RMS is not going to matter if your seeing is 1.5 anyway. That lines up with what I see. My average seeing is about 3” (in the summer, worse in the winter) and though I’m usually guiding at about 0.6 - 0.7” RMS in those conditions (2350mm FL), it has to go past 1” before there’s much direct effect on the subs. A bigger issue, that’s plaguing me right now, is getting disparate DEC and RA errors. Even if total RMS is rockstar, it won’t be round-star…. Regarding aggressive culling, to a point, I find more is better than best. Increased signal helps throughout processing, and particularly with BlurX, so any sacrifice in resolution can be regained. Cheers, Scott It's certainly a game of opposing factors. You can cull deeply and improve sharpness or you can cull minimally and improve S/N in the faint areas. On guiding though, I suspect that if RA and DEC errors are balanced, you can get round stars and still have a effect on resolution depending on the total guiding RMS. Meaning that the quality of any given subframe would be indicated by FWHM and roundness. Like

8.11

Tony Gondola

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Scott Badger:
MaksPower:
Hence guiding better than 0.5 arcsec RMS is not going to matter if your seeing is 1.5 anyway.

That lines up with what I see. My average seeing is about 3” (in the summer, worse in the winter) and though I’m usually guiding at about 0.6 - 0.7” RMS in those conditions (2350mm FL), it has to go past 1” before there’s much direct effect on the subs. A bigger issue, that’s plaguing me right now, is getting disparate DEC and RA errors. Even if total RMS is rockstar, it won’t be round-star….

Regarding aggressive culling, to a point, I find more is better than best. Increased signal helps throughout processing, and particularly with BlurX, so any sacrifice in resolution can be regained.

Cheers,
Scott

It's certainly a game of opposing factors. You can cull deeply and improve sharpness or you can cull minimally and improve S/N in the faint areas. On guiding though, I suspect that if RA and DEC errors are balanced, you can get round stars and still have a effect on resolution depending on the total guiding RMS. Meaning that the quality of any given subframe would be indicated by FWHM and roundness.

1.20 David Russell ... · view views · 3 likes · Share link
John Stone: So, I want to buy a longer focal length telescope to image Galaxies and PN and other smaller things. Could a Skywatcher CQ350Pro @ $3K or a iOptron CEM70 also at $3K guide at 0.3 rms and handle a galaxy scope? Hi John. I recently had the same questions as you, so I will detail my background, thoughts and solutions. I shot Nebula for a couple of years, with my Stellarvue 80mm and Takahashi FSQ106 refractors, before I decided I wanted to shoot Galaxies. I considered many OTAs, and I ended up getting the GSO RC10 as a specialist galaxy OTA. I shoot at 1484mm and F5.9 by reducer spacing. I considered a few mounts and bought a CEM-120, as nothing premium is actually available here in New Zealand. the CEM-120 was chosen mainly for the weight that I would be carrying, and not specifically for guiding specs. Something that does not get mentioned too much, that I feel is important, is the impact that one direction guiding can have on total RMS. I managed to bring down my total Guide RMS numbers significantly by enabling one direction guiding. My theory is as follows. if the DEC axis is guiding North and South, it will spend much time wobbling around, chasing seeing variations, often far more than reacting to genuine star movement. this is especially true if the guide exposures are very short, if the focal length is long, and if the seeing is poor. my solution is as follows. 1. get a Polar Alignment of something less than 30 arc seconds. at 1485mm and above accuracy with PA becomes important. 2. make sure the mount is balanced East heavy, so the RA motor is constantly engaged on the worm gear. 3. do a PHD2 guiding assistant run once you are on the target, and identify the North/South DEC drift direction. you may need to run GA for about 4 or 5 minutes to identify this if the PA is very good. 4. enable one direction guiding in the direction that goes against the DEC drift. So if the Guide star is slowly drifting North, then the one direction guiding would be in the South direction. 5. slow down those guide exposures, as you don't want to chase seeing fluctuations, but you do want to chase genuine drift. I now use 4.5 or 5 second guide exposures as standard, and I always use one direction guiding. if the PA is close to perfect you will find the needed DEC movements are very small, so the actual Dec guide pulses will be very small too. 6. watch the PHD2 DEC corrections, and if the plot goes over the line (significantly) reduce the DEC aggression setting. an advantage to one direction guiding, is that it almost completely nullifies DEC backlash, as the DEC axis is only heading in one direction, either North or South, and therefore there is no worm reversal. DEC backlash is basically eliminated, and thats a good thing, especially if backlash is considerable. Some experts have disagreed with my methods, but I have managed to get my CEM-120 RMS to very low numbers on nights of good seeing. Good Luck. Dave Edited ... Like

1.20

David Russell

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John Stone:
So, I want to buy a longer focal length telescope to image Galaxies and PN and other smaller things.

Could a Skywatcher CQ350Pro @ $3K or a iOptron CEM70 also at $3K guide at 0.3 rms and handle a galaxy scope?

Hi John. I recently had the same questions as you, so I will detail my background, thoughts and solutions.

I shot Nebula for a couple of years, with my Stellarvue 80mm and Takahashi FSQ106 refractors, before I decided I wanted to shoot Galaxies.
I considered many OTAs, and I ended up getting the GSO RC10 as a specialist galaxy OTA. I shoot at 1484mm and F5.9 by reducer spacing.

I considered a few mounts and bought a CEM-120, as nothing premium is actually available here in New Zealand.
the CEM-120 was chosen mainly for the weight that I would be carrying, and not specifically for guiding specs.

Something that does not get mentioned too much, that I feel is important, is the impact that one direction guiding can have on total RMS.
I managed to bring down my total Guide RMS numbers significantly by enabling one direction guiding. My theory is as follows.

if the DEC axis is guiding North and South, it will spend much time wobbling around, chasing seeing variations, often far more than reacting to
genuine star movement. this is especially true if the guide exposures are very short, if the focal length is long, and if the seeing is poor.

my solution is as follows.

1. get a Polar Alignment of something less than 30 arc seconds. at 1485mm and above accuracy with PA becomes important.
2. make sure the mount is balanced East heavy, so the RA motor is constantly engaged on the worm gear.
3. do a PHD2 guiding assistant run once you are on the target, and identify the North/South DEC drift direction.
you may need to run GA for about 4 or 5 minutes to identify this if the PA is very good.

4. enable one direction guiding in the direction that goes against the DEC drift.

So if the Guide star is slowly drifting North, then the one direction guiding would be in the South direction.

5. slow down those guide exposures, as you don't want to chase seeing fluctuations, but you do want to chase genuine drift.
I now use 4.5 or 5 second guide exposures as standard, and I always use one direction guiding.

if the PA is close to perfect you will find the needed DEC movements are very small, so the actual Dec guide pulses will be very small too.

6. watch the PHD2 DEC corrections, and if the plot goes over the line (significantly) reduce the DEC aggression setting.

an advantage to one direction guiding, is that it almost completely nullifies DEC backlash, as the DEC axis is only heading in one direction, either
North or South, and therefore there is no worm reversal. DEC backlash is basically eliminated, and thats a good thing, especially if backlash is considerable.

Some experts have disagreed with my methods, but I have managed to get my CEM-120 RMS to very low numbers on nights of good seeing.

Good Luck. Dave

Edited ...

8.11 Tony Gondola ... · view views · 1 like · Share link
I think that's an interesting approach and one I've been thinking for trying for while now. Glad to see that someone got good results from the method as it makes prefect sense. Like

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