what is wrong with my flat frames?

orly_andico · Post by **orly_andico** » Wed Jan 26, 2011 5:23 pm

i'm using Deep Sky Stacker. my flat frames are either of clear sky (with t-shirt) or white LCD monitor.

but it seems that when i add these flats to my stack of images, the vignetting gets worse!!

as an alternative, i did a DSS stack with no flats, then edited the final image (and a flat) in GIMP (photoshop-lookalike) and subtracted the flat manually from the final image there. and it was much better (although still didn't get rid of all the vignetting). so the "concept" seems OK but my use of DSS seems wrong..

i find the need to do this now as the WO-Flat2 shows quite heavy vignetting with my Zenithstar 70ED at the corners of an APS-C sensor.

cloud_cover · Post by **cloud_cover** » Thu Jan 27, 2011 12:14 am

Just wondering, if you inverted (white to black) the flats then tried them with DSS, would it get better? I have absolutely no experience but I would guess if the vignetting got worse then maybe doing what you're doing, but in reverse might help.

orly_andico · Post by **orly_andico** » Thu Jan 27, 2011 12:20 am

tried that before.. didn't do any good. something must be wrong with my workflow..

incidentally after raining all day i think it's clear..!

drat. spoke too soon. it was clear 30 minutes ago. sky always fills up with clouds past midnight these past few days.. and since my balcony faces west... i can't see orion till about 2 a.m.

Post by **weixing** » Thu Jan 27, 2011 10:05 am

Hi,
Since you shoot at home, why don't just get a big piece of white cardboard and shoot your flat on it??

Have a nice day.

orly_andico · Post by **orly_andico** » Thu Jan 27, 2011 10:29 am

Also tried using an LCD monitor displaying white as the flat frame (this is acceptable/normal... since LCD monitors have an EL backlight... there are companies which sell dedicated flat telescope caps which contain an EL panel).

The flat frame is "working" (I can even see the vignetting in the resulting picture). But it seems DSS subtracts too little of the flat resulting in still-obvious vignetting...

p.s. the problem with the flat cardboard is that interior lighting is not constant so the upper edge of the cardboard gets more light than the lower edge. OTOH if i put the cardboard flat on the floor so it gets constant lighting, my shadow falls on it when i take the picture from above...

timatworksg · Post by **timatworksg** » Thu Jan 27, 2011 1:13 pm

Too bright a flat frame can also be an issue. A dedicated light box is always the better solution. Though I have done flat frames with the white plastic cap in the adapter before my CCD and I pointed at a outdoor light source.

Do you take a single light frame or several? If several you could average them to make a Master Light frame and then calibrate into the image stack. I have read some AP-imager's use dark frames to subtract from the light frames to ensure clean calibration. Is color convert selected? Or any other boxes in DSS selected which could have effected the light frame subtraction?

Process all masters, subtraction and calibration before stacking/align. Then color convert and adjust colors accordingly. Long exposures can be finicky on a single process style, and software can be the same if something is a little different. Best is to try at best to stick to a particular set up, camera's and form a process style around that. Then you know what to expect from each different shoot.

I used DSS before but found it's calibration to auto and wanted some manual adjustments and refinements. I use MaxIm DL now for Masters, calibration and stack. Color convert is sometimes done in nebulosity or MaxIm. Final tweaks in PS CS3.

You could try to convert all FIT's/RAW files to TIFF. Then perform the calibration and stack. Sometimes you get pleasing results.