I'm going to buy the GSO RCA-300 Ritchey-Chrétien and put it on the new EQ8 with either my ED80 or ED120 on top as a guidescope. This will also enable me to keep the wide-field capability when I reverse the roles of the scopes.
I've checked with Bintel the weight load of either combination on the mount and even with cameras, they both come in under limits so that's all fine.
My question is:
Is the 120 or the 80 best for guiding on this big scope and mount? Why? I'd like to hear your thoughts.
Baz.
1. My current telescopes ED80 and ED120 on an EQ6Pro GEM.
2. Proposed GSO RC 300.
3. Proposed EQ8 GEM.
Last edited by bloodhound31; 08-08-2013 at 10:35 PM.
If I may comment, with gear this serious, I'd go off axis guiding. Sure, a refractor above will guide it, but why not go the whole hog and do it right?
If I may comment, with gear this serious, I'd go off axis guiding. Sure, a refractor above will guide it, but why not go the whole hog and do it right?
MMOAG.
I guess because I am really familiar with piggyback guiding and find it very easy to locate a guide star with my custom-built, independently adjustable guide-scope mount.
From what I understand about off-axis when I last attempted it, sometimes it can be VERY hard to even find a guide star in the FOV, depending on what you are imaging.
Unless your SCT has a fixed mirror - that can't shift, sway or flop even even a miniscule amount you may find guiding from a refractor whilst you image from a longer focal length SCT really challenging - I did! Differential flexure is a killer between scopes, but I still think many SCTs have slight mirror shifts that vary with elevation - which means you're only guiding option is on or off axis at primary focal length).
With a OAG I can easily do 30 minute subs on my C9.25 (I am sure the same would be true for an ONAG). Guiding at full focal length is brilliant! I tried both side by side and piggy backed quiding with a 80mm refractor the results were horrible - I couldn't even get 5 minute subs I'd be happy with some days.
I saw the new EQ 8 at Bintel a few days ago - looks great and has a really solid feel to it Baz - I think you'll be pleased with it.
If it were my option I'd go the 12" GSO RC and the 120mm ED - I like light grasp! I'd place an OAG on the GSO - and probably add a focal reducer (consider maybe the Lumicon models which have both). I would also invest in PC controllable focusers for both these scopes over time - having experienced now the Moonlight and Meade PC (+ JMI focus) controllable focusers - I really like the Moonlight - but both work well!
As for always getting a guide star, consider guiding with a sensitive enough camera - like a Lodestar, or go ON Axis Guiding with ForSight Innovations ONAG (Cold mirror) and be spoilt for choice.
Have you considered what imaging camera / focuser / filters / etc you'll be using? It might worth planning it out ahead of time to double check that everything is suitably matched and compatible.
Quote:
Originally Posted by bloodhound31
From what I understand about off-axis when I last attempted it, sometimes it can be VERY hard to even find a guide star in the FOV, depending on what you are imaging.
I'm only using an RC8 and have yet to find a patch of sky that doesn't have a suitable guide star for my Lodestar using 2 sec exposures in 2x2 binning. Most areas of the sky have pretty bright guide stars (0.05 to 0.5 sec exposures).
If your mount has a smooth PE curve (and the EQ8 looks like it does from what people have posted), then you can use even longer exposures without a worry.
An RC12 has over twice the light gathering ability as an RC8 so you'll be fine
Agree with what others have said. At a native 2432mm focal length, I'd be shying away from the guidescope idea. An off axis guider is the way to go. Save yourself from the headaches.
I use a long FL guidescope, and a Lodestar. Never once have I been unable to find a guide star in light polluted suburbia (but failed several times with the Orion SSAG).
I am going OAG (again - the SSAG was not good enough for the OAG I tried), but need to find one that will fit in with the imaging train I have (all screwed together - no compression rings etc). Teleskop-Service have one that should suit, and it is one of the THIN ones too.
Baz,
Persevere with the oag and invest in a loadstar, the improved sensitivity should cure your guide star availability problems. The guide scope route will be problematic irrespective of the aperture you throw at it..
The best guider I have found is the monochrome loadstar, the best price was from telescope express in Germany.
There are a couple of exceptional focusers you can put on your RC and on that list would be:
FLI Atlas (not to be confused with the pdf) almost the default standard.
Reginato (Italy) ... it looks good on paper anyway.
Clement compliant (USA)
Feathertouch (USA)
And last but not least, Planewave, (arguably the pick of the bunch)
Baz, what camera are you planning on using with this set up? If the sensor size is large then you'll definitely need a field flattener, in which case you'll need to also ensure that your camera + filters + OAG + anything else will fit within the back focus distance of the flattener, and that the focuser is up to scratch. A motorised focuser might also be very handy - especially given the steel tube.
Baz, what camera are you planning on using with this set up? If the sensor size is large then you'll definitely need a field flattener, in which case you'll need to also ensure that your camera + filters + OAG + anything else will fit within the back focus distance of the flattener, and that the focuser is up to scratch. A motorised focuser might also be very handy - especially given the steel tube.
Dave, my current cameras are an Orion Starshoot Deep Space colour CCD camera, DBK21AU618.AS planetary camera and a 5DMK II DSLR.
I'll also introduce you to Phil. He's a local guy, using a similar sized scope (12"), off axis guiding with arguably the best of the OAGs - the Astrodon MMOAG, and using a very sensitive guide camera to suit - the SBIG STi. I have one of these too, and chose it over the Lodestar as it has a mechanical shutter. Both are superb guiders, but costly.
P.S. I previously owned a Orion Deluxe OAG. You sure get what you pay for in this hobby...
Last edited by Logieberra; 09-08-2013 at 11:38 PM.
Not only consider a motorised focuser, consider a motorised, PC controllable one with a temperature sensor. My logic here is:
1. A steel tube will shrink as the temperature falls, so you will need to compensate to maintain focus as temperature varies (which is why I love Carbon Fibre tubes - almost no thermal expansion / contraction)
2. Focusing with say a Bathinov mask - or any focusing - I don't like to touch the tube at all - I want it rock steady and for folk to not even walk by it when I am focusing or imaging! So PC controlled is great!
3. If the tube has a read out of both its position and temperature then you should be able to chart and match temperatures to position.
4. If you use eventually use a mono camera and filters - then (not sure is this was refractors only or SCTs too) each filter will focus to a different position - so its handy tracking where each colour focuses at any given temperature.
The Moonlite focusers with High resolution Stepper motors, temperature sensing and Variable speed mini controllers (hand operated or USB controller v2) work very well!http://www.focuser.com/cgi-bin/dman....cgi&category=5
You would think that the following statement would be so bleeding obvious as to be unnecessary; the place for the oag is ahead of the focal plane of the main instrument (not the piggyback scope)
It beggars belief that there is at least one RC12 gathering dust (that I know of) because the owner is obtuse and deaf to reason specifically on this point.
You would think that the following statement would be so bleeding obvious as to be unnecessary; the place for the oag is ahead of the focal plane of the main instrument (not the piggyback scope)
It beggars belief that there is at least one RC12 gathering dust (that I know of) because the owner is obtuse and deaf to reason specifically on this point.
