Best way to modify a 90mm Refractor? - if at all

[Paulus]

Hi all,

I purchased a Meade 90mm x 800mm f/8.8 refractor very cheaply (£20) a few weeks ago. The focuser is 1¼" plastic rack & pinion, with a massive amount of movement in all directions. I've cut this down somewhat by putting 0.077mm x 25mm fibre glass PTFE tape along both surfaces, (if anyone wants any let me know, I had to buy 30m!).

After some consideration I thought I might change the focuser for the single speed 2" Crayford I've taken off the 8" Helios newt. If I extend the OTA from the present 600mm to 750mm & mount the crayford at the end the 50mm travel will give the 800mm focal length.

Now for the bits I'm not sure of (assuming my idea above is feasible), and would like your collective views on:

Firstly, the existing focuser has a diameter of 17/8" with a 1¼" ep ring at the end. The length of travel is 190mm, part way up the inside of the focuser tube there is a baffle ring with an inside diameter of roughly 1¼" (can't measure it at the moment), is this important and if so would putting a 2" focuser not work?

Secondly, as the focal length is 800mm why is the OTA only 620mm with a focuser with such a long travel, would it not be better to make the OTA 720 - 750mm long with a more stable focuser with less travel?

Please understand that I am not used to refractors, I've a ST80 for use as a guide scope, but this 90mm gave outstanding views of Saturn last month when the seeing was the best I've seen in a long time & I would like to use it without the problem of the focuser moving the object when you touch it.

My thanks for your indulgence and hopefully you advice, views, etc.

Paul

[Paulus]

Hi Moderators,

I think I've put this in the wrong place?

Would it be better in the DIY Astronomer section?

My apologies,

Paul

[Paulus]

Hi

I also have a DS-2090, unfortunately it cost me a bit more, but was a complete unit with goto and 5 eyepieces. I have had a couple of hours with it, but was also put off by the movement of the focusing tube. Although the views through it, to my beginners eye, were quite impressive for what it is.

I would love to see a couple of pictures of how you have corrected this and take you up on the offer of some PTFE tape. I am of course happy to pay something for it, if only to cover postage.

regards

keith

Hi Keith,

I'll put a couple of metres in the post to you Monday. I've taken some pics, but won't be able to sort them out until Tuesday I'm afraid.

You will need to undo 3 screws holding the focuser on the OTA & take off. then remove the screws on the adjuster plate to remove adjuster & spring-plate, then clean off all the grease.

Depending on how loose the focuser is will depend on putting on both surfaces, as I did.

Don't put the tape over the grooves in the outer tube, (I did that at first & the tape just crimped up or cut). My inner tube has a metal foil on it that had come off in a number of places so I removed as much loose as I could. I removed the grease with water soluble paint remover (blue stuff from B&Q) then washed the tube with washing liquid, rinsed & dried.

I placed tape on either side of rack overlapping on inside edge, which was folded into tube (this makes it a lot easier to put the tube back in without lifting the tape), stuck strips of tape with even spacing in the remaining gap, I think I made sure one was opposite the rack 1st then divide the rest, (I forget now, too many strong pain-killers make you memory somewhat erratic )

When you reassemble, use white Li grease on the rack & the focuser pinion, a touch of PTFE spray on the front edge helps insert the tube if you've taped both surfaces.

So that is about it, the main thing was to make sure that the tape on the outer tube folded over the front of the tube & the opposite for the inner if you need to do it as well.

Will sort out pics ASAP, I'll put a copy of this on the thread, as I still don't understand the reason for the baffle & so far no-one has commented on my plan, etc.

Cheers,

Paul

[Paulus]

These are the pictures of the focuser after application of PTFE tape.

Looking at the pics I need to spray the inside with Matt black paint.

Still no explination of what job the baffle does

Paul

[mdstuart]

Any of that tape left as I have just bought one of these..

I need to find some rings to attach it to a tripod!

Mark