Issues with short exposures and flats

[metastable]

Hey Robin,

I'm attempting to keep my setup minimal which means foregoing guiding and relying on shorter exposures and using Sharpcap to recenter when the images drifts enough pixels. I've noticed that when using flats and short exposures, I'm doing 15s, that I'm seeing artifacts when doing stretching, from what I believe flats are meant to correct. If I go with longer exposures these artifacts don't appear.

I know in a previous post I made it was mentioned that Sharpcap doesn't just sum up the stacks but averages. Could this be a side effect of that?

Thanks,
Alejandro

[admin]

Hi,

looking at that image I am seeing what looks like a ruler cut-off line running vertically about 25% of the way across and darkening in the corners, particularly bottom left and top left.

Flats should largely correct the corner darkening, but to do so accurately they need to be applied correctly, particularly if you intend to stretch the image in processing. There is a section in the documentation on getting good flat frame correction that covers the relevant points : https://docs.sharpcap.co.uk/4.1/#Gettin ... Correction

It's also worth noting that if your corners are very much darker than the rest of the frame (say 20% of center brightness or less) then you will struggle to get good flat correction in stretched images - in the case of a 20% brightness corner, the flat correction will be multiplying pixel values by about 5, magnifying the impact of any small issues in flats/darks/bias frames. These are then increased further by the stretching...

I can't think of anything specific that might cause the flat correction to be worse for shorter exposures than long ones unfortunately

cheers,

Robin

[metastable]

Hey Robin,

I should have been clear about the artifact. I'm not sure why that ruler line showed up in this frame, but I don't normally see that. If you look in the top left portion, there's a donut. I've highlighted it now. This only appears on the shorter exposures. I do also take Dark Flats as suggested by the link you gave me.

Thanks,
Alejandro

[oopfan]

Hi Alejandro,

I am replying based on my experience with "short" exposures. What is short? It's all relative. What's short for me is long for the next person depending on sky brightness, camera, and scope.

What does your sensor analysis say? Mine says 60 seconds so I know I'll have trouble doing 15-second subs. At 15 seconds I'll have troubles with raining noise and dust donuts. What is the cure? Following your sensor analysis minimum recommended exposure. It ensures that the sky background "bakes" into the chip. Many problems go away then.

Brian

[metastable]

Hey Brian,

I haven't run a sensor analysis. My 15s exposure was chosen based on the Smart Histogram which came back with 15.6s. This was some time ago, so I don't remember the exact options I had given it. I'm running a 533MC Pro btw.

Thanks,
Alejandro

[oopfan]

Alejandro,

My bad, I meant to say Smart Histogram. So you've got a fast scope and/or bright sky. So, if Smart Histogram recommended 15.6 seconds then that negates my concern. Sorry, I wish I could help but I am out of ideas. Except for one! I noticed that in the upper-left corner of your image you have a trail of red hot pixels. You might look into the possibility that you used the wrong dark frames.

Brian

[metastable]

Hey Brian,

No worries. Yeah, I'm using an 8inch Newt at f/4. I'm glad you pointed that out. Hot pixels seem to also be an issue with my short exposures. I should be able to setup my scope tonight and I'll take new flats and darks and even save out the individual frames. Maybe by sharing the actual data will help with diagnosing what might be the issue.

Alejandro

[turfpit]

Alejandro

If you have them, it would be helpful if you could post somewhere on the internet a light, dark & flat frame (1 of each) for download and inspection.

Dave

[metastable]

Hey everyone,

I managed to get some frames. I've placed them into a shared drive folder here https://drive.google.com/drive/folders/ ... drive_link. I'm also attaching an image of what I'm currently seeing with a heavy stretch. From what I can see, it seems to be an inverse of the flat frame. Very strange.

Alejandro

[oopfan]

Alejandro,

I'm not sure if it makes a difference, only because I am anal retentive about capturing everything in FITS, but I noticed that your Master Dark is a TIFF file and the others are FITS.

Brian