Author Topic: The Good the Bad and the RASA  (Read 177 times)

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RobertM

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The Good the Bad and the RASA
« on: Mar 13, 2021, 12:00:50 »
I’ve been very intrigued by the Celestron Rowe-Ackerman Schmidt design over the last couple of years and wanted to try one out for a while.  Having sold a lot of equipment recently I had some spare cash so when an 8” version came up on AstroBuySell located in Croydon I jumped.  Unfortunately one can only jump with both feet on the secondhand market as used equipment has no warranty (shouldn’t be allowed) !

Background: I suspect the previous owner could not get the scope collimated; well at least I would say could not get consistent collimation.   The scope is only 4 months old and the vendor suggested he adjust collimation to get it right.  Perhaps on reflection it should have been sent back but, anyway, it came into my ownership and with no warranty.

So the Good: It is nice and fast, the mirror rides on roller bearings on a very sturdy centre column and no focus shift (but see the Ugly bit) below.

The Bad:  Metal back focus distance is shockingly small - the gap between my ASI2600 and the corrector assembly, with a very short adaptor fitted) is about 7mm.  To compound the issue the camera is the same diameter as that assembly so I’m going to have to cut down some Allen keys to fit the gap.

The Ugly:  Well I could not get it collimated consistently, left and right sides of the meridian produced different results - this was a big problem !  I had collimated it using the recommended concentric donuts method, the big donuts were perfect either side of the meridian, very puzzling.  I had read an article on the CN website that smaller donuts would give higher precision so I used a very small out of focus donut and a webcam for speed;  the donut was oval !  At this point I called Ian King for options... option 1 send it back to Celestron in the states and option 2 have a look myself.  Option one was prohibitive on cost so I had to take it apart myself.

This is where it gets interesting...  after removing the focus mechanism (rubber knob and bearing mechanism) I found that I could shift the mirror using the threaded focus rod.  The mirror seemed to be pivoting on something at the back of the cell, all very intriguing !  At this point I realised that the rear cell needed to be removed - I was concerned that it may not go back aligned in the same way as f/2 is super critical on alignment.  After removal of the rear cell (the screws lock the cell into a specific place - phew) I could see that the front bearing was loose.  At this point various things were going through my mind (you can imagine) but after looking at the mirror cell and housing I can only conclude that the primary is adjusted on a workbench to a set configuration.  The mirror is attached to two roller bearing assemblies each with three steel roller bearings; one of the bearings on each assembly can be tightened to clamp on the central post and remove any play.  It was the front bearing assembly that was not making proper contact with the post.  The fix was simply to adjust the ‘clamp’ bearing till there was no play (I did the rear at the same time but that only needed about 1 mm of Allen key movement).  Reassembly was a reverse of disassembly and then a brief check with a webcam to prove all was well - concentric donuts near focus - magic !

I have some pictures showing the mirror assembly and offending parts below.

The front of the scope:


The Schmidt corrector without plastic ring:

This shows the four nylon tipped steel centring screws for the corrector (accessible from the outside).  Interestingly the hex adjustment screws have been capped off with a further set of hex screws, perhaps, to prevent accidental tampering.

Rear mirror cell:

This shows and focus rod locator (two prong didly).  It seems to me that this is a standard EdgeHD 8" mirror cell adapted for the RASA.

Rear of the mirror:

This shows the mirror, bearings and focus push rod.  Bearing adjustment Allen screws are in the top left bearing housing. The other three groups of three allen screws (two large and one small in each) are for main mirror collimation.  I would think the mirror is done on a jig outside the scope as there is no way of adjusting in situ.

Front of the mirror:

This shows the front bearings and adjustment Allen screws.

There are still some adjustments to do since collimation and camera need to be tweaked.  At least I can now image with it both sides of the meridian with absolutely no focus shift or mirror movement.  Focus is now very precise with no perceptible backlash.  All in all it looks promising, 60 x 60s M81/82 shows a minor collimation or sensor tilt/spacing issue still to resolve.

A successful outcome but not quite there yet.  Shocking QA from Celestrons Chinese factory !

Hopefully your next telescope will not need as much TLC !

Update 15th March:
Cooling fan doesn't work or rather the fan works but there's no connection from the jack socket at the back.  There is also quite a lot of slop in the corrector lens assembly within the housing (approx 2mm of radial movement) - not the lenses.  Two more things to look at...

Robert

« Last Edit: Mar 15, 2021, 14:42:25 by RobertM »

Roberto

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Re: The Good the Bad and the RASA
« Reply #1 on: Mar 15, 2021, 12:59:58 »
Robert

Great read and glad you opened it up and fixed it yourself!  There's nothing more satisfying that knowing you did it yourself and now know how it all works. Not surprised about the poor quality control but at least there was nothing broken or missing.
Looking forward to seeing your images with it; they have quite a following on CN.

Roberto

RobertM

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Re: The Good the Bad and the RASA
« Reply #2 on: Mar 15, 2021, 14:55:31 »
Thanks Roberto.

It was an interesting process and given me quite a lot of confidence in the design.  Incidentally I believe the mechanism could be adapted to other SCT designs and variants;  the engineering is way over what would be required for an 8" mirror and wouldn't surprise me if it was the same used in the 11" and 14" variants.

Think it's worthy of a CN post ?

Thanks
Robert

Roberto

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Re: The Good the Bad and the RASA
« Reply #3 on: Mar 15, 2021, 17:50:56 »
Absolutely; go for it.  They love DIY and repairs there.

Roberto

MarkS

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Re: The Good the Bad and the RASA
« Reply #4 on: Mar 16, 2021, 15:51:55 »
Very interesting!  It's always quite scary to take a precision instrument apart.  I've never seen the insides of a RASA before.  I'm glad you managed to fix the mirror issue.  Good luck for the remainder!

Mark