SharpCap Forums

Thanks Robin.
" In order to compare this sort of issue sensibly from one session to the next, you have to take care to use the same level of stretching in the image - even really good flat correction will show flaws if the image is stretched heavily enough, whereas mediocre correction can look quite reasonable on an unstretched or lightly stretched image" -

How can I ensure same level of stretch, or disable stretching?

Ehud

Whatever offset (black level) I use for my bias frames, I use the same value in my lights, darks & flats. See the last page of the doc. A real world example is here viewtopic.php?t=7696 , see post #9 (once you understand this you are on your way). It took a few years for me to get to that level of understanding. Following my own advice gets me images like this viewtopic.php?t=8322

This is not ideal but I took your master flat PNG and converted it to FITS format using PIPP. This is the resulting file displayed in FITS Liberator v3.
Note the Max value of 47,000. A figure around 30,000 (28000-32000) is what I aim for.


The answers to your problems lie in the Image Headers of your bias, flat, dark and light FITS frames.

Dave

Thanks Dave.
this means that if now I am trying to have the flats histogram in the middle, I actually need it to be a bit below the 50%?

Around 50% is good.

Next time you image, save everything as FITS as this will help debugging any problems. Draw up a table with bias, flat, dark, light down the left and offset, gain, exposure across the top and things will make sense (or not).

Dave

Hi,
I am still experimenting on clear nights.
M<any times the result is not smooth. I suspect it might be from the Dew Shield which I made from a black Yoga matt, and is not mounted exactly the same each time I put it.
Might it be a reason?

Hi,

yes, that could be it - if the dew shield sticks too far forward so that it interrupts some light that would otherwise reach the edges/corners of the sensor, then you will get a darkening of the image there that is *impossible* to correct using flat frames. That's because flats only deal with effects happening between the flat panel (front of the OTA) and the sensor. Anything outside the OTA/beyond the flat panel is not corrected.

I had a similar problem when I had two telescopes mounted side-by-side. One had a longer tube and because the front of the tube was further forward than the front of the shorter one, I got images dark on one side that flats could not correct. I eventually worked out the cause and adjusted the relative mounting of the two telescopes so that the problem went away.

cheers,

Robin

I had an issue with flats a while ago. I use a led panel for a light source and my corrected frames always had a circular over correction. I was putting led panel on end of a dew shield. I then removed dew shield and put screen directly on end of scope. This really helped. I also changed the histogram to 25% which helped further. But as was previously stated with extreme stretch some unevenness is seen. But much better.