Short Focal Length Refractor vs. 8" f/10 SCT?

[cloud_cover]

After some feeble attempts at photography and a disasterous, non-event at using an off-axis guider with my VMC, I'm now questioning the wisdom of using a 2000mm scope as my primary imaging platform, even though

1. In Singapore, skyfog pretty much saturates my D700 in about 1.5mins WITH a UHC filter in front. Saturates in 30s or under without
2. 2000mm even with the full field of an FX frame means mid to smaller DSOs only OR I'll have to mosiac. Objects like M42, M31 and the Rosette are completely out of the question. Even then, an object like Trifid has to be carefully framed to fall within the FX sensor and not be truncated by the short side of the rectangular CCD.

So far I haven't had much issues with jitter or trailing since I'm not "professional" about it and deconvolution seems to work wonders (but invalidates any flat frame data, a trade-off) and I haven't done any real "long exposures"

So I'm wondering whether I'm better off using a short focal length refractor or should I stick to the VMC + OAG setup. Sigh

Of course I love it that the VMC is just to resistant to dew that I don't even need to think about it.

BTW, if anyone as a nice imaging refractor to sell, I might be interested.......

[andeelym]

Use a f6.3 SCT focal reducer and you can get 1260mm. Just my 2 cents.

[cloud_cover]

Yup, and I have the Vixen one.
BUT it would mean the inability to use filters or the OAG (back focus is a mere 3mm) and I have to swap out my dual speed focuser for the set screw one which mars my diagonals and doesn't hold things that well
Sigh! choices.....

[orly_andico]

kevin,
what you need is an H-a, O-III, and S-II filter set. then you take take hours-long exposures even during full moon. i think you should reconsider the OAG.

you can barlow your finder to get around your SSAG not doing subpixel guiding.

people guide 8", 2000mm FL SCT's with 50mm finders (with subpixel guiding). a 3X barlow ought to get you in the range.

[cloud_cover]

Thanks for the thought
The only worry I have for the 80mm finder/guider is that it joins the main scope by a dovetail bracket, pretty much like to 50mm finder that you're buying (and haven't picked uo ..... ahem.....) so I'm likewise worried about flexure.
If not one of my thoughts is to use a 2.5x barlow (which I have) to achieve 750mm. But the problem is also that my scope itself is a 2000mm focal length scope..... not too sure even with the barlow if that's enough focal length.
Regarding the Ha, SII..... that would also mean a new camera.......

[kaomoo]

kevin,
what you need is an H-a, O-III, and S-II filter set. then you take take hours-long exposures even during full moon. i think you should reconsider the OAG.

you can barlow your finder to get around your SSAG not doing subpixel guiding.

people guide 8", 2000mm FL SCT's with 50mm finders (with subpixel guiding). a 3X barlow ought to get you in the range.

hi orly
does the so call emission nebular filter like the one from Baader UHC-S nebula filter work in place of 3 filters?

kao

[weixing]

Hi,

1. In Singapore, skyfog pretty much saturates my D700 in about 1.5mins WITH a UHC filter in front. Saturates in 30s or under without

So fast saturate?? Did you check the histogram? What ISO are you using?? I can get 2min of exposure(@ISO400) using my R200SS with broadband LPS filter and the histogram is only at the mid point.

Anyway, 2000mm focal length is very tough as you'll need a really stable mount and stable atmostphere to get the most out of it. I suggest you take a good look at Vixen R200SS...800mm focal length allow you to get 1min of exposure without guiding quite easily and you can basically ignore the atmospheric condition... Also, someone is selling one now at a very good price.

Have a nice day.