Background Gradient

[Somerled7]

Does anyone have any thoughts on this background gradient? These are 2 Raw images from a sequence of 75, which I have done a very quick and dirty stretch on just to show the problem. The first was taken at about 8pm and the background is fairly even. The second was at about midnight and there is an obvious band with the bottom right corner looking darker than the rest of the image. This banding appeared gradually over the last 25 images .  It's not cloud as its present over a long period.  I wondered if it might be dew/ice over part of the lens? It was quite frosty, and in fact this was the first time I was using dew heaters. When I packed up, I checked the lens and it looked clear to me, but maybe I didn't look hard enough. Another thought is light shining into the scope? Comparing all the exposures, it's the bottom right that is 'correct' ie it's the rest of the image that has lightened as the evening progressed.

[vlaiv]

3 minutes ago, Somerled7 said:

This banding appeared gradually over the last 25 images .

Maybe try to asses "speed" at which gradient is progressing. If it is at sidereal rate (exposure length x 15" = progression) then it could be due to tracking.

If it does not look like cloud then it is probably some sort of light shining down the tube (or in case of newtonian can sometimes be from other side if mirror is exposed for ventilation - but you don't have any spikes in the image so I guess not newtonian). It could be reflection of something rather than direct light - maybe piece of rain gutter reflecting street lamp or something? What is your environment like?

There is easy way to figure out if it is the light or shadow. Light will raise background values but leave stars the same intensity - shadow will block both background and stars. One is additive (adds light), other is multiplicative (only percent of light reached). Measure intensity of the same star over range of images to see what is going on (if you are really interested in that sort of thing ).

 

 

[Somerled7]

16 hours ago, vlaiv said:

Maybe try to asses "speed" at which gradient is progressing. If it is at sidereal rate (exposure length x 15" = progression) then it could be due to tracking.

If it does not look like cloud then it is probably some sort of light shining down the tube (or in case of newtonian can sometimes be from other side if mirror is exposed for ventilation - but you don't have any spikes in the image so I guess not newtonian). It could be reflection of something rather than direct light - maybe piece of rain gutter reflecting street lamp or something? What is your environment like?

There is easy way to figure out if it is the light or shadow. Light will raise background values but leave stars the same intensity - shadow will block both background and stars. One is additive (adds light), other is multiplicative (only percent of light reached). Measure intensity of the same star over range of images to see what is going on (if you are really interested in that sort of thing ).

 

 

Thanks Vlaiv. Thinking about it a bit more, I don't think it's dew or ice as the stars remain sharp across the image. As I mentioned, the dark corner is a similar exposure to the earlier images, so probably there's extra light shining on the left of the frame.  The moon was out and quite close to the Pleiades, so that's the likely culprit (though it was equally close all through the night!) To be honest, with the bright moon I was really only testing my new dew heaters - I wouldn't normally shoot so close to a bright moon.