12 replies to this topic

[dcm_guitar]

I'm not sure whether this should be in the beginner or advanced forum.  Since it's a flats problem, I'm going to put it here in the beginner area.....

 

 

I am working through a new telescope; a Tak E130D.  I had it out for my first images last night.  I took flats and some images of the Witch Head nebula.  Going through the data today shows the calibrated results look like the flats are over correcting.  

 

I have bias calibrated the individual flats (no dark calibration on the flats).  Then integrated the flats.

 

Here's some stats on my data:

 

 

Bias median value:  3.065^10-3

Dark median value:  3.07^10-3

Lights median value:  8.15^10-2

Flat median value:  4.25^10-1

 

The Bias and Darks are very close in median value, but the calibration images are correctly ordered in terms of median (and mean) values.

 

I generally calibrate with Bias and Flat during the image calibration step, then calibrate with darks during cosmetic correction.

 

So, far I've been scratching my head.  I can't figure out why my process that works so well for my FLT132 does not work at all on this data from the E130D.

 

I took a look at the histogram of the flat from my FLT132 and compared it to the histogram of the flat from my E130D (both from my ASI6200MM).  The histograms look VERY different.  I should also mention that I take my flats at gain 100 and offset 20.

 

Flat histogram from the FLT132 flat:

 

 

Histogram from the E130 Flat:

 

 

 

 

Also, the unstretched images of these two flats look very different.  While both the flat from the FLT132 and the E130D have the same mean and median values, the flat from the FLT132 looks like a flat, gray image.  The unstretched flat from the E130D looks like a stretched image; you can clearly see the vignette even though the image is still linear.

 

Obviously, the E130D is a LOT faster than the FLT132 (f/3.3 vs f/7).  For my individual flats on the FLT132 the exposure time is 0.99 seconds.  For my individual flats in the E132D the exposure time is 0.41 seconds.

 

So, what's going on here?  I can not get my lights correctly calibrated and remove the vignetting.  The statistics from the E130D flat looks good, but the histogram is very strange.  Thoughts?

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Edited by dcm_guitar, 25 January 2023 - 05:52 PM.

[Oort Cloud]

Can you post links to the FITS for each?

[dcm_guitar]

Can you post links to the FITS for each?

Hope this works....

 

Here's a link to a folder on my Google drive.  Inside is the integrated flat (lum_flat.xisf).  I also included 3 of the individual .fits of the flats.  Obviously, these are essentially identical.

 

https://drive.google...?usp=share_link

[idclimber]

Those are 3 flats. What we want to look at is one Dark, Bias, Flat and a Light. this way we can run a test calibration and inspect each file type. 

[idclimber]

It appears you are using an ASI6200 camera. The relatively small file size indicates you are binning?  The middle of the flats is right in the middle of the histogram at 35K. The edges drop quite a bit to about 15k.

 

Is this scope considered full frame?

[dcm_guitar]

Those are 3 flats. What we want to look at is one Dark, Bias, Flat and a Light. this way we can run a test calibration and inspect each file type. 

I keep a dark library based on bin and exposure time.  I keep a bias library based on bin size.  So, I don't have the individual .fits from my darks or my biases.

 

I idid upload a master bias (2x2 since this is the bin size I'm using with the E130D).  I also uploaded a master dark for 2x2 and 60 seconds.

 

I've been using these for a while with no issues.  All of my data from my FLT132 calibrates fine.  This is the first time I've tried to calibrate data from the E130D. 

[dcm_guitar]

It appears you are using an ASI6200 camera. The relatively small file size indicates you are binning?  The middle of the flats is right in the middle of the histogram at 35K. The edges drop quite a bit to about 15k.

 

Is this scope considered full frame?

Yes, 6200MM.  Yes, binning 2x.  Yes, this scope is full frame.

 

I've had no issues calibrating with these files using my FLT132.  It's the E130D where things are different.  The statistics for the flat data from the FLT132 and 6200MM and the statistics for the flat data E130D and 6200MM are essentially identical.  I aim for around 28k when shooting flats.  I use the NINA flat wizard.

[idclimber]

what does the calibrated light look like. Can you post that??

[dcm_guitar]

what does the calibrated light look like. Can you post that??

I added a directory in the original link called "lights".  In this directory I uploaded 2 files.  One is the un-calibrated image and the others is the calibrated image (bias and flat calibrated).

 

Thanks for looking!!!!

[idclimber]

What is interesting is the calibrated light shows pretty decent correction on the left side, but has obvious issues on the right. Is this a possible light pollution gradient?

 

[idclimber]

I came up with the same calibration result using your bias, flat and light. 

 

If you look at the uncalibrated light you can clearly see it is lighter on the right side. If I use the curser in PI I get about 5700 ADU in the right lower corner. I get about 4500 in the left lower corner. 

 

This is not seen in the flats. They are very uniform. Either this is a gradient caused by something in the sky like LP or a possible light leak. 

Edited by idclimber, 25 January 2023 - 07:51 PM.

[dcm_guitar]

I came up with the same calibration result using your bias, flat and light. 

 

If you look at the uncalibrated light you can clearly see it is lighter on the right side. If I use the curser in PI I get about 5700 ADU in the right lower corner. I get about 4500 in the left lower corner. 

 

This is not seen in the flats. They are very uniform. Either this is a gradient caused by something in the sky like LP or a possible light leak. 

I'm leaning towards light pollution.  I'm in Bortle 5/6 and trying to take broadband images with an f/3.3 telescope.  I wanted to use my darks library, so I used 60 second exposures.  I think I'm overwhelming the sensor with my craptastic skies.

 

Thank you for looking.  This is really helpful!!!!!

 

Tonight I'm going to take a set of exposures from 15 seconds to 60 seconds in 15 second increments.  I'll then calibrate the results and see what happens.  Because I don't have a 15 second dark calibration image in my dark library, I'll just calibrate them without darks to see what I get.

 

I'm pretty capable in mounting up my f/7 refractor and getting good data.  a really fast reflector is almost like starting all over again.

 

Thanks for the help!!!!

[Oort Cloud]

I'm leaning towards light pollution. I'm in Bortle 5/6 and trying to take broadband images with an f/3.3 telescope. I wanted to use my darks library, so I used 60 second exposures. I think I'm overwhelming the sensor with my craptastic skies.

Thank you for looking. This is really helpful!!!!!

Tonight I'm going to take a set of exposures from 15 seconds to 60 seconds in 15 second increments. I'll then calibrate the results and see what happens. Because I don't have a 15 second dark calibration image in my dark library, I'll just calibrate them without darks to see what I get.

I'm pretty capable in mounting up my f/7 refractor and getting good data. a really fast reflector is almost like starting all over again.

Thanks for the help!!!!

Hey, sorry I never got back to you, but I agree with Dave's assessment. Also, at f/3.3 I'm guessing the FOV is probably pretty wide...LP gradients are a function of both how much LP there is, and FOV. Optical speed doesn't really factor in, beyond the loose association that faster typically means wider. What matters is how much sky you're covering; the more sky (FOV) the more difference there will be from one side of the frame to the other.