Flats, what's going wrong?

[RayD]

18 minutes ago, DaveS said:

Now I'm totally flummoxed

This is a 30 sec Dark through the L filter. Well, not really *through* it, but with it in place. Just slightly stretched to make the gradient obvious, else it wouldn't show up on the JPEG.

 

 

There does look to be a light leak but it's off to one corner (Unless it's amp-glow, but this is a cooled CCD). No trace of the bright ring I was expecting. Will have to contact SX now I think.

 

I'd say that is an O ring problem Dave.  I suspect It would show differently on the flat, but you shouldn't have this at all on the dark, and it doesn't look like amp glow.

[DaveS]

Thanks.

I've sent a technical query to SX through their website, with a link to this thread, as I couldn't see how to attach images. Hopefully someone will see this and know what's going on.

[RayD]

Just now, DaveS said:

Thanks.

I've sent a technical query to SX through their website, with a link to this thread, as I couldn't see how to attach images. Hopefully someone will see this and know what's going on.

Terry is a good guy and has answered any query I've ever had in double quick time.  Let us know how you get on.

[ollypenrice]

My take: you can't take darks or bias on a telescope. Light will get in, though Lord knows how! Do them off-scope with a metal chip cover. Atik supply these but I don't know about SX.

The stark inner ring on your flats suggested, to me, that there might be dew or frosting somewhere. Vignetting produces a very gentle gradient whereas your flats had a radical change in brightness very locally.

The problem with twilight flats is that they invite light leakage from somewhere. I do flats in the dark using a panel to avoid this problem. A good number of our guests take the same view.

Use a master bias as a dark-for-flats. But for me flats take between half a second and three seconds.

Olly

[DaveS]

Thanks for the advice Olly, I've been thinking of getting a flat panel, now I'll push it forward.

I've been in contact with Terry, and he's puzzled too.

I wonder if the camera is dewing or icing up. I'll pull the camera off and check with the cooler on to see what happens.

[DaveS]

The plo thickens.

Following @ollypenrice  I took the camera off the 'scope to take some Darks and Biases with the camera covered and at working temp (-30).

As the camera was warming up I took the cap off and saw this

I think the camera will have to go back for new dry argon. Bother, and with winter targets too.

[sharkmelley]

It certainly looks like you have identified the cause.

Mark

[DaveS]

Terry thinks so too, he's emailed me a returns form, so I'll get the camera packed up and sent off.

At least with the moon getting in the way I won't be imaging for a while anyway.

[fireballxl5]

just seen this thread and the image of the chip above - I would say that the flats looks just like those taken with my SX-H35 camera before I fixed the problem, which was condensation/ice on the chip window. Fixed by fitting a dew strap to the camera body to warm it before turning the cooler on.

HTH, Andy

[DaveS]

I may do that, though the whole point of the dry argon filling is to stop the problem at source.

May still put a dew strap on my flattener though.

[RayD]

7 minutes ago, DaveS said:

I may do that, though the whole point of the dry argon filling is to stop the problem at source.

May still put a dew strap on my flattener though.

It should be Dave.  This would suggest the Argon has escaped.

Whilst I don't cool down to -30, I don't have this issue.  I did a few tests with my SX when I got it, and found that below -15 the difference was negligible (In fact I couldn't see any at all).  I tend to have mine now set permanently at -15, which means I can happily image all year round at the same temperature.

[DaveS]

Thanks Ray.

When I got the Trius one of the peltier stages wasn't working and I could only get to -10 or 11. After it was fixed and I could go to -30 I noticed a very great improvement, but as you say, beyond -15 or so there may be little change.

I did do some flats at -20 and noticed the same dark oval. I'm also wondering what "nasties" are lurking down in the black on my images taken lately.

[RayD]

2 minutes ago, DaveS said:

Thanks Ray.

When I got the Trius one of the peltier stages wasn't working and I could only get to -10 or 11. After it was fixed and I could go to -30 I noticed a very great improvement, but as you say, beyond -15 or so there may be little change.

I did do some flats at -20 and noticed the same dark oval. I'm also wondering what "nasties" are lurking down in the black on my images taken lately.

Yes it would be a shame if this shows up on them.  Well worth doing some tests when it comes back though as your sensor being different may have different results, but for mine -15 seems to be the optimum.  I got mine to replace my Atik 460 in Spain which, whilst great, wasn't able to get below 0 in the warm summer nights, which can often be up in the 20's.