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Which TV Powermate for imaging - 1.25" or 2"?


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How do you choose between 1.25" and 2" versions of the TV Powermate for imaging?

If I want a high multiplier, say x5, for planetary imaging at long F.L. with a small sensor camera, my guess is that the 1.25" model would be OK on account of the very narrow FOV. But suppose I want only x2 or x 2.5 with a DSLR or large sensor CCD camera? Is vignetting then a problem with the 1.25" models?

Does anyone know of a formula or general guidance about which diameter of TV Powermate is required to avoid vignetting, for any particular focal length, F/ratio and sensor size?

Adrian

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Thanks. Yes I read the advice on the TV site and it seems both sizes are an improvement on standard barlows for vignetting, but it doesn't really tell you at what point you should be moving up from 1.25" to 2".

Presumably there must be some wide field/ large sensor size combination at which the 1.25" causes unacceptable vignetting, and beyond which you have to go for the 2". (Otherwise why would they bother making two sizes.)

I was really trying to figure out what were the important parameters that define how bad the vignetting is, and what the cross-over point is between the 1.25" and 2" sizes.

Adrian

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It's a very fair question...

Don't know...the calculation on the TV site seem to refer to eyepieces rather than imaging.

If you consider the open aperture of the front lens in the Powermate, then I'd guess that the telecentric design would give a similar diameter beam back though to the imaging plane. This would infer for DSLR sized chips that the 2" version would be the go.

( I use both the 1.25 and the 2" versions of the powermate for solar imaging...but only with the smaller DMK41 sized chips - no problems there.)

Sorry.

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<<If you consider the open aperture of the front lens in the Powermate, then I'd guess that the telecentric design would give a similar diameter beam back though to the imaging plane. This would infer for DSLR sized chips that the 2" version would be the go.>>

Yes, I agree, would probably be pushing it with less than the 2".

Adrian

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I would not want to use a DSLR with powermate at all. DSLRs are great for DSO imaging, and Powermates or Barlows are counterproductive there. For planetary imaging, there is no point in smearing out the light over more pixels than a web-cam or specialist planetary camera offer, because you lose too much light per pixel. As a rule of thumb, F/20 to F/30 work best for planetary imaging, and the size of most planet images are catered for quite well by web-cams, except in the very largest scopes, in which case the larger-chip DMKs and the like are better. These are catered for by 1.25" powermates quite handsomely.

DSLR imaging using barlows just does not seem to make much sense. A 2" Powermate is useful if you need to barlow a 2" EP.

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Yes, there are two distict requirements here. My scope is 1000mm f.l. at f/5.3. For planetary imaging, I'll use a small-chip DMK camera and thought a 5x Powermate would suit. But I would also like to image some deep sky objects at longer focal length, using the OAG on my QSI583 camera or DSLR. In fact, a 1.5x multiplier would probably be ideal, but even x2 would still allow me to work at f/10 or so, and for these objects, I would want to use the wider field of the non-planetary cameras ...... hence the question.

Adrian

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