Hello! I’m new to this amazing hobby and have a few questions that I’m struggling to find answers for. I’m imaging with a Celestron NexStar 8SE + f6.3 focal reducer on the alt/az mount (I just got the wedge and auto guiding, but haven’t had a clear night to test it!). With the alt/az, I limited my imaging to 8 seconds and would try to acquire at least 500 subs for stacking. The camera is a modified Canon EOS 1100D (h-alpha mod). Processing consists of DSS and photoshop. I’m saving up for Pixinsght! My scope is collimated and focus is checked throughout the night with a Bahtinov mask. My questions are: 1. Am I doing something wrong that’s causing the bloated stars? 2. Is it even possible with this setup to achieve a nice background star field? 3. If these stars aren’t fixable in image acquisition, what’s the best way to reduce their size without distorting them? I’ve been using RC-Astro’s StarShrink, but have to be careful because they’ll sometimes leave little rings behind. Any advice or suggestions will be so very much appreciated!!   Please also check my nebula images on my Astrobin. I can’t get them to load here because the files are too large.
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They look a lot like the stars I would get with my 8se + nexstar before I switched to a better mount. I can now get reasonably good stars with the 8se - and no reducer - and running much longer exposures (see my images for a few examples), but the nexstar mount was hopeless for me…
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Heather,
This is a hobby that really benefits from OCD. Everything counts and it's the sum of the parts that make up the final result. Here are some things that can contribute to less than perfect results.
1) Optical quality. First, the base optics need to be of high quality and second they need to be well aligned. Using a reducer can work but in general, reducers are likely to produce poor results at the edge of the field. My recommendation is always to start without a reducer to get everything dialed in. That reduces the number of variables when you need to figure out why things aren't as sharp as they should be. The other thing that affect optical quality is component spacing. Have you confirmed that the sensor in your camera is properly positioned with respect to the specified back working distance? Finally, it doesn't take much misalignment to seriously degrade optical performance. Be VERY careful to make sure the secondary is properly aligned. A misaligned secondary will produce coma and I don't see much coma so that may not be an issue with your system.
2) The number one thing that causes poor imaging is a focus error. Using a B-mask is a great way to focus and if you are using it properly, you should be getting pinpoint stars. Zoom way in on the pattern and very carefully center it to insure perfect focus. Thermal drift can cause your scope to change focus significantly so it's important to check focus frequently. Your images looks mostly out of focus to me.
3) Guiding errors can cause enlarged stars. Your stars should be circularly symmetric over the field and clearly that's not the case so you've most likely got a guiding related error. Polar alignment can cause this sort of problem with longer exposures. Mechanical vibration can also cause elongated stars so make sure that the scope is mounted on a solid surface.
With the short exposures that you are using, you should definitely see pretty sharp stars in the background when everything is properly adjusted and the guiding is good. Keep in mind that really poor seeing can also cause soft star images but even on an average night an 8" telescope should be able to deliver sharper images than what you are showing here.
John
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The world with an SC telescope is very different from that with a small APO refractor telescope. The reason is not so much the quality of the optics, but simply that the resolution is entirely different. The cause could be poor tracking or inaccurate focusing, but one of the likely reasons is poor seeing. While stars are about 2-3 pixels in size with an APO refractor telescope regardless of the seeing, with an SC telescope, the star size can vary between 3-10 pixels depending on the seeing, even if everything else remains the same. The F-number of the telescope also limits how small the stars can be.
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8 sec is too short for any good images. If you are going to go with the AZ mount you will need a rotator such as the pegasus so you can get 1-3 min exposures. If polar aligning is not a big obstacle, then look for a wedge for your mount. that would be cheaper and arguably better. I see a number of them on cloudy nights. I like the AZ mounts on a wedge because there is no meridian flip.
And like others said, you need good focusing. Best would be to use an electronic focuser such as the ZWO. This is an expensive hobby you have chosen…
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I've had somewhat decent success with 8 seconds on the nebulae and bigger galaxies. But I did just get the wedge and an auto-guiding rig installed. Tonight will be the first light with all the new stuff. I'm hoping getting longer, 3-ish minute subs will make a big difference. We shall see!
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Good luck, I think you will be pleasantly surprised.
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I third the recommendation to ditch the reducer, and if there's no other corrector, then backfocus is no longer a factor, right? I agree with John that it 'looks' like focus is the primary issue, but to Tommi's point, I've also had seeing bad enough to produce similar results….and as seeing deteriorates, so does focusing and guiding, a vicious circle….
For improving thing with processing, BlurXterminator is going to be you best bet for both correcting and then reducing stars plus bringing back detail in the target. Also, fwiw, I stretch my stars (separate from the target) using GHS and play with the Local Intensity adjustment to soften them around the edge so they don't look so much like dots.
Cheers,
Scott
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I'm also completely new to this hobby and using a comparable setup: NexStar Evo 6 and a Canon R7. I also purchased the wedge for longer exposures, and the SSAG. The SSAG does an incredible job at correcting tracking issues. It really helps to have good stars. So definitely give it a go! I also recently considered back-focus, and added a spacer to my imaging train. This helps correct stars on the edges. You may want to check if yours is appropriate. BlurXTerminator from RC Astro is also really good for post-processing: https://www.rc-astro.com/software/bxt/It's a PixInsight plugin. If you don't use PixInsight, maybe there's other similar software.
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@Scott Badger I definitely need the reducer. There’s no way I would get any data with exposures even up to 5 minutes. The back focus is actually dead on with the reducer, t-adapter, t-ring and camera. Here’s Orion before I got the reducer.  And after (exposures and integration time are the same). https://astrob.in/9xkgls/0/ |
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@Jean-David Gadina I would LOVE to get a SSAG, but it’s a bit out of the budget right now. I also confirmed that my back focus is dead on with the reducer, t-adapter and camera.
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@Rostokko It definitely sounds like it’s the fork arm alt/az that’s the culprit. I just got a wedge and auto-guiding that I’m testing out tonight. I’ll post the results (if I get everything to work, of course).
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For the post-processing steps, try doing a background extraction first (with GraXPert or DBE/ABE), and then run BlurXterminator with 'Correct Only' selected. That always helps my star bloat, especially with my 8". Then I run SPCC followed by a full BlurXterminator again to sharpen stars and non-stellar. There are a number of experts who I've picked this process up from and it really works wonders.
Hope it helps.
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Heather Charron:
@Jean-David Gadina I would LOVE to get a SSAG, but it’s a bit out of the budget right now. I also confirmed that my back focus is dead on with the reducer, t-adapter and camera. Sorry, I thought you were mentioning the SSAG. I just got a wedge and auto-guiding that I’m testing out tonight. What kind of auto-guiding do you use then? Good luck and clear skies!
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I’m using a 60mm guide scope with a Svbony sv305 pro and PHD2 to run the autoguiding. Tonight was actually my first night with the wedge and autoguiding setup. Prior to this I was just tracking in sidereal with the NexStar alt/az mount. This kept my subs limited to 8 seconds before the stars started to trail. Things didn’t work out great tonight unfortunately. I was battling clouds and wind and couldn’t get auto-guiding to run for more than 3 minutes before it would lose the star. Had to eventually accept the fact that I can’t control Mother Earth and that there will be more clear nights eventually.
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Hello Heather! I feel very happy every time I see a woman getting started in this hobby. As you mentioned this is a great community and there is a lot of people with tons of experience willing to share their knowledge. I started in 2015 with a very basic equipment: a cheap 6” reflector (it cost me only USD 99 :happy-4  and an entry level German Equatorial mount (no guiding, no autofocus, no auto anything). In the beginning I was thrilled about my images and little by little I started adding and/or making changes to my setup. I changed my cheap reflector for a small refractor which I was never able to flatten (or so I thought). A couple of years later I was very frustrated and decided to change optics and got an 8” Edge HD, which is the scope I currently have. SCT’s are great “do it all” telescopes. They are very good for planetary and Moon imaging and can be used for deep sky imaging with two important caveats: (1) even with a reducer they are painfully slow, which means you need long exposures to get the best results out of them (2) adding a Hyperstar lens makes these scopes a lot faster, but its collimation is not for the faint of heart (I had one…) The thing that I never liked from my SCT is how big stars come out. Like others said, collimation and backfocus are crucial to get the best results. There are ways to make those big stars less distracting in post-processing. Currently I rely a lot in Russ Croman’s BlurX and PixInsight’s Morphological Transformation for that. I suppose there are ways to do the same with other post-processing programs. I’ve been tempted to change my scope once again, but the reason I am still undecided is that after so many years with my Edge, I feel I know how to get the best out of it in spite of all it’s challenges. The one thing I admit is that I still really don’t like are the stars I get. Every time I see images taken with refractors and reflectors that are properly collimated and flattened I wish my stars look like them, but I am learning to “live” with the ones I get. Best of luck Heather and never give up. I look forward to see your progress in AB. Clear skies, Wanda
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Here’s Orion before I got the reducer.
The stars all show severe coma, especially on the right side. Most likely your collimation is way off. That would explain the lack of sharpness with the reducer.
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There's some good advice being given here Heather especially from Azhar Rana. I'm doing the same thing. Check my results and I'm using a reducer https://www.astrobin.com/full/dq852w/0/This is with a minimal amount of cropping just to eliminate stacking artifacts. Stars are round and fairly small to the edges. Mike
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Victor Van Puyenbroeck:
Here’s Orion before I got the reducer.
The stars all show severe coma, especially on the right side. Most likely your collimation is way off. That would explain the lack of sharpness with the reducer. That was before I added the reducer and before I learned how to collimate. I had the telescope for about 2 weeks when I took it.
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Since you are using Photoshop, one thing you can do to rescue the data you already have is to use the Spherize filter. Use the elliptical marquee tool to draw a small selection circle around a bloated star. Then go to Filter -> Distort -> Spherize. Move the slider all the way to -100. Voila! A smaller star. Repeat as many times as you need to or until your patience runs out. This should be one of the last things you do, as the image needs to be converted to 8 bits for this filter to work (at least in my older version of Photoshop - newer versions may allow this filter to work on 16 bit images).
Arnie
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Arnie:
Since you are using Photoshop, one thing you can do to rescue the data you already have is to use the Spherize filter. Use the elliptical marquee tool to draw a small selection circle around a bloated star. Then go to Filter -> Distort -> Spherize. Move the slider all the way to -100. Voila! A smaller star. Repeat as many times as you need to or until your patience runs out. This should be one of the last things you do, as the image needs to be converted to 8 bits for this filter to work (at least in my older version of Photoshop - newer versions may allow this filter to work on 16 bit images).
Arnie StarShrink from Russ Cromann is cheap and does this automatically…for all the stars. Software tools can fix a lot of defects in an image but it’s always better to fix the optical-mechanical issues before resorting to software fixes. John
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John Hayes:
Arnie:
Since you are using Photoshop, one thing you can do to rescue the data you already have is to use the Spherize filter. Use the elliptical marquee tool to draw a small selection circle around a bloated star. Then go to Filter -> Distort -> Spherize. Move the slider all the way to -100. Voila! A smaller star. Repeat as many times as you need to or until your patience runs out. This should be one of the last things you do, as the image needs to be converted to 8 bits for this filter to work (at least in my older version of Photoshop - newer versions may allow this filter to work on 16 bit images).
Arnie
StarShrink from Russ Cromann is cheap and does this automatically…for all the stars.
Software tools can fix a lot of defects in an image but it’s always better to fix the optical-mechanical issues before resorting to software fixes.
John Thanks for this! I already use his GradientXterminator and StarXterminator. I didn't know about this one. Downloading now!
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