Flats Over-correcting?

[NGC7789]

What is the light source behind the fabric? How far away is this light source (unless is 93 million miles  )? How much space is there between the fabric and the first glass element?

I ask this last question because I once added a dew shield. Out of laziness I just stuck the EL panel I use for flats on the end of the dew shield which moved the panel away several inches. It totally threw off my flats presumably due to reflections or shadows caused by the dew shield.

[iksose7]

You may have just hit the nail on the head there!

I leave my refractors dew shield fully extended when taking flats, this may be the issue as it leaves about 15cm between the lens and the light source. I will have to wait for a break in the weather to go out any try some with it retracted.

Its funny, if this is the cause, i actually learned this lesson a couple years back with my EF200L lens, that flats without the lens hood on were the way to go. Somehow when i bought my frac i just didnt carry that info over, although my DSLR didnt seem to mind too much.

I'll take some new flats asap and update how i get on!

[jkmorse]

I think you are absolutely right.  I do not have this problem with my CDK12.5, which has no dew shield, but I have been shooting mostly with my e130D lately and there I use a dewshield.  If the damned weather ever clears here in Houston, I will see if that solves things.

Best,

Jim