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What is wrong with my star shape? William Optics GT

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#1 JanD

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Posted 02 July 2017 - 04:38 AM

Dear CN community,

 

A while ago I bought a used William Optics GTF81 APO refractor (the five element variant with integrated flattener). It is in very good shape and in general I am very happy with it.

 

Nevertheless, I always had some problems with my star shape. I focus usually with a Bahtinov mask and the nebulas/galaxies seem to be in good focus, but still they seem very soft and sometimes weird in shape, see here:

 

Unbenannt-1.jpg

 

This is especially visible on bigger/brighter stars.

 

I am still very new to astrophotography, so I always thought that these weird shaped and soft stars are because of my lack of experience in processing with Pixinsight, or maybe because my guiding is not perfect. But when I compare my stars with pictures of other photographers on Astrobin or Flickr, I see the same effect. Brighter stars always seem blown out and very soft. And those photographers seem way more experienced in processing than me, judging from the overall quality of their pictures.

 

See examples:

Canon T3i: https://www.flickr.c...ro/31943774773/ (Credit: Andrew Klinger)
ZWO ASI 1600: https://www.flickr.c...in/photostream/ (Credit: Andrew Klinger)
Starlight Xpress Trius SX-694: https://astrobin.com/206663/B/ (Credit: karambit27)
SBIG ST-8300C: http://www.astrobin.com/209579/0/ (Credit: Antonio.Spinoza)

This seems to be independent of the imaging camera (CCD or DSLR). Do we all do something wrong with processing? Is there something wrong with our telescopes? Or is this just what you get with refractors in general, or the expected quality of the GT series?

 

I would be happy to hear some opinions from experts.

 

Thank you and best regards from Switzerland,

Jan


Edited by JanD, 02 July 2017 - 04:49 AM.


#2 james7ca

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Posted 02 July 2017 - 07:43 AM

Probably better to post/ask in an imaging thread rather than just "Refractors."

 

Stars can be pretty difficult. You need good optics, good seeing conditions, and good technique during both capture and processing to get what I'd term "nice" looking stars. Given that almost all of my images are taken under red/orange zone light pollution and with relatively inexpensive CMOS cameras I don't think my results would qualify as "expert," but here are links to two of my images that I think look fairly good (for star color, definition, and shape). One was taken unguided on a Celestron AVX mount with a nice but probably not great 80mm APO and a relatively inexpensive, uncooled CMOS, one-shot-color camera and the other was captured guided with an A-P Mach1GTO, a Tele Vue NP127is, and a cooled CMOS mono camera (so, a wide range in equipment from low-mid range to high-end, but still with relatively inexpensive cameras).

 

The Trifid Nebula done on an unguided AVX mount:  https://flic.kr/p/LsJxPn

 

The Dumbbell Nebula done on a Mach1GTO with a mono CMOS camera:  https://flic.kr/p/KwugRG

 

Now, not all  of my images look like this and I have plenty of examples that are worse but having said that I can comment on what I see in the samples you provided.

 

Your clips from the globular clusters look pretty good to me. The image on the lower left (stars and nebula) looks like you may have had a guiding issue or maybe they show optical aberrations from focus or because of problems with the field coverage (is that clip from the edge of the image?). Then, there are the two sample galaxies. Here, something does look a bit odd with the stars. It's hard to tell exactly what happened and I actually think there is more than one issue with those images. Part of it could be from the processing (artifacts from a star mask?) and part could be from optical issues (focus, tilt, lens aberrations?). One thing I might ask is how often do you check or redo focus? Every thirty minutes might be a good place to start (depending upon the temperature and other conditions).

 

So, now the question becomes can you do better and the answer to that is most likely, "Yes, you can." How? Well, practice, experience, more care and better technique, and sometimes just plain "luck" (it also helps if you have good sky conditions, seeing being an often overlooked factor in producing what might be termed "world class" images).

 

In any case, as you classify yourself as being "very new to astrophotography" I'd say that you don't seem to be doing that badly. In fact, although I've been using some pretty good equipment for nearly three years now I don't think I've yet taken an image that I would term "perfect" or flawless, so I'm still trying myself and I do hope to get better (with time and continued opportunity).

 

It can be hard, but don't give up, and I think you will see continued improvements with time.



#3 Goofi

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Posted 02 July 2017 - 09:21 AM

+1 to what James shared.

 

Two other comments I'll add:

 

First, aperture matters in astrophotography for a lot of reasons, one of which is star size. Basically, the larger your aperture the smaller your stars (relative to nebulas and other extended objects).  In an a 300mm class scope the stars are not nearly as prominent than in an 80mm class scope.

 

Second, processing choices can really make a difference. Since you are using PI, try doing your stretch with the MaskedStretch process; it generally gives smaller stars without blown out cores ... at a cost a a little less contrast across the image.  Here's a thread from the Pixinsight Forum on it.



#4 JanD

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Posted 03 July 2017 - 04:20 AM

Probably better to post/ask in an imaging thread rather than just "Refractors."

 

Stars can be pretty difficult. You need good optics, good seeing conditions, and good technique during both capture and processing to get what I'd term "nice" looking stars. Given that almost all of my images are taken under red/orange zone light pollution and with relatively inexpensive CMOS cameras I don't think my results would qualify as "expert," but here are links to two of my images that I think look fairly good (for star color, definition, and shape). One was taken unguided on a Celestron AVX mount with a nice but probably not great 80mm APO and a relatively inexpensive, uncooled CMOS, one-shot-color camera and the other was captured guided with an A-P Mach1GTO, a Tele Vue NP127is, and a cooled CMOS mono camera (so, a wide range in equipment from low-mid range to high-end, but still with relatively inexpensive cameras).

 

...

Hi,

 

thank you for your feedback. You have some very nice looking stars!

To be honest, I set the focus once and then lock it. I never re-check, maybe I should start doing this. Thank you for your other advice which I will surely take into account.

 

+1 to what James shared.

 

Two other comments I'll add:

 

First, aperture matters in astrophotography for a lot of reasons, one of which is star size. Basically, the larger your aperture the smaller your stars (relative to nebulas and other extended objects).  In an a 300mm class scope the stars are not nearly as prominent than in an 80mm class scope.

 

Second, processing choices can really make a difference. Since you are using PI, try doing your stretch with the MaskedStretch process; it generally gives smaller stars without blown out cores ... at a cost a a little less contrast across the image.  Here's a thread from the Pixinsight Forum on it.

 

I never thought about the relation of the aperture with the stars. But it makes sense. Thank you for the advice with the masked stretch, I will read more about this.

 

Best,

Jan



#5 dkeller_nc

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Posted 03 July 2017 - 08:31 AM

About focusing - Realize that all scopes will change focus with temperature changes.  The effect is especially noticeable with scopes that have aluminum tubes and fast optics (<F6).  I'm guessing that from your location information you're likely at relatively high elevation.  That's great for reducing the distortion effects of the atmosphere, but the thinner air doesn't hold heat the same as the air in the lowlands, so you will typically see bigger swings in nighttime temperature than would otherwise be the case.

 

So carefully letting the scope come to ambient temperature, and then re-focusing on a schedule is important.  For those of us with autofocusers, it's not unusual to build in re-focus points 4 or 5 times during a 2 or 3 hour imaging session.

 

If you want to figure out whether this might be affecting your results, run your subexposures through the SubFrameSelector script in Pixinsight.  Then plot the FWHM of each frame in consecutive order.  I suspect you may find that the first few subs have a roughly equivalent FWHM, and then after that you'll see them slowly increasing.



#6 KBALLZZ

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Posted 05 July 2017 - 02:36 PM

I'm glad I stumbled across this post! A few of my images are linked by the OP (under Andrew Klinger) and I can confirm that my stars do not focus correctly. They are all soft and flaring off to one side. My biggest concern is that the "softness" of the stars was a lot less appearant during my first light with the WOGT81. I also use the FLAT6A (0.8x reducer/flattener).  

 

A few things I need to troubleshoot before making any claims are playing with my backfocus (I am sub-mm perfect according to WO specs, but worth a shot), imaging without the FF/FR, and making sure screws are tight on the scope. 

 

I hope this is not a collimation issue as I would have to send it away. Just to clear any doubt, I am OCD about focusing and spend a lot of time with and without the bahtinov mask (and using SGP HFR value), so I'm positive that my "best" focus has soft stars with a small flare off to one side.


Edited by KBALLZZ, 05 July 2017 - 02:37 PM.


#7 ChrisWhite

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Posted 05 July 2017 - 03:05 PM

Those are classic WO "crunchy stars". It's caused by the design of the scope and technique won't fix it fully. Some copies are really bad and some are not so bad. The newer designs improved the iron cross stars, but the Pentagon shape and crunchiness didn't get better. You could try using an aperture mask. It will slow the scope down a little, but star shape will tighten up.

#8 KBALLZZ

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Posted 05 July 2017 - 03:43 PM

Those are classic WO "crunchy stars". It's caused by the design of the scope and technique won't fix it fully. Some copies are really bad and some are not so bad. The newer designs improved the iron cross stars, but the Pentagon shape and crunchiness didn't get better. You could try using an aperture mask. It will slow the scope down a little, but star shape will tighten up.

That's unfortunate to hear, the fast optics were the selling point for me. Ah well, at least deconvolution does great work tightening up the details.



#9 ChrisWhite

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Posted 10 July 2017 - 09:57 PM


Those are classic WO "crunchy stars". It's caused by the design of the scope and technique won't fix it fully. Some copies are really bad and some are not so bad. The newer designs improved the iron cross stars, but the Pentagon shape and crunchiness didn't get better. You could try using an aperture mask. It will slow the scope down a little, but star shape will tighten up.

That's unfortunate to hear, the fast optics were the selling point for me. Ah well, at least deconvolution does great work tightening up the details.

Yeah, I had two WO design scopes and didn't like the stars much. Otherwise they are great little scopes. I ended up with an esprit 80.

#10 keithlt

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Posted 11 July 2017 - 01:06 PM

it took me some time to dial in the oem ff/fr with my gt81fd. also that and changing out the stock focuser to a moonlite helped.




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