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Deep sky imaging with large telescopes - Pointless?*

Askar FRA500 Acquisition Deep Sky NGC6888
andrea tasselliMichael E.John NobleArun HAnderl
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Michael E. avatar
Michael E. avatar
I know the title may sound provocative, but recently I compared two images of the same target (NGC6888) that I captured with two different telescopes. 
One is my beloved Zeiss Meniscas, a Maksutov with 180mm/1800mm, and the other is my new acquisition, the Askar FRA500 refractor with 90mm/500mm. 
I used the QHY268mc camera and the Optolong L-Extreme NB filter for both. 

I was curious about how much worse the resolution would be in the image captured with the smaller refractor.  
To make the comparison fair and possibly even push the resolution a bit further (to avoid undersampling) I applied 2x drizzling to the Askar. More on this later. 

Here are the two images. The image captured with the small Askar was cropped and rotated so that they can be compared easily.  
Both images were processed in a similar way and yes, I did use BlurXterminator.  

Askar_cropped.jpg
Zeiss.jpg

For reference of the field of view here also the entire image of the refractor in full resolution (unfortunately max. 5MB allowed here) 
Askar_drizzled_fullres.jpg

Could you tell which image was captured with which telescope without being labeled?  
In other words, I was surprised the small Askar's image seems to match the resolution of the larger Zeiss. And it's not like the Zeiss produces blurry images! 

I know it certainly all comes down to guiding and seeing conditions! 
Unfortunately I cannot remember the conditions then plus the two imaging sessions occurred on different nights in May (Zeiss) and September (Askar).  
Also, the exposure times of the subs were different, with 108 x 10' (Zeiss) and 136 x 5' (Askar), which of course doesn't help the Zeiss either besides being very heavy (17kg equipped). 

Given the mediocre seeing conditions here in Switzerland (which I cannot change) and the not always optimal guiding results of my mount  (this at least money could solve - I'm usually somewhere between 0.8 - 1.2" RMS), I wonder if I should continue using the Zeiss for deep sky imaging. 

==> Has anyone else made the same comparison and reached the same conclusion? 
==> Do you think that using a better mount would make a significant difference? 

* Please note that by mentioning large (big aperture) scopes, I explicitly refer to resolving power, not the light-gathering ability, where a telescope with a larger aperture (regardless of the F-ratio) should always provide a benefit if the same field of view is being compared (but I might be wrong here - perhaps the effects of oversampling work against my understanding). 

---------------------------------

As mentioned earlier, I would also like to know your opinion on the applied 2x drizzling (yes, I dithered :-)) with the equipment I used.  
Although the Askar's image might only be slightly undersampled with 1.55"/px, I wonder if drizzling still benefits from a "real" improvement in resolution.  
The Zeiss is quite oversampled with 0.43"/px, so there is no need to apply drizzling here. 

Here are two heavily cropped images of a section of M31. I only applied NoiseXT and BlurXT for processing, so no pretty pictures here :-) 
M31_compare.jpg

And another cropped comparison with the above mentioned NGC6888 
NGC6888_compare.jpg

Here M31 also in fullres for comparison:
M31_Drizzled.jpg
M31_not_drizzled.jpg

I have the impression that the resolution indeed benefits from drizzling although mostly noticable on stars. 
Or is it just an artificial improvement by increasing the number of pixels? What do you think?  
I also observe that the drizzled image appears to be darker, as spreading the same amount of light information to more pixels is a tradeoff. Does that make sense? 

==> What are your thoughts on this, and do you occasionally apply drizzling on deep sky targets as well?  


Sorry - perhaps I introduced too many topics at once... 


cs, michael
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andrea tasselli avatar
andrea tasselli avatar
On the comparison images one (the left side) is clearly the best in terms of resolution but both are heavily affected by processing and jpeg compression which makes an honest comparison difficult. Plus, all this processing doesn't help either.

All this to say that in my experience no amount of drizzling or deconvolution is a substitute for actual aperture/resolution.
John Hayes avatar
John Hayes avatar
This isn't how to compare the capabilities of different scopes.  You need to compare raw data taken under identical conditions--ideally at the same time, using the same sensor sampling.  Even then, you might not see a huge difference unless you make sure that focus is perfect and the seeing is good (below about 1.5").  Using the shortest possible exposures will help to mitigate seeing fluctuations.  Put both scopes on a tight double (~1.0"-2.0") and take a bunch of exposures with each scope.  Then compare the best images with each scope.  M13 might also be a good target if you can get the exposures right.

If you really doubt the benefit of aperture, pick any of Adam Block's images taken with the 32" scope on Mt Lemon and try to match his results with your 90 mm scope.  In that case, you can even compare processed results if you like.

John
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Arun H avatar
Arun H avatar
Michael:
==> Has anyone else made the same comparison and reached the same conclusion? 
==> Do you think that using a better mount would make a significant difference? 

* Please note that by mentioning large (big aperture) scopes, I explicitly refer to resolving power, not the light-gathering ability,


If you are talking about purely resolving power and not light gathering ability - that is a convolution of seeing, guiding, and diffraction. The diffraction limited resolving power of a 90mm scope is ~1.5 arc seconds. So it is certainly within the realm of possibility that bad seeing and bad guiding can contribute to the overall loss of resolution to the point where there isn't much difference between that and a larger scope. And it would be even more probable that that is the case if the mount is not suited to the increased weight of the heavier larger aperture scope. Your guiding numbers are high for a 1800mm focal length scope (your 180mm should theoretically resolve at 0.77 arc seconds, and your guiding numbers are obviously not  in line with that, not even counting excursions which are likely much larger than the RMS values), so I am not surprised you are not seeing much difference in detail. 

From my experience, I see clear and repeatable increase in detail with a 200mm reflector versus a very good 92mm refractor. Yes, the mount matters, a great deal.
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Kevin Morefield avatar
Kevin Morefield avatar
To put it simply, if you are seeing limited with the smaller scope, getting a bigger scope will not improve resolution.  

Many, many people use scopes that are larger than necessary for the typical seeing they experience at their imaging site.  

That said, your seeing would need to routinely be pretty bad to not benefit from a scope larger than 90mm in aperture.  There may be other issues at play limiting your resolution.   And, as others have said, this is not the way to compare the resolution on the two scopes.  

Kevin
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Michael E. avatar
Michael E. avatar
Thank you for your replies.
Indeed that's not how to compare the theoretical abilities of two scopes.
I am certainly aware of the fact that my tiny refractor will never be able to produce the same level of detail in the images of i.e. the moon (with short exp./lucky imaging) which I took with the larger Maksutov, even if I drizzeld the hell out of it - physics can't be tricked.

However, I have realized that bad seeing conditions might be less (I know guiding then get's worse too) apparent at deepskyimaging than when imaging bodies in our solar system - at least that was my experience. In the end the stars might appear round but are blurry.
So the tricky part will be to determine if the night will satisfy the potential of the larger scope. The precision of the seeing forecast is not always accurate and you only find out when the scope is already set up.
@Hans: indeed, there is a significant improvement.

That being said, I'm already looking foreward to using my Mak with the newly installed autofocusser and, perhaps one day, also a new mount?? So, to be continued… :-)
GoldfieldAstro avatar
GoldfieldAstro avatar
When we first started getting into imaging years ago it was with a 5" APO and KAF-8300 and at the time the absolute best seeing that we thought we got was 2" because that is the best that we could achieve with the gear that we had. This ultimately was due to the gear combination that was being used. Imaging at 1.22"/pixel was really what was limiting detail but we didn't mind. After cycling through a few more refractors and cameras over the years thinking that we had pretty average seeing we purchased a RH200 to make the use of the seeing conditions that we thought we had.

Not long later we purchased an older Mewlon 250 with the idea of using it primary for visual but attempted some imaging with it and to our surprise our seeing was considerably better than we'd ever imagined. So far the best we've managed is an exposure measuring at 1.05" but with deconvolution we've been able to resolve sub-arc second features. It's normally around 1.4-1.8" with patches lower. Sometimes long patches lower.

@Kevin Morefield really has hit the nail on the head though. If you are seeing limited with your telescope then there really isn't much that you can do about it. You may very well be limited by your mount (higher guiding RMS) which isn't entirely uncommon with some of the cheaper mounts. Sometimes even the guiding setting cause havoc.

I would suggest to continue using both. Use the Zeiss when you want to image something smaller because it does do a better job and use the refractor when you want a wider vista.
Karl Perera avatar
Karl Perera avatar
It's always difficult to compare two images taken with different equipment on different nights. To compare we need to keep as much as possible the same and then we can justifiably compare.
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Howard Richard avatar
Howard Richard avatar
What mount are you using? Is it up to the task of the heavier scope?
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Matthew Proulx avatar
Matthew Proulx avatar
You call 0.8"-1.2" mediocre lol. I am at 2-3" average and I still can get a better image with my 300mm RC than my Tak120 could ever dream of.
Michael E. avatar
Michael E. avatar
What mount are you using? Is it up to the task of the heavier scope?

It's a Zeiss 1b Mount that is more than 30 years old and was manufactured in East Germany. 
It has been modified to be used with stepper motors. Although it is generally of high quality, it was not intended for astrophotography. 
When the conditions are good (i.e., good seeing and no wind), it can produce quite good guiding results with an accuracy of well below 1". 
However, as you already assumed, the results are mostly better with my smaller scope. Therefore, a more suitable mount would certainly help improve the performance and show more of a difference.
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Michael E. avatar
Michael E. avatar
Matthew Proulx:
You call 0.8"-1.2" mediocre lol. I am at 2-3" average and I still can get a better image with my 300mm RC than my Tak120 could ever dream of.

That is indeed astonishing. According to the book, this result is quite difficult for me to comprehend unless you are referring to planetary photography or generally mean lucky imaging.
Matthew Proulx avatar
Matthew Proulx avatar
Michael:
Matthew Proulx:
You call 0.8"-1.2" mediocre lol. I am at 2-3" average and I still can get a better image with my 300mm RC than my Tak120 could ever dream of.

That is indeed astonishing. According to the book, this result is quite difficult for me to comprehend unless you are referring to planetary photography or generally mean lucky imaging.



I do neither of those two types of imaging. It's not astonishing at all, it's simple physics. A 300 mm aperture has 9 times the light grasp of a 100 mm aperture when imaging at the same resolution. You might not notice much on bright objects but when you start getting to the fainter stuff and into 3nm narrowband on galaxies it's not even comparable. The amount of time you would need to show the same detail is unrealistic.
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andrea tasselli avatar
andrea tasselli avatar
I would add that the resolution potential of a 300mm aperture scope, assuming diffraction limited optics, is nearly 3 times that of a 120mm. When performing deconvolution the difference will show through by the wagonloads.
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Michael E. avatar
Michael E. avatar
Your statements regarding the light gathering ability and theoretical resolution of telescopes make complete sense to me. 
However, I'm still curious about how the larger 300mm telescope is able to outperform the 100mm telescope only in terms of resolution, considering the mentioned guiding accuracy of just 2-3". Maybe your guiding accuracy is an understatement. :-)
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andrea tasselli avatar
andrea tasselli avatar
I guess Matthew meant seeing, not guiding accuracy. But I could be wrong…
Matthew Proulx avatar
Matthew Proulx avatar
Michael:
Your statements regarding the light gathering ability and theoretical resolution of telescopes make complete sense to me. 
However, I'm still curious about how the larger 300mm telescope is able to outperform the 100mm telescope only in terms of resolution, considering the mentioned guiding accuracy of just 2-3". Maybe your guiding accuracy is an understatement. :-)

My guiding accuracy is around 0.3", seeing however is 2-3" average. If by 0.8-1.2" you said earlier was guiding then.. disregard that. That's pretty bad by any standard when using larger aperature and is your limiting factor.
Michael E. avatar
Michael E. avatar
Thanks for the clarification. This way it makes sense to me.

PS: I suppose that was precisely the answer I was seeking. So even with average seeing, a large telescope combined with a suitable mount is definitely not pointless!
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Jerry Gerber avatar
Jerry Gerber avatar
Michael:
Thank you for your replies.
Indeed that's not how to compare the theoretical abilities of two scopes.
I am certainly aware of the fact that my tiny refractor will never be able to produce the same level of detail in the images of i.e. the moon (with short exp./lucky imaging) which I took with the larger Maksutov, even if I drizzeld the hell out of it - physics can't be tricked.

However, I have realized that bad seeing conditions might be less (I know guiding then get's worse too) apparent at deepskyimaging than when imaging bodies in our solar system - at least that was my experience. In the end the stars might appear round but are blurry.
So the tricky part will be to determine if the night will satisfy the potential of the larger scope. The precision of the seeing forecast is not always accurate and you only find out when the scope is already set up.
@Hans: indeed, there is a significant improvement.

That being said, I'm already looking foreward to using my Mak with the newly installed autofocusser and, perhaps one day, also a new mount?? So, to be continued... :-)

There are some things in life where bigger is better.   Computer monitor screens.  Musical keyboards.   Telescope aperture for resolving galaxies 200 million light years away.

I decided after purchasing an Esprit 100ED refractor and an Edge 8 that for me, the extra focal length isn't worth the time I have to spend collimating, waiting for seeing conditions to be good enough, focusing without a focus lock and not getting the crisp sharp stars and contrast I am getting with the refractor.  So I bought the Askar 130mm with a FL of 1000mm and decided to limit my focal length to 1000.  This is a hobby for me, it's not my life's work.   There are thousands of objects to photograph with the 4 focal lengths I have with 2 scopes (358mm, 550mm, 700mm and 1000mm with reducers).  That's enough for me.  I'll leave the small galaxies to others who can afford the time and money to build an observatory with a permanent setup and to NASA, who can afford to put telescopes in space.  I am happy setting limits on this pursuit, otherwise it will be an endless rabbit hole of more and more time and money.  I like setting limits.  Creativity and productivity, in my experience, have always depended upon setting limits.   One man's meat is another man's poison.
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Joe Linington avatar
Joe Linington avatar
Jerry Gerber:
There are some things in life where bigger is better.   Computer monitor screens.  Musical keyboards.   Telescope aperture for resolving galaxies 200 million light years away.

What do you mean, I love my little MODX 61. Wait, that's my top, smallest keyboard in a 2+1 stage rig. OK, bigger is better.
Jerry Gerber avatar
Jerry Gerber avatar
Joe Linington:
Jerry Gerber:
There are some things in life where bigger is better.   Computer monitor screens.  Musical keyboards.   Telescope aperture for resolving galaxies 200 million light years away.

What do you mean, I love my little MODX 61. Wait, that's my top, smallest keyboard in a 2+1 stage rig. OK, bigger is better.

I have the MODX 61 too Joe, it's a fantastic synthesizer.  I've always limited my studio keyboards to 61 keys,  It has no effect on sequencing at any octave.  When I want 88 keys I play the piano! 
John Noble avatar
John Noble avatar
I'm sure many of you suffer from the same urges as I do to buy new stuff. So this post kicked off a frequent urge I have to buy myself a 10" f4 Newtonian. Why because I live in a light polluted suburb of Detroit an I'm looking for something with fast optics to help me go a bit deeper into planetary nebula and fainter fuzzies.  Anyway right now I do most of my imaging between 200 mm and 700 mm at f4/f5 so my logic goes surely something at 1000 mm and f4 is going to be the next step. Then  I thought let's look at the data I have from two years of imaging here in Michigan/Ohio in the back yard and at dark sites and, see what it tells me. It told me exactly what my astro head has been telling my astro heart for the last year. We rarely have conditions here which justify greater pixel resolution than that which I already get from my favorite set up which is the WO FLT 132 at f5 (1.15"/pixel).

I took a whole bunch of data and measured the FWHM in PI for all the scopes and mounts I've used and it was pretty clear the FLT 132 at f5 performed the best over multiple targets even compared to using it at f7 (they are probably the same with the seeing etc limiting). The Edge HD8 that I had came second to the WO but probably because I didn't have it optimized etc etc but then I didn't have to optimize the FLT it came as is (that's why its twice the price). The one thing I did find interesting was that my old CGME was actually doing pretty well not as good as the CEM 70 but by no means as bad as I thought.

Anyway if I lived somewhere with better seeing I'm sure I'd get significant benefit from a longer focal length but for my location it just isn't going to worksmile

So I'm going to leave that Skywatcher Quatro 10" on the shelf at Agena for someone else who can really use it.

CS

John

P.S As far as the musical instruments are concerned I'm drums man myself and have recently found more joy in a 4 piece jazz cocktail kit than pretending to be Neil Peart (RIP my hero) - bigger is not always best that's for sure.
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Joe Linington avatar
Joe Linington avatar
Jerry Gerber:
Joe Linington:
Jerry Gerber:
There are some things in life where bigger is better.   Computer monitor screens.  Musical keyboards.   Telescope aperture for resolving galaxies 200 million light years away.

What do you mean, I love my little MODX 61. Wait, that's my top, smallest keyboard in a 2+1 stage rig. OK, bigger is better.

I have the MODX 61 too Joe, it's a fantastic synthesizer.  I've always limited my studio keyboards to 61 keys,  It has no effect on sequencing at any octave.  When I want 88 keys I play the piano! 

I only play live, no studio work really and my rig is an 88 key weighted Casio (PX-350, before the most recent disaster of a keybed) under my MODX 61. It is midi up to the MODX on channels 9-16 so I make all of my sound out of the incredible MODX and it is essentially 1 giant keyboard with the MODX working on ch1-8 and the 88 on ch9-16. I also have a Roland VR-730 73key waterfall keyboard usually on my right side for the organs and classic Roland patches one might need. Anyway, off topic so back to the main show.
Arun H avatar
Arun H avatar
John Noble:
Anyway if I lived somewhere with better seeing I'm sure I'd get significant benefit from a longer focal length but for my location it just isn't going to work


The reason to get a 10 inch f/4 would not be for its longer reach or higher resolution, but for its light gathering capability and hence ability to combat light pollution. SNR, per arsecond of sky, depends directly on aperture. For years, I struggled to take meaningful broadband images with small refractors from Bortle 6, and was never happy. Results with a 8" f/4 are much better. A 10" f/4 would be better still.
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John Noble avatar
John Noble avatar
Arun H:
John Noble:
Anyway if I lived somewhere with better seeing I'm sure I'd get significant benefit from a longer focal length but for my location it just isn't going to work


The reason to get a 10 inch f/4 would not be for its longer reach or higher resolution, but for its light gathering capability and hence ability to combat light pollution. SNR, per arsecond of sky, depends directly on aperture. For years, I struggled to take meaningful broadband images with small refractors from Bortle 6, and was never happy. Results with a 8" f/4 are much better. A 10" f/4 would be better still.

Arun, thank you so much for commenting. I want to be sure I understand.

I currently use a 5" refractor at f4 and in my head I'd simply said f5 vs f4 is essentially 50% more exposure time to get a similar SNR. If I understand you correctly you are saying there is also an aperture factor to be considered? Can you quantify that for me? Is it as simple as the 10" gathers 4 times more light than the 5" so I'd need 4 times more exposure for the same f ratio etc etc.

This could open up a whole new can of worms!!!!

Thanks again

John
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