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

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.

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

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

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

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.

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.

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.

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...

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.

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

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.