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APM 8" f6 Refractor Has Newton Rings

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#1 Tom Duncan

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Posted 03 April 2021 - 06:53 PM

Recently got a highly modified 8" f6 APM "Deluxe Achromat" OTA and I'm seeing Newton rings in the center of the elements, which to me mean they are touching. I'm thinking this is not a good thing. 

 

I'm also seeing what looks to be two element spacers floating around between the elements, but also three where I'd expect them to be. I can see what looks to be the glue residue of the two floating spacers at the edges but they are in wierd places, one right next to one that is in the correct position and another all by itself. Makes me wonder what kind of aberrations APM was trying to address. 

 

Your thoughts? 

 

Tom Duncan 

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#2 The Ardent

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Posted 03 April 2021 - 07:00 PM

Are the Newton rings centered? 

 

How is the star test? 

 

These answers may be helpful.



#3 petert913

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Posted 03 April 2021 - 07:01 PM

If it is a new telescope purchase, I would contact the manufacturer right away.  Does not look good to my eyes.



#4 Richard O'Neill

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Posted 03 April 2021 - 07:13 PM

The lenses don't have to be touching to exhibit Newton rings, just very close.



#5 Tom Duncan

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Posted 03 April 2021 - 07:25 PM

The lenses don't have to be touching to exhibit Newton rings, just very close.

I was wondering about that, I seem to remember that being the case. 

 

Are the Newton rings centered? 

 

How is the star test? 

 

These answers may be helpful.

I have yet to star test this scope so I'll get back to you after I do. The Newton rings are slightly off-center.

 

 

If it is a new telescope purchase, I would contact the manufacturer right away.  Does not look good to my eyes.

It is an older scope now, at least 12 years or so (serial number 8) but bought new from APM and highly modified by the previous owner, so I don't see any recourse to APM.

 

Tom



#6 GalaxyPiper

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Posted 03 April 2021 - 07:52 PM

If the Newton rings are off center, that tells me that the loose spacers are allowing the lenses to get closer at that spot.

 

Pick a very windy day and take it apart and collect the spacers flying out and put them back where you think they belong.

 

Then carelessly slap the two lenses back together before tightening everything down with excessive force till you hear the glass start to crack.

 

OH wait, this is April third...Nevermind, just be careful, and use good judgement.



#7 Mirzam

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Posted 03 April 2021 - 08:09 PM

I really don’t know if this applies to your case, but I had a lot of trouble with a similar vintage APM triplet lens cell that was very poorly designed.  In particular, the rear retaining ring was held in by 3 tiny set screws that had insufficient holding power to prevent the ring from loosening.  This in turn allowed the rear element to tilt and de-center. The fact that two of the three spacers have “escaped” and now are floating around in the middle of the lens suggests that something has loosened.  The fix that I eventually came up with was to drill and tap the cell to add much larger set screws and increase the number from 3 to 6.  So far so good..

 

JimC


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#8 Tom Duncan

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Posted 04 April 2021 - 08:49 AM

I really don’t know if this applies to your case, but I had a lot of trouble with a similar vintage APM triplet lens cell that was very poorly designed.  In particular, the rear retaining ring was held in by 3 tiny set screws that had insufficient holding power to prevent the ring from loosening.  This in turn allowed the rear element to tilt and de-center. The fact that two of the three spacers have “escaped” and now are floating around in the middle of the lens suggests that something has loosened.  The fix that I eventually came up with was to drill and tap the cell to add much larger set screws and increase the number from 3 to 6.  So far so good..

 

JimC

Thanks for relating your experience, I'll look into the integrity of the design/build when I separate the elements to get at the spacers. 

 

Tom



#9 Jeff B

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Posted 04 April 2021 - 02:06 PM

In conventional achromats the inner radii are spherical and either match, or are very close to each other.  With a small airgap seeing Newton rings is normal, especially with fast achromat where the internal radii are very steep..  You can see them in the picture of my old Jaeger 6" F5.  Since the surfaces are spherical and match each other, the spacers can be placed just about anywhere between the elements but it's best, of course, to place them at the edges, outside of the light path if possible.  The real concern is that they are of the same thickness and equally spaced, or close to it, to avoid tipping of one element relative to the other.  That's why you typically see three of them as they form a tripod between the elements.

 

To center the newton rings in the Jaegers, with the lens face up and the retaining ring a little loose, I tapped around on the side of the lens cell as this helps to self-center the elements on top of each other.  When the newtons rings were centered, I gently snugged the retaining ring down, making sure the rings stayed centered.  I took me a couple of tries but it went fine. 

 

Please take some pictures as you take it apart.  I suspect you have an excellent objective in there somewhere.

 

Jeff

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#10 Tom Duncan

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Posted 06 April 2021 - 09:58 AM

In conventional achromats the inner radii are spherical and either match, or are very close to each other.  With a small airgap seeing Newton rings is normal, especially with fast achromat where the internal radii are very steep..  You can see them in the picture of my old Jaeger 6" F5.  Since the surfaces are spherical and match each other, the spacers can be placed just about anywhere between the elements but it's best, of course, to place them at the edges, outside of the light path if possible.  The real concern is that they are of the same thickness and equally spaced, or close to it, to avoid tipping of one element relative to the other.  That's why you typically see three of them as they form a tripod between the elements.

 

To center the newton rings in the Jaegers, with the lens face up and the retaining ring a little loose, I tapped around on the side of the lens cell as this helps to self-center the elements on top of each other.  When the newtons rings were centered, I gently snugged the retaining ring down, making sure the rings stayed centered.  I took me a couple of tries but it went fine. 

 

Please take some pictures as you take it apart.  I suspect you have an excellent objective in there somewhere.

 

Jeff

Thanks Jeff, appreciate your input. As there are 5 spacers (three in position, two loose) my concern is where the other two go. I think I can see slight glue residue next to one of the spacers but why they would put two side by side is a bit of a mystery. That residue is more likely to be from the spacer next to it that moved a bit which would suggest two of the spacer positions had two spacers, suggesting wedge, supported by the off-center newton rings. Once I get it apart I'll inspect the spacer positions closer. 

 

Tom


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#11 Jeff B

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Posted 06 April 2021 - 03:54 PM

Good luck Tom.  As you see in my picture, a well put together objective will also have multiple spacer tabs between the cell's aft lip and the back surface of the back element (called radius 4 or R4) to support the lens assembly and prevent glass-to-metal contact.  With my old Jaegers, there were six of them.  With my Istar lenses, there are typically three, aligned with the three used to space the elements.

 

Jeff



#12 Tom Duncan

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Posted 06 April 2021 - 04:09 PM

Good luck Tom.  As you see in my picture, a well put together objective will also have multiple spacer tabs between the cell's aft lip and the back surface of the back element (called radius 4 or R4) to support the lens assembly and prevent glass-to-metal contact.  With my old Jaegers, there were six of them.  With my Istar lenses, there are typically three, aligned with the three used to space the elements.

 

Jeff

Interesting, makes sense but I hadn't run into that before...perhaps more common on very large (6"+) refractors?

 

I can understand their purpose but in my case how would they have gotten between the elements? 

 

Markus Luidi PM'd me in response to this thread: "There should be no flying around spacers. If the optics still have 3 spacers, 120 degree separated, then that is enough. You should careful, open the cell and blow out the 2 loose spacers."

 

Tom




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