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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
