MPCC and compression-ring focusers

[pmrid (Peter)]

Using a MPCC with a Moonlite focuser has become a PITA for me lately. The problem was with the width of the flange on the threaded end of the MPCC. It is just a fraction too wide - which has meant that when I try to insert the MPCC into the focuser with the locking ring flush against the lip of the drawtup, the compression ring is just high enough to foul the little flange. So when you try to tighten up the 3 screws on the focuser, it forces you to withdraw the MPCC afew mm in order to get the compression ring to sit properly. The reult has been the MPCC and with it the imaging train behind it is not necessarily sitting true - resulting in some image tilt.

My cure is to put the MPCC in the lathe and turn off about 1.5mm of that flange leaving only enough to act as a backstop to the lockring (about 1mm). The result is I can sit the MPCC in the focuser with the flange flush up to the lip and therefore giving me a stable and flat connection for my cameras.

The attached drawing is not to scale but it probably says what I have done.

Might be of interest to others so here it is.

Peter

[Peter.M]

I noticed that moonlite are offering "low position compression ring" as an option on their drawtubes now. It has the compression ring recessed slightly in the drawtube I assume to hold accessories like the mpcc around the ring you are talking about.

I will be getting one for my new scope in the new year for the same reason.

[peter_4059 (Peter)]

Here's my solution to the same issue

http://www.iceinspace.com.au/forum/s...highlight=mpcc

[Tandum (Robin)]

Pete,
Do you have a GSO coma corrector in the kit? I bought one for the newt and it is longer, with more lenses, and a wider indent than the baader one. I thought the 3" would take 3 months to arrive but I've since been told, 2 weeks so this was money down the drain I guess. Once the 3" shows up I'll sling it up to you. It is made in taiwan not china.

It comes with a 2" nose adapter on top of it for eyepieces, but should also have an 48mm to 42mm adapter as shown on the GSO website. That adapter is missing and I'll chase that up with andrews. Every time I buy from andrews, something is missing or not right. Anyway, Backfocus is 65mm to 85mm from the final lens surface, so it's pretty relaxed. I have no idea how good it is yet, I still have no computers in the dome. It gives a 1.1x amplification/barlow effect from what I've read but it has 45mm clear path through the thing.
robin.

[pmrid (Peter)]

Didn't I sell you a GSO reducer a while back? I thought I had one once anyway, and I don't have it now.

I've been having a further play with the lathe and the MPCC and have turned off a ring/collar thingy that fits very closely around the MPCC - it's only 3mm wide though so it acts as a flat plate against which the imaging train can sit securely so I don't have to worry about how badly those 3 compression ring screws throw the MPCC out of orthogonality.
It was just too darned hot today to open up the obs and I see outside that there is a wet ring around the moon so I'm guessing it'll be a day or 2 before I get to test it out.

Peter