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Old 07-08-2013, 08:44 PM
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Astronomik CCD filters and SBIG FW

Received the Astronomik LRGB filter set I bought today. Fitted them to my SBIG CFW-8A filter wheel - well, tried to anyway.

There is no way these were going to screw all the way down. Seems the Astronomik thread is different to the SBIG thread. Is this normal???


Concerned now that I won't be parfocal with this set, as they all seem to tighten up in different positions etc. Was hoping they would bottom out on the FW face, but no luck there.

Really NOT handy of Astronomik NOT to put some knurling or slots on the edge to help screw these in too (like Baader and even Orion does). Cannot complain - got them for a great price, and they seem to be top quality.

Ideas?
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Old 08-08-2013, 11:17 AM
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If it's any consolation, my 1.25" Astronomiks were the same in my new SX filterwheel, a couple didn't screw all the way in either and I was a tad concerned too but nahhh all good

Mike
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Old 08-08-2013, 12:11 PM
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Shiraz (Ray)
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ditto for the 1.25s in my Atik wheel - mildly annoying, but they work fine.
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Old 08-08-2013, 09:29 PM
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OK, I'll give it a crack.Thanks!

Mike, your blasted curse hit here today - the Westerlies struck, and not even worth considering imaging, even though the sky is perfect. GRRRRRR
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Old 08-08-2013, 09:59 PM
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naskies (Dave)
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Quote:
Originally Posted by LewisM View Post
Concerned now that I won't be parfocal with this set, as they all seem to tighten up in different positions etc.
Isn't parfocality associated with the relative thicknesses of the filters? Provided that the filters are orthogonal to the axis of light, a few mm either way shouldn't make a difference (if I remember my first year optics lectures correctly).

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Mike, your blasted curse hit here today - the Westerlies struck, and not even worth considering imaging, even though the sky is perfect. GRRRRRR
Wind? What wind? I love guiding with adaptive optics...
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Old 08-08-2013, 10:34 PM
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Dave,

Thanks.

I would ASSUME they are parfocal irrespective of distance, but as you said, orthogonality concerns me as the filters almost seem cross-threaded.

And this assumes Astronomik made the glass perfectly flat... which I don't doubt they do

Only one way to find out - use it.
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Old 09-08-2013, 10:25 AM
jase (Jason)
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I wouldn't be phased by it that's for sure. Parfocal is a little overrated in my opinion. Even with filters of very precise thickness and a well corrected instrument that focuses all wavelengths at a single focal point, you'll still find a shift between the different 'parfocal' filters. Luckily, these shifts are typically within the tolerance of most optical systems critical focus zone. The only exception to this is if you have a fast instrument with a tight critical focus zone of maybe 25 micron or less, you run the risk that the shift pushes beyond the zone and focus becomes soft.

All this quickly becomes a moot point when you've got software automatically doing a focus routine post filter changes, or even better have filter offsets configured so that the focus automatically moves a set amount for a given filter in the light path to ensure focus is within the zone again.
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Old 09-08-2013, 10:34 AM
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I focus manually
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Old 09-08-2013, 11:23 AM
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I focus manually
Hardcore. You'll give in eventually.
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Old 09-08-2013, 11:49 AM
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Hardcorp, soft wallet
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