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Televue x5 Powermate


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i keep thinking about getting a x5 Televue Powermate, for some real close ups of proms and AR regions. i have used my x4 imagemate a few times and it does a fair job, question is will the 2,000mm focal length and at F50 be too much for it?

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I think a 5x powermate would be total overkill then.  By the time you reach f/20-ish the resolution of the camera and OTA will be pretty much matched so beyond that you'd not be adding detail, just size.  You could as well do that in processing.  I'd stick with a 2x or at most 2.5x if you're using the C9.25.

James

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Yes, I'd say so.  The target focal ratio is based on the pixel size of the camera and as both the C9.25 and PST have the same focal ratio, the same multiplier barlow/powermate should work for each.

James

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It's a result that I didn't really expect myself, to be honest, but put simply you can calculate the resolution of the camera (that is, how much sky corresponds to one pixel on the camera) based on the pixel size and focal length.  The limit of resolution of the telescope can also be calculated based on the aperture.  The "optimal" arrangement of the optics is when the camera can capture on one pixel the smallest thing the telescope can resolve.  By setting the two equations equal to each other and rearranging things the maths basically works out that the two are equal when the focal ratio is some figure dependent just on the pixel size of the camera (actually, it's also dependent on the wavelength of light involved, so for the full visible spectrum you have to approximate a bit whereas for H-alpha you could be fairly strict).

For a camera such as the ASI120MM with a pixel size of 3.75um it works out that the optimal focal ratio is around f/20-ish.  Some people like to go a little less, some a little more, but that gets you in the right ballpark.

James

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I concur with the above. I use a x2.5 powermate with my st102m and my solarscope. Thats difficult enough to focus, anything more is overkill and even more difficult.

Sent from my iPhone so excuse the typos!

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I have been toying with the idea of a X2.5 barlow. With my scope it's at f/8 to begin with and I think between the camera and barlow it pushes it close to f/20. I have always found the x2 gives the best results for planetary and the x3 is really at the limit. I can't even get a workable close up on a sunspot at x3.

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thank you for all the replies.

i have given the wrong info on the camera used with the PST, i have been using the DMK21 with the PST, NOT THE asi120mm.

I have also widened the discussion in another thread here http://stargazerslounge.com/topic/227883-resolution-in-arc-seconds-per-pixel/ to try and understand better the maths behind the relationship between the camera and OTA.

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The limit is about 1 arc second per pixel and some imagers suggest 2 arc second per pixel. Anything below that is beyond what the atmosphere will support and won't give any more resolution and most likely less. Once you get over a certain F number any errors in the optics become increasingly obvious, even if the atmosphere lets you go higher.

The other problem is the amount of light coming through a high F number system and this leads to an increase in gain (more camera noise) or increase in exposure time (greater chance of blurring due to the atmosphere).

My Lunt 60 is fine with a 1.5x Barlow, will support a 2x Barlow if the seeing is good and this weekend I tried a 3x Barlow, although the results are not really that much better it did pull it off.

On my night time scope (C11, so should be the sameas your C9.25) I use it at prime focus and with a 2x TV Barlow. I have a 3x TV Barlow which takes very good seeing to use to get anything any better. I did have a TV 5x Powermate for a previous scope but sold it when I got the C11, there was no way I could ever had used it.

Robin

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I would agree. The very short exposures used while snapping the frames will freeze seeing quite well, so the actual aperture is generally limiting, unless seeing is really horrendous. I find the rule-of thumb values work well on the sun. I might want a 4x telecentric for my F/6 for white-light solar (or just plug a quark into that) for optimal matching to my DMK21. An ASI 120 would be better off with a 3x (which I have)

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Why the concentration on F ratio rather than focal length?

If you make the initial assumption that you want the camera to be able to capture the smallest thing that can be resolved by the OTA and no more then it just falls out of the maths really.  You end up with an equation relating focal length, aperture and camera pixel size that rearranges neatly to give focal ratio as a function of pixel size.

If you just said "I want 1 arcsecond per pixel resolution regardless of the OTA" then you probably would only consider focal length as it relates to pixel size.

James

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Agreed with that... Following James last sentence, if you were concentrating on resolution, then if you double the aperture you doyble the resolution, hence the plate scale needs to decrease so as to still get at max resolution, image gap adjacent resolved other image.

Inpractice for our kind of apertures seeing is probably limiting resolution, edpecially daytime when the suns up n heating the air. Generally on that front early morning sun viewing tends, at least for me, to be best. The sun gets just above the worst of the gunk, and hasnt yet at that point started serious turbulence.

Sent from my iPhone so excuse the typos!

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In fact, if you look at it a different way, once you fix which OTA you're using, thereby fixing the aperture, the focal ratio does effectively relate directly to focal length because changing the focal length (unless you choose to use an aperture mask) is the only way to change the focal ratio.

What specifying the focal ratio actually does is give a generic way of specifying when the camera and OTA resolution are matched without needing to know what OTA you're talking about.  It's one of those Brian Cox "that's what I like about physics" moments where to tell you how to arrange things so your camera matches your scope, I don't actually need to know anything about your scope :)

James

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I am not at all sure seeing limits resolution for scopes in the semi-affordable class in H-alpha, like LS60s or LS80s. On solarchat, most people agree that going beyond 6" aperture does not necessarily add much unless seeing is exceptional. Some great results are however obtained at 8" and above (especially in H-alpha, where seeing effects are much smaller than in white light).

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I am not at all sure seeing limits resolution for scopes in the semi-affordable class in H-alpha, like LS60s or LS80s. On solarchat, most people agree that going beyond 6" aperture does not necessarily add much unless seeing is exceptional. Some great results are however obtained at 8" and above (especially in H-alpha, where seeing effects are much smaller than in white light).

I've certainly seen it suggested that there are no features visible in white light that require a resolution better than that afforded by a 120mm OTA.  I've not really given Ha much thought in that respect I have to admit.

James

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