Problems generating my flats

I'm no expert, but I think this is a reflection in the light path

What are you using to take the flats with? That camera is tricky with light sources. I'd get that filter drawer off there. Try some heat and strap wrench to loosen it.

Adam Block:
Only H-alpha (and perhaps SII)?
That means it is wavelength dependent and likely an interference effect. If so, it *is* in your lights- but it would be hard to see since the sky  brightness is so dark in H-alpha (and red in general). If you see it at all wavelengths, then scattered light from a cylindrical surface (like an adapter). 

The coatings on the chip or glass cover may indeed have this pattern. It honestly doesn't look symmetric enough to me to be scattered light.

Hi Adam! Thanks so much for taking the time to help me. I'm going to take shots with all the filters, both narrowband and wideband, to confirm if the circles appear in all of them. I managed to get my HA master to come out without those two circles in the center by using a library of flats from a year ago. These are flats I took at a different temperature than the current ones and with the camera in a different position. I'm going to try removing the filter drawer and give it a try. CS!

What are you using to take the flats with? That camera is tricky with light sources. I'd get that filter drawer off there. Try some heat and strap wrench to loosen it.

Hi Scott! Thanks so much for your suggestions. I use an iPad as a light source for my flats and haven't had any problems so far. I'm not sure if the filter drawer was already in place when I took my flats without details. My memory is failing me. =). I'm going to follow your suggestion and remove it and try again. CS!

m? I had trouble with reflections and light leakage taken that way. I then started taking them in a completely dark room and problem solved. Also, what are your exposure times, a few seconds or very short?

Hi Scott! It’s true what you say....I’ve already had 5 hours of HA with the current setup ='(. I first took the flats outdoors after finishing my session, it was still dark although in the middle of the city. Then I tried as you say: inside a darkroom but the problem continued although I remember I had the laptop screen almost in front of the telescope, I don’t know if the light coming from the laptop screen could be affecting me, I will do the test again but being careful that it doesn’t shine light on the tube. My flats time is 7.75 s. Thank you very much for your support. CS!

That's a flat in Ha (Sii can look very similar) that looks perfectly “normal” for that camera and sensor. The 294 is known for being very finicky to calibrate; you can find more information on CN.

Hi, Marcelof, thank you very much for your comments. That's right, the 294mm flats are very special, although I only had problems at the beginning. After that, I haven't had any problems until today, when I see circles in the center of the flat, which means they also appear in my final stacked HA master. I'll check online for your suggestion. Best regards! CS!

Médéric Hébert:
Hi Médéric. Thank you so much for taking the time to share your feedback. I'll try removing the extra filter drawer and see what happens. CS!

Hi Médéric! Thanks for your feedback! Fortunately, the problem was solved! The problem was caused by the least expected thing: the light source. I used a 2024 iPad mini for my flats and remembered that the last successful flats I had generated were with a regular iPad. I did remove the filter drawer, but the problem persisted. I tried with a white shirt and the iPad mini on top, and at different brightness levels, but the problem persisted (by the way, only in Ha and SII). Then I tried with the regular iPad, which, because it's so old, I only used it for flats before acquiring the mini, and surprise! I don't really know why my flats came out badly with the iPad mini. But well, in the end, the problem was solved, and I'm calm because it wasn't my setup. Thank you so much for your support and that of everyone who took the time to write to me with their suggestions. CS!

Flat wrong with the new iPad mini  ='(



Flat OK with iPad regular  =)

Arun H:
Hi - you should know that displays such as iPads can have significant illumination gradients themselves.

To see this, you can take a set of flats in one orientation, rotate 180 degrees, and take another set. Dividing each by the median and subtracting them will show significant differences. Simply using diffusers like a t-shirt etc. is not adequate to force uniformity over the entire panel/display.

These gradients will complicate the overall gradient that must be removed by ABE, DBE, etc.

It is worthwhile to invest in a flat panel of known uniformity.

Hi Arun! I didn't know that! I really appreciate your comment. I'll look into getting a light source like the one you mentioned. CS!