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oobydooby

Minor repair to solar 'scope is a major problem

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Hi all, hope this is in the correct topics.

I have a Lunt 35mm solar 'scope and one of two screws and the spring holding the etalon have been lost.  I have contacted Lunt in America who asked for photos of the missing items which I sent over two weeks ago but all I get in return are ads for their products.  The screw is very fine (see images) and no-one seems to have any suggestions.

I am desperate to find a solution as the telescope is totally unusable. Suggestions or advice would be most appreciated.

 

full sccrew position.jpg

IMG_9277 -1.jpg

full scope.jpg

screw.jpg

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I assume the dimensions in your drawing should be mm rather than cm?

I'd guess the bolt would be relatively easy to replace if you know the exact size.  ebay is quite a good source for that kind of thing in small numbers.  The spring will perhaps be more tricky.

James

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Posted (edited)

I presume that should be millimetres not centimetres ? and screw diameter must be more than 1mm

Screw should be easy enough to get of EBay etc, a better pic of the spring would be good, I have a selection box of springs so may have one

Dave

Edited by Davey-T

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Posted (edited)

I expect they are UNC screws being US made. Can you post a close-up picture looking  straight down on the screw and spring lying next to the ruler as it will be easier to give you the right size then as we can measure the thread pitch. The thread diameter looks to be around 2.5 to 3mm so your drawing is a bit off. :smile:

Alan

Edited by symmetal

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Yes I suspect as Alan it will be imperial so either UNC (Unified National coarse) or if a fine thread may be UNF (Unified National fine).

If you can find the diameter of the threads accurately as there are a number of screws this size with very little difference in diameter that narrows it down tremendously. If UNC or UNF then dimensions are in inch but if you can give in mm then we can convert. You will ideally need a pair of calipers or micrometers.

Then knowing the diameter we need the tpi or threads per inch, or in 25.4 mm.

My immediate suspicion is it is either UNC #4-40  (40 threads per inch) or UNF #4-48(48 threads per inch) but that is just going on the 2.5 to 3 mm diameter suggested by Alan.

Steve

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@oobydooby.   Just to clarify, is the lost screw and spring part of a pair set, the other set being the ones pictured against the rule?. You should be able to at least get the dimensions from Lunt despite the instrument being discontinued. If all else fails I have a large collection of screws and springs and should be able to match them, I could even repair the unit if sent to me and return it in working order.   😀

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Thanks for all replies

@PeterDrew  and all - have contacted Lunt and they should know the required dimensions that I need but I see no point in keep on asking if the are not replying. The screw and spring are the matching pair as you asked.  I do have a spring that comes from a small pen which fits so the screw is all I need.  I have some small computer screws which fit and are the same size except for the thread.   If I could get a small die cutting set I could either cut the hole or the thread of the screw, but as it's so small I cannot find any of that size.

Thanks for the offer of repairing the unit, I assume you mean the whole telescope?

To all I have added close ups as suggested but cannot disable flash so shadow is a bit of a problem.  I should have quoted millimeters.

 

IMG_9285.JPG

IMG_9286.JPG

IMG_9290.JPG

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That actually look like it could be metric, I would guess M2 x 10 mm.

If you look at the diameter of the threads it looks to be bang on 2mm. I blew one pic up in paint and turned the thread round 90 degrees against the ruler. The pitch best I can see if I blow the photo up is 0.4 mm which is M2. These are readily available and I probably have some if you want me to post one to try. That just leaves the spring (if it is an m2)

image.png.98e4f0dbea41b7a34d303ac1e999b099.png

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I only seem to have screws as small as M2.5 that I can find at the moment (I am sure there are about somewhere, if I ever find I would send some with pleasure) but I would be tempted to try a M2 screw, maybe as Neil says off ebay (might have to get 5 of them but should be cheap. Go for stainless, I would think that at this size they will all be full threaded even if not stated as such.

Steve

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I have some of these 10mm in stainless metric if you have trouble finding them.

Dave

bolt.png.1fd94b99fd5e00c4bc2fa3dc6a0aab0d.png

 

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Posted (edited)

Small screws you say?  😂

For a model I’m building.

48C2367E-508D-4D79-9EF9-E551DEE29CA7.jpeg

0F9B6F5A-7923-4B49-BDB4-91B74FB2402E.jpeg

Edited by johninderby
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Hi all as suggested I have ordered 5 m2 10mm screws which cost less the £2 and as I was desperate as I am doing outreach next week I paid for next day postage. About 12 quid in total.  Hopefully this will be the right size and I can get set for a few days of sunshine even at the lowest solar cycle for a long time.

I will let you know how it works out. Thanks again.

Tony

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Posted (edited)

 

Late to this thread, however - I’ve had a Lunt 35 Ha for 10 years and I’m familiar with those screws and springs.  Their purpose is to return the front etalon cell to the non-tilt position after using the tilt tuner.  Most of the time the tilt tuner is not needed, the best view being with the front cell in the non tilt position.

So at the last resort if you cannot find suitable replacement screws and springs is to simply tape the front cell in the non tilt position. Most of the time you will still get the best view.  Also with the tape covering the tiny gap, dust cannot enter. That’s how I know about those screws and springs because I dismantled to clean the 2 element objective.  Dismantling in this manner does not involve disturbing the etalon, that comes off as a fixed unit.

If it were me I’d repair if I could but tape as mentioned if no suitable parts could  be found.

Ed.

Edit - without the 2 screws and springs the etalon cannot fall off because it’s also held by the oblong brass plate with 4 screws on the opposite side to the tilt tuner control.

Edited by NGC 1502
Extra info

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2 hours ago, NGC 1502 said:

 

Late to this thread, however - I’ve had a Lunt 35 Ha for 10 years and I’m familiar with those screws and springs.  Their purpose is to return the front etalon cell to the non-tilt position after using the tilt tuner.  Most of the time the tilt tuner is not needed, the best view being with the front cell in the non tilt position.

So at the last resort if you cannot find suitable replacement screws and springs is to simply tape the front cell in the non tilt position. Most of the time you will still get the best view.  Also with the tape covering the tiny gap, dust cannot enter. That’s how I know about those screws and springs because I dismantled to clean the 2 element objective.  Dismantling in this manner does not involve disturbing the etalon, that comes off as a fixed unit.

If it were me I’d repair if I could but tape as mentioned if no suitable parts could  be found.

Ed.

Edit - without the 2 screws and springs the etalon cannot fall off because it’s also held by the oblong brass plate with 4 screws on the opposite side to the tilt tuner control.

Slight disagreement here.  When I first bought the telescope I found that to observe the proms I needed a greater tilt than when viewing the details on the inner face of Sol, so I think I still need a screw.   No sniggering at the back please! 🤤

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Doesn’t make much sense...

the surface detail and the proms should be visible at the same setting.

unless there there are significant Doppler shifts this should be the case.

 

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On 15/08/2019 at 10:01, Merlin66 said:

Doesn’t make much sense...

the surface detail and the proms should be visible at the same setting.

unless there there are significant Doppler shifts this should be the case.

 

What is the point of having an etalon that is adjustable then?

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3 hours ago, oobydooby said:

What is the point of having an etalon that is adjustable then?

 

It’s for when the doppler shift of the prominences or part of the prominences are very different from each other.

The majority of the time the tilt tuner is not needed and the best view will be with it in the untilted position.

Hope you get it fully sorted, Ed.

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Still waiting for the screw, if it fails to arrive tomorrow I will be demanding my money back. Ten quid for next day delivery that takes three days is not on!

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There’s also a mechanical need for etalon tuning.

The F-P etalon used relies on a very fine gap between the elements, usually around 80 micron.

In the Lunt and Coronado this is an air gap and in the Daystar Quark it uses mica as a spacer. Temperature and atmospheric pressure can change the effective gap, so the etalon must be retuned to maintain the nominated central wavelength.

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I sent a complaint to the e-bay seller today and the total, both postage and cost of item has been refunded.  If the screws are delivered I will keep them for a week or so and if I don't get a message detailing how they intend collecting them, at their expense, I shall be inclined to assume they will not want the added cost of sending them back and will keep them.

In the meantime I have re-ordered via Amazon the screws at a much better price and with free postage.

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