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Old 14-09-2021, 01:31 PM
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Stonius (Markus)
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What are the best options for putting filters between the secondary and the focuser?

To be clear, I'm looking for something that precedes my imaging train / eyepiece on my 16" UL dob.


I'd also like to avoid having to cut the truss tubes down to accomodate a longer focuser assembly, which is why I'm thinking something on the inside of the tube, like the Astrocrumb things might be the go? I think Moonlight have an accessory too, but it means buying a whole new focuser (my scope currently just has a GSO focuser with a ZWO EAF).



The advantage is that it's independent from the imaging train so the same filter can be used visually and for photography without disassembly.



The idea is that it will perform several functions;
  • An IR filter can be used in conjunction with the W47 in the imaging train to get UV. Essentially, it allows me to stack those two filters, and still use the W47 on its own to help check focus.
  • a 2" UHC filter can be used on any eyepiece without having to worry about screwing it into each one, or even whether it's a 2" or 1.25" EP; One filter size for all my EP's
  • Having them all mounted in a slider or wheel of some sort means no faffing around in the dark.



Does anyone know what the options are? I can't seem to scare up much info on the internet.


Markus
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Old 14-09-2021, 02:23 PM
JA
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Hi Markus,

I believe your scope has a 2 inch focuser, so if you used a 2 inch Plain Male to M42 T2 camera adapter, which sometimes have an INTERNAL M48 x0.75 thread then you could mount your 2 inch filters to the M48 internal thread and possibly the camera/eyepieces somehow at the other M42 end with the right adapters etc, (excepting 2inch eyepieces of course)..

Something like this....
https://www.aliexpress.com/item/4000...c8eea85d2a0a78

and possibly one of these mounted to the rear of the above adapter in order to mount 1.25 inch eyepieces...
https://www.bintel.com.au/product/lu...v=322b26af01d5

Best
JA
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Old 14-09-2021, 03:00 PM
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Stonius (Markus)
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Thanks, JA,


It's an interesting idea, and I do have a 2" focuser, though I think the 2 inch part of the first adapter would have to be on the *outside in order to mesh with the threads of my focuser.


I'm not sure of he tolerances involved, but I assume there is some leeway there for the clear aperture of the focuser to be reduced by screwing something inside it.


it would seem to be a way of mounting things there, at any rate. Everything else I've seen relies on precisely aligned screw-holes which makes me nervous. Maybe I should look at using your adapter idea and getting someone to print something up on a 3d printer
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Old 14-09-2021, 03:16 PM
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Merlin66 (Ken)
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Have a look at the Neumann filter drawers.
https://www.google.com/search?client...+filter+drawer
I use the version with M42 (T thread) either side and both 1.25" and 2" filter slides.
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Old 14-09-2021, 03:28 PM
JA
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Quote:
Originally Posted by Stonius View Post
Thanks, JA,


It's an interesting idea, and I do have a 2" focuser, though I think the 2 inch part of the first adapter would have to be on the *outside in order to mesh with the threads of my focuser.
Hi Markus,

The way I was thinking of it (and it depends on your focuser) is that most 2inch focusers, even if they have some threads, will almost always have a PLAIN/Smooth 2 inch bore in order to accommodate 2inch eyepieces, often held in place by a thumbscrew or two or sometimes a collet type clamp. Anyway you simply slide the 2inch adapter (The 1st adapter above -holding the 2inch filter in the M48x0.75 internal thread) into the focuser and lock it in place with the thumbscrews. That leaves an m42 x0.75 male thread sticking out at you. To this simply screw on the 2nd adapter (M42x0.75 Female to 1.25inch) adapter and you have a system which can support 1.25 accessories/eyepieces etc. Hopefully things haven't been moved that far away that you can still achieve focus. That you would need to test before hand by knowing the lengths of the adapters involved are where things come to focus.

You can also leverage that M42x0.75 male thread for imaging as many cameras support that thread in either male or female form, so perhaps an adapter would be required. It depends on what cameras you have or intend to use, anything with APSc sized sensors or below would be fine, but full frame would vignette and push you to look at larger thread sizes.

Best
JA

Last edited by JA; 14-09-2021 at 04:09 PM. Reason: typo
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