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Old 15-09-2021, 05:51 AM
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OneCosmos (Chris)
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Eyepiece projection vs prime imaging

I’m certainly enjoying my visual astronomy these days and only very occasionally mourn the move from imaging because I still find looking through the eyepiece a more visceral experience.

The views through really high quality EPs is so astounding and with the 1200mm f/8 APM refractor the planets and moon are a sight to behold and one is always tempted to believe you could capture just that view. Yet, as far as I know, nearly everyone shuns eyepiece projection in favour of prime focal length imaging. Is that because EP projection is doomed to sub-par results perhaps due to the natural aberrations in EPs that the camera is less forgiving of than the eye? Curvature at the edge for example wouldn’t be an issue for planetary imaging would it? You’d end up cropping anyway.

So the question is with a 1200mm slow f/8 refractor I’d either need a 4x or 5x Barlow for prime or use EP projection. Does anyone here have any success with the latter and if so, which camera are you using? I’m not really interested in going mono.

I should add that my garden has simply incredible seeing most nights and Jupiter and Saturn can be sharp as a razor at 350x and more!
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Old 15-09-2021, 01:17 PM
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PRejto (Peter)
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Here is at least one example of EP projection:

https://www.cloudynights.com/topic/7...io-2021-09-10/

I'm no expert in planetary imaging but I suspect that using a 4x barlow may prove very difficult! I noticed for example that a user of a C14 Edge uses a 2.5 barlow and achieved terrific results. The larger aperture would support this resolution and short exposures due to the greater light gathering ability. At 5,000mm FL on your scope I suspect you would need longer exposures (less lucky seeing!) and rather perfect seeing. I was able to take some satisfactory images with my TEC140 and 2-3X barlows but these couldn't compete with large aperture mirrored OTAs.

Peter
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Old 15-09-2021, 03:06 PM
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multiweb (Marc)
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Don't you have a large aperture dobs already? Why are you playing with a refractor?
Stick a zwo on your light bucket.
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Old 15-09-2021, 10:07 PM
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OneCosmos (Chris)
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Quote:
Originally Posted by PRejto View Post
Here is at least one example of EP projection:

https://www.cloudynights.com/topic/7...io-2021-09-10/

I'm no expert in planetary imaging but I suspect that using a 4x barlow may prove very difficult! I noticed for example that a user of a C14 Edge uses a 2.5 barlow and achieved terrific results. The larger aperture would support this resolution and short exposures due to the greater light gathering ability. At 5,000mm FL on your scope I suspect you would need longer exposures (less lucky seeing!) and rather perfect seeing. I was able to take some satisfactory images with my TEC140 and 2-3X barlows but these couldn't compete with large aperture mirrored OTAs.

Peter
Thanks Peter. Yes I reckoned using prime focus would need a significant Barlow and shutter speeds would be very slow. That’s what made me think of EP projection instead. The image always seems so bright with an eyepiece that sometimes you even need a filter so I thought magnification plus brightness was the winnimg combination.
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Old 15-09-2021, 10:09 PM
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OneCosmos (Chris)
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Originally Posted by multiweb View Post
Don't you have a large aperture dobs already? Why are you playing with a refractor?
Stick a zwo on your light bucket.
Lol. Yes I have a 28” dob but views of the planets through a refractor just seem so crisp and sharp.
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Old 16-09-2021, 04:56 AM
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Originally Posted by OneCosmos View Post
Thanks Peter. Yes I reckoned using prime focus would need a significant Barlow and shutter speeds would be very slow. That’s what made me think of EP projection instead. The image always seems so bright with an eyepiece that sometimes you even need a filter so I thought magnification plus brightness was the winnimg combination.
I did a lot of eyepiece ptojection in the days of film using my 6” f5.5 (840mm f.l.) Telescope and a Vixen 5mm (0.965”) ortho. I later switched to using Clave 10mm and 6mm eyepieces.

Using the Vixen 5mm ortho and the particular eyepiece projection adapter that I had, I got 10920mm effective focal length working at F72. Exposures of planets varied from 1/30s to 1 sec and were done manually by first putting a black card box (or black velvet) over the aperture, locking the shutter open then slowly taking the box off and covering again counting the exposure in my head all the time making sure the telescope was not bumped. Exposures of 1sec and 1/2 sec were easy to judge and common with film speeds between 100 to 400 ISO. 1/4s and less was more difficult to accurately state the exposure but the end result was what counted and if it meant a very brief ‘unknown length’ exposure so be it. It was all trial end error and to a few of rolls of film to get right.

Using a Barlow was not an option as e.f.l. would be nowhere near long enough. Technically using a Barlow or Powermate is not called prime focus since prime focus is primary lens/mirror focal length only.

Note at f72 the image soon becomes very dim hence the longer exposures. All captures were single shots and could really only be called good on nights of exceptional seeing. Most of the time detail captured was nothing like that seen at the eyepiece visually. (Completely opposite to what processed images can show now).

I had a fixed distance eyepiece projection adapter and changed the eyepiece if I wanted different magnification but only for the Moon I would use less than maximum sometimes. Variable distance ones allow changing of the e.f.l. without changing eyepieces.
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Old 16-09-2021, 09:52 AM
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multiweb (Marc)
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Originally Posted by OneCosmos View Post
Lol. Yes I have a 28” dob but views of the planets through a refractor just seem so crisp and sharp.
ah... but it's only a perception because of the image scale. A 30s video stream on a 28" would surprise you I reckon once you stack it and apply a little wavelet sharpening to it. You should try it.
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Old 18-09-2021, 09:26 AM
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Yeah I’m with Marc…dunno why you’d fiddle around with a baby scope when you have a beast

Planetary is all about resolution, which comes from aperture, as I’m certain you already understand.

The refractor will give “pleasing” results on an average night, whereas the beast won’t, it’ll show you the truth of the signal and what the atmosphere is doing to it. The refractor is limited by the resolving abilities of a 6”. There’s no escaping that. On a good night the beast will eat the ‘frac for lunch and truly soar from a detail perspective…

Just my 2c of course, but if you want to give it a go while the gas giants are still well placed, gimme a yell
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