Problem with data or calibration

[alinderbaum]

Last night I captured 81s x 180 subs of M81 with what appears to be really good data. I took my flats as I have been for 2 years now, same with bias and darks. The result shows several issues that I have never experienced, and I am having a hard time understanding what the cause is.

With the exception of some airplane lines, I have not seen anything wrong with either the lights, or calibration frames. Again, I have not experienced any issue in the past year and a half or so and am at a loss. Does anyone have insight they can share? In somewhat laymen's terms as I am still relatively new to astrophotography.

Also, after cropping and applying DBE to get a better look, I am noticing many large red dots/groups of red dots on the right side I have not seen before. They do not appear to be hot pixels (or look like what hot pixels have looked like in the past).

For stacking, I used DSS.
Altair 269C camera
AVX mount
PHD2 for guiding

Here is a link to the stacked image, flats, dark flats, master dark, master bias and a couple of the individual light frames. Thank you!


https://1drv.ms/u/s!AhE2Rt0-5SmlgcNn2TF ... Q?e=n6cnzq

[admin]

Hi,

you have a glow on the right hand side of your master dark that isn't there in the image frames - that is causing the dark area on the right when you stretch the 'autosave.fits' stacked image, and I am thinking it may also be part of the cause of the red dots due to incorrect dark subtraction in that area.

The glow doesn't look like sensor glow (if it was, it should show up in the light frames too). I suspect you may have had a light leak when capturing the darks.

cheers,

Robin

[alinderbaum]

Robin,

Thank you for your insight. In regards to the cause of this, I recently had a session and ran across a similar issue. I am following the same process I have for the past couple of years and this seems to be a new issue.

In the link below are: a light frame, flat, dark flat, master bias, 2 dark frames for comparison and a stacked image using only the GOOD darks (see below description).

In the below link are two dark frames taken about 6 minutes apart in the same sequence. I notice a glow on the bottom left on one but not on the other. These were taken in a pitch black garage with the cover on the telescope. Do you have any insight as to why there is such a stark difference between the two?

To the final image issue.....I thought the dark issue above was the issue to the awful results in the attached stacked image, but even when I removed these problematic dark frames, I am still getting poor results. I have looked at all of the images in PixInsight and everything looks OK and I do not know how to evaluate the issues otherwise. Any thoughts? I am using the latest version of SharpCap.

https://1drv.ms/u/s!AhE2Rt0-5SmlgclXK9L ... w?e=X0GyV0


Thanks again for all you do and your response.

Aaron

[alinderbaum]

Robin,

As a follow up to my post from a few minutes ago, I restacked multiple times, using only bias, then only dark, then only flat...etc. The red glow in the upper left did not show up until I added the flat. My flat looks similar to all my other flats I have ever taken and they have worked well. This is perplexing to me....I guess I do not have the knowledge to troubleshoot this.

[admin]

Hi,

so it does start to sound as though there might be an issue with the flat frames if you are gettting an anomalously dark (or light) corner when you add them into the processing mix. One thing you can do to test is to try captureing another set of flat frames (and dark flat). Then when you have got *two* master flate files, try to apply one to the other - if both flats agree on how much the different areas of the image are dimmed then you should get an almost perfectly uniform image. If you get anything with significant deviations in then one flat or the other is wrong. I do this when testing flat frame correction in SharpCap - create a flat frame, apply it and then keep flat illumination in place and check I get a good uniform image even when I change exposure.

I also noticed that there are lots of zero value pixels in some of your frames (dark flat for instance). This isn't good, and it means you should choose a higher black level setting to get rid of those if you continue to use gain 575. However I think this is a separate issue and unrelated to the flat correction problems.

cheers,

Robin

[alinderbaum]

Thank you, Robin.

Is there a black level you recommend using? I have looked at a lot of sources and cannot find much, if anything, on this. I see some postings in the forum but am not able to make a correlation to my issue. Is this a setting that needs to be changed each session, each type of capture, etc?

On the flat frame issue, do you happen to know what may have caused this? One challenge about this hobby (other than being a beginner and not understanding 90% of everyone's recommendations:) ) is identifying the cause of an issue to keep it from happening again. I have never had an issue with the flats (once I learned how to take them)....any thoughts as to what may cause this - and how to prevent it? Is it related to the software update to version 4? I always chuckle that when things are working great and I do not change anything in the process, about once a year a problem arises out of the blue that freezes everything and it is like starting this hobby all over again!

Thanks again for your feedback and insight.

Aaron

[admin]

Hi Aaron,

the simple way to set the black level is to set the camera up to expose a dark frame (including setting the gain/exposure that you intend to use for imaging). Once you have captured the dark frame, look at the histogram and adjust the black level so that the histogram peak is not touching the left hand edge of the graph. Now that you have your black level value set, use it for all the captures - lights, darks, flats, dark flat, bias - to avoid potential problems. For advanced use, you might change it when capturing the flats / dark flats / related bias frames to a new value that is used for all of those captures, but then you run the risk of forgetting to change it back.

The subject of where the flat has gone wrong is harder. There are some fairly typical flat problems (like under or overcorrecting in every dark area) - these are usually caused by either failing to take dark flats/bias frames with the flat capture or failing to use dark frames with the light capture. From what I saw of your images it looked like just one corner was causing issues, which doesn't really fit that pattern. It's possible that something changed between flat capture and light capture that caused the problem - for instance some dew on the sensor or sensor window or the camera being rotated or misaligned somehow.

sorry not to be of more help here,

Robin

[alinderbaum]

Robin,

You have been very helpful and that is much appreciated. I want to recap before I truly panic.

The session for which this thread was created was 'normal' by all my typical standards - no dew, no camera rotation, no cooling temp changes, etc. All was very consistent. Tonight, I went and took another set of flats. When I looked at the flats in PixInsight, they look exactly like all other flats I have taken these past 2 years. Dust spots in the same place, everything looks exactly the same as every set of flats I have taken on some 20 targets. There is nothing at all in the corner (the corner where the issue is on the stacked frame).

As you saw, lights, darks and bias frames all look good as well. Other than the Sharpcap update (to 4.0), nothing has changed in my set up or image capture process. Is there anyplace you recommend posing this problem to? I imagine if the folks here cannot ID a problem, nobody can. I truly enjoy this hobby, and it is not in my nature to give up, but I do not know what else to do. It is like I have been tying my shoes for 50 years the same way and now all of a sudden they will not tie and everyone I speak to is telling me to tie them the same way I am tying them

[admin]

Hi,

I have just spent some time looking over the images that you have shared again to remind myself of the situation. I am fairly sure that you must have an intermittent light leak - looking at the 'bad' dark frame that you shared, all the symptoms are there

Capture.PNG (232.34 KiB) Viewed 1156 times

You can see the grid pattern which comes from the bayer sensor matrix over much of the image, along with the triple peak in the histogram. Oddly the top left seems not to be badly affected compared to the rest of the frame.

It seems odd that you would have flat correction issues and coincidentally have a light leak - more likely that the two are connected. If you have light leaking during the capture of darks or lights then that would have an impact on the dark corrected image, which would in turn be affected by flat correction. For instance a light leak in the light frames would make the background brighter even after dark correction, then the flat frame correction would be applied to that extra brightness, meaning it would be more apparent in the areas where the flat does more 'work' to brighten the image.

If you still have previous raw data that worked correctly with your 2 year old flats, then another approach to try to track this down would be to compare the latest frames (dark/light/etc) that don't work with the older ones that do - examine them closely in an application like FITS Liberator where you can look at average brightness, brightness range, individual pixel values, etc and see if you can spot *how* the new frames that give problems differ from the older good frames. Once you have identified those differences, hopefully going from there to the likely cause will be a smaller step.

cheers,

Robin