I have two questions I hope you can help me with regarding flats. Please take a look at the attached flats for two of my filters. What would cause this? Are the filters defective? Both are Astrodon 3nm. The OIII at 3 nm is OK. I am using an Alnitak FlipFlat to illuminate flats. I have noticed that if I take a flat at a given exposure (say 3 seconds) and a given brightness level (say 14), then repeat this five minutes later, the histogram peak (Maxim DL) will have shifted to the left, i.e., its value will have dropped. It slowly drops over time. Is this an issue with the light source being inconstant? Or with the CCD response changing over time? Thank you. Alan
Hi Alan, please supply FITS images. We can't tell much of anything from a JPG. The FITS gives us all the details on the exposure, etc. What camera?
(I suppose its an 11000 or 11002). Tell us also what the optical setup is. The Ha flat could be a reflection problem as well.
Colin, Links to FITS images Sii https://1drv.ms/f/s!AtHc9a_3_v7tgsNc63gyfb6wLUUtYA Ha https://1drv.ms/f/s!AtHc9a_3_v7tgsNc63gyfb6wLUUtYA Files too large to upload. Camera STXL-11002 with FW8G-STXL filter wheel. Tak FSQ-106, Gemini rotator focuser, filter wheel and camera. Alan
Try again. Sii https://1drv.ms/u/s!AtHc9a_3_v7tgsl5G8kQHOLAc0T4dA Ha https://1drv.ms/u/s!AtHc9a_3_v7tgsl6xOn9Okj8nBw5xQ
I found them in the original upload... lots of stuff there. Are these unmounted filters ? (eg glass, no aluminum holder) - there's a bit of vignetting (I think normal for FSQ on that camera), but it seems a bit asymmetrical. My impression is that you might have stray light hitting the side of the flipflat or some other reason for uneven illumination. Is the observatory dark when you use the flip flat? Is the flip flat partly open / maybe a light leak? Is this an EL flipflat or an LED lit one? It may need a few seconds for it the power supply to stabilize at the brightness level. Maybe @Doug will have further thoughts.
Flats can be tough. Any kind of light leak will make them fail. Even what looks like a trivial thing with the telescope tube will make a mess of it. That is especially true for narrowband flats. Make absolutely sure there isn't any light shining on the camera, filter wheel, focuser, etc., in case light finds a sneak way past. The front of the telescope must be completely illuminated. If you don't cover every square inch of the front aperture, including some distance outside the glass (because light from the sky is a cone), then it won't work.
Gentlemen, Thank you for your replies. The filters are unmounted. The vignetting has always been slightly asymmetrical, even with the telescope pointing up and an ELP resting on top. I concluded that is the nature of my optics and accepted it. Below are links to flats taken last night for all of my filters, one after the other in the order listed. Note that L/R/G/B and OIII were fine but the Ha and Sii are not. Conditions were the same for all filters - there must be something wrong with these two. Unless you can think of something else, I am prepared to pull the cover off the filter wheel and examine these filters for any physical issues, e.g., tilted, not properly seated, etc. I would like to eliminate everything else first. I made sure the FlipFlat closed completely against the dew shield. L https://1drv.ms/u/s!AtHc9a_3_v7tgsokaKQ1G_bSKcxr3w R https://1drv.ms/u/s!AtHc9a_3_v7tgsolSebVNMH-fW0i8Q G https://1drv.ms/u/s!AtHc9a_3_v7tgsomcEGhea1sh7H3ow B https://1drv.ms/u/s!AtHc9a_3_v7tgsone5h9IQwOLdBJNg Ha https://1drv.ms/u/s!AtHc9a_3_v7tgsooSZE2EQSRbvjcXw OIII https://1drv.ms/u/s!AtHc9a_3_v7tgsopvUiKDOS3Xy0Dkw Sii https://1drv.ms/u/s!AtHc9a_3_v7tgsouF7UWQniUGzpmwQ The Sii flat is from the previous evening - I can't find the one I saved tonight but it looked the same. Alan
