Any tips for diagnosing reflections?

[codemonkey]

Never had to deal with these before so any advice would be appreciated. In particular, tips on how I might exacerbate the issue would be great. I can't actually see the reflections in SGP, and, in a single sub, can only faintly see the top left one in PixInsight with a auto stretch, but as you can see, they become very apparent when stacked and stretched.

[bojan]

Those look like reflections from some shiny rim... are there any inside or outside your optical system?
I had something similar once (it was caused by external stray light source)

[codemonkey]

That seems likely. I've changed a few things recently:

1) Added a flip-flat - have imaged since using this, but only in narrowband
2) Collimated my focuser
3) Refocused my OAG
4) Started on a new target, pointing to a different area of the sky
5) Put all my gear in a dome - have imaged since using this, but only in narrowband

I'm suspecting this might be the flip-flat reflecting some LEDs. I have two LEDs on circuit boards on my focuser, one on the laptop and one on a power source.

[ericwbenson]

That's easy to check, put black tape over the sources.

If it is not those, try a more controlled experiment by imaging a field with an isolated bright star (1st-3rd mag for example) just outside the FOV. Then take more subs as you move the star further away. The bright star will allow you to see any tube/imaging train reflections in short subs so that you can diagnose it quickly (without having to stack a bunch of long exposures).

I had bad arc/reflection problems on my CDK20 a few years ago, once fixed (flocking inside corrector lens cell and on the shiny carbon fiber truss tubes) I could not detect any funny rings from the star Altair just outside the FOV (see attached).

Best,
EB

[Atmos]

First off you may be best working out what side of your scope it's coming from, 5s exposure and a dum torch will show you that. From there it gives you a rough idea on what side of the imagine train it may be on.

[Shiraz]

suggest that you take off the camera and look down the tube - should be easy to see which component is diverting light into the field. do under room lighting or with a torch aimed at the aperture and from various positions around it to pick up reflections from out of the field.
http://diffractionlimited.com/flat-fields-stray-light-amateur-telescopes/

[codemonkey]

Thanks for your help everyone, appreciate it! Will see how I go this evening.

[codemonkey]

I noticed that I'd gone on auto pilot and had rotated my OAG about 90 degrees which is what I think was causing that shadow on the right. Hadn't really thought too much about that, because that's where a gap in the amp glow on the ASI 1600 is, so it's usually darker there, but it shouldn't have been that dark on subs this short. I'm usually careful not to put the OAG there because I know it obscures the sensor a bit in that position.

So, I fixed that before I did anything. Then I verified that the flip-flat can cause similar issues by shining my torch on the flip-flat panel and seeing reflections come up in SGP.

So, not sure whether it might have been light reflecting off the prism on the OAG, or the flip-flat, or maybe both, but I'm hoping it was one of the above. I've removed the flip flat because I was planning on using it, essentially, as an automated lens cap when I start going "semi-remote" but I don't need it right now. Fingers crossed I can actually get some usable data tonight.

Thanks again everyone!

In other news, looks like I've mostly fixed my "tilt" - focuser collimation. Thanks to Colin for his posts on how to sort that out. Previously it was bad enough that I'd get as bad as 4" stars in one corner, while another was 2.5". Attached is FWHM map (in pixels, not ") from this same data (about 30mins of L data integrated). Looks like the tilt is almost gone but I still have what I assume is some curvature from not-quite-perfect flattener spacing. Still, a big step in the right direction!

[codemonkey]

Well, that was a nope. Pointed back to NGC 1808 / 1792 and the reflections were still there, which means whatever it is, it's been in my imaging train for a while, I just haven't encountered in before. Odd. One calculator suggests that, based off the size of the ring on the bottom left, the thing causing the reflection is likely to be around where my flattener is.

In other news, I now have autofocus working again and filter offsets set up for narrowband, which I've not done before. Autofocus is not only working, but seems to be working the best I've ever had it, and (knock on wood), consistently. At least I got something productive done, even if I didn't end up imaging like I'd hoped.

[Atmos]

What are you using for auto focus?

[codemonkey]

Auto focus is done using SGP. Hardware is a dodgy kit I built myself, using a hacked up version of someone else's arduino code; their code was buggy as hell, but I'd never written anything for an arduino before so I used it. In retrospect I would have been better off starting from scratch, but eh, it works for the most part.

Not sure why, but the ASCOM driver (not written by me) seems to forget settings sometimes. Key one is focuser speed, which impacts on the backfocus settings in SGP. I lost those so I had to figure it all out again. I'd been putting it off because I just wanted to get some imaging done for once. This time I actually recorded the settings so if (when) it happens again I'll just be able to set it straight back to what it should be.

SGP's auto focus routine is not ideal for narrowband. It often picks up very few stars and some brighter areas of nebula, the latter of which confuses it and causes it to totally stuff up the focus more often than not. Focus with L and then using filter offsets is a lot more reliable.

[JA]

Hi CM,

If I was trying to solve something like this issue, I would set the camera to the highest possible gain in order to try and obtain real time vision through the telescope, even if it means substituting your ASI1600 camera with your guidescope/planetary cam for this higher gain. This very act of substitution may lead you down a path to the solution if the problem remains/disappears with the other camera.

After you have real time (or close enough) vision, then start to play with bright objects/torches near the edge of field of view, or removing this/that adapter, using a piece of say black velvet or black matt paper to obscure reflections form one area, (spider support, tube lips etc..) held in place with plasticene or similar.Any experiments will be easier in real time, rather than going through the process of imaging, etc... for the result, although this may ultimately be the final test when you have a way forward.

Good Luck with a solution

Best
JA

[Peter Ward]

I agree with Ray here.

Don't over complicate it.

Just point the scope at the sky in daylight, (or use a torch) look down the baffle tube (with the camera removed) and note the location of any bright rings.

Those surfaces will need flocking.

[codemonkey]

Thanks for the help everyone! I think I found two things that are likely causative.

First off I took off the camera, opened up the dome and pointed it at the sky. My rather costly precise parts adapter that threads into my drawtube and connects to my Celestron OAG is like a black mirror. Super reflective. Rather unimpressed I have to say, given the cost.

In addition to that, I noticed my dew shield had fallen back quite some way, presumably due to the crappy locking mechanism for the dew shield on the Esprit, and the weight of the flip-flat.

So I suspect the dew shield falling back allowed more stray light in which then reflected off the crazy-reflective precise parts adapter.

Megan (my wife) tells me we have some blackboard paint, which I believe some people use to resolve these reflections?

[Shiraz]

I just tried some (Dulux) on my OTA and it is pretty impressive at grazing angles - even without a roughening agent
http://www.gerdneumann.net/english/instrument-building-parts-teile-fuer-den-fernrohrbau/totmatte-schwarze-optikfarbe-deep-black-optical-paint.html

[codemonkey]

Awesome, I'll give it a shot, maybe tomorrow. Thanks again Ray :-)

[codemonkey]

Well, I tried to image briefly last night. The seeing was absolutely terrible, and the reflections were still there.

Tried to use the blackboard paint, but looks like it's no good. All gummed up.. I did randomly find some adhesive-backed black velvet stuff which looks like it might do the trick. Cut off some strips and stuck it in two spots that looked reflective and to the human eye it made a huge difference. Let's just hope it makes the same impact on the camera sensor and I've got everything that was causing issues.

Fingers crossed for some clear skies so I can test it out, and hopefully get back to imaging NGC 1808 + 1792 before they drop down too far.

[Shiraz]

be interested to know what went wrong with the blackboard paint Lee - had been thinking of using more on my system, but not if it gives trouble.

hope the velvet works out well.

[Camelopardalis]

I'd be interested to know what hot spots you found Lee, I wonder if SW made the same bungle with the 100 too :question:

[codemonkey]

Probably nothing wrong with the paint as an idea, just the stuff I had was an old spray can of it that was gummed up and wouldn't spray.



Nothing that I noticed with the Esprit, all the reflections that I saw were on adapters and stuff. I did notice some reflections around the edge of the lens on the flattener, but I'm just hoping that wasn't what was causing my issues... one forecast suggests a few clear hours early this evening so hopefully I can confirm my issues are resolved tonight.

[Camelopardalis]

Sweeeet! Hope it's the adapter :thumbsup:

[codemonkey]

Looks like that was the case; finally got out last night and got some imaging done -- no reflections!

Down side is my focuser issues persist afterall. Getting very nice v-curves in SGP, but the focus is way off. Last night I checked my first sub in PI before walking off and found the FWHM was over 3" even though guiding was going good and the seeing seemed pretty good. I then manually focused and got it down to 1.8"

[Atmos]

I am going to try SGP again tonight, never been hugely impressed by its focus accuracy but I'm more than willing to give it another shot. FocusMax V4 and Maxim do a very nice job quick sharp.

[Shiraz]

That's odd. I find that SGP invariably does a better job than I can - mind you, 1.8" is a pretty rare beast down this way.

does the reported HFR from the final focusing run in SGP look reasonable? - SGP will give you a warning if it does not get back to the best focus point and if the v curve is OK, cannot think of anything else that would upset the final result. If it cannot get back to the best focus point, I think that would mean that the backlash setting needs tweaking.

[codemonkey]

Yeah, once I get the backlash dialed in (of which I have a lot), it's usually fine as long as it doesn't confuse nebula with stars, but there was no nebulae in this image. It didn't give me a warning, the v-curves were very nice, but the focus was just quite off...

Same as you, it usually does a better job than I can do myself. Quite strange.

[Camelopardalis]

I've resorted to the tried-and-tested manual Bahtinov dance in the dark. At least I'm happy with the focus then.

[codemonkey]

I think I can get it better manually with the morotized focuser than I used to with a bhatinov mask, but I could be wrong. Been a long time since I used a mask.

Last night it seemed to be behaving itself. Got a nice v-curve and verified focus with PI. And then I got clouded out 10mins later...

[Camelopardalis]

Yeah decent seeing can make all the difference :)