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Old 21-12-2008, 02:39 PM
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QHY8 Flare Reduction

Hi All,

The QHY8 is good value for a cooled CCD but it has some issues, one of which is an intense circular flare around bright stars in long exposures. This can mess up images of the HH and Pleiades to name but two... and we obviously bought a cooled CCD so we could take long exposures huh?

Discussion with other IISers led me to the cause: it's an internal reflection.
Incoming light reflects off the CCD cover slip onto the front filter, then subsequently back into the CCD.

Simply removing the front filter is not a solution, as this filter is also a seal to prevent moist air entering the CCD chamber. Remove the filter and the CCD will ice up within seconds.

The solution is to move the filter as far away from the CCD as practical, and/or find a filter with superior antireflective coatings.
There's a limit to how far we can move the filter. Move it too far and the air volume inside the QHY8 increases to the point that the cold finger cannot remove the moisture and the CCD ices up. Perhaps a desiccant plug is called for but that's another subject entirely.

I tried two solutions and both work OK.

1) Replace the QHY8 nosepiece with a 2-inch T-adapter and a Baader UV/IR filter. Image attached. The Baader filter is supposed to good A/R coatings, and the 2-inch nosepiece moves it away from the CCD.
This solution was good optically, the flare was reduced and had a larger diameter. But I had issues with air leaks and CCD icing. Possibly the 2-inch nosepiece held too much air for the QHY8 cold finger to remove. Also, the 2-inch nosepiece with the Baader filter attached was too long to fit properly into the focuser, it didn't sit flush and I had camera tilt issues. So...

2) Add T-extenders between the QHY8 body and the original nosepiece. Image attached. I got the T-adapters from a Meade variable T-adapter that came with a 3.3 focal reducer. This is my best solution so far. Flare is greatly reduced in intensity, although it's larger in diameter. The assembly is reasonably airtight and the camera fits accurately into the focuser.

Images attached:
1) QHY8 with standard nosepiece and T-extender in the background
2) QHY8 with 2-inch T-adapter nosepiece and Baader UV/IR filter.
3) QHY8 with T-extenders between the body and the standard nosepiece.
4) HH image with standard nosepiece
5) HH image with extended nosepiece

NOTE the ugly blue PVC tape around the QHY8 body is an attempt to seal the housing against dew.

Cheers,

Rob
Attached Thumbnails
Click for full-size image (QHY8_original.jpg)
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Click for full-size image (QHY8_BaaderUVIR.jpg)
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Click for full-size image (QHY8_original_extended.jpg)
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Click for full-size image (B33_horsehead_08092901_small.jpg)
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Click for full-size image (Autosave_ps02_contr_iis.jpg)
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  #2  
Old 21-12-2008, 03:01 PM
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Looks good...

I tried the 2" T-thread adapter with my Ha filter, but my T-thread adapter did not seal up at all.... With the variable T extender + standard nose piece, do you then screw the 2" t thread adapter onto that to insert into the focuser? That would provide enough extension to then not require an extension tube in order to focus I think....

**Meade variable T extender added to shopping list

Cheers mate
Alex.
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Old 21-12-2008, 04:59 PM
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Quote:
Originally Posted by AlexN View Post
Looks good...

With the variable T extender + standard nose piece, do you then screw the 2" t thread adapter onto that to insert into the focuser? That would provide enough extension to then not require an extension tube in order to focus I think....

**Meade variable T extender added to shopping list

Cheers mate
Alex.
Yep, I thread the 2" T-adapter into the nosepiece as shown in the attached image. As I recall, I still need a 50mm extender but try it without one first. The less bits, the better.

I'm not sure if the Meade variable t-adapter is available as a separate item, however t-extenders are readily available, for example these ones from myastroshop are 40mm long and they also supply shorter lengths.
http://www.myastroshop.com.au/produc...sp?id=MAS-407D
The Meade one I'm using is about 45mm long.
**Just check the length of the threaded section before you screw it into the QHY8 body, if the threaded section is too long it'll hit the ccd and that would be 'bad'.**

Cheers,

Rob
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Old 21-12-2008, 05:20 PM
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Fantastic.. Cheers Rob...

I only need a 50mm extension to get focus with my M102, and I've still got 3cm or so to go on my focuser draw tube... If I got a 45mm extension+the nose piece then the 2" adapter, I reckon that would get me VERY close!! Even if I got a 45mm and then a 15mm screwed together that should give me plenty, and be less likely to flex than the GSO extension tube...

Cheers mate.
Alex.
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Old 21-12-2008, 08:12 PM
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I like it Rob, simply solution that solves the problem, thanks for sharing. That what it's all about isn't share info.
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