MMOAG Issues?

[rally]

Mark,

Just playing around and looking at options onthe Helical focusser model 7315

Borg make a number of different helical focussers
http://www.sciencecenter.net/hutech/borg/focuser.htm
But I was puzzled by the measurements on the thread, so I measuerd them all.

The Nosepiece on the RGH is removeable and the internal M42 thread on the RGH allows you to attach the RGH directly to the Helical focusser (just remove the thumbscrews).

This possibly cuts out around 10mm of back focus

Why not try that and see if it works ?

The only down side is you would need to remove the cable when screwing it in and locking it off at a given angle might be slightly more difficult.
But that could be achieved by using some of that internal shim.

Rally

[el_draco (Rom)]

This is an interesting thread as I am currently debating over which OAG to get. I'm pretty sure I want an Astrodon because of the build quality but I am not sure whether I should get the MOAG or the MMOAG. Currently running a C11 OTA with a 2.5 inch Moonlite focuser.

I've read all I can about the two versions, but cant decide conclusively. It seems the MOAG stand alone comes with a more adaptors than the MMOAG for playing around with the spacing of components. At this point, I gather that the back focus on my C11 will not pose a problem in focusing with the HF, but I may get issues when I upgrade imagers in the future, and perhaps swap OTA's to a RC. I am leaning towards an SBIG ST-8300 but not sure yet. I doubt I'll get up to the massive CCD set-ups but then, the price is always coming down....

Question is, does the extra diameter of the MMOAG and shorter profile make significant difference compared to the MOAG and does the MMOAG provide more in terms of future proofing? I'd appreciate any feedback.

Rom

[wysiwyg (Mark)]

Rom,

My main reason for getting the MMOAG was its large aperture, as I use a STL-11K I needed something with a bigger opening. I also had to consider its thickness as the FSQ has very limited backfocus.

As far as build quality, its actually not that good when you assess it close up.
I think the use of bayonet type adapters is a bad design and when there are big imaging rigs attached to it like the STL-11K with 8CFW, there is bound to be flexure issues. I just dont see how 4 small little grub screws are sufficient for the sort of forces that are being exerted on them.
Bayonet style couplings are also very inaccurate as far as alignment, hence add more error into the imaging train. They eat into the soft aluminium creating burs, especially if you are dismantling the rig often.

Then there is the prism and helical focuser design. I had to throw the helical focuser out as it was useless, I could not achieve focus with a sbig remote guide and ended up using a TAK compression mounting instead which just happen to fit.
The depth of the prism is adjusted by adding spacers, they do provide you with a couple with the MMOAG, but I had to get extras as I need to go much further.

All in all I got it to work, but am I really happy with it? NO.
Too much time was wasted with spacers and adapters and Astrodon knowing exactly what type of set up I had, still managed to provide me with the wrong information and spacers.

How does it perform once it works?
Just like any other OAG system, it works on my set-up which is an FSQ-106. But I cannot confirm its functionality on other scopes etc.

Is it worth the money? NO.
If you dont have back focus issues, go with something else. Not sure what as I have not bothered looking but I am sure you can find better made OAG systems elsewhere.

The coupling mechanism in my opinion is crucial in any equipment of your imaging train, and if its bayonet, then its rubbish. I even considered getting one professionally machined using conical fittings which would assure alignment every time and provide for a more rigid imaging train.
But that would cost an arm and a leg :-)

Does it do its job? YES
Are you getting bang for buck? NO

I would be very surprised if there wasn't anything better out there.
The only one that I am aware of that comes close to what you might want is the Mitsuboshi OAG
http://www.sciencecenter.net/hutech/mitsub/oag.htm

Hope this was useful.
Good Luck!

Cheers
Mark

[rat156]

I'll provide a counterpoint to most of what Mark has said now, not to create an argument, but because my experience has been almost totally the opposite.

I have the MOAG, not the MMOAG, but both use a v-groove or dovetail adapter to connect to the filterwheel. I find that this is an excellent method to attach the camera to the OAG, it doesn't provide any flexure at all, the four grub screws ensure that. Yes, the inside of the V gets marked, but who cares? If you're really worried, get the plastic tipped grub screws. The nature of the V-groove means that as long as it's cut perpendicular to the main axis it'll hold the system square, as long as you tighten the grub screws properly. By this I mean just nip them up, then progressively tighten them until snug.

I have also found Don Goldman a pleasure to do business with, very helpful. I didn't have the focus issues Mark had mainly because I also have an AOL, so the setup is different.

I think it's a great product.

Cheers
Stuart

[rally]

I'll add to this, I think the MMOAG is a solution to a problem, but it has its strengths and weaknesses.
Its greatest benefit is that its wide and its takes up the least backfocus - it is almost the only OAG you can use with the older FSQ106's.

It uses the same sort of mounting system as many other OTA accessories (4 grub screws that screw into a groove on whatevere it adaptes to), but the problem is with a very heavy camera like the STL11000m and 8 filter wheel this mounting system is barely up to the job.
I can understand why they have done it that way - it allows any rotational positon to be attained which a simple screw on type fitting will not - ie once tightened it can end up anywhere !, but its less than ideal if you have a huge amount of weight hanging off it.

The method of setting the prism position is limiting and awkward, and necessarily involves a recollimation/realigning the prism each time, which means you have to remove the MMOAG off the OTA to do it.
Neither the helical focusser nor the fixed 1.25" adapter supplied can be used with a Remote Guide Head - they simply will not allow you to reach focus on the very systems it is supposed to be used with - that is sheer nonsense.
Also the helical focusser is OK (assuming you can reach backfocus) but it is sloppy in the helical threads from new - hardly something you want in a guide camera !
I couldnt use it anyway and ended up using a 1.25" EP compression fitting the same as Mark did.

Cheers
Rally

[Doomsayer]

I've also just integrated a MMOAG into my setup. Its rigidity and security of connection between the camera and scope is excellent. Getting the guide camera to focus is tricky. I also have short back focus behind my 12.5" f6.7 RC. An STL6303e now rides at the rear. I use an ST402 for guiding. I couldn't use the MMOAG helical focuser, which does have some slop and is too deep. I also had to use the long STL-MMOAG apapter to reach focus with the STL and ST 402. My guider stars are round.

I have also elected to lower the prism position a bit more - the 6303 chip is quite a bit smaller than the 11k so there is space. This has been done because of my plans for squeezing in an image rotator.

OAGs have always had compromises - I've been using them on and off since the the early 90's often without success but more frustration. The MMOAG is so far the best I have used by far - even though there are challenges.

guy

[AlexN]

I haven't had personal experience with the MOAG or MMOAG, but I'll add this...

Rom..

I used to use a C11 for imaging, with a Lumicon 2" Newtonian OAG and had no problems with it what so ever.. Focusing is tricky the first time, but I used a parfocalising ring and once I found good focus on the guider I simply put the ring in the right spot and tightend it up...

I can absolutely, 100% guarantee that you will not run out of backfocus on the C11... Not even if you added 80mm of extension, then the OAG, then adaptive optics, then a filter wheel then the imaging camera. Obviously you would want to watch your imaging train length, as if you decide to move from the SCT to a refractor you may well run out of backfocus quickly. If your intention is to go from the SCT to an RC telescope, then have no fear, most RC's have a ton of backfocus, and practically anything is possible. This is especially true of the GSO RC's. They have about 11" of backfocus...

The MOAG and MMOAG both seem to be great devices, Mark is sure having some difficulty however.. I think unless you are using a really really large sensor, the MMOAG and MOAG might be overkill. The ST8300 sensor is smaller than a standard DSLR, the MOAG and MMOAG are designed for sensors twice as large, like the KAF16803. This is not to say that you can't use one, just that it may be more than you require at the moment, or even any time in the future...

Rom If I were you, before going the whole hog and buying such an expensive OAG, I'd be getting a hold of one of the cheaper units, like the Orion Deluxe OAG.. See if its going to do for you what you want.. see if you like the results before spending up big on an item that may be very difficult to sell if you find its not to your liking..

Personally, I love OAG's.. I do not see myself moving back to a separate guide scope at any stage for any reason...

[rat156]

A word of caution re backfocus and the GSO RC's. I have the 8" version which comes with three screw in extensions. When imaging with an SBIG ST-10, CFW10, MOAG, AOL, I have just enough room for everything behind the focuser (Moonlite). So even though there is lots of backfocus you can use it all up!

Cheers
Stuart

[el_draco (Rom)]

Thankyou all for your views, I've done everything except machine these units in my head and I've read a heap about ways of dealing with the many issues associated with these gadgets. Seems most have a "fix" that is dependent on many scope specific factors, but doable.

From my readings, it seems that I am going to need an OAG particularly if I am going to play around with narrow band imaging, (Guidestar visibility issues), and do it, atleast initially, with a C11, (mirror shift problems), but I get the distinct impression that there are two camps of thought in relation to OAG verses seperate guidescopes.

Having not used one before, I am going to follow the advice given by Alex and grab an Orion OAG to experiment with; of the "cheapies", it seems the easiest to play with, and I can always sell it later on.

I'm about to start 8 weeks holiday and have a world of learning to complete in that time. Relation from the states is coming out just after the "commercialised pagan ritual" and will bring me an OAG and half a dozen other things I can now budget for, so it's going to be fun...

Rom