Another flats help topic

[Ibbo!]

WO 132 QS! 689 with lum filter.  300 secs binned 2 x 2

The flat deals with the dust bunies and what I think is the shutter (maybe not) but introduces what looks to be vignetting.

 

My thoughts are the stupidly long extension I have to use might be the problem any other suggestions wecomed.

If i get another clear spot I will try the 102.

Flat

 

With flat applied

 

Without flat

 

 

 

[happy-kat]

Flats deal with dust bunnies and vignetting.

300 seconds for a flat is a very long exposure.  You have several dust bunnies but there is a strange mottling pattern with out the flat and on the fiat and on the combined.

[Oddsocks]

Need much more information to be able to make a diagnosis.

Which software is being used for image calibration?

Which calibration data is being used, bias frames, dark frames for lights and dark frames for flats or bias frames used in lieu of darks for flats? 

Was the camera cooled to the same temperature for both lights and flats?

What was the light source for the flats acquisition and was an IR blocking filter being used in the image path?

When you mention ‘Stupidly long extension’ are you talking about the USB or power cable to the camera? In either case, a long cable should not degrade data as USB data protocol is not analogue in nature, only binary logic, the data contents can not be easily changed by bad cabling, only communication errors can occur leading to failure to download any image at all.

External electrical Interference over long USB cable runs or camera power supply issues can cause image defects in-camera but these would usually show as bars, lines or dots in the image, not as whole-image problems such as a failure to correctly calibrate a set of lights with a set of flats.

If you mean by 'Stupidly long extension' a Mechanical coupling between the OTA and camera then yes, this could be the cause, especially the brightening ring in the vignetted field, this is often down to poor blackening of the internal surfaces of the camera coupling extension, if the coupling is just black anodised and not baffled then some wavelengths of light are reflected off the coupling walls more than others, IR is particularly bad for this since normal anodising dyes do not absorb IR, appearing almost white. In this case the light source used for the flat has to have exactly the same spectrum as the night sky else when the flat is used for calibration the result will not be a good match and residual uncorrected artefacts in the vignetted field will remain. An easy a inexpensive fix is just to paint the inside of any couplers with high temperature mat-black barbecue paint fro B&Q etc, as this is a pigment paint it absorbs IR quite strongly.

This web document is a good explanation of the problems caused by anodised surfaces.

http://diffractionlimited.com/flat-fields-stray-light-amateur-telescopes/

As always with these sort of image calibration issues, it’s not often possible to make a diagnosis, other than a guess, from in-forum jpg images, it will be easier if you put a FIT format copy of the calibration masters, bias, flat, dark and the resulting image of the uncalibrated registered and integrated lights (four images in total) in a shared Dropbox or Google Drive folder.

 

[Ibbo!]

Flats are 18 secs (also tried a 8secs originally but used more paper to reduce light)

Sub is 300 secs

maxim for data collection and stacking (also APP with same result)

Yes both cooled to -25°C

No darks but yes bias frames used

2 LED panels that i have used before successsfully

 

 

Sorry the stupidly long extension is the adaptor needed for the 132 to get to focus (as they are short OTA tubes so a bino- viewer can be used without a barlow)

 

 

 

 

 

[Oddsocks]

4 minutes ago, Ibbo! said:

Sorry the stupidly long extension is the adaptor needed for the 132 to get to focus (as they are short OTA tubes so a bino- viewer can be used without a barlow)

Probably then it is down to internal reflections off the walls of the extension, especially if it is just plain anodised aluminium.

When you replied I was just completing an update of my first post, so have a reread as I have added some extra info and web link.

At the moment there is a bug in the calibration engine for APP and the software author is re-writing this part of the application, Maxim should have been fine though.

William.

[Ibbo!]

Thanks for the answers.

 

i have put the Tak 102 back on the mount for the time being and will see if i can find my 4" s/s tube extension I made a while back.

Its not neat but it extends the tube at full dia for 4" and it is/was blacked on the inside.

 

Now that you mention it I have in the distant past (like 15 years ago)had the reflection problems on the internal "shiny tubes" which I had forgotten about.