MMOAG Issues?

[wysiwyg]

Hi Guys,

This is the first time I am attempting to use an OAG, so I am not sure what is normal and what to expect as far as FOV.
I have a MMOAG from Astrodon which I am using with my FSQ106N and STL camera. I have the right adapters, there is no question there.

The first time I mounted it I noticed that the stars were oval, at closer inspection I was was getting a football shape FOV as in the attachment. Hence I thought I need more prism spacers to move the prism further into the light cone.
Ordered some sapcers and fitted them but no chnage in the shape of the firld of View. I still get a football.
Is this normal, or should you be getting a nice round circlular FOV?

I manually manipulated the prism holder until I got a nice circular FOV, but the prism was in the center of the opening. This would surely obscure the mani imaging chip.
Also noticed that there is a baffle inside the FSQ, and when looking through the eyepeice holder of the OAG you can clearly see it.

Just wanting comments from people who have had experiences with OAG's and what sort of FOV do you get.
Keep in mind that with this sort of FOV I was getting oval stars using the STL remote guide head, yet using an eyepiece I was getting nice round stars.

Any comments are appreciated.

Cheers
Mark

[rat156]

Mine's the same as this, I use it without a problem.

Cheers
Stuart

[wysiwyg]

Thanks,

Does that mean your stars are round when guiding?

Mark

[rat156]

If I focus them, yes. Out of focus, they tend to go a bit funny, but these are at the edge of the field.

Cheers
Stuart

[wysiwyg]

Thanks Stuart!

You have been a great help :thumbsup:
I will confirm this once there are some clear skies.

Mark

[gregbradley]

Hey Stuart,

I have a MMOAG and when I tried to use it with an ST402ME guide cam I couldn't get it to focus.

Someone else said they had to replace the focuser extension with the shorter one supplied. is that what you had to do?

Greg.

[wysiwyg]

Greg,

As you know the MMOAG came with a helical focuser and a shorter 1.25" nosepiece holder.

Could not attain focus with the helical focuser, distance is just too far.
The 1.25" nosepiece holder was better but still could not get focus.
The remote guide head was bottoming out and could not get any closer, which meant that I had to increase the distance between the STL imaging chip and the prism by getting a longer STL-MMOAG adpater.

I ended up getting the .5" adapter from Astrodon, this way I have plenty of room to manoeuvre the RGH to get focus.
Lost .5" of back focus so I have another .5" left, but thats not a big deal as the CFZ of the FSQ is 55 microns.

Need to make up spacer(s) that will go around the RGH nosepiece so when RGH is inserted into the MMOAG it gets it near focus every time.

Mark

[rally]

Just to add to Marks comment,

The other alternative was to modify (by shortening) the nose piece on the RGH.
By shortening (turning off) the end of the nose piece by a few mm, the same end result (achieve focus) was possible, but that would have meant the RGH nose piece was no longer standard.
This is because the RGH nose piece is longer than the 1.25"EP standard adapter and bottoms out at the base leaving bare RGH nose pice exposed at the top.

So for others in a similar situation but possibly without any further backfocus in their system, this may have been the only solution.

eg when using a FLI-PDF focusser.

Rally

[rat156]

There is a small lens in the bottom of the 1.25" port of the MOAG. This is to extend the focal length to a similar distance to that of the STL, or in my case an ST-10 and CFW. When I insert the RGH I have to leave it a little bit proud (sticking out) of the helical focuser. If I remove the lens I may not have to do this. As an alternative there is another lens that can go in there that shortens the focal length. All the info is on the Astrodon website.

Greg, I'm not sure how the backfocus distance of the RGH differs from the ST402, do you have to go closer, or further away?

If closer then remove the lens at the bottom of the helical focuser. Further away, check that it's there first, then buy a 1.25" eyepiece extender.

Cheers
Stuart

[wysiwyg]

Stuart,

The helical focuser supplied with the MonsterMOAG does not have any sort of barlow lens at the bottom of it, its simply straight through.

The helical focuser supplied with the MOAG does.
There is no mention of the removable barlow lens on the MonsterMOAG specifications.

However, there is mention of an Optional f/-250 lens to extend focus by ~ 12 mm, just saw that now, I am pretty sure that was not there when I order my MMOAG, but I could be wrong.

Cheers
Mark

[rat156]

I'm not sure it's a barlow as such. Here are some pics of the MOAG with and without the lens in question.

I'm sure that the same goes for the MMOAG.

Cheers
Stuart

[rat156]

More information at
http://www.astrodon.com/articles_faq/moag_off-axis_guider/

Cheers
Stuart

[gregbradley]

Thanks Stuart.

I'll have to check it out. I doubt it has a lens though. Perhaps that is what is needed most.

Cheers,

Greg.

[wysiwyg]

Stuart,

Well, I finally have the MMOAG issue sorted, I am getting nice round stars with the additional distance from the STL to MMOAG spacer.

Not enough distance to use the helical focuser though, which is a shame because getting focus by sliding the RGH in and out is very difficult.
I know the focus does need to be perfect but it does not assist in repeatability.

May need to purchase the lens from astrodon to gain another 12mm or so and then I might be be able to use the helical focuser.

Thanks Stuart :thumbsup:

[rat156]

Hi Mark,

Why can't you use the helical focuser?

If you have to move the RGH in and out to get to focus, then set the focuser to about half way, then slide the RGH in or out to get focus again, then the focuser works as a fine focus.

Cheers
Stuart

[wysiwyg]

Stuart,
My initial problem was that I could not get the rgh close enough to the prism to achieve focus. The helical focused is just too long, I had to use a shorter 1.25" nosepiece holder which can only slide thr rgh in and out.

Mark

[rat156]

The threads on the top of the MOAG focuser thread straight into the RGH (or at least mine do), so you could mount it sans adapter if you needed.

Cheers
Stuart

[wysiwyg]

Stuart,
mine does as well, it's still too long!
Tried everything unfortunately it's not possible unless you increase the distance between the stl and the mmoag using a longer adapter, but that options chews up my backfocus, and I have only about a half inch left.

Mark

[rat156]

Sounds like you need the lens to shorten the focus issue to the RGH. Then you can gain a bit of backfocus back, although if you're not using it for anything, who cares.

What scope is this on BTW?

Cheers
Stuart

[wysiwyg]

Stuart,
unfortunately the lens sold for the mmoag according to don is an unmounted one and it needs to be glued in. It just gets better and better.
The scope I have is a FSQ-106N, they are renowed for their lack of backfocus.

Cheers
Mark

[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]

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]

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]

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