11 replies to this topic

[RandallK]

I thought I just mastered taking my first flats about 3 weeks ago, and in fact my flats did remove the vignetting from my image frames taken with the ATIK 383L (Kodak 8300 chip with 1.25" filters). I stretched the newly processed light frames after applying flats and I couldn't stretch to reveal any vignetting on all four corners like I did without taking flats so I deemed that to be a success. But a few nights ago, the skies were "clear" but the relative humidity was 97%. After a 4 hour session of imaging the Cave Nebula I brought my scope in to the basement and took all the flats and applied them. The next morning I started to process them in Nebulosity3 as I always do and my processed subframes still had vignetting despite applying flats and they almost looked like the original lights before applying the flats. The only thing different between the session 3 weeks ago and the one a few days ago was heavy dew. I was using my 80mm apo with dew shield so I didn't bother to check the lens for any moisture. I'll have to make sure to check next time we have a heavy dew.

 

So my question is, "Can a very high humidity (it was 97% according to my outdoor weather station sensor) increase the effect of vignetting, as opposed to a dry night?"  Any answers greatly appreciated.

[Ron (Lubbock)]

You might have frosted over your chip and didn't notice it while imaging.  Hope that is not the case.  The sensor sometimes frosts over from the corners inward if there is water vapor inside the camera.  If so, better change the dessicant pronto.  Frost can permanently damage the microlens array in certain chips.  In addition, the dessicant takes 3 days or more to work in the 383L+, from personal experience.

[Lenbo]

Never saw that effect with dew. I have had the frosting effect but it is usually pretty apparent at startup. I have extra desiccant tabs and bake them on low heat for awhile. I cycle out every few days if necessary till it clears up. Len

[RandallK]

Never saw that effect with dew. I have had the frosting effect but it is usually pretty apparent at startup. I have extra desiccant tabs and bake them on low heat for awhile. I cycle out every few days if necessary till it clears up. Len

Hi Lenbo,

Yeah, I've seen the frost on the edge before. I just give the camera a bit more time to cool down, then it disappears. The frost only seems to occur on the first 3 frames or so, then it disappears so I go for an extra 3 frames at the beginning of imaging. But failing to reduce the vignetting has got me stumped. I will give it another go when we get clear skies again and a lower humidy. I'm sure that must be the culprit as everything was the same as before when my flats did a perfect job of eliminating the 4 corner vignetting. This is not related to the frosting issue, I'm positive.

Edited by RandallK, 03 October 2014 - 11:36 AM.

[Madratter]

It certainly can if there is light dew on the mirrors/lenses you might not have noticed.

Edited by Madratter, 03 October 2014 - 12:00 PM.

[Mitchell Duke]

Can you post a picture of the image affected?

[RandallK]

Can you post a picture of the image affected?

Hi Mitchell,

 

Here are 3 frames of the Cave Nebula. One of the original Red Light frames, one of the original Red Flat frames, and one of the original Processed Red frames after flat frame application. They almost look the same. I did the processing in Nebulosity3 and then a gave it a go in Deep Sky Stacker and got the same final result. Randall

 

Edited by RandallK, 03 October 2014 - 05:58 PM.

[Mitchell Duke]

It looks circular, could it be the sensor catching the OTA walls?

[RandallK]

Funny, it worked like a hot **** on my previous image...same set-up but much lower humdity. Guess I'll have to wait for the next clear sky and get more results.

 

In the mean time, here is a cropped "final" of that vignetted Cave (Sh2-155) Nebula.

 

Thanks for all your input.

[TxStars]

With the humidity that high moisture can form on the filters.

[RandallK]

With the humidity that high moisture can form on the filters.

Thanks,
Thats another good point. My filter wheel is very much enclosed with no light getting in and maybe things began to sweat.
It was rather stupid of me to go out in 97% humidity. I shouldn't go above 60 to 65 I would think. Thanks again for your idea.

[TxStars]

I use a cut off point of 95% for my imaging equipment.

For visual dew heaters and hair dryers can get you by.