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2.5x powermate w/ binoviewer focus position?

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#1 betacygni

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Posted 03 March 2021 - 01:04 AM

I’m trying to come up with a solution to get my dob to focus with my zeiss binoviewers, with very little focuser travel to spare. I can just reach focus using the televue 2x amplifier, but I’m looking for a standard Barlow/powermate configuration. I might be wrong in this, but since the zeiss unit uses mirrors instead of prisms, my thought is standard glasspaths/amplifiers made for binoviewers would induce aberrations if the system didn’t have the expected prisms.

For my refractors I’ve been using the astrophysics BARADV optical element screwed into the quick changer, but I run out of in focus travel with this configuration on the dob.

The last option I can think of (while still keeping around 2x mag) is the 2.5x powermate with the T2 adapter. Anyone have the measurements to how close this binoviewer configuration is to being parfocal to a mono viewing eyepiece?

#2 Spartinix

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Posted 03 March 2021 - 01:18 AM

The possible issue with GWC's has been discussed in various threads here and IIRC, the consensus is that there are no visible aberrations.

 

I've used the unit with a 2.5x TV PM, but have not measured yet in comparison with mono.

From what I've read, the TV 2x amplifier should work fine, so I doubt the PM will be better in your case.
Another solution might be a Siebert 1.3x corrector (no more 2x of course...), or decrease the distance between primary and secondary mirrors.


Edited by Spartinix, 03 March 2021 - 01:20 AM.


#3 scarubia

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Posted 03 March 2021 - 09:22 AM

1) You can use a 2x 2" Powermate. Unscrew the chrome barrel and couple it with Televue's PTR-2200 PMT-2200 T-Ring Adapter, then screw that into your BV.  Magnification 2x.

 

2) Add T2 extension ring(s) between the BARDAV and the bino nose piece to get the BARDAV lens deeper into the light cone. Magnification TBD

 

3) A little known and never mentioned option is APM's 2 inch coma-correcting 1.5x telecentric Barlow. The nose section can be unscrewed and attached to your bino via a T2 adaptor.  An expensive option, but how much is getting 1.5x with coma correction worth to you? 

 

1 and 2 work very well. I will be experimenting with the APM unit soon.


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#4 Spartinix

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Posted 03 March 2021 - 10:01 AM

The APM has 96mm outward focus shift so I assume it would give more than 1.5x.

But probably a fine option indeed.



#5 scarubia

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Posted 03 March 2021 - 10:27 AM

The mag is calculated at 1.5x with the focus shift at 96mm. The outward focus shift is specific for spacing between the lenses for optimal coma correction.  Given the 3" long nose piece into the incoming scope light cone, it should come to focus in my scopes. 



#6 faackanders2

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Posted 03 March 2021 - 09:08 PM

Why not try to see if you could get a denkmeier OCS to work?


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