Camera Labs

I've ended up looking up possible future scope purchases again tonight. High on my shortlist is going SCT with Hyperstar. Not cheap by any means, but it's by far the most affordable "fast" optical system.

I recall Greg has has one, specifically on the C11 giving a 560mm f/2.0 system. As ball park indication only, that could be yours for around £2400 at current prices. If I were to go this route I'd probably look at the next size down to shave a chunk off the cost.

Now the bit that is confusing me is the Takahashi Sky 90, which Greg also has. Basic spec 500mm f/5.6, but I recall Greg saying in the past the focal reducer is essential for correcting imaging, giving f/4.5 at... around 400mm. But this isn't cheap either. In a quick look around, you're not going to get change from £2000 for this either. You're not going to call this a budget alternative.

On the big assumption the small difference in focal length of these are not deal breakers, why choose the Sky 90 at all over the much increased speed of the HyperStar system? Centre obstruction of the C11 is under 12% so the speed loss from that isn't really significant.

Possible factors:
Weight? C11 is 12.5kg, Sky 90 is 3.2kg - so mounting would be a lot easier there?
Image circle - Hyperstar 27mm diagonal (good for APS-C), Sky 90 is 45mm (good for full frame)

Anyway, to answer it myself, I looked at Greg's book I got ages ago. It's making more sense this time round. The difference in focal length is somewhat significant. And if you have a sensor big enough to make use of it, the Sky 90 would image significantly bigger areas.

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.

And the other thing is the use of narrowband filters. They don't work at f#2 and they can't be put in the Hyperstar without pulling the whole optical train apart. The field of view of the Sky 90 is twice that of the Hyperstar BUT as the Hyperstar is 5x faster you don't gain with FOV arguments. Contrast is better with the Sky 90 - I got the dwarf galaxy by Regulus with the Sky 90, I am pretty sure I wouldn't see it with the Hyperstar (but I haven't tried).
If I had thought it all through better, instead of the mini-WASP array I would have bought a 14" Celestron with Hyperstar, would have been a lot cheaper too - too late now.
Greg

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Nexstar 11 GPS, 2 x Sky 90, M25C, MaximDL, Photshop CS3, Noel Carboni's Photoshop actions, 7 foot Pulsar fibreglass dome, Canon 40D, 100mm macro lens, 28-200mm zoom lens, 17-55mm f#2.8 zoom lens, 100-400mm zoom lens, 1.4x converter, 2x converter.
http://www.newforestobservatory.com/
http://www.flickr.com/photos/12801949@N02/

Hi popo,

Whatever you get you'll love to bits and my needs aren't your needs...

I am so glad I didn't get a light bucket. My TEC 140 may not be the biggest 'scope in the world nor, at f/7, is it the fastest but add a field flattener (I have a BIG sensor), filter wheel and camera to it and it's 13 or 14 kg - I prefer to keep the imaging gear permanently attached unless I want to go visual for some odd reason. If you don't have a permanent observatory then that's quite enough weight to manhandle on and off the mounting. On a sample of one I'd say decide what weight you can comfortably lift and then aim for half of that to allow for manœuvring the 'scope on and off the mounting safely while still keeping half a hand free to tighten the bolts etc.

One issue with the Hyperstar system I seem to remember Greg flagging somewhere is the regular need for collimation unless, again, it is housed in a permanent observatory. I'm sure Greg will chip in if I've remembered wrongly.

One topic that raises disagreements is focal ratio. For a given entrance pupil and pixel size a faster f-number is undeniably better at picking up the faint stuff although, within those same constraints, the resolution decreases as the f-number goes down. However, for a given entrance pupil if the pixel size is allowed to increase or decrease in proportion to the f-number then, if sensor noise per pixel is constant, you can do the math yourself and work out whether a smaller f-number is any faster!

If I had the luxury of a permanent observatory complete with climate/humidity control I would have been right to go for a reflector. Without those a refractor is far more practical IMHO and there are numerous images out there on the wibbly wobbly web that prove you don't need a huge light bucket to get fantastic results and a great deal of pride in ownership.

Bob.

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OM-D E-M5 + ED 12-50mm, H-PS14042E, H-F007014E, L-RS014150E, M.ZUIKO DIGITAL 45mm 1:1.8.
Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.

Thanks for the replies. Much to think about there.

Good point on the filters. I currently use the astronomik 12nm ones at f/2. Their web site only claims performance to f/2.8, but in discussions with their tech support they say they are uprating them for f/2 operation. I guess a wider bandwidth one at 12nm might have more relaxed requirements than tighter ones.

As for the optimal choice, this is ever complicated... so basically I have to weigh field of view (related to sensor area) and imaging speed. My goal is to get much more resolution than I currently can with DSLR imaging. And mono CCDs of any size seem rather more expensive than I'm planning on spending this year on top of all the other stuff needed around it...

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.

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One other thing I did learn was not to be afraid of dealing with reputable companies in the States. In addition to TEC I used OPT, Precise Parts and Starizona and got excellent service and saved a shed load of cash in the process despite the cost of courier shipping on the big items and import duty on everything. Worth checking prices once you do decide what to go for.

Bob.

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OM-D E-M5 + ED 12-50mm, H-PS14042E, H-F007014E, L-RS014150E, M.ZUIKO DIGITAL 45mm 1:1.8.
Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.

I was kinda wondering that... if I go for the Hyperstar, there seems to be a £1 = $1 conversion factor based upon one UK reseller. Given a dollar rate of 1.58 at moment, adding in 20% VAT that's still some 25% possible saving over buying locally. Shipping might dilute that somewhat but the items I'd get are realtively small, so I hope wouldn't be too significant considering the higher value of items. Are there any other fees I need to be aware of? I know couriers may change a customs handling fee. Any duty?

Ooh, precise parts are exactly what I was looking for earlier, as I was thniking about some day finishing the FD-EOS conversion I started on the 50mm f/1.2 over a year ago...

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.