Questions about LRGB imaging

[Benjamin (Ben)]

Thinking of taking the next step with LRGB imaging, mostly to take advantage of the less noisy cooled mono cameras and to speed up image acquisition. Plan at this point is a ZWO package with the asi1600mm-cooled, and the mini-filter wheel with the 1.25" filters including the Ha filter. I've been using a TOAG for guiding recently, although also have a finder guider. This would be a mobile not a fixed setup. So a few newbie questions.

Initially I was planning to use the f7.5 ED80, using the .85x FF/FR to bring it down to f6.4. Using the finder guider (no TOAG) it seems I'd need a 28.5mm spacer (between the wheel and the FF/FR), with M48 threads at either end. My think was to get a 19mm spacer (or .75 inches, of which there is Blue Fireball product) and a 10mm spacer, so that with other setups I could swap out the 10mm spacer and use the TOAG. Or could I just keep the TOAG in place (so no extra 10mm spacer) and just not use it? Would that introduce any issues with the image acquisition (stray light)? Guess I could just cover it?

EDIT: just realized all the spacers come with the camera (right?) but still curious about ditching the 11mm spacer and keeping the TOAG in place even if not using it?

Given the ZWO LRGB filters are close to par-focal with this setup (please correct me if I'm way off here) would the ED80 be more forgiving with focus? Could I conceivably get away with not refocusing after each filter change? My focuser is not yet motorized and if I don't have to I'd rather not. Could small differences in focus effectively be processed out with this scope/focal length perhaps?

The plan then is to motorize the focuser on my F5 8" Newt (same spacing as I'm using a coma corrector) and adjust focus automatically (with appropriate cables - FCUSB or similar) through SGP with each filter change. Does that sound feasible? Can SGP do it all?

Sorry for all the questions, many of which will be answered once I actually have the stuff to play around with but I guess I'm just trying to estimate the hole I'm about to jump into, in terms of $ and time learning new processs and software. The DSLR is still good for long while I'm sure but I'm happy to pay and learn a bit, if, in the long run, it makes everything a little more efficient.

[rustigsmed (Russell)]

hi ben,

you can keep the OAG in there if you like i'd probably move the prism out of view although i'd prefer to use a oag, rather than the finderscope to avoid flexure. but it would be preferable to have an additional spacer instead but the OAG will work.

SGP can focus well. I would be refocussing on every filter anyway, its needed due to temp change anyhow. i usually do it once every 45 mins.

[glend (Glen)]

Don't complicate the setup, a finder guider will work well with that scope. I would try that first, and it eliminates the OAG in the spacer equation. Don't firget the Rule of 1/3 when calculating spacing, ie the filters and any cover glass on the way to the sensor extends the focal length by 1mm for every 3mm of added glass. The LRBG filters are 2mm thick, the cover glass AR window ahead of the 1600 sensor is 1mm, so that equals 3mm, so you add 1mm of extra spacing.

[Camelopardalis (Dunk)]

The ASI1600MM-Cool is a very versatile camera, IMO.

I don't like their filter set, personally. This is mostly because the OIII line is captured solely while the green filter is in play, so OIII features show up as bright green in your images where it should be more teal/turquoise. To achieve this, some other filter sets overlap the green and blue so that both capture OIII photons. ZWO may have changed this, of course.

A motorised focuser really makes a difference in tracking temperature changes in the Brisbane winter, I find my scope can go out of focus in 30 minutes going from warm day to cool night...