Another FLATS problem - Samyang 135mm / ASI071

[fireballxl5]

Hi SGL'ers,

I celebrated first light with my newly acquired Samyang 135mm lens at the weekend using my ASI071MC Pro OSC camera. There is also an IDAS D2 LPR filter in line and I tried shooting at f/2 to see how the lens coped.

So, I managed to get lots of subs which I've been processing this week using darks, bias and flats as I would my mono CCD/refractors.

The problem seems to be the flats, which have a strong wave-like variation across the frame, most noticeable in the corners - see extracts below, stretched for effect.

The first shows the flat.

The second is a corrected stack (flats, darks & bias).

Finally, the partially corrected stack (darks and bias only).

I've tried using 3 different light panels for the flats with the same result. I've even rotated the panel above the lens whilst continuously taking images (flats) and the pattern remains stationary, implying that it's the camera, lens or filter. Thing is, the raw, unprocessed subs don't have the variation suggesting the these components and combination are ok.

What do you think that this could be due to? The filter, the fast aperture, but why only when taking flats? I know that some NB filters do not work well below f/3 or f/4, so could it be related to this?

Any help or advice would be very much appreciated🙂

FLAT:

STACK - CORRECTED WITH FLATS:

 

STACK - NO FLATS:

 

CS, Andy

[symmetal]

I assume you've cooled the camera as that looks very much like ice forming on the sensor. Try taking flats without cooling and I expect they will be fine. My 071 had the same problem the first time I used it. The flats show it clearly, but individual subs look fairly normal. It seems the 071 sensor chamber is not as airtight as other cameras and the desiccant tablets quickly get saturated.

When not in use I keep my 071 in an airtight food container with a big bag of desiccant, and remove the screw on the side of the camera (under the black plastic sticker) which allows outside air to enter the chamber. This allows the internal tablets to transfer their moisture to the large bag outside. The humidity meter in the container reads 9% all the time.

I haven't had any icing problems since. I've had it in use for several nights in a row with no icing issues, but when it looks like there will be no imaging for a while I put the camera back in the food container. 🙂

Alan

Edited October 20, 2020 by symmetal

[fireballxl5]

5 minutes ago, symmetal said:

I assume you've cooled the camera as that looks very much like ice forming on the sensor.

Nice call Alan, that could be the cause. The camera was cooled indoors for the flats.  I'll check the desiccant tablets and retry the flats. 

 

[symmetal]

1 minute ago, fireballxl5 said:

I'll check the desiccant tablets and retry the flats. 

I did initially order a replacement set of tablets from FLO and opened up the camera to replace, them but icing occurred again when I next tried to use the camera after a couple of weeks. It's not worth taking the trouble to open the camera to replace or dryout the tablets, and potentially let in dust. The little screw on tube with a couple of tablets which Zwo used to include with the 1600, which fitted on the access hole I mentioned above, isn't supplied with the 071, or other cameras now I believe, but the access hole is still there on the side of the camera. The little tube was not effective as the volume of desiccant it contained was not enough, which I why I assume it's not included now.

Alan

[fireballxl5]

24 minutes ago, symmetal said:

isn't supplied with the 071

Yes,  just opened but no tablets.  Looks like I'll be adopting your process,  thanks👍

[fireballxl5]

just to confirm, I have re-done the flats at 0degreeC whilst checking that there was no evidence of the problem and they seem to work ok. So it does appear to be the result of icing/fogging on the sensor. 

I have the same problem with my SX-H35 CCD (though the symptom is very different - small black spots on the images) and have found that pre-warming with an anti-dew band before cooling works, so I may try this next time.

Thanks for the suggestion Alan

CS, Andy