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Need help with same smudge pattern in flats with all filters

Stellarvue SVX102T Flattener Acquisition Deep Sky Troubleshooting
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Robert Lowenthal avatar
Robert Lowenthal avatar
I’m seeing the same pattern on all of my flats: LRGBSHO (see pics below). I’m not talking about the vignetting or the shadow from the OAG prism, but rather the pattern that you can see in the Ha photo. The “smudge” is most prominent in Ha and Sii, but I can see hints of it in the other filters as well. (See pics for example flat images - L, H, O in order)

Equipment:
QHY600M
Chroma LRGBSHO 50mm filters
SVX102T telescope with flattener
QHY OAG
Gerd Neumann Aurora Flat Field light panel

I did the following troubleshooting process…

1. Rotated camera to r/o something in the telescope / flattener - Same pattern on flats
2. Cleaned  front of sensor AR window (thinking it was a smudge on the glass) - no change
3. Reversed Ha and Sii filter sides (the 2 filters where the “smudge” is most notable) (front -> back) in the wheel - no change
4. Moved Flat Field panel to different spots - same pattern
5. Tried a standard LED tracing panel and repeated taking Flats - same “smudge” pattern 

The only remaining possibility that I can think of is that there is either a smudge on the inside of the AR glass window, or on the sensor itself. 
Before I try to open up the camera and try to clean the inside of the AR window and/or the sensor (which really scares me), I’m checking in with the Astrobin community for help / guidance.

Thank you,
Robert
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andrea tasselli avatar
andrea tasselli avatar
My bet: it's the lens.
Oskari Nikkinen avatar
Oskari Nikkinen avatar
The most important question here is: Does it calibrate the lights? If it does, no action required.

If the smudge pattern stays in the flat calibrated lights then move on with the troubleshooting. Internal reflection of some kind is a possibility, perhaps from something shiny either inside the tube or in one of your imaging train adapters.
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Robert Lowenthal avatar
Robert Lowenthal avatar
Oskari Nikkinen:
The most important question here is: Does it calibrate the lights? If it does, no action required.

If the smudge pattern stays in the flat calibrated lights then move on with the troubleshooting. Internal reflection of some kind is a possibility, perhaps from something shiny either inside the tube or in one of your imaging train adapters.

Oskari,
I haven’t tried calibrating the lights yet. I’ll do that today.

I was thinking about ignoring it (in hope of it calibrating out), but I was imaging around Gamma Cygni last night and noted a prominent (irregular) diffraction pattern around the star, that disappeared after a meridian flip. So I think the “smudge” was causing a reflection pattern around the star, and after the meridian flip the star was now projecting onto a different part of the sensor (where there is no smudge). This leads me to think that I need to address the issue, as I doubt the diffraction pattern will calibrate out.
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Tony Gondola avatar
Tony Gondola avatar
Unless there's in issue with your calibrated stacks there's no reason to worry about what your flat frames look like.
Robert Lowenthal avatar
Robert Lowenthal avatar
Update:

I took the camera off the telescope and ran a flat with the panel over the camera. The smudge is not present on this flat, with the telescope removed from the equation!

So it looks like it’s a reflection from the telescope. I’m really happy I didn’t try to clean the sensor!

I looked back at flats I took when I had an ASI2600MM on the telescope, and the flats look fine (no smudge). So why would I have the reflection with the QHY600MM (full-frame sensor) and not with the ASI2600MM (ASC sensor)?

How do I troubleshoot from here?

If the smudge were to be on one of the lenses in the telescope, I would expect it to shift when I rotate the camera. But it doesn’t. This is an SVX102T, so the entire focuser assembly rotates with the camera… I’m not sure if that is important, but I think it’s the only part of the telescope that changes with rotation of the camera.

I’m going to put the ASI2600MM on the telescope and see if the smudge is there. If it is, then it’s possible that I touched the flattener lens while changing out cameras. If not, then I’m stumped.  I’ll try this and report back.
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Robert Lowenthal avatar
Robert Lowenthal avatar
Update:

There is no smudge when the ASI2600MM is connected. Ha flat looks good. No smudge.

So the smudge only occurs with the combination of the QHY600M and the SVX102T. I could try connecting the QHY600M to a different telescope and see how that looks.

I’m going to run FBPP on last night’s subframes and flats and let you know how that turns out.
Robert Lowenthal avatar
Robert Lowenthal avatar
Update:

I tried the QHY600M with an ASKAR FRA400… same, exact smudge.

So, my QHY600M shows the same flat pattern with the SVX102T and the FRA400. No smudge when not connected to a telescope.

GOOD NEWS  IS THAT I JUST RAN FBPP AND IT CALIBRATED OUT NICELY!!! 😃

So, it may be a moot point now, but I’m wondering how / why the smudge pattern shows up when connected to a telescope, but not when the flat panel is placed on the imaging train when disconnected from a telescope? I’m also wondering if anyone else with a QHY600M notices this issue?

In the end, all is well…

Thank you all,
Robert
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Tony Gondola avatar
Tony Gondola avatar
Glad it worked out Robert…