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Diseased M13... any ideas?


MikeWilson
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Dear all,

My M13 looks rather ill. Would you say that this is a result of poor focus, or something else awry while stacking? I'm testing out a CLS filter, although it seemed to perform alright on another very faint target from earlier...

post-18683-133877612105_thumb.jpg

I'm hoping someone else has seen something similar before?

All the best,

Mike

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Stars that look like rings seem to indicate a focus problem - but I have very little experience with stacking software. That said, the rings don't look symmetrical enough for a focus error on a well collimated scope. Can you determine if there is any ring-like star features on the single frames? It really doesn't look like coma, either - that tends to get worse as you move away from the center of the field, and I don't see any evidence of that either. :)

The only other optical thing I can think of is dew. That might account for both the ring shaped stars and the inconsistency in the shapes. Dew droplets tend to be of a very consistent size as they form, and your rings are all about the same size; as opposed to say, dust particles, which have a Gaussian random distribution that shows more variance in distortion effects.

If it isn't an optical problem, I think maybe you need some of the software boffins here to help you out. I'm out of my depth with that stuff.

Good luck to you!

Dan

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Keep an eye on the cooling. I found that if I push the cooling too hard, the sensor window can dew up (and I got a result exactly like the effect you have posted). Also, the heatsink on the 8L is marginal...if you push the cooling power over 70% (according to the QHY forums) then heat can leak back onto the cold side of the TEC as the heatsink cannot shift the heat load.

I now have a small dew strip wrapped around the extension tubes to keep the air inside above the dewpoint. I've not tested it yet, but I can't see it not working.

Finally, make sure that the check box for Detect and Clean hot Pixels is not ticked in the Cosmetic tab of DSS....it can black out the centre of stars, giving a halo effect.

HTH

Edited by Zakalwe
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Finally, make sure that the check box for Detect and Clean hot Pixels is not ticked in the Cosmetic tab of DSS....it can black out the centre of stars, giving a halo effect.

I agree - looks like over-aggressive DSS cleaning to me.

NigelM

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It would be interesting to see a single sub for comparison - otherwise I would have immediately suspected focus or dew. Last night there was quite a heavy dew as I found to my cost when my dew heater controller played up.

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Thanks for the advice, chaps.

Looking at single subs doesn't tell me much but they don't look perfectly clean. I'm suspecting that it could therefore be dew. I restacked all the 180second frames without applying any cosmetic and found the stars still had an odd shape to them.

If it were dew, I wonder which 'side' of the glass protecting the CCD it was on. Or indeed it could have been in the filter wheel maybe? It's quite an 'open' design (where you rotate the wheel) so it's conceivable that warm air could have gotten in and cooled on the glass of the filters. Has anyone else ever gotten dew on (in?) a filter wheel before?

I'm going to follow Zak's suggestion and maybe up the temperature a bit. I might try for -15 this time instead of -25 and look for a 2" eyepiece dew strap to go between the camera and the filter wheel.

Thanks again, I'll keep an eye on it for tonights subs to see how they turn out.

All the best,

Mike

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-25 worked fine for me Mike. Once I pushed it to -40 and it started to dew up. However, mine is a "closed" system the field flattener screws onto the spacers, which screw onto the CCD. If it was more open, then I guess the chances of dewing are much higher.

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-25 worked fine for me Mike. Once I pushed it to -40 and it started to dew up. However, mine is a "closed" system the field flattener screws onto the spacers, which screw onto the CCD. If it was more open, then I guess the chances of dewing are much higher.

Right. (And thanks for the tip on temperature - I may well keep it at -25, there is a noticable difference between -15 and -25 for noise).

And I suspect that the introduction of a filter wheel (it is quite open) allows moister night air to get in much easier than it would otherwise have done between the elements in the closed portion of the system.

I wonder if it's possible to add dessicant in the filter wheel. I might explore the possibility of cutting a hole in the filter wheel itself - putting a semi-permeable membrane across it filled with dessicant.

Or a 2" dew strap might just work if applied between the camera and the filter wheel.

All the best,

Mike

Edited by MikeWilson
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Mike, SteveL had some sensor dewing Issues with his QHY8, when he had one that is. I don't remember him posting anything like that result you have their though.

If he doesn't join the thread, a PM to him might open up a solution as to what might have caused it.

Ron.

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Mike, SteveL had some sensor dewing Issues with his QHY8, when he had one that is. I don't remember him posting anything like that result you have their though.

If he doesn't join the thread, a PM to him might open up a solution as to what might have caused it.

Ron.

I'll do that - although the QHY8 sensor dewing problems are supposedly resolved in this model (and it helpfully comes with a way to dry out the sensor).

I suspect that the dew (if it is dew causing the problem) is getting in via the filter wheel...

Mike

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Mike, SteveL had some sensor dewing Issues with his QHY8, when he had one that is. I don't remember him posting anything like that result you have their though.

If he doesn't join the thread, a PM to him might open up a solution as to what might have caused it.

Ron.

I think that dewing was fairly common on the QHY8. They increased the thickness of the sensor window on the 8L to specifically address the problem. I guess though, with a -25% sensor on one side and moisture rich air on the other that dewing is a very real possibility.

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I think that dewing was fairly common on the QHY8. They increased the thickness of the sensor window on the 8L to specifically address the problem. I guess though, with a -25% sensor on one side and moisture rich air on the other that dewing is a very real possibility.

I suspect that the only way to prevent this would be to apply some slight heat to outside of the optical train somewhere. I might look at getting some form of dew heat strap and applying it to the filter wheel somehow... or perhaps a 2" eyepiece dew heater might work wrapped between the camera and the filter wheel.

Mike

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Ok, here's what's new:

1. Tonight's RH is down from ~80-90% to about 60%

2. I took the dew strap from the ST80 guidescope and wrapped it around the filter wheel, taking care to apply the warmest section near the opening of the wheel.

3. Proper flats were taken using an EL panel (arrived today).

4. A dessicant tube was attached to the sensor air cavity of the QHY8L.

8 x 60 seconds later, then this:

post-18683-13387761267_thumb.jpg

post-18683-133877612678_thumb.jpg

I suspect that the problem was dewing, but was it internal (sensor side) dewing, the glass window (external dewing), filter dewing? I don't suppose I'll find out...

All the best,

Mike

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Mike, if you keep the desiccant tube connected to the camera when you store it, then the air inside the sensor chamber will not dew. I am guessing that your issue was dewing on the outside of the chamber, especially as your filter wheel is open to the elements.

I tried my idea of warming the extension tubes tonight. I was running at -35 without any sign of dewing, so it certainly seems to have helped me.

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