15 replies to this topic

[Steve_M_M]

I am posting this here for future reference.  I bought an Astroscan a few weeks ago.  As you can see from the attached picture, the rubber tube that rolls against the focus tube had a large divot in it making the focuser skip, not engage,  each time that area came in contact with the focuser tube.  I think this is a fairly common issue, especially after a long period of non-use.

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Edited by Steve_M_M, 07 April 2020 - 10:35 AM.

[Steve_M_M]

My first attempt at a repair was to simply order a new rubber tube.  The ID is about 7mm and the OD is about 12mm.  First I tried ¼” ID by ½” OD.  That tube arrived, it is red too, and was .32 ID and .56 OD.  Not quite right although I did get it to work.  Next, I found tube that was exactly 7mm ID and 12mm OD.  $12/ft.   Yikes.  But, hey, in the spirit of restoration….That tube arrived and was 5.5mm ID and 11.6mm OD.   Close, but a smaller OD was not going to work well.  So, I gave up and used the modified ¼” x ½” tube.

 

Last weekend, the toilet in our rental home started to leak.  I discovered an actual crack in the porcelain.  I had always wanted to try Flex Seal liquid rubber, so I ordered some.   It worked great.  Then, on Sunday, a light bulb went off in my head that maybe I could actually repair the focuser tube with flex seal.  I used a tooth pick to add drops off the tip and slowly added just enough to fill the divot.  I let it dry for 48 hrs to make sure and tested it this morning.  It works great!!  And appears to have “molded” into the existing rubber.  No way this is coming off.  Focusing is smooth across the entire travel.

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Edited by Steve_M_M, 07 April 2020 - 10:39 AM.

[Couder]

For some reason plumbing things seem to work with astro things.....

As for the rubber seal: We had a Victorian home a few years ago. On the ceiling were large medallions with ornamental plasterwork pieces in the middle. The house had been turned into apartments around the turn of the century (1900 or so) so who knows what happened, but some of the ornamental plaster was broken. I took a good one, used the rubber to make a mold, and cast new ones from plaster. 

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[clamchip]

Is it possible to move the rubber tube over to contact the un-used area?

I'm just wondering because that's what I've done with the Edmund gear type

rack and pinion focusers.

Edmund focuser shafts are just a 2-1/2 inch long pinion gear. When the

brass pinion teeth are worn you can loosen the knobs and slide the pinion

over for some nice fresh teeth. The rack hardly ever shows wear being

hard chrome plated.

 

Robert

Edited by clamchip, 07 April 2020 - 10:53 AM.

[markb]

Great tips!

My Astroscans are within a year of going back to 'work' as my schedule settles and my kids move to 'real' jobs (I distributed 4 Astroscans within the family), and I was concerned about aging issues even beyond the primary foam horror.

I've never used the FS product and this is a far better result than I would have thought.

It seems appropriate in this thread to mention the other wonder product that keep saving my bacon with my Astro equipment, E6000 'glue'.

Adhesion way beyond that of RTV, glues more diverse surfaces then J-B weld and has a vital characteristic that JB weld does not share, flexibility after cure.

The flexibility is small but enough to prevent cracking and joint failure. it is probably the only thing I've ever used that properly adheres to plastic. I made attachable USB micro/mini and micro/C adapters using a split ring with fabric loop E6000 glued to the plastic adapter body. No joints of failed yet, and I never lose my adapters now.

It provides a reversible, yet solid bond on bolts and adapter plates as well.

[Steve_M_M]

Is it possible to move the rubber tube over to contact the un-used area?

I'm just wondering because that's what I've done with the Edmund gear type

rack and pinion focusers.

Edmund focuser shafts are just a 2-1/2 inch long pinion gear. When the

brass pinion teeth are worn you can loosen the knobs and slide the pinion

over for some nice fresh teeth. The rack hardly ever shows wear being

hard chrome plated.

 

Robert

 

There is only about 3/32" space on each side of the rubber tube before the telescope housing.  My option here would have been to cut the rubber tube in 1/2 and then line up a good part to contact the focuser tube.  I could have secured that piece down on the pinion rod so it did not move side to side.  That would probably work.

[psybiggs]

I just picked up a used Astroscan and had a similar problem with the focuser, the rubber was worn down where it make contact with the eyepiece tube.  I wrapped electric tape around the rubber, where it's worn down, two layers thick, and it seems to do the trick.

[markb]

I came back to this thread while checking a like and realized that my easy fix for loose Astroscan focusing tube rollers is not in this thread. 

 

I have never encountered the badly deformed tubing shown by the OP, but my UHMW fix should work with that problem as well.

 

Loose focusers are easily fixed using UHMW self-stick tape of defined thicknesses, .005 mil (.007 mil with adhesive) and, most frequently, .003 mil UHMW (.005 mil with adhesive.

 

No roller mod nor roller removal required.

 

The roller can be left in place to do the fix. I simply leave the roller in place, cut narrow strips of tape approximately 25-30 mm long, (IIRC- the tape should be about the same length as the black portion of the rib) and carefully place them on the tube guidance rib across from the roller. Generally a single thickness of .003 mil tape that's the trick. I guide the tape strip by temporarily sticking one end to the tip of a small cuticle scissor, etc. Try not to overlap the red plastic lip at the top as it may be a tad smaller and diameter, messing up the fit.

 

 

 

I mention the thicknesses without adhesive because those are the dimensions used in describing the 3M tape versions. 

 

The giant spools that the 3M sells are respooled by Tapecase and sold on Amazon.

 

 

 

Any time I am tempted to use teflon, I use UHMW tape that's the coefficients of friction are essentially the same. Brown Kapton insulating tape, interestingly, also has virtually the same coefficient of friction, and is even thinner although harder to work with.

 

Missing focuser tubes can also be easily replaced with 3D printed tubes. They work remarkably well despite the amount of flex. 

Edited by markb, 02 May 2024 - 02:56 AM.

[Tom Corbett]

I didn't want to start a new thread as I am having a similar problem.

 

The INSIDE of the rubber tube is not gripping the metal bar on the focus knob. The scope is new to me. Found after about a decade in a garage, in its red bag the whole time.

 

Any suggestions?

[markb]

I never had this issue.

 

So these are the things I would try, with my fingers crossed.

 

First I would wash both the bar and the inside of the rubber tubing with dish detergent and dry it thoroughly. That might be enough depending on the dimension change.

 

 

You can try my UHMW tape fix on the internal ribs of the tower since that will tighten everything up and may increase the force of the rubber against the metal. 

 

The increased smoothness was a definite benefit as well.

 

 

One of the problems with dealing with these focusers is the tolerances are surprisingly tight, one reason even .003 tape with .002" adhesive strips can change things too much and put on both sides of the tower. IIRC I was limited  to just applying to one side.

 

 

Any attempt to actually change the diameter of the metal bar internally with tape etc will run into the problem of leaving no gaps, since gaps could cause a bump in the focuser as it is moved 

 

 

If the UHMW tape doesn't work, or you want to do it without it

 

I would try putting a thin layer of slightly gritty toothpaste (not the gel stuff) on the metal bar, let it dry overnight to a gritty powder, to see if that increased friction enough. 

 

EDIT I'm deleting this one because of the danger of powder getting into the body in the primary, since the powder may be pushed through to the end of the rubber and therefore into the body. 

 

 

One problem is that the bar has to slip inside the rubber in order to reassemble and one day disassemble the focuser, so regular glues are out. 

 

 

Except, possibly, old fashioned rubber cement, but I think this one is risky. Put a small amount inside the rubber  at the end and slide in the metal through it while the glue is still slightly damp, it's not particularly strong and normally you can break what little bond there is if necessary.

 

Good luck.

 

 

 

These scopes can perform extremely well since the mirrors, or at least most of them, are very good. 

 

Unfortunately, this may be complicated a bit if the foam has disintegrated under the mirror. There are CN threads on correcting this, and it is not a pleasant process. The retaining rings are extremely strong, access is extremely tight, and the mirror has to be pressed just a bit against the foam to insert the retainer. Kind of a recipe for disaster and I did not enjoy doing the two I completed.

 

On my next foam fix I'm going to try the solution suggested by one user here who crammed thin strips of white polyethylene foam through the threaded insert at the rear under the mirror. It worked for him and I'm going to try it in the future.

 

 

I and others have threads here on collimating them using tape as shims under the front glass. Not a true collimation but far far better than most of them came out of the factory. Gary Seroniks method, and you should be able to find his original web page on it. I strongly suggest using a different method for checking the change, however.

 

There are several ways to check collimation, I have found a quick way on the Astro scan is to view the mirrors from the window end, the edges of the reflections should appear concentric.

 

One of mine was molded well enough that starting collimation was spot on. Two others were awful before collimation, but remarkably sharp and crisp afterwards. Luck of the draw, really, on a given Astroscan as it came off the line.

 

 

I have used this concentric reflection method for a long time doing a preliminary SCT collimation before fine-tuning, and I was surprised to find it works very well on the Astroscans. Ones collimated with an artificial star show concentric reflections. I'm not a newt guy but this has worked very well for me and greatly simplifies collimation since everything is done in daylight. Because the tape method will necessarily work in increments, it will take a bit of luck for a perfect combination but a very close match works fine. 

 

You can simply confirm collimation on a star or artificial star, although, the increase in sharpness will jump out at you if you start with a poorly collimated one.

 

The only thing I can collimate with a Cheshire is my refractors. IIRC the AS has a dropped diagonal, more than I can deal with a Cheshire.

Edited by markb, 02 September 2024 - 06:23 PM.

[markb]

There is perhaps more information here 

https://www.cloudyni...er-tube-safety/

 

Links for the tape are included as well as information as to the thickness ratings.

[Tom Corbett]

Thanks for the reply. I'm going to try cleaning, reassembly, and adding a piece of electric tape (or two) to the focus tube for tonight.

 

It hasn't done this before, but last night was also 20F cooler than any other time I'd been out.

 

I'll update with the results.

[Tom Corbett]

One strip of electric tape the length of the focus tube has worked out. It is likely not the permanent solution.

[markb]

As long as you have no bumps in the focus action when you get to the joint in the tape, you likely do have a permanent solution, great job.

 

 

I do have another suggestion for others in this situation, possibly the least dangerous (no powder or glue)- cut an appropriate length of electronic equipment shrink wrap tubing and slip it over the center half of the bar. It should shrink tight without any seam under heat. Hair dryers should work, if you use a lighter don't be in a rush, it'll shrink eventually.

 

My only concern might be the thickness of the tubing but thin ones are available. Also, the tighter the fit before shrinking, the less thickness increase when it shrinks.

Edited by markb, 05 September 2024 - 09:03 PM.

[Pluto 134340]

That tubing used in the focuser looks suspiciously like something from the auto parts store. Too bad we don't have a former Edmund factory worker to tell us from where Edmund sourced their parts and materials. 

[markb]

It's no picnic trying to source the correct tubing for the focuser tube either

 

I've never found a viable source, so I have 3D printed my replacements. There is a CN remember now selling them on eBay IIRC.