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Truly Awful Star Test Of Coulter 17.5" In Truss Dob

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#1 Tom Duncan

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Posted 28 February 2020 - 09:17 PM

I have a DobSTUFF truss DOB scope with an original Coulter 17.5" mirror in it and the star test is AWFUL! I should have taken a picture of it with my cell phone at the eyepiece but didn't think of it at the time and the scope is now stowed away with rain coming soon. However I would describe it as a deformed walnut. Kinda oval shape with the edges in and out. "Rings" is a term that doesn't apply. 

 

The first thing I thought of was a bad secondary (it happens, see this thread) but once I looked at how the mirror was attached to the cell I think this is the source (see the attached photo). It looks like the mirror is glued to the arms of the cell with silicone or some such. If that stuff doesn't age evenly I would think there is a possibility of enough pushing and pulling between the multitude of attachment points to cause the problem. 

 

However once I pry the mirror from the glue spots how do I retain it on the cell? 

 

Any thoughts? 

 

Tom Duncan 

Attached Thumbnails

  • 20200228_151811a.jpg


#2 fcathell

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Posted 28 February 2020 - 09:26 PM

This will do it!  I had similar problems with a thin 8" mirror that was stressed due to being RTVed to a spoked mirror cell many years ago. The uneven stress/support on a 17" mirror would have to be much worse.

 

Frank

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#3 Barlowbill

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Posted 28 February 2020 - 09:41 PM

Get a bigger hammer, Tom



#4 J A VOLK

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Posted 28 February 2020 - 09:56 PM

those are thin mirrors - mirror must float or astigmatism easily introduced.   Just emulate a traditional cell such that the mirror can move very little laterally, and some clips just above the surface so it does not accidentally fall forward.  Some people loosely wire the triangles together.


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#5 TOMDEY

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Posted 28 February 2020 - 10:47 PM

I had a superior Coulter 17.5 and built my own 9-point cell that actually worked nicely. Back in those early days, many people blithely glued their mirrors to cells or even wooden plates, lying on carpet, etc. That will bend the heck out of an otherwise good mirror... terrible practice. I'm guessing that folks think that squishy adhesive will somehow locate the mirror but not distort it... unfortunately this is not the case.    Tom

 

Oh, to your point --- although the mirror could well be inferior --- gota fix or replace that cell before jumping to any conclusion whatsoever. I know Jim is no longer with us, but he did indeed produce some good and a few superior 17.5 inchers. 


Edited by TOMDEY, 28 February 2020 - 10:50 PM.

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#6 Pinbout

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Posted 29 February 2020 - 08:59 AM

I’d used nylon cap screws heads on the triangles as support points and make a wire rope sling using linear. Ratings and stainless steel shoulder bolts.

 

Sometimes for the triangles I make a plastic ring to keep the triangles oriented correctly.

 

you’ll need something to keep the mirror from tipping over, hook eyes on screws are typical 

 

EC94DBEF-2121-434C-B250-874743D1BF05.jpeg


Edited by Pinbout, 29 February 2020 - 10:15 AM.

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#7 Starman1

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Posted 04 March 2020 - 07:35 PM

Danny,

I recommend rubber or plastic sleeves on the 3 mirror retaining clips.

Otherwise, bounce in the mirror during transport might scratch or chip the mirror.

(Yes, I realize that's an unfinished cell--just pointing it out to the OP who will remount his mirror.)

For a thin 17.5", I also recommend triangles at the ends of each support bar, for a total of 18 points.

A wire sling is the easiest way for him to support the edge of the mirror, though, because it takes the least space around the edge of the mirror.



#8 Pinbout

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Posted 04 March 2020 - 07:59 PM

 

recommend rubber or plastic sleeves on the 3 mirror retaining clips.

yes they are clear but they are there 

 

heat shrink


Edited by Pinbout, 04 March 2020 - 08:03 PM.

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#9 DAVIDG

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Posted 05 March 2020 - 10:57 AM

  I recommend that you do a Foucault test of the mirror. You can do it while the mirror is in the scope. All you need a simple Foucault or Ronchi test so no measuring zones or anything like that. A very simple tester which is easy to make is all you need. You just want to see the figure on the mirror. It will tell you what the present condition of the figure is and if you do any modifications to how the mirror is mounted it with show if you improved the figure or not. You won't be guessing as to the  problem  as if it really is a mounting issue or the figure is bad or combination of both and you won't have to wait for the stars or check out the results of any modifications.  

   I also recommend you test the diagonal.  Many times amateur blame the primary when it is the secondary and it is more common then what many think. Just because it said to to be 1/10 flat doesn't mean it really is. TEST !  Here is a picture of a diagonal that came out a 10" Newt that was donated to our club. The scope is made from high end parts. The story goes it lived in closet for years and wasn't used. I tested the primary and the figure is very good then I tested the secondary. It was a mess. Many waves from flat. One of my club members is refiguring it under my direction. The picture shows the results after  about an hour of figuring. Still not flat but a HUGE improvement  from were it was. I'm sure the poor quality of diagonal was the reason that the scope never got used and was pushed to the back of a closet. I'm also pretty sure that the diagonal was advertised to be flat to at least 1/8 wave if not better. 

 

             - Dave 

 

 

diagonal bad.jpg


Edited by DAVIDG, 05 March 2020 - 01:33 PM.

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#10 Pinbout

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Posted 05 March 2020 - 08:02 PM

heres a gso 2ndry

https://www.youtube....5PT43k&index=13

 

a coulter 3"

https://www.youtube....5PT43k&index=15

 

astrosystems

https://www.youtube....5PT43k&index=14

 

 

a 4in 2ndry

https://www.youtube....5PT43k&index=12


Edited by Pinbout, 05 March 2020 - 08:04 PM.

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#11 Augustus

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Posted 05 March 2020 - 10:50 PM

I would bet it is the secondary or maybe the cell. Coulter's bad rep largely comes from the Murnaghan days and the often-poor cells many folks used with the early Dobs.



#12 Starman1

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Posted 05 March 2020 - 11:41 PM

I've worked on several of the earliest Coulters.
Sorry, but their bad reputation was well deserved. From mirror cell to secondary support to bearings to tube, to inability to cool or collimate, they were just total junk.
If any of them had a good mirror it would be like winning the lottery.
The large size equivalent of the department store 60mm refractor.
Today's inexpensive dobs are so much better.
IMO, of course.
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#13 Steve Dodds

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Posted 06 March 2020 - 04:10 PM

With Coulters a good rule of thumb is:  The older it is the better it is.  As the years went by instead of raising their prices they dropped quality, by the 90's I don't even think they were testing them.


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#14 pierce

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Posted 06 March 2020 - 04:17 PM

my red tube 10" f/4.5 coulter is decent.    I've never seen a decent 13 or 17.5



#15 lphilpot

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Posted 06 March 2020 - 07:02 PM

I put a Coulter 10" f/~5.6 in a home built truss Dob, but it had a good (Novak) primary cell. I never had it tested, but it wasn't obviously bad. I got some very detailed views of Jupiter through it. Had a Telescopics secondary. That was in about 1991.


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#16 dallen24

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Posted 06 March 2020 - 08:09 PM

I got a new 13.1" Coulter about thirty years ago.  First light was awful, so I sent the mirrors off to Galaxy optics for testing.  To test, they had to strip the coatings.  The main mirror was a tad under-corrected, but the secondary was junk.  Turns out, the secondary came from bad batch so Coulter sent Galaxy a new one.  While Galaxy corrected and coated my mirrors, I installed a Kenneth Novak mirror cell and spider.  Also installed a 2" crawford focuser.  First light was fantastic.  Not sure, but the main mirror has something like a 1/40 wave front error.  Ever since, my 13.1" Coulter has been a real valuable asset at all our club open houses.


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#17 piaras

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Posted 07 March 2020 - 09:23 AM

Bought a 13 in 1989 and still have the mirror, it is in the box that a Zambuto replacement came in. There is nothing left of the original scope in use today.

Pierre



#18 Pinbout

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Posted 07 March 2020 - 12:32 PM

 

The main mirror was a tad under-corrected, but the secondary was junk.

this

 

but each case has to be individual assessed



#19 Cary

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Posted 07 March 2020 - 12:37 PM

I've never seen (tested) a Coulter mirror that I would install into a telescope without first re-figuring it.  If it is an original condition mirror, this is probably part of your problem.


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#20 Steve Dodds

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Posted 07 March 2020 - 01:39 PM

I have seen ONE! out of about 30-40, and it was from about 1980.


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#21 Tom Duncan

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Posted 11 March 2020 - 01:10 PM

Hi folks, been away from the thread for a while, didn't realize I had so much response. 

 

So I managed to cut through all 18 blobs of silicon with an oiled hacksaw (not fun), then cleaned off the leaves of the cell (pic attached), reinstalled the cell in the rocker box, laid down some cloth over the leaves and set the mirror down on the assembly inside the scope. As it turned out Capella was perfectly placed overhead and right in the middle of small hole between the trees at my house. Even with really bad seeing conditions I could tell the star test had nice round rings on both sides of focus, nothing like the walnut shape it had before (not just a severe oval, in and out all around the edges, really nasty, worst I have ever seen). So I thinking the silicone blobs were shrinking at various rates (depending on how much silicone was there in each blob) and thus distorting the mirror. I'm in contact with the maker of the scope to find a fix. 

 

If I do a sling I'll have to make/get a box for the mirror I think as the sling only holds in one axis. Or not? 

 

As to the secondary, yes that can be the problem. I had an old Celestron/Vixen orange tube Comet Catcher a while back with a bad secondary (link noted in my original post). 

 

As I don't have the best skies in suburban Chico (California Central Valley north) I won't be able to do a definitive star test but I can see the mirror is at least OK now. 

 

Thanks all for your input. 

 

Tom 

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  • DSC_0002a.jpg


#22 Starman1

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Posted 11 March 2020 - 01:21 PM

I suggest attaching small round polyethyene washers to each corner of the triangles and letting the mirror rest on those.

The easiest way to support the mirror edge is with two small posts (you can figure out how to attach them to the cell)

with screw holes for 1/4" x 20 nylon screws.  The nylon screws (one per post) press against the mirror edge in the front-to-rear Center of Gravity.

and they are 90° apart on the bottom edge of the mirror.

This will help you figure out where to have the screws hit the mirror edge:

http://www.cruxis.co...ecalculator.htm

 

Looking at your cell, two "L" brackets could work as the mirror edge support posts and take up very little room.

 

A wire sling would also work, but then you'd have to figure out how to anchor the wire on each end to give a 180° wrap around the bottom edge of the mirror.

You might not have enough space for the posts.



#23 Pinbout

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Posted 11 March 2020 - 01:50 PM

 

I suggest attaching small round polyethyene washers to each corner of the triangles and letting the mirror rest on those.

i like black nylon cap screw heads...

 

 



#24 Starman1

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Posted 11 March 2020 - 02:36 PM

i like black nylon cap screw heads...

That would be easy.


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#25 pierce

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Posted 11 March 2020 - 03:08 PM

Re cutting silicone blobs, I've had good luck doing it with a high E guitar strings and two wood handles. My Coulter, the mirror was on neoprene foam pucks, glued to the mirror with Duco cement and something like coarse cheesecloth. I cut through the foam with a wire, then removed the Duco with acetone.
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