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Analysis...How can I further improve?

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#1 SL63 AMG

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Posted 10 December 2012 - 07:06 AM

I just finished spending about $5K to improve a $4K telescope and I am finally starting to see decent results. I know, it sounds silly but I really wanted to learn how to image with a long focal length RC before spending 20-50K on a high end telescope.

I am hoping for some feedback on how I can further improve my system...or have I exhausted all possibilities of capturing better data?

The telescope is an AstroTech 12" RC (AT12RC). It is an F/8 2432mm FL telescope. It seems like a decent inexpensive RC with which I can learn without the expense of an RCOS or PlaneWave.

I really struggled in the beginning using a 3.5" FeatherTouch focuser. I was never happy with the thumb screws used to attach it to the telescope and the compression clamps used to attach the camera to the focuser. I felt like I could never really get everything aligned and properly collimated. In fact, I really didn't like the focus I achieved, it was difficult to setup FocusMax with it and I couldn't frame an object without the camera changing tilt and collimation being off.

I finally broke down and purchased a 3" Pyxis rotator and an FLI Atlas focuser to go with my FLI ML8300 and CFW2-7 filter wheel. I think the zero tilt adapters are going to help a lot and everything threads together from start to finish.

When I purchased the custom adapters to fit it all together, I also purchase an adapter for the Takahashi collimation scope so that I could screw it into the exact same place the camera and filter wheel would attach.

I believe this will help me achieve better collimation and maintain it. It allows me to remove the TAK scope and attach the camera and filter wheel without introducing tilt and finding my collimation to be off significantly.

It was a real bear to find the sweet spot in mirror adjustment to reach the 288mm back focus of the system, particularly because the AT12RC uses two parabolic mirrors. I finally managed to get the right back focus so when the telescope is collimated focus is reached at almost exactly the center of the Atlas focuser travel (which has little room to play).

Now I have my system collimated and FocusMax working with the Atlas but I am not certain if there is more I can do or even how to do it. I took the image below and measured it in CCDInspector and discovered the following:

Current Conditions
==================
Cloud Cover: None
Transparency: Average
Seeing: Poor

Measurements
============
FWHM 5.8px
Aspect ratio 4
Contrast ratio 45
Air mass 1.18
Curvature 35.8
Scale 0.46 "/px
Tilt in X -0.5px
Tilt in Y -0.4px

3D Curvature Chart
==================
Min FWHM 5.71px
Max FWHM 6.74px
Curvature 35.8%
Total Tilt 1% SWEET!!!
Collimation 7.8px
Stars Used 163

I have little to no experience with CCDInspector. My image appears good to me, but I have an untrained eye. I think I have taken a cheap telescope and turned into a cool toy for imaging galaxies.

Is there anything I can do to further improve this system or have I nailed it and now I can just go crazy getting data?

Here is a single 600s luminance sub of NGC2403, unguided at 2432mm FL F/8.

I'll post a full resolution image of a calibrated stack of 15 600s subs as soon as I finish acquiring the remaining data (about 40 minutes from now)

Posted Image

#2 Footbag

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Posted 10 December 2012 - 10:09 AM

or have I nailed it and now I can just go crazy getting data?



+1

#3 SGT500

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Posted 10 December 2012 - 10:19 AM

You nailed it!!! Go nuts getting some awesome data. Looks like a sweet setup!

#4 SL63 AMG

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Posted 10 December 2012 - 10:25 AM

What about the curvature? Is it normal or can I do something about it? It seems high to me.

Also, the collimation is off by 7.8px. Is it worth further tweaking? Last time I recall having tweaked it and I could never get it better and only made it worse most of the time but I didn't have the TAK scope then either.

#5 J.P.M

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Posted 10 December 2012 - 10:41 AM

FWHM 5.7 is highish, Maybe the focus is somehow off, it can be the seeing effect though.

Pixel scale 0.46 is very much over sampled, even more so, if the seeing is really so bad. It means over magnified image, without any real gain for details and you'll be wasting valuable exposure time. Looks like a place for a good focal reducer/flattener, it also helps with the curvature.

#6 Peter in Reno

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Posted 10 December 2012 - 10:58 AM

Dave,

I have mixed feelings about CCDI. The FWHM using CCDI can be misleading. One time I was imaging a DSO and started with okay seeing and gradually seeing got worse, I measured FWHM of different subs with CCDI and it picked the worse sub (the one with bad seeing and the stars were badly bloated) with the lowest FWHM. Huh? So I didn't buy CCDI since it was a trial version.

You can measure individual and unsaturated stars with PixInsight. I believe it's called Dynamic PSF. You can pick each star and it will reveal many information about each star. I would trust this one.

You said the seeing was bad, I would wait until the seeing is better and possibly recheck/tweak the collimation again. Your image appear to be a little soft possibly due to bad seeing and focusing is difficult under bad seeing. I usually quit imaging if I see the stars too bloated and it's not worth keep on imaging. It's difficult to judge your image due to JPG compression.

I agree with JPM about getting a corrector/flattener if you want to flatten the FOV.

Peter

#7 SL63 AMG

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Posted 10 December 2012 - 10:59 AM

Thanks for the feedback.

The seeing conditions last night were poor so I am not surprised by your response. It is possible that I don't yet have FocusMax optimized. I am just learning FocusMax.

I would really prefer not to use a reducer. I have an ASA N10 that will work at 1620mm and 900mm focal lengths. Besides, in February, I hope to be using a PL16803 with this setup.

Do you have a recommendation for a field flattner for this system? How will that change my back focus? The Atlas focuser has less than a half inch of travel.

#8 SL63 AMG

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Posted 10 December 2012 - 11:12 AM

Ok, well I have learned a bit here.

I was having trouble with FocusMax, likely due to the poor seeing.

Please explain to me about oversampling. I am just learning about cameras and pixel sizes. I know my FLI ML8300 has 5.4 micron pixel sizes and the PL16803 I have my eye on is 9.0 micron pixel size.

So how does all this equate to over and under sampling? Could you tell it was oversampled by looking at the image or just by the camera and resolution?

I was pretty excited :jump: when I saw that galaxy open on my monitor with nice diffraction spikes on the stars.

Now I'm sad to learn it's not that good :(

I'll try again tonight. I think the seeing is going to be better.

#9 Peter in Reno

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Posted 10 December 2012 - 11:33 AM

Hi Dave,

You can get lots of good information about image scale and sampling through Google.

Peter

#10 SL63 AMG

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Posted 10 December 2012 - 11:50 AM

Hi Dave,

You can get lots of good information about image scale and sampling through Google.

Peter


LOL... I am aware of Google. I can also Google Mitochondrial DNA but that doesn't mean I'll understand what I am looking at and how it applies to what I did last night. :foreheadslap:

I was hoping for a layman's explanation from the guy that said I oversampled the galaxy.

#11 Peter in Reno

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Posted 10 December 2012 - 11:55 AM

The sampling rate is a personal preference. My image scale is 0.47" and I get decent results. If the focus and guiding is tight, then small image scale should be fine. But if the seeing condition is less than ideal, I typically avoid imaging whether the image scale is 0.47", 1", 2", or even 3".

I know you don't guide and I have seen great results from your images so at 0.46" should be fine if the seeing allows it. The stars tend to bloat a little bit more and DSO looks a little softer at small image scale under bad seeing condition.

Peter

#12 SGT500

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Posted 10 December 2012 - 12:08 PM

Hey Dave,
I don't have much time to respond here, but ill give you as much insight as I can for now in a short time. .47 arc seconds per pixel is slightly over sampled. If your seeing is around 2 arc seconds or below I think that sampling will give you some excellent resolution with that scope. If you were coming up with raw FWHM numbers in the high 5 range that means your resolution last night was a little under 3.5 arc seconds. That is decent... Obviously the better your seeing is, the better your resolution will be at that image scale. The one thing I want to add is that deconvolution routines work particularly well on over sampled data. So even though you are over sampling, the data your capturing isn't going to waste. You cAn take advantage of your sampling with deconvolution. Trade off is that deconvolution will amplify noise and artifacts in your images. As a result you will have to obtain high SNR data and many subs. Looks like that won't be a big problem though as I see you usually gather plenty of time on each imaging run and you have some nice dark skies.... Moral of the story is that with your equipment you should be able to get some nice results even though the data is a bit over sampled. 9 micron pixels might be a bit better in theory, but I would try it out with the 5.4 microns that you have now. That image you posted looks beautiful. Nice spikes and the resolution looks good as well even with poor seeing. Looks to me like a very promising setup

#13 David Ault

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Posted 10 December 2012 - 12:08 PM

Dave,

You might take a look at Starizona's guides. They have one on Nyquist Sampling and how it relates to CCD astrophography that might help explain the concept better. It certainly explains it better than I could.
http://starizona.com...vtheorynyq.aspx

Regards,
David

#14 Leonardo70

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Posted 10 December 2012 - 12:36 PM

Small pixels is not the good choice for your oversampled resolution. Try to use a 9nm and you'll find a great improvment (but i'm not sure you can use a 16803ccd with your RC, there isn't a flattner for that size). Or use the small pixel when you have a very very good seeing.. in this case you need to had under 4x your resolution in terms of seeing => 0.46" x 4 = 1.84" or better and in association to deconvolution for have the best results (my opinion). I hope this help you.

All the best,
Leo

#15 rigel123

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Posted 10 December 2012 - 01:09 PM

Send me your equipment to try out for a year or two and I'll send you a comprehensive report of what I think you need to do to improve upon it. :grin:

#16 JWalk

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Posted 10 December 2012 - 01:11 PM

Looks good to me!

#17 Hilmi

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Posted 10 December 2012 - 01:32 PM

Looks very good, don't bother with peeking at statistics if your image looks very sharp and the image looks good. I'm no expert, but if you need to look for a mathematical analysis of the image just to know if there is room for improvement in your system, then you are probably getting to the area of diminishing returns.

Try binning your FLI8300, that will give you effectively the same results as having larger pixels. I have binned my STT-8300 4x and imaged 10 minutes exposures and I have not faced the blooming which people complain about in 8300 chips when binned.

#18 Peter in Reno

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Posted 10 December 2012 - 01:46 PM

Dave,

After re-reading your original post, I didn't realize your image is a single Luminance sub. It will look much better when you stack it. Still, it's a good idea to measure FWHM of each star of each sub to check if the focus, guiding and seeing is good. There's always room for improvement. On good seeing nights, you can measure for best FWHM and use it as a reference for future imaging. Or go back to your old raw subs and find the best FWHM of unsaturated stars and use it as a refernce. You can throw out subs containing large FWHM before stacking. If your future imaging shows an increase in FWHM, you can investigate what went wrong.

Binning Luminance is generally not a good idea because it will lose details. Binning color is fine like you always do.

Peter

#19 SGT500

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Posted 10 December 2012 - 02:07 PM

Binning Luminance is generally not a good idea because it will lose details. Binning color is fine like you always do.


Hey Peter, I don't mean to be a nit picker, but I don't necessarily agree with this. Binning only reduces the image scale by increasing pixel size. It will make the image smaller but will not necessarily result in the loss of details. Nice quality images can still be taken by binning the luminance 2x2. I think this might have been what you were saying. Maybe I mis interpreted.
-Sal

#20 korborh

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Posted 10 December 2012 - 03:17 PM

Your FWHM is 2.67" which is good, however I would try to bring it under 2", given your location. That would be a significant step in added resolution, assuming seeing allows it.

What is the FWHM of your focus stars? It will tell you if your collimation and focus is optimized as best as possible. If it is much smaller than 2.67", then you know its in the tracking or other factors like focus shift, flexure etc. limiting your long exposure FWHM/resolution.

#21 Peter in Reno

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Posted 10 December 2012 - 03:41 PM

Hi Sal,

I am saying is if you can guide tight and focus well to get low FWHM of stars, then I do not see an issue of imaging at less than 0.5" if the seeing allows it. I don't like to image under bad seeing even if the image scale is between 1" and 2" because the results would be mediocore. It's a personal preference. Dave lives where there's very little to no light pollution and the seeing is normally good which is why I think he should stick to 0.46" image scale.

Peter

#22 Peter in Reno

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Posted 10 December 2012 - 08:02 PM

Your FWHM is 2.67" which is good, however I would try to bring it under 2", given your location. That would be a significant step in added resolution, assuming seeing allows it.

What is the FWHM of your focus stars? It will tell you if your collimation and focus is optimized as best as possible. If it is much smaller than 2.67", then you know its in the tracking or other factors like focus shift, flexure etc. limiting your long exposure FWHM/resolution.


Dave,

In your original post, you said FWHM is 5.8 pixels. I and maybe others thought it was in arcsecs. To convert to arcsecs based on your image scale is closer to 2.67 arcsecs which is pretty good despite bad seeing. That's better than my typical images in my area. FWHM in arcsecs is usually more meaningful than in pixels.

Do not discard your Luminance images since the FWHM of the stars are very good. I am curious what's your typical FWHM of the stars in excellent seeing condition?

Peter

#23 SGT500

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Posted 10 December 2012 - 08:03 PM

Hi Sal,

I am saying is if you can guide tight and focus well to get low FWHM of stars, then I do not see an issue of imaging at less than 0.5" if the seeing allows it. I don't like to image under bad seeing even if the image scale is between 1" and 2" because the results would be mediocore. It's a personal preference. Dave lives where there's very little to no light pollution and the seeing is normally good which is why I think he should stick to 0.46" image scale.

Peter


I agree fully, i think in good seeing he will be able to get away with 1x1 binning and make some really nice images.

#24 SL63 AMG

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Posted 12 December 2012 - 03:47 AM

Dave,

You might take a look at Starizona's guides. They have one on Nyquist Sampling and how it relates to CCD astrophography that might help explain the concept better. It certainly explains it better than I could.
http://starizona.com...vtheorynyq.aspx

Regards,
David


David,

Thank you. This was helpful for me to understand under and over sampling. It makes sense now and after reading the article and also reading the help files for CCDInspector to learn what is meant by aspect ratio and curvature with respect to the over all measurements taken across each sub I can now better understand the numbers.

Comparing subs from each night as the seeing has progressively improved I am obtaining better aspect ratios and lower curvature numbers for each sub. It was important to understand how CCDInspector averages these values across the entire image. Incidentally, my FWHM's are lower tonight also.

Current Conditions
==================
Cloud Cover: None
Transparency: Above average
Seeing: Average to Good

Measurements
============
FWHM 3.3px or 3.05"
Aspect ratio 6
Contrast ratio 33.36
Air mass 1.31
Curvature 10.4
Scale 0.46 "/px
Tilt in X -0.0"
Tilt in Y -0.0"

3D Curvature Chart
==================
Min FWHM 3.07px
Max FWHM 3.24px
Curvature 11.4%
Total Tilt 1%
Collimation 2.6px
Stars Used 121


Considering all that I have learned, I guess I am pretty happy with the data I am getting out of this system. I have always been pleased with the results of my ASA N10 astrograph but I was skeptical about this AT12RC.

For anyone considering an AT12RC for imaging I highly recommend that you consider an FLI Atlas focuser. It has made a huge difference in the stability of the system with regard to flexure and stable collimation.

I am currently gathering RGB data binned 2x2 at 10.8 micron pixel sizes for NGC2403 and the results look awesome!

Thanks for all the feedback folks. I really appreciate your insight as I continue to gain more knowledge in my pursuit of capturing excellent data.

:bow:






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