Fixing tilt on a GSO 10:1 focuser

[kinetic]

GSO make a great Crayford style focuser that is quite a nice piece of engineering but mine seems to be let down by having quite a large amount of tilt on the main drawtube.
When imaging with my QHY-8 this shows up as annoying elongated stars, emphasised by the short focal length 12" F5 Newt.

I measured this tilt with my digital verniers and it was almost a whole millimetre out. (Pic 2)
I pulled the focuser apart to see if I could minimise or eliminate this tilt.

Firstly I removed the 1:1 knurled focuser wheel with a 4mm allen key. I also removed the two knobs on the underside, being careful to remember which threaded holes they came from. They are different lengths and have different functions each, as Pic 3 shows. The longer is the Lock Screw and the shorter is the tensioning/ grip screw.
Removing these allows the remaining shaft and 10:1 assembly to come out to the right. Pic 4 shows this and also shows the small black bush which I was careful not to lose! :)
The drawtube can now be removed and I flipped the base plate over to reveal the 4 x bearings which cradle the drawtube in position. Pic 5. It is this curved plate which must be shimmed one end to
minimise or totally eliminate the tilt.

Pic 6 shows the shim I made up sitting next to the curved plate. For my problem the shim had to sit just under the right hand end of the plate shown in Pic 6. I made the shim out of none other than a Guinness beercan :)

Pic 7 shows it all back together, the reassembly is just in reverse. I also removed all of my white and black texta markings on the drawtube which noted where each camera came to focus.
I fitted a small 34mm section of old tape measure blade, bent and flexed to match the drawtube curvature. Stuck on with 5 min Araldite. Makes a nice focus point index.

Steve

[Alchemy]

Interesting fix, the question is will it work in the field, I had done some surgery on my tube, so from memory I had a minimal amount of extension with the regular GSO focuser the low profile version just didn't have enough support. If you are using the 12 inch tube you will find by applying a bit of weight to the end of the focuser the tube will distort. I beefed up mine with a stainless flange, but it needs to be big and rigid, I felt in the end that an off axis guider or an SBIG type inbuilt guider would eliminate flexure from a side by side setup, I found 6 min exposures was as long as I could reasonably expect without some movement, I did try various aspects of angle of the focuser in relation to the direction of travel.... Didnt help much. Being as I have been through what you are doing, I will be most interested in your progress

Clive

[asimov]

Good onya Steve. I've done fix-it's like this before. Well done.

[kinetic]

Nobody noticed the tape measure was on the wrong way around! :D
With only 4mm or so showing and the other 30mm obscured it reads
34mm That's not good!:rolleyes: First night out last night but only with the
smaller DSI II chip. Stars looked crisper to focus but the seeing was
bad. This CCD never shows off-axis elongated stars anyway, even working
at F5.
Hopefully by Thurs nite I should have a test shot with the QHY-8.

Steve

[multiweb]

Very clever mod Steve. Will do this on mine(s) too. :thumbsup: Do you think drilling and tapping two holes with small grab screws at the top of the barrel body would work? (i.e. make it adjustable. Like a push/pull to tilt the plate holding the 4 bearings inside?

[DavidU]

Nice write up Steve. I'm surprised how much out it was.

[kinetic]

Marc, I had thought of doing this. This is a first try fix, if it removes the
elongated stars on the QHY8/ MPCC then I'll be happy.
Still yet to test it! Another low passing through here until at least Tues.



So was I David. On glancing down the OTA one day I couldn't believe how
obvious it was and why I'd missed it for ages.

Steve

[jayscheuerle]

Focuser tilt is irrelevant to star shapes. The only important aspect of the focuser is that the movement in and out follows a straight line. There's nothing special about a 90° angle. Lowrider dobs are purposely tilted quite a bit. - j

[kinetic]

Hi Jay,

On my fast (F5) Newt the elongation on the large QHY-8 chip is quite
pronounced even when only slightly tilted off axis.
This could be because of the chip itself being tilted off axis slightly within
the QHY-8 housing, tilted off axis due to the focuser, incorrect spacing
of the MPCC or a combination af all three.

The MPCC problelm shows as coma and is almost nil on my setup now.
The CCD off axis tilt is also minimal as I have proved it is not that by
imaging with a slight rotation around 360 degrees on successive nights.
So the only culprit must be the focuser and , as the OP of this thread
shows, it was quite noticeable and I'm amazed I missed it for so long.

Steve

[multiweb]

In a perfect world with a big fully corrected imaging circle, yes... to some extent. But not in practice. Tilt will introduce a whole bunch of new problems.