Flats - Somethings wrong here...

[JonHigh]

Hi All, 

I hope you can help? I decided to upgrade my ASI585MC to a Altair Hypercam 26c and last night was its first outing! I use SharpCap Pro and APT as well as Siril for stacking DSO images.

Sharpcap to do my Polar Align and flats (As I understand it better in Sharpcap) And use APT for image acquisition. I have yet to work out how to use Flats Aid in APT as I have no idea what figures to enter!

My concern is, and never happen previously on the 585 was that after processing my Flats inc biases the result I got looked like the Moon was superimposed. Can anyone shed light on this as flats are meant to cancel these things out?

I have attached an unprocessed plus a Background extraction from a Siril stack for your perusal. Also a rough processed version too.  16x3min subs, Gain 120. Thanks 

Edited April 29, 2023 by JonHigh

[Robert Hayes]

Hello. I believe its the filter wheel. It should rotate in one direction so the filter stops in the same place every time. If the filter goes bi directional ie clockwise and counter clockwise to get the nearest filter it can create these artifacts. I belive Adam block has a video on it. Or it maybe James from the DSO imager channel.Clear skies.

Edited April 29, 2023 by Robert Hayes
More info.

[JonHigh]

Hi. thanks for replying so quickly. I do have a filter holder but it was empty

[almcl]

The 'crater' effect can be caused by the dust moving relative to the sensor between the time the lights were taken and the flats.  Is this a possibility for your acquisitions?

If you blink btween a stretched flat and a stretched light, can you see any movement?

Edited April 29, 2023 by almcl

[JonHigh]

I took the flat frames directly after the lights. I guess that might be a possibility. I will check your suggestion out tomorrow. Thanks. If this was caused by something out of my control then fair enough. I know that flats can cause issues so was hoping it was down to user error. lol. 

Out of interest and maybe a stupid question... At 120gain in APT, is that the same value in Sharpcap? 

[michael8554]

Blobs of dew on the Light but not on the Flat.

Or vice-versa

Michael

[tomato]

The 3D button effect is caused by a slight shift in register between the flats and the lights. If your optical train is still intact then I suggest you retake the flats. If you set up and take down every session I would recommend you take flats at the end of the imaging run before dismantling.

If the dust buttons are only affecting the background areas, you can take a starless image into your photo editing software and remove them with the clone tool. A tad unethical perhaps, but it works.

[ollypenrice]

28 minutes ago, tomato said:

The 3D button effect is caused by a slight shift in register between the flats and the lights. If your optical train is still intact then I suggest you retake the flats. If you set up and take down every session I would recommend you take flats at the end of the imaging run before dismantling.

If the dust buttons are only affecting the background areas, you can take a starless image into your photo editing software and remove them with the clone tool. A tad unethical perhaps, but it works.

That's what I'd do. I don't feel removing artifacts is unethical when all you're affecting is background sky.

Olly

[tomato]

Thanks Olly, I try to tread carefully on this topic, knowing the lengths folks go to to improve peripheral stars with tilt adjustment when tools now exist to correct these in software. But I suppose these are are real astronomical entities and not background artefacts, there must be a common sense balanced approach in there somewhere.

[JonHigh]

Hi. 
Unfortunately, I have to dismantle my gear after each session. However i do my dark and flats at the end of each session. Controversially I don’t mind making slight adjustments/cloning to sort out artefacts in post here and there if it’s just  background issues but I would rather not with emission nebula so I really have to work at this. It’s just annoying that other than moving the scope to take the flats nothing else was changed. I wonder if the T-shirt I was using dropped lint on to the lens, as there seems to me more in the flats than on the light frames. Need to check. 
 

Also. Any ideas what figures I should  enter in APT with regards to doing flats and biases with this camera? Or can I carry on using SharpCap with the same gains as in APT. Are they transferable or should I just stick to one or the other? 

[Stickey]

Just saw the images. The blobs are quite strange to me. Could they be dew drops on the sensor window,  or on something optical close to the sensor, with a little bit of light leakage coming from the side?

As regards a white T-shirt dropping lint onto the lens, that should be far enough away from the sensor to be unfocused and not visible. 

Just some thoughts. 

Hope you find the cause soon. 

 

Regards, 

Mike

Edited May 1, 2023 by Stickey

[JonHigh]

Ah. Okay. May I need to turn the dew heater up on the sensor. Will give that ago tonight as it seems clear for a couple of hours. Thank you. 

[symmetal]

By the size and the sheer number of 'craters' i would also go with dew on the sensor window. The 26C does have a variable setting for window heating I believe, so make sure it's turned on. If they're on the sensor itself the spots would be in focus and if on a filter, which you're not using anyway, they would be significantly larger.

They possibly changed size or position between imaging and taking flats leading to the cratering effect.

As long as the camera gain is the same you can use any software to take the flats, the gain is specific to the camera and not the software.

Alan

[tomato]

The 3D effect is definitely characteristic of a shift in the sensor between the lights and the flats, but the root cause of the spots might be be dew. How close to vertical was the scope pointing during the session? In my experience, moisture droplets can impact the sensor more readily when the OTA is in this position, but they always appear as dark smudges on the image, being so close to the sensor.

[JonHigh]

Hi All.

Thank you so much for helping me out. Sorry for the delay in replying - life got in the way!!! 

So was out the other night and imaged M51 but ran in to camera power issues after the meridian flip so only got twelve usable subs to stack but happy with the result so far. Will be out again once clear skies adorn us again as much more integration time is need but it will do for now... So taking your suggestions onboard I ran the heater on medium and seems to have almost sorted it out.  The scope was pointing almost upright so next time out, I'll put it on the high setting and see. Being 2:30am at the time and done with that night, I realised that the flats exposure were much too long so I am now certain that could have been the part of the cause. Taking images of dew!!! 🙄. I shortened the exposure time and they came out so much better plus there wasn't as much dew!  So that's great. I also tackled the light leak which may be entering from the focuser, so I have a very temporary solution which seemed to have worked (Dark paper wrapped around the end of the scope extending down to the reducer!). I will continue using Sharpcap for PA and Flats and APT for Acquisition. 

Thank you.

 

Edited May 3, 2023 by JonHigh