Problem with Flats

Hi All

Happy new year to everyone!!

I am having a few issues with flats at the moment, I am using a program called Sequence Generator Pro which is very good for creating flats, you specify the ADU required and it does all the exposure sequences to work out the exposure time etc.

I am having difficulties getting flats with my SII filter though, as you can see, the first image is a HA flat with an ADU of 27k, the 2nd image is an SII flat with an ADU of 27K also, but there's a huge white area in the SII flat that I cannot explain or get rid of, anyone have any ideas?

I understand why the SII is darker, and that is because the Bright white section is affecting the "Average" for the whole image, but can't understand why it is only affecting the SII filter!!





Regards
Simon

I don't know what I've done, but the SII flats are coming out as I would expect, I am wondering if there was some condensation on the SII filter at the time....weird

Hi Simon,

You are right the SII is darker than the H-alpha and should require more exposure time to reach just under half of your total ADU on your CCD. I’ve heard of the software you are using, but I’ve never tried it. There for I can’t say if it’s unreliable or not. Can you manually set the ADU in the software?
Also check to see if the filter is not at an angle, and that’s its nice and flush in the filter wheel. Check also for Dew as you said. Sorry I can’t be of more help but I have never seen anything like this before.
Let me know how you get on.

Carl.

Out of curiosity is there much difference between a Ha and SII flat? Do you need to take them to mainly deal with different dust particles on the filters or do the differing wavelenghts cause a slightly different flat frame?

Simon - what data do you need to feed SGPro in terms of the CCD chip? Not that that has possibly anything to do with the images you're seeing but it might be one place to look.

I've seen similar behaviour taking flats through various filters - I saw it when there was either condensation on the filter or when I was taking sky flats and there was passing cloud. I got rid of the sky-flat problem by investing in a light-box. Avoiding condensation is hard.

One other possibility - is the filter itself OK? No sign of any coating peeling off etc?

Some food for thought anyhow.

Albert - flats for each filter predominantly for differing dust donuts but I suspect there are possibly enough optical differences that a different filter will have some kind of effect on the optical path.

Dave

Hey Dave

Because it detects the camera, it says that the ideal ADU is between 25k and 30k, so I opt and set it to 27k

I'm still having problems with my Flats for OIII and SII, they seem to punch a black area in my images, the HA flats are perfect though

The filters look fine, they are Baader filters and I have the reflective coating facing outward from the camera, the only thing I can think of is condensation on the filters themselves, might tape in a couple of silica gel packs to the inside of the FW, it's sealed at both ends by the camera and by the field flattener. StarlightXpress are sending me a replacement motor for the EFW as it became "Sick"