Formula to calculate useful focal length?

[swannies1983 (Dan)]

Is there a formula or recommendation for the maximum useful focal length for a given aperature when imaging with a toucam? I have been using my 8" f/6 Newt to image Saturn. I only have a 2x barlow but would like to get a bigger image scale. The question is, do I get a 3, 4 or 5X powermate/barlow? I see a lot of people use a 5x Powermate but I don't think I have the light gathering power with my 8" Newt. So I'm debating between the 3x or 4x. Will I have enough light gathering power when using the 4x or would I have to really crank up the gain and brightness settings?

[[1ponders] (Paul)]

Using an 8" SCT (f/10) I find using a 2.5x powermate (f/25) no trouble at all. It is a bit dim but 1/25 sec exp @ 10fps is OK.

[sheeny (Al)]

G'Day Swannies,

There are more qualified people than me to answer this, and I'm sure you'll get a ore definitive answer from the Dennis or 1ponders.

f/6 x 5 gives you f/30. That's the same focal ratio as I get from my C8 with a 3x barlow. That's certainly in the ballpark for planetary imaging. However, the more magnification you aim for, the greater the effect that seeing has on your images - you need stable conditions to make use of high magnification. So an 3 or 4 barlow/powermate maybe useful more often.

Hopefully some of the more experienced guys will jump into this thread to help you out with a bit more info.

Al.

[sheeny (Al)]

Thought you'd be watching, Paul!

Al.

[[1ponders] (Paul)]

Yep

[Dennis]

Hello,

I haven't stumbled across a formula, but it seems that on nights of very good seeing, working anywhere from F20 to F40 can produce excellent results. I tend to operate my C9.25 F10 at F25 using the TeleVue x2.5 PowerMate when the seeing is better than 6 or 7. This might happen say, 6-12 times per year.

I can operate my Vixen 4" F9 refractor at F45 on nights of exceptional seeing using the TeleVue x5 PowerMate. This might happen say, 3-5 times per year.

I have used the TeleVue x5 PowerMate on the C9.25 to give F50 for imaging the Trapezium using my ST7E CCD camera but this was really pushing the C9.25 way too far. I’ve only tried this a couple of times and got away with it due to the sensitivity of the ST7 and the good tracking of the Tak mount, plus a lot of help from deconvolution and stacking tools in image processing software.

Cheers

Dennis

[Karls48 (Karl)]

Have look at http://www.lrgb.com/