Telescope types for Astrophotography

[alaskacam]

Hello. I'm starting to wade my way into astrophotography. I've had pretty good luck so far. Like many of you I have the desire to set up a more permanent amateur observatory in the backyard. I'm looking for some pros and cons contrasting high-end refractors and high-end short reflectors. Say the difference between a Planewave CDK 12.5 and a Takahashi 150F. I'd be curious on feedback contrasting flexibility, image sharpness, overall image quality with a good CCD and good auto-guided mount. I'm not set on these two models, but more use them to illustrate my question.

Thanks,

Rob

[Ben Ritchie]

What's your local seeing like? One advantage to a high-end reflector (e.g. a 12" Ritchey-Chrétien) is that you're matching a well-corrected field of view to high resolution - the (much) longer focal length means that you're sampling at under 1"/pixel for many CCDs. Couple that with a large CCD - STL-11k or similar - and you get a wide field of view AND high resolution. Under great seeing that's a brilliant combination, but under typical UK conditions (and I realize you're down-under) there's much less point, because there's no point sampling at 0.5"/pixel when you have 3" seeing.

Both options require a pretty decent mount, a 6" refractor isn't light and has the weight predominantly at the ends (lens cell and focuser/CCD). I'd ideally look to AP900/1200 for both options, but you would be just about ok with the CGE/G11 for the refractor (although i'd consider it undermounted on that).

The reflector requires collimation, the refractor doesn't. On the other hand you'll probably need a flattener with the refractor and a RC/CDK probably won't unless it's a very large CCD.

In short, no bad option if you can afford the price of admission - the refractor is probably the better all-round option though if you're interested in visual too.

[alaskacam]

Thanks for the comments Ben. Not sure what the seeing measurement is here most nights, but it's not great. We live at sea-level, right on the Indian Ocean so the atmospheric column is usually quite active, especially in the Winter when we have the best nights. Transparency is usually excellent as we have 1000s of miles of ocean in front of us. Light pollution is low to moderate as we live about 25-30 miles from Perth. Another 100 miles out and you could almost read a book by the light of the Milky Way!!

The down-side is that we'll probably be in SE Texas by the time I actually get the new setup configured. Not the best place for a backyard observatory.

Rob

[roundycat]

I agree with Ben, I use a 10" LX200 and even on nights of relatively good seeing I never approach the capability of the camera/scope. I also have a TMB 152 which is half the focal length of the LX and gives me brilliant results most of the time. My view is that you can forget relatively expensive big scopes unless you are using it above 5000 feet. I overlook farmland and the North Sea so my seeing is not usually that bad.

[alaskacam]

Thanks guys. I now understand the seeing specs. I jumped in full stop. I bought a used 2.3m Sirius dome, takahashi TOA-150B on a tak EM-400 temma2m mount. I'm most interested in DSO and this seemed a great setup for that.

Rob

[ollypenrice]

You'll love it. I use a TEC140 refractor on an EM200. It could be faster (F7) but it is a peach to use and doesn't need perfect seeing.

Olly

[NickK]

Only way to undo bad seeing is to use a reflector with an active flexible second mirror with a wave front analyser. The mirror has voice coils attached and can vary the curvature of the mirror across both axes of the mirror. These adjust the image in real time as the exposure is being captured to reduce the refractive differences in the atmosphere.

Budget kits for these are in the £10-20K bracket and that's just the low granularity mirror, wavefront sensor and controller.

Starlight AO are different - they are to provide further accuracy on tracking the image (ie the guide star) between the mount movements. Usually 8-10 times per second. The do perform basic atmospheric correction but apply the correction to the entire image (in reality the image may be affected by different refractive indices over the image that change over time). At 1/10th the cost they are still a good option for longer focal lengths in reasonable seeing conditions to help cope with some of the atmospheric churn.

[ollypenrice]

Only way to undo bad seeing is to use a reflector with an active flexible second mirror with a wave front analyser. The mirror has voice coils attached and can vary the curvature of the mirror across both axes of the mirror. These adjust the image in real time as the exposure is being captured to reduce the refractive differences in the atmosphere.

Budget kits for these are in the £10-20K bracket and that's just the low granularity mirror, wavefront sensor and controller.

Starlight AO are different - they are to provide further accuracy on tracking the image (ie the guide star) between the mount movements. Usually 8-10 times per second. The do perform basic atmospheric correction but apply the correction to the entire image (in reality the image may be affected by different refractive indices over the image that change over time). At 1/10th the cost they are still a good option for longer focal lengths in reasonable seeing conditions to help cope with some of the atmospheric churn.

Agreed. The best SCT images I have seen have been done with the moving glass lollipop devices.

Olly