Does Barlow position matter (Coronado SMiii 70mm)?

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OregonAstronomer
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Does Barlow position matter (Coronado SMiii 70mm)?

Post by OregonAstronomer »

Hello again,
I have another question for which my previous research failed to find an answer, so here I am again! I am currently using a 1.25" Televue 2x Powermate in the diagonal of my SolarMax III 70mm Ha scope for higher resolution imaging of prominences. The barrel of the Powermate, however, is twice as long as the barrel exiting the scope's diagonal/blocking filter assembly. I insert the barlow with the video camera as far as it will go without touching the blocking filter and tighten the holding screw as much as possible, but since the barlow is not sitting flush in the diagonal it's hard to know if it truly square with the optical axis. With my other scopes, I can put the barlow directly in the focuser BEFORE a diagonal, but the SolarMax has a 2" fitting on the objective side of the diagonal. If I get a 2" Powermate can I put it between the objective/etalons and the diagonal? Then I could securely seat the ZWO camera in the diagonal just like when I am doing full disk imaging. Is there a preference for putting the barlow before or after the diagonal?
Thanks so much again,
Arnie


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Re: Does Barlow position matter (Coronado SMiii 70mm)?

Post by Montana »

Nobody wise has answered this so I will take a crack at this myself.

I would say no, if you put a barlow or powermate before the blocking filter you will bend the parallel light grating produced by the etalon. You will then not have the correct wavelength passing through the blocking filter. So it wouldn't work. You could try a telecentric like a TZ3 or TZ4 but I wouldn't like to say it would work.

You need to have the barlow etc between the blocking filter and camera. This is only my own logic, I could be totally wrong. Have you tried it?
My 2x powermate sticks out a lot but it has never proved to be any problem.

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Re: Does Barlow position matter (Coronado SMiii 70mm)?

Post by marktownley »

After the diagonal for me. Don't worry if its not sitting square as long as the image is in focus across the field.


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Re: Does Barlow position matter (Coronado SMiii 70mm)?

Post by Bob Yoesle »

I've used the TeleVue 2x Big Barlow ahead of my BF30 straight-through blocking filter without ill effect, and believe either position will work. The issues become that of more magnification as the image distance increases, and possible vignetting if the Barlow restricts the converging light-cone from the objective or subsequent optics. This doesn't seem like a very probable consequence for the SM70 (actually 60 mm clear aperture etalon/etalons) with a 2 inch Barlow, but might be with a 1.25 inch BF diagonal ahead of the Barlow - especially if it is a small BF5 or possibly a BF10.


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Re: Does Barlow position matter (Coronado SMiii 70mm)?

Post by OregonAstronomer »

Hi!
Thanks for the information, everybody. The 1.25" 2x Televue actually rocks slightly back and forth in the diagonal unless I really crank down on the set screw, which is something I'd rather not do long term. The 2" Powermate is currently on back-order. I think for now I will shim the 1.25" Barlow to keep it tight in the diagonal until I can get the 2" model (which I will also use for nighttime observing/imaging). Once I can get the 2" model I will try that with a straight-through modification, which I was planning on doing anyway.
Thanks again,
Arnie


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