Guiding - Off-axis vs piggyback CCD camera?

[AndyUK]

Hi there - I'm exploring auto-guiding options on a CG5 mount fitted with a SW Synscan GOTO uograde and considering an off-axis guider as possibly the best option.

As cost obviously cost comes into the equation, my other option is to use my SW ST102 as a possible guide scope, which I reckon would also fall within the payload limit of the mount (with a Celestron 100 ED refractor and a Canon EOS 300D). However, from what I've read, would an Off-Axis guider be a lot simpler to set up and also remove any possible flexure issues?

As auto-guiding is very new to me, I'm also wondering if, as a starter, I could possibly get away with using my unmodified Toucam II 840 as a guide camera, or would I really need to get a better camera to perform this function? Again, as cost is obviously an issue (and I'm a total beginner in this area!), I've looked at an Opticstar AG-131C COOLAIR as a possible option - Does anyone have any experience with this as a guide camera (or maybe I should budget a little more for something more "future-proofed"?)

[ollypenrice]

Because the 100ED will be very slow for imaging (because of its focal ratio) and because it has a longish focal length, you will need good guiding. It may not be an area in which to cut corners! Used with an OAG in a slow scope I would doubt that a webcam based device would be all that likely to pick up stars easily, but I may be wrong.

I wonder if you could pick up a second hand Atik 16ic? I use a couple of these and find them ideal because stars are always 'just there' without hunting. They are very sensitive so hang onto guide stars if some wisps of cloud pass over.

Olly

[AndyUK]

Cheers Olly - I must admit, I hadn't considered the slow speed of the scope...

I really wanted to buy an 80mm f6/HEQ5, but that would have cost twice as much as the 100ED/CG5, added to which I didn't want to wait too long else I'm sure the funds would have been grabbed by my wife for some other "higher priority" project (like a new carpet or something..!)

I doubt that these 2nd hand Atik's come up that often, but I'll obviously try and keep my eye out for one... and in the meantime continue normal viewing and trying to keep myself satisfied with lunar shots (and may be see if I can get any guiding use at all out of the Toucan with an OAG).

(Why oh why do I always pick such expensive hobbies?!)

[jgs001]

As you already have the ST102, why not use that ? I'm not using a webcam as a guidecam, but I was imaging at 1200mm (f/15 using a 2xTC) with my C80ED last night, and guiding without issues with an ST80 clone and a QHY5v (small CMOS sensor) quidecam. In fact in experiments, just to see if I could really... I was able to hit 20 minute subs at 1200mm (C80ED and a 2xTC).

[AndyUK]

Hi John - Yes, that was my other option, but I was a bit concerned that the weight of the C100ED + ST102 (+ DSLR + guide rings / plate etc) might either be getting close to overloading the CG5 or introduce flexure issues... both of which I reckoned would be circumvented with an OAG... but obviously with my slow scope (as Olly noted), maybe the OAG route isn't a very good one for me at this time after all.

On the assumption that I can get that lot on the CG5, as guide rings / plates are comparatively cheap (certainly a lot cheaper than an OAG!), maybe I should just see how I get on with piggybacking and see what I can squeeze out of the Toucam (if anything). If I do then start getting frustrated in not being able to grab hold of guide stars, I can first look at upgrading to the Atik, and then maybe look at the OAG route later on...

By the way, I see that the Atik's are the same price whether colour of mono - Is there any particular benefit of having a mono?. Okay, as a pure guide it obviously wouldn't need to be colour, but in the event that I might like to try and use it for planetary imaging, it might be nice to have the option...

[jgs001]

I wouldn't worry about guiderings plates etc to start with, just bolt the dovetail of the ST102 on the top of the tube rings of the C100ED... you might need to buy a set of tube rings and a dovetail for the C100ED though if it came with a slamshell. The other thing I'd suggest, if you haven't replaced the focuser... is a teflon tape mod... takes the slack and slop out of it nicely. The same goes for the ST102.

[AndyUK]

Cheers again John... But what's a "teflon tape mod"?? I had a look on google but couldn't really find out too much about it?

I haven't replaced the C100ED focuser, but it's certainly something I'm seriously considering. It's actually possibly a higher priority than getting a guider as I'm also considering a 4x Imagemate but it's tricky enough achieving reasonable focus using a 2x barlow with the Toucam on my 130P Explorer (and I know that even that's pushing the scope's magnification boundary) let alone trying to use it on an f9 scope (but that's a whole different subject!).

I believe it's possible to replace the R&P focuser with a GSO dual-speed crayford though which would make things a LOT easier. However, from what I've read I believe that I also need to get an adapter to attach the crayford and also a finder dovetail base... I've only seen them advertised in the US though (total cost c. $190 + p&p + customs), but again, I certainly intend asking more about this to see if I can get it in the UK...

My C100ED does have rings, but my ST102 has a REALLY tiny dovetail (it came with an AZ GOTO mount), but I'll certainly have a crack at seeing how I might be able to attach it...

I haven't actually had the ST102 that long - I really only wanted the mount (but the OTA came with it) as I knew it would also work with my 130 Explorer (and, as I hoped, the GOTO seriously resurrected my interest again!) but the focuser on that certainly needs some attention as it's REALLY sticky - I know I need to strip the old grease off and reapply, but maybe your "Teflon tape mod" will improve it even more...

Thanks again...!

[jgs001]

Andy, for about £6 you buy some teflon tape and stick it to the focuser drawtube to pack it out and remove the slop. I've done this with both my fracs... it's no substitute for a decent Crayford, but if the budget is really tight....

Anyway... With my konus, it was so bad, that with a 2x barlow and my spc900, I had to move the mount to adjust for the slop trying to focus. With teflon tape fitted, I get barely any movement with a 5x barlow and the spc900. The same is true on the C80ED. I focus with 10x liveview on the 450d, and the movement in the star was very noticeable before I applied the tape. Now it's barely visible.

As for the ST102, gotcha... you'll probably want a pair of rings and a long dovetail then, you need to make sure that it's locked firmly to the C100ED with no chance of movement, and whilst that SLR dovetail does have a tripod screw, it's not going to cut it as it will allow the scope to twist.

[AndyUK]

Ah... Now I understand - It's not as complicated as I was expecting...! Mine don't seem to be too sloppy at the moment, but then again I haven't done too much high mag work / imaging to really find out...

Interesting to read that you stack 2x with a 5x for the DSLR - Assuming I do get a 4x, I'll have to experiment with that (although I suspect it'll be pretty dark in there!). Thanks again John - Great help...

[jgs001]

Wasn't a stack of the pair... that would give x10... I did stack a pair of 2x and a 10x for 20x, but I was being silly with the SLR on Venus... it worked but was a tad precarious ...

[AndyUK]

Blimey - Must have been quite a balancing act! (And do you have a copy of any images taken...?) I've never really tried much with Venus to date as it's simply too small with a 2x barlow, even using the Toucam.

In fact it was only last night that I managed to get what I consider fair images of Saturn and Mars using the Toucam on the 130 reflector - Nothing to shout about mind, but a minor milestone for me. An orange dot with some slight dark smudges (possibly using a little imagination?) and a blue dot with a line through it and a VERY slight shadow...

A LONG way to go, but looking forward to the journey... as far as my budget will take me (and without upsetting my wife!)

[jgs001]

I can't find the thread anywhere... but I'll PM it to you...

[Tim]

Hi,

OAG will give you better results for sure, but you would need a good very sensitive guidecam, like an SX Lodestar or DSI Pro series (look for Sony Exview HAD chip) for best results from an OAG.

Side by Side is much simpler to set up, might be cheaper, but if you get a decent guidecam and an OAG then you can likely use both your tubes for imaging with a simple swap of the OAG/camera combo from one OTA to another.

A lot of it depends how serious you are, and on what type of targets you want to aim for, but as a general rule of thumb, off axis guiding will give you better results and offer better long term adaptability to your imaging, but might not necessarily be the cheapest option.

Whatever option you go for, I would recommend getting a decent guidecamera. The really sensitive ones can cope with light drifting cloud, and really increase your choice of suitable guide stars.

Cheers

TJ