Refractors vs SCTs

[Rodd]

I am bored. Its perpetually c;loudy and I have no data to process...I have to do something.  I would like to relate my recent astrophotographic experience, talk a bit about telescopes (if you can't use them, look at them, and if you can't look at them, talk about them!).   My experience sort of goes against the grain, you might say.  I image with high end refractors (Takahashi FSQ 106 and TOA 130, as well as a Televue np101is that I haven't used in years), and a Celestron Edge HD (C11Edge).   I use the same camera for deep space on all scopes (except the TV) so it simplifys the discussion.

The Celestron is by all accounts in a leauge far below the Taks and TV--most would say its not even close.  But here is the strange thing....I LOVE IT!  I am finding imaging more enjoyable and I think the data is of higher quality.    I have been struggling lately with getting everything to work, and its always cloudy, so my output of complete images is much reduced, but the images that I have created with the C11Edge inspire me to want to image more.  Well, I always want to image more, but there seems to be a little extra exceitement.    Yes, some things are a bit more tricky--like hunting for targets if the GOTO doesn't put them in the FOV.  But this hasn't been nerly as bad as I remember it being once the scope is aligned.  I might spend 5 minutes looking for my first focus star of the night.  After that--subsequent stars/targets are put near the center.    Yes, guiding is more difficult--but the mount does the work, not me.  Once the guide settings are optimized and teh system is balanced--guiding is sufficient.  I am luck to have a good mount, I guess.  This issue might well be much more of a problem if I had an inferior mount.  I am ure it would be for me, for I am not a tinkerer.  I hate troubleshooting and tweaking.  It gives me stress.  Others, who like that aspect of this endeavor, would have no issue.  

Seeing is definitely more an an issue, I can't argue that (and conditions in general).  But I am usually able to guide at seeing with teh OAG.  So id seeing is 2.5", my FWHM is near 2.5".  I find that anlything under 3' can render nice images.  Sure, when data is under 2" you can definitely see it.  Seeing is rarely over 3' I find, and when it is, it is usually very obvious that imaging is not in the cards.   I guess this aspect of the comparison is a wash--guiding with the refractors is much more consistent and even--and the focal lengths are such that it is not as critical (except with the TOA native).   Lately after midnight everything is literally soaking wet.  I leave everything out so the sublight can dry it in the morning.  Once or twice my corrector plate fogged up at the worst of times--which tends never to happen with a refractor.  But 20 secs wusing a hair drier solved the problem.  

The consensus is shooting with an SCT at F10 is horrendously slow.  This I do not concurr with.  Before I installed the OAG, I used 20 and 30 sec subs to render images at F10.  It is amazing how much information is in a 20-30 sec sub at F10...becuase of the aperture.  Aprture seems to be the key.  When all else is equal--same focal length, same pixel scale, same everything, aperture is synonimous with speed.  You'll get no more detail shooting at .7 arcsec/pix with the big scope  at F7 than you will shooting at the same pixel scale and focal ratio with the refractor.....but the details you get will be 2 things,1) identical, and 2) FASTER.  I am finbding the data cleaner, and the images more realistic.

Gain might have something to do with it....I started shooting gain 300 when I was taking 20-30 sec subs.  Stars were not blown and images came out decent (granted, it was a paing becuase I neede many hundreds of subs).  I noticed the stacks of 300-400 subs were clean as can be.  It really made processing more enjoyable.  I stuck with gain 300 beacuse my expoosure time for narrowband stayed teh same (300sec) and my exposure time for RGB went to only 120 sec.  Yes, more stars are blown, but I plan to take 10 sec subs to fix that.  Stacks shape up much more quickly than they did with the refractors.  In less than an hour I can have a somewhat nice red stack with the C11Edge--even of nebulae.  For example, I shot the Wizard Nebula with the red filter and 120 sec subs teh other night as a test--waiting for my target to rise.  I only collected 20 subs I think but the stack looks amazing (the target was low, seeing was poor so the FWHM is almost 3--so not great)--but its clean and the signal is amazing. With the refractors, I need much more data to get decent stacks.  I shot the Rosette Nabula in red with a refractor (FSQ native I thin), and a 3 hour stack was in serious need of more data.  I never shot at gain 300 with the refractors, so that could be a factor.

Dont get me wrong--I love my refractors, but I found I had a tendency to want to crop out regions of interest as stand alone images.  With the C11EDge--that is not necessary.  I found that with the refractor data I was never satisfied--I tended to ober stretch to compensate.  Maybe my processimng has improved (hopefully), but I feel it is something more.  F7.7 with a pixel scale of .7 (TOA 130) is tough.  It takes a lot of data and a lot of work to get a galaxy looking nice.  With the C11Edge at F7 and a pixel scale of .4 binned to .8, its like a dream.  

Yes--beginners should start with a refractor....if for no oher reason than to be amazed when they jump to a big scope!  For all the other reasons commonly stated as well--it is easier to get the data (but is it really all that much easier?). 

Well--there you have it.  I have an FSQ 106 that is on its way to Japan (Tak America could not fix the problem), and a TOA 130 being sent to Texas for cleaning and TLC, and.............I DON'T CARE!!!!   I am having a blast (when its clear).  Call me crazy 

So--by way of advice I will say to anyone thinking about big aperture....there is a way to do it without sucumbing to aperture fever.  If you have a mount that can handle the weight (and frankly, my TOA 130 weighs as much as the C11Edge), and you can guide consistently well,and you are able to acquire data consistently with a refractor--do not fear the deep end of the pool.  It is very rewarding.  If seeing is always very poor, you might not want to try--that is the single caveat I will use.

To give an example of images taken with the refractors vs the C11Edge--I have two that are on point

1) The Firewroks Galaxy, and 2) IC410.  

C11Edge--bicolor--only 40 min of OIII and 5 hours of Ha--NEED MUCH MORE!

 

FSQ 106 and TOA 130 (Ha)--20 hours of data,

 

C11Edge--using 30 sec unguided subs--about 12 hours

 

TOA 130, about 12 hours

[nfotis]

I know this is tangential, but may I suggest going to NASA or other institutions and download Hubble and other telescopes data for exercising your processing workflows?

This should keep you busy enough on cloudy nights... 🙂

https://hla.stsci.edu/

https://archive.stsci.edu/missions-and-data/hst

https://www.youtube.com/watch?v=1XR1DQRO69E

 

N.F.

[Rodd]

3 minutes ago, nfotis said:

I know this is tangential, but may I suggest going to NASA or other institutions and download Hubble and other telescopes data for exercising your processing workflows?

This should keep you busy enough on cloudy nights... 🙂

https://hla.stsci.edu/

https://archive.stsci.edu/missions-and-data/hst

https://www.youtube.com/watch?v=1XR1DQRO69E

 

N.F.

It is very complicated to get a full set of data.  I have processed 10-12 galaxies where the data sets were pre organized. So that was fun.  Not as fulfilling as I had hoped.   But thanks for the links.  Maybe I can manage to get an image out of them 

[nfotis]

You may want to check this tutorial from Cuiv on hubblesite, in case you didn't see it already:

 

N.F.

[Rodd]

5 minutes ago, nfotis said:

You may want to check this tutorial from Cuiv on hubblesite, in case you didn't see it already:

 

N.F.

A video. That will be easier then wading through instructions. Thanks 

[newbie alert]

I'm doing the same thing but not with premium scopes but still loving it... my 8 inch was my first proper scope and I have several targets lined up ready for it, inc the tadpoles 

[Deadlake]

11 hours ago, Rodd said:

I am bored. Its perpetually c;loudy and I have no data to process...I have to do something.  I would like to relate my recent astrophotographic experience, talk a bit about telescopes (if you can't use them, look at them, and if you can't look at them, talk about them!).   My experience sort of goes against the grain, you might say.  I image with high end refractors (Takahashi FSQ 106 and TOA 130, as well as a Televue np101is that I haven't used in years), and a Celestron Edge HD (C11Edge).   I use the same camera for deep space on all scopes (except the TV) so it simplifys the discussion.

The Celestron is by all accounts in a leauge far below the Taks and TV--most would say its not even close.  But here is the strange thing....I LOVE IT!  I am finding imaging more enjoyable and I think the data is of higher quality.    I have been struggling lately with getting everything to work, and its always cloudy, so my output of complete images is much reduced, but the images that I have created with the C11Edge inspire me to want to image more.  Well, I always want to image more, but there seems to be a little extra exceitement.    Yes, some things are a bit more tricky--like hunting for targets if the GOTO doesn't put them in the FOV.  But this hasn't been nerly as bad as I remember it being once the scope is aligned.  I might spend 5 minutes looking for my first focus star of the night.  After that--subsequent stars/targets are put near the center.    Yes, guiding is more difficult--but the mount does the work, not me.  Once the guide settings are optimized and teh system is balanced--guiding is sufficient.  I am luck to have a good mount, I guess.  This issue might well be much more of a problem if I had an inferior mount.  I am ure it would be for me, for I am not a tinkerer.  I hate troubleshooting and tweaking.  It gives me stress.  Others, who like that aspect of this endeavor, would have no issue.  

Seeing is definitely more an an issue, I can't argue that (and conditions in general).  But I am usually able to guide at seeing with teh OAG.  So id seeing is 2.5", my FWHM is near 2.5".  I find that anlything under 3' can render nice images.  Sure, when data is under 2" you can definitely see it.  Seeing is rarely over 3' I find, and when it is, it is usually very obvious that imaging is not in the cards.   I guess this aspect of the comparison is a wash--guiding with the refractors is much more consistent and even--and the focal lengths are such that it is not as critical (except with the TOA native).   Lately after midnight everything is literally soaking wet.  I leave everything out so the sublight can dry it in the morning.  Once or twice my corrector plate fogged up at the worst of times--which tends never to happen with a refractor.  But 20 secs wusing a hair drier solved the problem.  

The consensus is shooting with an SCT at F10 is horrendously slow.  This I do not concurr with.  Before I installed the OAG, I used 20 and 30 sec subs to render images at F10.  It is amazing how much information is in a 20-30 sec sub at F10...becuase of the aperture.  Aprture seems to be the key.  When all else is equal--same focal length, same pixel scale, same everything, aperture is synonimous with speed.  You'll get no more detail shooting at .7 arcsec/pix with the big scope  at F7 than you will shooting at the same pixel scale and focal ratio with the refractor.....but the details you get will be 2 things,1) identical, and 2) FASTER.  I am finbding the data cleaner, and the images more realistic.

Gain might have something to do with it....I started shooting gain 300 when I was taking 20-30 sec subs.  Stars were not blown and images came out decent (granted, it was a paing becuase I neede many hundreds of subs).  I noticed the stacks of 300-400 subs were clean as can be.  It really made processing more enjoyable.  I stuck with gain 300 beacuse my expoosure time for narrowband stayed teh same (300sec) and my exposure time for RGB went to only 120 sec.  Yes, more stars are blown, but I plan to take 10 sec subs to fix that.  Stacks shape up much more quickly than they did with the refractors.  In less than an hour I can have a somewhat nice red stack with the C11Edge--even of nebulae.  For example, I shot the Wizard Nebula with the red filter and 120 sec subs teh other night as a test--waiting for my target to rise.  I only collected 20 subs I think but the stack looks amazing (the target was low, seeing was poor so the FWHM is almost 3--so not great)--but its clean and the signal is amazing. With the refractors, I need much more data to get decent stacks.  I shot the Rosette Nabula in red with a refractor (FSQ native I thin), and a 3 hour stack was in serious need of more data.  I never shot at gain 300 with the refractors, so that could be a factor.

Dont get me wrong--I love my refractors, but I found I had a tendency to want to crop out regions of interest as stand alone images.  With the C11EDge--that is not necessary.  I found that with the refractor data I was never satisfied--I tended to ober stretch to compensate.  Maybe my processimng has improved (hopefully), but I feel it is something more.  F7.7 with a pixel scale of .7 (TOA 130) is tough.  It takes a lot of data and a lot of work to get a galaxy looking nice.  With the C11Edge at F7 and a pixel scale of .4 binned to .8, its like a dream.  

Yes--beginners should start with a refractor....if for no oher reason than to be amazed when they jump to a big scope!  For all the other reasons commonly stated as well--it is easier to get the data (but is it really all that much easier?). 

Well--there you have it.  I have an FSQ 106 that is on its way to Japan (Tak America could not fix the problem), and a TOA 130 being sent to Texas for cleaning and TLC, and.............I DON'T CARE!!!!   I am having a blast (when its clear).  Call me crazy 

So--by way of advice I will say to anyone thinking about big aperture....there is a way to do it without sucumbing to aperture fever.  If you have a mount that can handle the weight (and frankly, my TOA 130 weighs as much as the C11Edge), and you can guide consistently well,and you are able to acquire data consistently with a refractor--do not fear the deep end of the pool.  It is very rewarding.  If seeing is always very poor, you might not want to try--that is the single caveat I will use.

To give an example of images taken with the refractors vs the C11Edge--I have two that are on point

1) The Firewroks Galaxy, and 2) IC410.  

C11Edge--bicolor--only 40 min of OIII and 5 hours of Ha--NEED MUCH MORE!

 

FSQ 106 and TOA 130 (Ha)--20 hours of data,

 

C11Edge--using 30 sec unguided subs--about 12 hours

 

TOA 130, about 12 hours

Well I going to cause trouble and say the colour from the 130-TOA is just amazing compared with Edge.
But you would expect that from possibly one of the best colour corrected scopes ever made for the amateur market. 

[newbie alert]

30 minutes ago, Deadlake said:

Well I going to cause trouble and say the colour from the 130-TOA is just amazing compared with Edge.
But you would expect that from possibly one of the best colour corrected scopes ever made for the amateur market. 

Maybe but isn't narrowband false colour ? 

[vlaiv]

39 minutes ago, Deadlake said:

Well I going to cause trouble and say the colour from the 130-TOA is just amazing compared with Edge.
But you would expect that from possibly one of the best colour corrected scopes ever made for the amateur market. 

Except color in the image has very little to do with optics and color correction of the telescope.

In fact pure mirrored systems will simply always be better color corrected than any refractor no matter how well color corrected - because they don't need color correction in the first place.

If your scope is sufficiently color corrected - well, then color is down to sensor used and processing. It stops being related to used scope altogether.

[Rodd]

1 hour ago, Deadlake said:

Well I going to cause trouble and say the colour from the 130-TOA is just amazing compared with Edge.
But you would expect that from possibly one of the best colour corrected scopes ever made for the amateur market. 

Don’t confuse saturation with color.  The amount of color in an image is really processing choice.  It’s a personal thing.  I happen to think the images above from refractors are way over saturated. They don’t look as real to me.  That’s what I meant when I said   It could be my processing has improved.  I just  think the edge images are better images.  

Edited October 6, 2021 by Rodd

[Rodd]

1 hour ago, vlaiv said:

Except color in the image has very little to do with optics and color correction of the telescope.

In fact pure mirrored systems will simply always be better color corrected than any refractor no matter how well color corrected - because they don't need color correction in the first place.

If your scope is sufficiently color corrected - well, then color is down to sensor used and processing. It stops being related to used scope altogether.

Exactly. 

[vlaiv]

1 minute ago, Rodd said:

I just  TV think the edge images are better images.  

There is simple explanation for this:

- larger scopes gather more light and speed of telescope is not tied to F/ratio of telescope as is usually believed - speed is "aperture at resolution". Once working resolution is set - and is often set by seeing + guiding (and use of binning if there is need for that), then aperture wins

- larger aperture produces sharper images for same conditions. Sharpness of the image is combination of three factors - seeing, guiding and aperture size. If first two are kept the same (and they are provided that mount can handle both small and large OTA well) - again aperture wins. Granted - difference that aperture makes will vary depending on other two - in poor seeing, well poor seeing will mask both guiding errors and aperture size and difference will be minimal, but in good seeing with good mount - aperture will make difference that is visible

[Rodd]

2 minutes ago, vlaiv said:

There is simple explanation for this:

- larger scopes gather more light and speed of telescope is not tied to F/ratio of telescope as is usually believed - speed is "aperture at resolution". Once working resolution is set - and is often set by seeing + guiding (and use of binning if there is need for that), then aperture wins

- larger aperture produces sharper images for same conditions. Sharpness of the image is combination of three factors - seeing, guiding and aperture size. If first two are kept the same (and they are provided that mount can handle both small and large OTA well) - again aperture wins. Granted - difference that aperture makes will vary depending on other two - in poor seeing, well poor seeing will mask both guiding errors and aperture size and difference will be minimal, but in good seeing with good mount - aperture will make difference that is visible

All true. Unfortunately a comparison like this is complicated by the fact that processing skills improve with time (I hope).  Some of the image quality difference is due to processing.  

[Rodd]

3 hours ago, newbie alert said:

I'm doing the same thing but not with premium scopes but still loving it... my 8 inch was my first proper scope and I have several targets lined up ready for it, inc the tadpoles 

Very nice. You have split the “neck double”. Not commonly seen.

[Rodd]

1 hour ago, vlaiv said:

larger aperture produces sharper images for same conditions.

I assume you include pixel scale in with "conditions"?   An example would be a 12 inch scope shooting at 1.5 arcsec/pix and a 4" scope shooting at 1.5 arcsec/pix.  Sharpness will be the same (provided 1.5arcsec/pix is not beyond the resolution of a 4" optic (would that be the Dawes limit, or something else?).  The only difference in this case (all else being equal) would be speed of capture (and color correction if one is a refractor and one a mirrored scope).  True?

[vlaiv]

33 minutes ago, Rodd said:

I assume you include pixel scale in with "conditions"?   An example would be a 12 inch scope shooting at 1.5 arcsec/pix and a 4" scope shooting at 1.5 arcsec/pix.  Sharpness will be the same (provided 1.5arcsec/pix is not beyond the resolution of a 4" optic (would that be the Dawes limit, or something else?).  The only difference in this case (all else being equal) would be speed of capture (and color correction if one is a refractor and one a mirrored scope).  True?

No, pixel scale is not included in conditions.

I'm strictly speaking at focal plane of telescope - which one produces sharper image.

In your example - 12" at 1.5"/px vs 4" at 1.5"/px - 12" will produce sharper image - smaller star FWHM, if both scopes are diffraction limited.

There are however cases - usually vastly under sampled - that will result in virtually the same images. Say we have 12" at 8"/px and 4" at 8"/px - they will produce virtually the same image as image resolution is determined solely by (under) sampling here.

Square pixels produce something called pixel blur. In normal cases it is very small and overpowered by other blurs in the image, but with large under sampling - it overpowers all other blurs (similar to seeing - in very poor seeing, it is the seeing that dominates and hides guide errors and aperture size).

[Rodd]

5 minutes ago, vlaiv said:

In your example - 12" at 1.5"/px vs 4" at 1.5"/px - 12" will produce sharper image - smaller star FWHM, if both scopes are diffraction limited.

How can teh resolution be higher if the pixel scale is the same?

[Andrew_B]

4 hours ago, newbie alert said:

Maybe but isn't narrowband false colour ? 

Colour correction still matters for narrowband. The main problem is spherochromatism where the degree of spherical aberration varies with wavelength so you could have a scope that's well corrected in green (as they normally are) but shows severe SA in red or blue.

If it was just longitudinal chromatic aberration then it wouldn't be a problem provided you refocused for each filter.

[vlaiv]

7 minutes ago, Andrew_B said:

Colour correction still matters for narrowband. The main problem is spherochromatism where the degree of spherical aberration varies with wavelength so you could have a scope that's well corrected in green (as they normally are) but shows severe SA in red or blue.

If it was just longitudinal chromatic aberration then it wouldn't be a problem provided you refocused for each filter.

How severe does it need to be to significantly impact DSO images with long exposure?

It will certainly impact planetary performance, but 130mm scope has critical sampling in OIII at 0.4"/px and above images are probably taken at at least x3 that so 1.2"/px or more. Given 2" seeing, is there going to be significant difference if say spherical aberration is as poor as 1/2 wave?

[newbie alert]

3 hours ago, Andrew_B said:

Colour correction still matters for narrowband. The main problem is spherochromatism where the degree of spherical aberration varies with wavelength so you could have a scope that's well corrected in green (as they normally are) but shows severe SA in red or blue.

If it was just longitudinal chromatic aberration then it wouldn't be a problem provided you refocused for each filter.

How's that work as you can assign any of the filters to any of the rgb channels.. Hubble is ha to green, so to my mind that's red end of the spectrum to the green channel

 

[Rodd]

5 minutes ago, newbie alert said:

How's that work as you can assign any of the filters to any of the rgb channels.. Hubble is ha to green, so to my mind that's red end of the spectrum to the green channel

 

Just guessing but I think whats important in this case, would be the ratios of the colors to each other.  If a perfectly corrected scope has green at 500 (just chose this value--its not correct--just an example), and a poorly corrected scope has green ad 440, it will affect the palette differently.  But more importantly, I think that in a poorly coorected scope, the colors come to fiocus at different points - which would make for inferior sharpnness even if one could use Pixel Math to modify the palette.

[newbie alert]

46 minutes ago, Rodd said:

Just guessing but I think whats important in this case, would be the ratios of the colors to each other.  If a perfectly corrected scope has green at 500 (just chose this value--its not correct--just an example), and a poorly corrected scope has green ad 440, it will affect the palette differently.  But more importantly, I think that in a poorly coorected scope, the colors come to fiocus at different points - which would make for inferior sharpnness even if one could use Pixel Math to modify the palette.

Would you visually see that in a image thou

Not everyone uses pixinsight 

[Rodd]

26 minutes ago, newbie alert said:

Would you visually see that in a image thou

Not everyone uses pixinsight 

Ouch!   Even with PI it would no doubt not be seen by others.  But if one was used to working with a certain well corrected scope and then switched and began processing data sets from a poorly corrected scope, it would probably be noticed if the person was a fairly experienced processor and not working by complete trial and error.  

 

 

[Deadlake]

18 hours ago, Andrew_B said:

Colour correction still matters for narrowband. The main problem is spherochromatism where the degree of spherical aberration varies with wavelength so you could have a scope that's well corrected in green (as they normally are) but shows severe SA in red or blue.

If it was just longitudinal chromatic aberration then it wouldn't be a problem provided you refocused for each filter.

Yes, best colour correction and the the TOA can be used with a full frame as well make's it a great imaging scope.

Fastar on the Edge will speed up the scope, however it also has its own limitations, such as blue star bloat.

All machines have plus/negatives. Not that I'm an expert on imaging scopes.