Collimation and focuser tune-up of an ES ED80 frac

[SkyHiker]

I recently moved my AP rig over to my Explore Scientific fracs, an ED12CF and an ED80. The hardest part is generally the EAF focuser but fortunately, the bracket fit perfectly, so all I had to do was get four 25 mm long M4/0.70 screws from the hardware store, and it fit almost out of the box. I made the last couple of mm fit by pushing a quarter underneath the bracket as I did not have 4 washers to make it fit perfectly. This worked for both the ED12CF and the ED80 because they have the same stock focuser.

First off, here is the problem; look at the elongated stars at the lower right. This was for the ED80, the ED127CF had round stars. Clearly there is some misalignment that is caused either by the focuser or by the lens (that I will not touch except possibly for the pull-push alignment screws). It can also be caused by the ES field flattener, I have the 45 mm backfocus version (the current one is 55 mm).

 
There have been a lot of negative comments about the ES stock focuser but IMHO that is not justified. The key is to identify all 6 adjustment screws, 4 of which are hidden. The two visible ones are the regular tightening screws on the outside, no need to discuss those. The first pair of hidden screws are underneath the focuser housing (the part that contains the turn screw). When taking the 4 screws out (they may have Loctite on them, so the first time is the hardest), there are 2 adjustment screws on both side that you can tighten. If you can get the rotation collar off, there are two more screws there that push down on the Teflon sliders inside. I was able to get it off for my ED127CF IIRC but today, not for my ED80. However, if you take the handle off there is a big enough hole that, when turning the rotation collar, they show up, and you can adjust them through that hole. Note that these screws are so far up that they only have effect if the focuser tube is underneath them. You may have to get an extra ring to make it so, depending on your focus position. Anyway, using these 4 hidden screws and the 2 visible ones, everything can be made as tight enough to carry a hefty camera rock solid. Try this first before spending a fortune on a Moonlite.

To fix the alignment I cut a paper disc that fit right the outside rim of the lens and marked the center as accurately as possible. Then I inserted the Howie Glatter laser in the focuser with the 3 extension rings that I needed. The more rings the better for this, I presume. I loosened up all 4 screws of the focuser collar. There is one meant to be used to rotate the collar that can be operated with your fingers. The other three are thumb screws. I don't know if these thumb screws are meant to be tightened, or if they are adjustment screws. As the whole focuser was flopping all over the place when the screws were loose, I tightened them all while looking at the laser dot on the paper disc. It took a while but eventually I got the laser focused exactly on the dot to the millimeter. Just 1 mm makes a significant difference, considering that an APS/C sensor is 25x16 so that''s a substantial part (the camera is about equally far away from the collar as the lens).

The last part that can cause tilt is the ES FF that, in my case, could not be pushed up against the outer ring to align it. I had to pull it out 15 mm and tighten it with the screws, which can cause tilt, I think. I made a little cardboard collar 14 mm wide and slid it tightly around the FF. Then I pushed it up against the ring. That should hopefully be aligned well now.

I haven't tried it yet but will ASAP. The last thing that I may need to do is to try other ring configurations on the camera because I am not sure if the 45 mm backfocus is cast in stone. It worked for my ED127CF at F/7.5 but my F/6 ED80 may need a different distance.

[Juno16]

Thanks for the detailed information Henk.

Have you had a chance to try it out yet?

I have the 2" r&p on my ES RD102 Essential Series refractor and it was kind of wonky when I first received it (mabey about 6-7 years ago).

I emailed their tech support and they sent me this:

Rack and Pinion Focuser Adjustments.docx

(633.76 KiB) Downloaded 26 times

It worked very well especially after I removed the focuser and disassembled it for cleaning and re-assembled and lubed with Superlube.
It still works very well now, but I do still have a very tiny bit of tilt that is due to some very small movement of the focuser in non-consistent directions (as measured by ASTAP). The tilt is so small that I really am not worried about it. The view isn't worth the climb to me. I have to really dive in to notice anything and it is tiny.

The stars in your image are significantly impacted. Looking forward to your testing and good luck!

[SkyHiker]

Good news. Before dark I took some flats to see if the collimation placed the stretched image in the center. Not quite. I then put the flats in a loop while adjusting the thumb screws until the light was centered. I trust that better than a laser collimated method, and it is easy to do.

Taking some M31 images the stars were still hopelessly oblong but at least there was better symmetry in the corners - all pointing out, equally sized. I then added another ring of 10 mm on the FF et voila, perfect stars.

[JayTee]

Nice work Henk,

Just for reference what's the new distance from the sensor to the ff?

[SkyHiker]

Nice work Henk,

Just for reference what's the new distance from the sensor to the ff?

Thanks JT! The distance is now 50 mm. It made a huge difference, see attached images. These were all shot last night.

I am not going to post them separately because I always have the same objects. Most people cycle through objects, I cycle through telescopes and telescope configurations.

[JayTee]

Excellent Henk,

All 3 images (which are excellent) show nicely shaped stars out to ALL the edges.

Nicely done,