Master Flat... well almost

[Earl]

I managed to get to this last night, but as you can see there is a line down the one side so there is still something not quiet right any ideas?

QHY8L, TV reducer Meade 80mm Triplet and a CLS CCD filter.

[Jessun]

There a are many equally spaced lines there across the whole image. Never saw this, hope someone here can help! What does a master BIAS look like?

[Earl]

Bias and darks also have lines.

[Jessun]

Ok, my laptop doesn't show line on that dark. Could it be odd columns? If they come up in the BIAS and DARKS chances are good that once applied to the FLAT subs that the master flat will be fine and still 'work' on the LIGHTS. At least that's what I find on my ugly line on my cam.

[Earl]

this is the dark stretched

[FrankieValley]

There is a line on the left, from top to half way down. That looks like maybe a problem with the sensor. The other evenly spaced lines could be the same or they could be caused by interference/noise. Try changing cables, psu's, etc to eliminate any interference/noise.

[Earl]

I have just done a fresh set of masters.

the PI tutorial says you do not subtract the bias from the Darks as this is done during the calibration process for lights anyhow. im not convinced as the Lines in the Bias and Darks should not show up in flats.

Dark and Bias have been stretched to show the details

[Earl]

Im runing a fresh calibration, making a bias subtracted Master Dark, and seeing if this solves the problem.

[pereloup]

First post!

That's why we use bias frames in our calibration: to remove that kind of signal from the light frames! I have a QHY8L and I have the same lines in my bias. If you substract the bias signal in your calibration process, the lines will be removed and the final image will be OK. Remember: substract the bias from the flats and the darks from the lights but don't substract the bias from the lights because the darks already contain the bias.

On the other hand, I still have problems with my flats, they overcompensate the vignetting. I tried skyflats, t-shirt flats, lightbox flats with always the same overcompensation. Do you have the same problem?

Stephan

[Earl]

Hi Stephan, welcome to the board, I would be very interested to see a few images you have done your self.

What software do you use in the calibration process?

I have really been struggling to get good flats, my last set seemed ok, but i still suffer from the bias lines in them.

Im starting to ponder if im calibrating correctly.

[harry page]

Hi

Do not worry about these lines as they are part of the chip

Thats why you would subtract a bias or dark from the flat and Light frame:hello2:

Harry

[Earl]

If i use DSS it creates this as the master flat, which does not have the lines other than 1, which says to me I am calibrating in PI incorrectly, even though I follow the tutorials to the letter.

[Earl]

This is a Single Flat which I have debayered and it shows no lines at all, so Calibration is definitely adding lines.

[pereloup]

What software do you use in the calibration process?

I use Nebulosity. I don't use PixInsight so I can't help you with that.

The calibration formula with darks and bias is

Final image = (Lights - Darks)/(Flats - Bias)

The Darks include the Bias so you don't subtract the Bias from the Lights.

Hi Stephan, welcome to the board, I would be very interested to see a few images you have done your self.

Below is one of my best image of the M81 Group of galaxies taken with the QHY8L.

Stephan

[Earl]

I have created masters using Astroart and now my results have no lines

[harry page]

Hi

I like AA for calibration , simple to use

Harry

[jaspalchadha]

what offset and gain do you use for ur images Earl

[ollypenrice]

Another Astro Art stacker and calibrator. The great thing about AA5 (or one of them) is the coherent control it gives you. You can tell it what you want it to do. So, flats; put all your raw flats in the images section, put your master bias in as a dark for these 'images' and make a master flat. No messing about. No wondering what it has done with your bias, your darks, etc, in an all in one operation. You can also look at your master flat and decide whether or or not you believe it!

Over correction of vignetting is something that plagues one of the setups here but none of the others. I wish I could tell you what the solution was. One thing that does help is exposing the flats to a low value, around 15000 ADU. It may be that something in the lightpath is not as black as it looks. It turns out that some matt black finishes are very reflective in UV. Great!! Just what we need.

AA5 also gives you the option of ditching darks and using a Defect Map (a dark with 2000 ADU clipped off the black point. It's in the instructions.) You put this in a special box on the calibration page and substitute a bias for the normal dark on the same page. With the very noisy KAF11000 chip this (with hot pixel filter applied) gives me super clean images from very short stacks. I do mean clean, too. No fixes needed.

Olly

[Earl]

what offset and gain do you use for ur images Earl

Its quiet a while ago, I cant remember sorry

[Mav359]

Hi Earl

Don't know if you got this sorted or not but i had a similar issue when i first got the camera. Once i set the Debayering settings manually i never saw it again.