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kz1bob

Focusing Issues when adding filter wheel

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Hi All,

New challenge I am running into. When I add a filter wheel on my Bresser AR102s I run out of travel when trying to focus in. I have removed all the extensions and still can't focus in. Am I out of luck or is there something I can do? I am reading up on Focal Extenders am I heading down the right path?

Thanks for your help.

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Hi

You can measure approximately where the focal point of the objective is supposed to be just with a tape measure - 600mm from the centre of the objective lens. That might help you to see what's what. I see the scope comes with a diagonal. Without that fitted I imagine there would be something like 40mm of space - that should be enough for a filter wheel - depending on your camera requirements. Some filter wheels have a 'nosepiece' which can be removed/substituted. Maybe you could post a pic of your scope with everything attached. You probably would want a field flattener or reducer anyway. Reducers tend to have stringent backfocus requirements i.e. 55mm to sensor plane, whereas some field flatteners give you a lot more.

Louise

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You could resort to using a binoviewer OCA to gain additional back focus.  Some folks shorten their OTA in this situation.

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If the thickness of the filter wheel matches or in this case exceeds the length of the extension tubes you have removed I cannot think of much, or at least not a lot that is easy. Mind you I am a bit surprised that the filter wheel is that thick but not overly familiar with the assorted options. Also those scopes are kind of aimed at AP so little chance of removing whatever items there are there. Bresser have already taken bits out to enable a DSLR to reach the focal plane. To Bresser it is easier to aim at AP+DSLR and add a 2" extension tube in for visual as that keeps more or less everyone happy.

I assume that you are not using the 1.25" adaptor as that takes up just under 20mm of length. If you are then it equally means your attachment is 1.25" so you would need a 2" adaptor of whatever sort in place of the present 1.25" item. I have a Bresser 102S also. But it would give another 18-20mm to you.

Will warn you that if you get it to JUST focus then filters will wipe up about another 1mm, refractive index and glass thickness. Just don't want you to barely squeeze in, then add filters and start all over again.

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5 hours ago, kz1bob said:

Hi All,

New challenge I am running into. When I add a filter wheel on my Bresser AR102s I run out of travel when trying to focus in. I have removed all the extensions and still can't focus in. Am I out of luck or is there something I can do? I am reading up on Focal Extenders am I heading down the right path?

Thanks for your help.

There are some thin filter wheels available.

A Barlow might help

Dave

Edited by Davey-T

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Um, sorry Bob, thought you were setting up for imaging so assumed it was camera not able to focus. Quick-change filter drawers (teleskop express) are quite good and only use 15mm.

Louise

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I had presumed the same, that the travel was to allow a DSLR to be used. The 102S comes with 2 extension tubes, one at 25mm the other at 37mm.

As it is said "removed the extensions" that means at least the 25mm has gone and I would guess the focuser would have had some inward movement with the 25mm (even a little) so the filter wheel must be up around 30mm thick or more. Have to go look up filter wheel specifications as that or more might be normal.

If it is for AP then I guess that buying another filter wheell is not a great idea - more cost - also a barlow tends to increase image size and in AP the normal is to reduce it to get a brigher image.

Will say the initial post does not say AP and half implies visual but the use of a filter wheel is usually in relation to AP.

Any idea how close to focus you are getting? One bit mad idea if it is literally 1mm or 2mm

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Ah looking at other topics by the op, it does look like this is indeed an imaging setup using an old Olympus dslr?

Louise

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3 hours ago, Thalestris24 said:

Ah looking at other topics by the op, it does look like this is indeed an imaging setup using an old Olympus dslr?

Louise

The OP might want to look into getting a dedicated astro camera with much less back focus requirements.

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Hi All,

Thank you for your comments and assistance.

Still fairly new to this. I am using my DSLR now for AP (will upgrade to a CCD/CMOS once I am more confident in what I am doing), however this latest came about when trying to get my new guide camera (Altair GPCAM 0130C) to actually 'see' anything. We have had a very bright moon the past few days so have managed to get it to show and focus on the moon, however, point it at something resembling a star and just get a black screen. I have read their forum and the recommendation is to keep playing with the software (adjusting gain and exposure time) and focusing until I get my "eureka moment". I had aligned the guide scope with my main tube during the day focusing on the door to a farm house about 3 miles away. The software (using Altair and SharpCap) worked fine during the day so was hoping I was at good starting point for when it got dark. Still haven't managed to see any stars with the guide cam.

The issue with the main focus came up when I had the guide scope focused on the moon, and the main scope focused. I then added the filter wheel and couldn't focus in. I removed the extension tube and was able to just about focus on the moon with the focuser dialled all the way in.

I am getting there slowly, just hope I get my eureka moment soon :-) 

Thanks for all your advice and help. It is greatly appreciated.

 

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Hi again

What guide scope do you have? Guide cams can be tricky to focus as the focus position is quite critical.

Louise

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