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Old 03-10-2010, 05:43 PM
Alchemy (Clive)
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Halo reflections, scope or camera???

After a good imaging night last night, some early processing of images left me a little disappointed, so I am looking for some feedback.

The problem is similar to here.....

http://www.iceinspace.com.au/forum/a...se.php?a=59632

This is an old image but the problem is the same, all stars brighter than about mag 5 give me the halo as seen in the image.

Last night I had a go at m45 and given 7 stars brighter than such it was a complete dud.

The relevant details are qhy8 and wiliams optics flt132,
Most of my imaging does not include bright stars, horse head and the flame are probably the exceptions, as was the corona shot, the other 2 were done with another scope and camera. So it's a little difficult to ,ake a determination as to the cause ( I figure it's internal reflections but is it camera or scope) all other stars are fine so I'm not throwing gear away just yet.

Does anyone have any input from what they have used, and perhaps a solution, if it's the camera is there another ir filter setup? If it's the scope ..... Well I'm open to ideas.

Longer term I'm hoping to get a mono camera to do some b/w Ha imaging which would drop the stars intensity anyway.

Thanking you for suggestions in advance.

Clive
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Old 03-10-2010, 07:41 PM
ericwbenson (Eric)
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Hi Clive,

The halos are of course caused by a double reflection (1st towards the sky then back onto the sensor). The 1st reflection is almost always caused by the CCD/CMOS sensor itself. They actually have a lot of shiny metal bits on them that are very good reflectors. (If you ever get the chance to shine a low power laser onto a sensor you would be amazed at the checker board pattern coming back!) The second reflection is the killer, but you might be able to do something about it.

Measure the diameter of the halo in mm. Find out your f/ratio, distance from offending surface to ccd is the fratio times the diameter divided by two (since the light travels the distance twice).

If the offending surface is curved (e.g. the back of a Petzval) the formula won't work. You would need to know the surface curvature and od more math in that case. Better to swap the imaging train to another scope to diagnose that case.

Halo problems in recent years with LRGB filters have been due to filters with leaky IR spectrums. They didn't block the NIR wavelength up past 1000nm very well and/or their anti reflection coating wasn't very AR. Many CCDs are still sensitive at those wavelengths and recorded up the NIR reflections along with green or red or whatever. In these cases the reflection originates off the filter itself. I think this is not your case due to the OSC camera.

HTH,
EB
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Old 03-10-2010, 08:45 PM
Alchemy (Clive)
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Doing a quick bit of math

The onscreen image @ 25 % has a width of 205 mm halo diameter is 33 mm
Chip is 3032 pixels wide and each pixel 7.8 micro
33/205= .16097
3032 x .16097 x 7.8 = 3.80687 mm
F ratio is 7
F ratio times diameter /2 is
7 x 3.80687 /2 = 13.324 mm

Therefore it has to be the ir filter on the camera as thats got to be the distance...... Lovely bit of maths you've given me there Eric. Looks like the problems identified, next question is there a better quality filter that will not reflect so much light or alternatively moving the filter further away will reduce it's effect by distance squared by spreading it further, I prefer a new filter first
Anyone know of a filter solution ????

Thanks again Eric

Clive
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Old 03-10-2010, 10:50 PM
ericwbenson (Eric)
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Isn't the filter deposited right on the CCD chamber window? If that is the case it can't be moved, only replaced. Checking the QHY8 webpage, many of the pics there have a halo too. Perhaps they (QHY?) can replace the window with one with a better coating? Or you could add a UV+IR blocking filter in front, if the offending radiation is NIR. For example the SBIG ST-4000XMC has a custom coated CCD chamber window, see here:
http://www.sbig.com/sbwhtmls/special..._st4000xcm.htm
and here:
http://www.sbig.com/sbwgifs/baader_uvir_filter2.gif

EB
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Old 04-10-2010, 09:51 AM
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Geoff45 (Geoff)
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Hi Clive
Not sure exactly what the setup is with the IR blocking filter. Is this a screw-on/screw-off filter, or is it part of the camera? If it's something you've added, then do you really need it? I would have thought that the Bayer matrix filters on the sensor would have blocked IR.
Geoff
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Old 04-10-2010, 12:34 PM
Alchemy (Clive)
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The filter is a 42 mm screw on affair, it is needed as without it the chip will ice over really quickly, I have it straight against the camera, but I do have a series of spacers to give the correct distance for the Williams 68 mm flattener, I removed the train last night and can swap the filter to the end of the spacers, which will give me another 20 mm or so, at the moment I'm not sure if the Williams flattener is airtight or whether such a large plennum will introduce enough moisture to cause icing.

I had a search through old posts for an hour or so last night and it appears to be a common problem with the qhy8 , the filter glass is way more reflective than an astronomic one I have floating around at home. I am toting with the idea of getting another standard filter ffrom gamma and knocking out the glass and getting the astronomic one cut down and fitted. That seems my beat option at the moment.
I did pm a couple of others as to their experiences and they have been quick to reply, but it seems were stuck to some degree.

Thanks for all your input, at least I have identified the cause and it's not the scope causing it which was my worst fear.

Cheers Clive
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Old 04-10-2010, 12:45 PM
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multiweb (Marc)
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Try a bit of acetone on the edge of the thread ring that holds the glass on the nosepiece with a syringe then once dissolved, heat it up with a hairdryer. You'll crack the glue seal then you can remove it or put it back as needed.
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Old 04-10-2010, 07:48 PM
Alchemy (Clive)
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Exclamation

I think I have found just what I need,

http://www.optcorp.com/product.aspx?...1157-862-11417

On the astronomic 2 inch I have the reflections are WAY less so this should be prefabbed ready to go.

Will see if I can get one from one of the Aussie suppliers.
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