Flats over-correcting

Fede

Member
Mar 16, 2021
5
0
Hi,

Yeah, I know a lot of people had this problem but searching on forums I could not resolve this issue that still ruins my integrations. I have made many attempts, taken flats and dark flats several times (with different ADUs and without touching camera and focusing of course) but with no success. I have taken lights and flats/darkflats with two different profiles in SGP but one is a copy of the other (same camera drivers, same settings). I calibrated flats with the master darkflat and then lights with master dark and master flat. I can't figure out how to resolve this issue. I'm new in astrophotography and I'll really appreciate any help.

Here is an example (after a fast ABE). I had to increase Log sensitivity in StarAlignment process otherwise, when I calibrate lights with flats, it can't align frames well and I get strange trails with stars and the central object.

integration_ABE1.jpg

Some details:

Acquisition software: SGP
Camera: ASI 294MC Pro
Drivers: ASCOM
Lights and calibration settings: gain 121, offset 30

I share some lights, darks, flats and dark-flats in a Dropbox folder.



Thank you so much
 
Last edited:

bulrichl

PTeam Member
Nov 2, 2016
1,121
135
La Palma, Canary Islands
Hi,

often an erroneous calibration workflow is the cause when flats are overcorrecting, but this is not the case here. Your flats are not usable.

This could be caused by reflecting parts (e.g. extension tubes) in the optical train. I suggest you to take off the camera, view through the telescope from the camera side against an illuminated white area and look out for reflections. Reflective surfaces in the optical train have to be removed, e.g. by exchanging a reflecting part, or painting with a suitable matt black paint, or pasting up with self-adhesive black velour.

Bernd
 

Fede

Member
Mar 16, 2021
5
0
Hi Bernd,

I have a new Newtonian telescope ad I use a LED table to take flats. Is this an incorrect way to take them?

Thank you so much.
 

bulrichl

PTeam Member
Nov 2, 2016
1,121
135
La Palma, Canary Islands
The flat frames that you captured are exposed correctly.

For flat frames, a uniformly illuminated field is necessary. So if your LED table generates uniform illumination, it is suitable.

If you used a dew shield when capturing the light frames, capture the flat frames with the dew shield in the same position. The LED table should lie flush against the dew shield. However, flexible dew shields might cause problems with the flat frames.

As I wrote, I suppose that in your case internal reflections cause the problems. If my assumption is correct, it is easy to detect and put things right.

Bernd
 

Fede

Member
Mar 16, 2021
5
0
Hi,

Thanks for your answers, I tried to take a shot of newton tube illuminated by the LED table (I took it with a smartphone, so the collimation is not good). I don't know it for sure, but I don't see any reflection that could corrupt my flat frame. Could I have your opinion about it too?

Regards


20210318_125210.jpg
 
Last edited:

bulrichl

PTeam Member
Nov 2, 2016
1,121
135
La Palma, Canary Islands
Viewing your image, I cannot judge the situation since I have no experience with newtons.

What is on top of your image - is it the coma corrector?

There seem to be reflections from the coma corrector (?) and from the edges of the secondary mirror.

In my view, nothing should protrude into the tube, and the edges of the secondary should be blackened. Maybe there are still more issues. Some reflectors are not shielded from light incidence from the rear of the main mirror (because of vent holes). Probably it could help if you looked into the tube from the open side as well.

The following link might give you an idea of what can cause your issue:

Perhaps someone experienced with newtons can chime in here?

Bernd
 

Fede

Member
Mar 16, 2021
5
0
Hi,

could these light reflections I see in frames be generated by an out-of-collimation telescope?
 

bulrichl

PTeam Member
Nov 2, 2016
1,121
135
La Palma, Canary Islands
No, I don't think so. Bad collimation primarily will produce distorted star images, especially in the image corners.

In your M 51 image, the star images near the image center are OK, but at large distance from the image center, coma is visible. However, the distortions seem to be symmetrical, so this is probably not caused by bad collimation. It might rather indicate a wrong distance (it is essential to achieve the correct distance!) of the camera sensor from the coma corrector. However, this is not related to stray light and reflections.

Here is another link that might be helpful to you: https://diffractionlimited.com/flat-fields-stray-light-amateur-telescopes/ .

Bernd
 

AstroDude

Active member
May 2, 2017
37
0
Hi,

could these light reflections I see in frames be generated by an out-of-collimation telescope?
i have the same problem but mine is inverted , center is overcorrected and edges are good.
ZWO294MC with ES 8 inch F3.9. all i can offer is try to find those light leaks bouncing off the entire image train and make sure your flats are 25000/30000 ADU with 2-3 sec expo , no less. im still fighting it and will be darkening everything inside the scope. Right up to the 294MC.
im using the Baader Mk II , not sure if is the culprit.
Mitch.
 

Attachments

Fede

Member
Mar 16, 2021
5
0
Hi,

Thank you for your response.
I don't know how to find those lights. Now, I'm wondering if they can be caused by light pollution (I live in a big city) or by the LED table that I use to generate flats. I read that in some cases it can generates gradients. I use a gray paper to dim it.

Fede
 

akoenig

Member
Jan 27, 2017
15
1
There’s a great video by Adam Block on how you can correct this issue with Pixelmath.
Another thing you could try, is to take your calienation files in as dark an environment as possible, if you’re not doing so already, and, this has helped me a lot recently, to take a higher number of calibration subs. Maybe 50 plus.

CS,
Allen