Flat frame question

[LindV]

Was trying out the ZWO ASI2400MC Pro on the SW Esprit 120ed, after my imaging session with M31 on a moonless night, i was finishing up with my flats. When i was loading them up on my laptop, i noticed my flats look a little different to my usual flats with another camera. Will this still be usable for calibration? Attached image is unstretched.

Edited September 16, 2023 by LindV

[Elp]

Have you tried calibration with it?

[michael8554]

What is the light source ?

What exposure, it's underexposed ?

Michael

[alacant]

On 17/09/2023 at 00:31, LindV said:

usable for calibration

Hi

It looks unusual, but we can't tell for certain because we don't have a light frame against which to divide, nor do we know the offset etc...

If you're looking for a more definitive answer, maybe post -links to- originals of all the frames you intend to use for calibration.

Cheers 

Edited September 18, 2023 by alacant

[Ouroboros]

On 16/09/2023 at 23:31, LindV said:

Was trying out the ZWO ASI2400MC Pro on the SW Esprit 120ed, after my imaging session with M31 on a moonless night, i was finishing up with my flats. When i was loading them up on my laptop, i noticed my flats look a little different to my usual flats with another camera. Will this still be usable for calibration? Attached image is unstretched.

That looks decidedly odd. In fact if you’d have told me that was a photo of the underneath of the dog’s bowl I’d have believed you.

How did you take them? I usually wait till morning, point the telescope at a uniform bit of sky, away from the sun, throw a piece of white cloth over the end of the telescope and fire off 60 shots of suitable exposure. I have a light panel too, which I use at night. 

[Ouroboros]

Is the image circle too big for the scope?  This is a full frame camera isn’t it?

[LindV]

On 17/09/2023 at 04:29, Elp said:

Have you tried calibration with it?

I did and the images turned out worse than without flats.

11 hours ago, alacant said:

Hi

It looks unusual, but we can't tell for certain because we don't have a light frame against which to divide, nor do we know the offset etc...

If you're looking for a more definitive answer, maybe post -links to- originals of all the frames you intend to use for calibration.

Cheers 

Yes, i figured it looked off as my usual flats never looked like that, but that was with an ASI533MC Pro with a smaller sensor. I will try to upload my other calibration frames when i get home from work.

10 hours ago, Ouroboros said:

That looks decidedly odd. In fact if you’d have told me that was a photo of the underneath of the dog’s bowl I’d have believed you.

How did you take them? I usually wait till morning, point the telescope at a uniform bit of sky, away from the sun, throw a piece of white cloth over the end of the telescope and fire off 60 shots of suitable exposure. I have a light panel too, which I use at night. 

I usually take my flats after my imaging session with a white t-shirt and a light panel. ASIAir calculates the exposure time, i believe for this flat was around 330ms.

10 hours ago, Ouroboros said:

Is the image circle too big for the scope?  This is a full frame camera isn’t it?

Yes it is a full frame camera. The Esprit 120 with flattener (no reduction) is 44mm image circle, while the sensor for the ASI2400 is 43.3mm.

My current image train starting from the focuser is Focuser -> Flattener (Needs 75mm back focus) -> M62 to M48 Adapter (20mm) -> M48 Spacer (16.5mm) -> M48 to M54 Adapter (2mm) -> ZWO Filter Wheel (20mm) -> Camera (16.5mm to sensor with tilt plate)

Could the M48 spacers be the issue not being wide enough for full frame?

Many thanks and much appreciate all the replies!

Edited September 18, 2023 by LindV

[Elp]

Do your lights show a similar pattern without flats applied? As 48mm is larger than the diagonal of the full frame 43mm I wouldn't have thought it that, if anything it'd likely just show a uniform vignette at the corners, this looks like some sort of obstruction, near or out of focus.

Is it normal for your flats to be so short in duration? Just asking as I darken my flat panel light for the histogram peak to be around third to halfway as they're usually a few seconds long.

Edited September 18, 2023 by Elp

[woldsman]

You said you used a filter wheel with the MC camera. First thought is that the orientation of the filter is wrong & is causing the ring pattern as the rings are close to the edge.

Edited September 18, 2023 by woldsman

[LindV]

41 minutes ago, Elp said:

Do your lights show a similar pattern without flats applied? As 48mm is larger than the diagonal of the full frame 43mm I wouldn't have thought it that, if anything it'd likely just show a uniform vignette at the corners, this looks like some sort of obstruction, near or out of focus.

Is it normal for your flats to be so short in duration? Just asking as I darken my flat panel light for the histogram peak to be around third to halfway as they're usually a few seconds long.

That makes sense, my original thought was some sort of obstruction but just couldn't figure out if it was the problem or where it's stemming from.

I do notice the lights showing similar pattern without flats which i've never seen before when i used an ASI533 camera. Attached is M31 i was testing out recently and i do notice the rainbow banding around the galaxy like some sort of reflection within the tube somewhere.

15 minutes ago, woldsman said:

You said you used a filter wheel with the MC camera. First thought is that the orientation of the filter is wrong & is causing the ring pattern as the rings are close to the edge.

I think i have the orientation correct, currently have the flat side facing the telescope and the ZWO lettering bulky end facing the camera. Correct me if it's incorrect. 

Edited September 18, 2023 by LindV

[Ouroboros]

1 hour ago, LindV said:

I usually take my flats after my imaging session with a white t-shirt and a light panel. ASIAir calculates the exposure time, i believe for this flat was around 330ms.

I use the air too. I’ve only ever tried flats as short as about 1 second in duration, and they worked fine.  I used to worry about this because there were some discussions of forums of them needing to be at least about three seconds long. I’ve never tried exposures as short as 330ms. As I say, 1 second has worked for me. 

[LindV]

17 minutes ago, Ouroboros said:

I use the air too. I’ve only ever tried flats as short as about 1 second in duration, and they worked fine.  I used to worry about this because there were some discussions of forums of them needing to be at least about three seconds long. I’ve never tried exposures as short as 330ms. As I say, 1 second has worked for me. 

I'm wondering if it's because the light panel i'm using is too bright hence it calculated and used a shorter exposure time. I will try to lengthen the exposure time and see if it makes any difference.

I was doing some more research and i'm wondering if i attached my camera incorrectly to the filter wheel. Currently using an M54 adapter to attach the camera to the filter wheel WITH the tilt plate still attached. But i found this image stating the tilt plate needs to be removed and camera screwed onto the EFW. I will do some tests once i'm off work to see if that is the issue. It does seem plausible now that i'm seeing this image and the tilt plate is probably spacing the sensor too far from the filter.

EDIT: I may have misunderstood the diagram and probably need to keep the tilt plate on. I'm now wondering if it's the M54 ring i'm using to attach the camera to the filter wheel causing the obstruction. I may need to take the ring out and directly screw the camera onto the filter wheel back plate instead.

Edited September 18, 2023 by LindV

[Ouroboros]

You’re probably on the right track there, @LindV.  Rule of thumb is to have filters as close to the sensor as possible. I hope it works. Good luck.

[Elp]

Best thing to do if you've got the spacers is to introduce or in your case take one item out of the train at a time and take a flat.

[ollypenrice]

The outer rings on the flats are pretty much the light spectrum, so I wonder what might create that. A diffraction grating consists of elongated parallel lines which will open up white light into a spectrum. Is there, perhaps, such an object replicated in your light path? The rim of a filter or coupled adapters, for instance. If your light source for the flats is close to the objective, might it illuminate the inside of the dewshield or the lens cell in a way that light from the sky does not? Might it illuminate something circular which then works as a diffraction grating? This is just spectulation but how about extending the dewshield fully and moving the light source further away?

Olly

[alacant]

Do you have an m42 thread/adapter/spacer anywhere in the imaging train?