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Achromatic Lens Centering...again

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#1 oldmanastro

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Posted 25 March 2016 - 08:01 PM

After a long time I finally decided to remove my CR-6 refractor objective lens and try to reduce the undercorrection that borders between 1/4 to 1/3 wave. The lenses were easy to remove from the cell and I marked everything including the position of the spacers. The original spacers are made of some material that looks and feels like duct tape. The thickness of the spacers is .2mm according to my digital caliper. My first try was with foil paper (about .03mm) spacers. Testing with an artificial star revealed that I had better spherical correction but the images were awash in blue. The next set of spacers were made from car trimming tape that has exactly the same width as the original spacers but with a .06mm thickness. Star tests with these spacers revealed a still improved spherical correction that was now clearly in the 1/4 wave area but the out of focus image showed the purple point besides a red point and the diffraction rings just coming out of focus more prominent on one side. Since the telescope was very well collimated, the only reason for this could be lens decentering. So, back I went and took the lenses out. Very carefully repositioned all spacers to make sure that they were all equidistant from the lens border and made sure that the crown element was as well centered as I could make it. Result... Now the blue dot out of focus is in the center with a hint of the red dot behind it. The telescope now snaps into focus and chromatism is very well controlled, in fact better than ever for this scope. There should be centering screws for all achromatic objectives bigger than 4 inches. How much three threaded holes with small adjusting screws can cost?

 

Clear Skies

Guido Santacana


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#2 gunut

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Posted 25 March 2016 - 08:29 PM

optics manufacture doesn't want people messing around inside the product.....



#3 turnerjs085

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Posted 25 March 2016 - 09:38 PM

Most fraunhofer objectives are actually possible to get centered by touch, if the lenses have been edged to the same size. The decenter tolerance is much looser than a triplet (a couple thousandths of an inch at f10 iirc). If the lenses don't have appreciable wedge, or if the rotational alignment is already known, it is often possible to stack the lenses by feel then carefully tape them up with kapton tape before installing in the cell.

Adjustment screws are nice though. I also have always wondered why they aren't included. Collimation screws I can see leaving off, but it seems like it would be faster on the production side to have centration adjustments...

Jeremy

#4 oldmanastro

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Posted 25 March 2016 - 11:57 PM

Most fraunhofer objectives are actually possible to get centered by touch, if the lenses have been edged to the same size. The decenter tolerance is much looser than a triplet (a couple thousandths of an inch at f10 iirc). If the lenses don't have appreciable wedge, or if the rotational alignment is already known, it is often possible to stack the lenses by feel then carefully tape them up with kapton tape before installing in the cell.

Adjustment screws are nice though. I also have always wondered why they aren't included. Collimation screws I can see leaving off, but it seems like it would be faster on the production side to have centration adjustments...

Jeremy

The C6R is probably one of the most messed around telescopes in the market due to their usual variation in optical performance. The moment a manufacturer includes adjustments for collimation, the owners will try to get it as best as possible. Changing spacers to correct under or overcorrected objectives has been the norm with these Syntas with much more leeway for overcorrected ones. In my case I also needed to do some cleaning for fungus spots between the lenses so I went ahead with the spacing experimenting and, except for the centering at first, it came out ok. I think Synta uses .2mm spacers in all their C6Rs. Maybe about 25% of them or less come out well with this spacing but a lot come up with over or under correction. I don't know which one is more common. My adjustment was limited due to the fact that I had to reduce the distance of the elements but a reduction from .2 to .06mm made it better. It would be nice to see what experience others may have with these objectives.  I also have to inquire more on the significance of the violet and red dots right in the center of the extrafocal image. Usually the dot is just violet and turns crimson as one comes into focus. Theymust be related to the different focal points of violet and red in these achromats. 

 

Guido



#5 junomike

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Posted 26 March 2016 - 08:09 AM

 

Most fraunhofer objectives are actually possible to get centered by touch, if the lenses have been edged to the same size. The decenter tolerance is much looser than a triplet (a couple thousandths of an inch at f10 iirc). If the lenses don't have appreciable wedge, or if the rotational alignment is already known, it is often possible to stack the lenses by feel then carefully tape them up with kapton tape before installing in the cell.

Adjustment screws are nice though. I also have always wondered why they aren't included. Collimation screws I can see leaving off, but it seems like it would be faster on the production side to have centration adjustments...

Jeremy

The C6R is probably one of the most messed around telescopes in the market due to their usual variation in optical performance. The moment a manufacturer includes adjustments for collimation, the owners will try to get it as best as possible. Changing spacers to correct under or overcorrected objectives has been the norm with these Syntas with much more leeway for overcorrected ones. In my case I also needed to do some cleaning for fungus spots between the lenses so I went ahead with the spacing experimenting and, except for the centering at first, it came out ok. I think Synta uses .2mm spacers in all their C6Rs. Maybe about 25% of them or less come out well with this spacing but a lot come up with over or under correction. I don't know which one is more common. My adjustment was limited due to the fact that I had to reduce the distance of the elements but a reduction from .2 to .06mm made it better. It would be nice to see what experience others may have with these objectives.  I also have to inquire more on the significance of the violet and red dots right in the center of the extrafocal image. Usually the dot is just violet and turns crimson as one comes into focus. Theymust be related to the different focal points of violet and red in these achromats. 

 

Guido

 

When I had a chance to get a Chromacor for my (now sold) C6R, I wanted to be sure it was the correct one as there were different versions  made to correct for under-correction or over-correction.  Seeing as It was Winter and I didn't have a chance to actually Star test my C6R, I emailed Markus Ludes and asked him which one I would need.  He replied that most of the Synta C6R were under-corrected and I would most likely need the O-1 Chromacorr.  I eventually tested the C6R in the spring and he was correct.

I'd expect most mass manufactured telescopes to be under-corrected as time is money and keeping them any longer on the machines means less productivity.

(The Chromacorr did indeed greatly help the C6R in both CA and SA correction).

 

Mike


Edited by junomike, 26 March 2016 - 08:15 AM.


#6 turnerjs085

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Posted 26 March 2016 - 08:29 AM

The colored dot is indeed the focused light of that color. Changing the spacing can slightly change the color correction maybe to the point that the violet and red focus simultaneously, depending on the original designed color curve.

When you did your SA evaluation, did you use a colored filter to tune it for best green correction?

Jeremy

Edited by turnerjs085, 26 March 2016 - 08:31 AM.


#7 oldmanastro

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Posted 26 March 2016 - 10:15 AM

 

 

Most fraunhofer objectives are actually possible to get centered by touch, if the lenses have been edged to the same size. The decenter tolerance is much looser than a triplet (a couple thousandths of an inch at f10 iirc). If the lenses don't have appreciable wedge, or if the rotational alignment is already known, it is often possible to stack the lenses by feel then carefully tape them up with kapton tape before installing in the cell.

Adjustment screws are nice though. I also have always wondered why they aren't included. Collimation screws I can see leaving off, but it seems like it would be faster on the production side to have centration adjustments...

Jeremy

The C6R is probably one of the most messed around telescopes in the market due to their usual variation in optical performance. The moment a manufacturer includes adjustments for collimation, the owners will try to get it as best as possible. Changing spacers to correct under or overcorrected objectives has been the norm with these Syntas with much more leeway for overcorrected ones. In my case I also needed to do some cleaning for fungus spots between the lenses so I went ahead with the spacing experimenting and, except for the centering at first, it came out ok. I think Synta uses .2mm spacers in all their C6Rs. Maybe about 25% of them or less come out well with this spacing but a lot come up with over or under correction. I don't know which one is more common. My adjustment was limited due to the fact that I had to reduce the distance of the elements but a reduction from .2 to .06mm made it better. It would be nice to see what experience others may have with these objectives.  I also have to inquire more on the significance of the violet and red dots right in the center of the extrafocal image. Usually the dot is just violet and turns crimson as one comes into focus. Theymust be related to the different focal points of violet and red in these achromats. 

 

Guido

 

When I had a chance to get a Chromacor for my (now sold) C6R, I wanted to be sure it was the correct one as there were different versions  made to correct for under-correction or over-correction.  Seeing as It was Winter and I didn't have a chance to actually Star test my C6R, I emailed Markus Ludes and asked him which one I would need.  He replied that most of the Synta C6R were under-corrected and I would most likely need the O-1 Chromacorr.  I eventually tested the C6R in the spring and he was correct.

I'd expect most mass manufactured telescopes to be under-corrected as time is money and keeping them any longer on the machines means less productivity.

(The Chromacorr did indeed greatly help the C6R in both CA and SA correction).

 

Mike

 

Thanks for this info Mike. I read that most telescopes are designed for some undercorrection but Syntas have so much variation that I did not know which one was more prevalent. When the C6R arrived the Chromacors were already gone. I wsh that they were still available.

 

Guido



#8 oldmanastro

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Posted 26 March 2016 - 10:33 AM

The colored dot is indeed the focused light of that color. Changing the spacing can slightly change the color correction maybe to the point that the violet and red focus simultaneously, depending on the original designed color curve.

When you did your SA evaluation, did you use a colored filter to tune it for best green correction?

Jeremy

I totally forgot about it !! but the violet was so prevalent in the first spacing trial that even with a better correction this spacing was not acceptable. The second trial tests need to be repeated since I was interrupted by the arrival of a tropical shower that tend to come out of nowhere. I will use the green filter on my next second test. Thanks for the important reminder.

 

On the colored dots I think that since they do not coincide completely there may still be a tiny bit of decentering, so after the next test is finished, the lenses may be out again. One thing that I found is the very small tolerance between the lenses and cell. One has to be very careful to avoid getting the lenses stuck.

 

Guido



#9 KerryR

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Posted 26 March 2016 - 03:24 PM

I spent a LOT of time tinkering with the element spacing on my C6R. The first time, I used a green and yellow stack (I don't think the yellow did anything, but it was a suggestion I'd heard somewhere). I got good results. The next time, I used a Baader Solar Continuum filter. While I improved the green correction using the filter, I think I got too far out for the other colors, and while the scope performs really nicely in green, it's no longer any fun in white light. I need to go back and do the correction again a little more conservatively, and use a standard green filter instead of the narrow pass filter.

 

Even with the filters in place, I find analysis of the interference pattern difficult to interpret, and that makes things difficult to do with certainty.

 

I haven't used the scope in years. I need to revisit all this some time.



#10 oldmanastro

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Posted 26 March 2016 - 04:55 PM

I spent a LOT of time tinkering with the element spacing on my C6R. The first time, I used a green and yellow stack (I don't think the yellow did anything, but it was a suggestion I'd heard somewhere). I got good results. The next time, I used a Baader Solar Continuum filter. While I improved the green correction using the filter, I think I got too far out for the other colors, and while the scope performs really nicely in green, it's no longer any fun in white light. I need to go back and do the correction again a little more conservatively, and use a standard green filter instead of the narrow pass filter.

 

Even with the filters in place, I find analysis of the interference pattern difficult to interpret, and that makes things difficult to do with certainty.

 

I haven't used the scope in years. I need to revisit all this some time.

There seems to be a sort of compromise zone where the spacing gives as good spherical correction as possible with also  good color correction. The .03mm spacers corrected SA a lot but brought about a degree of violet that I don't want. The .06mm spacers achieved the result I wanted. Now I just need the weather to cooperate.

 

Guido



#11 orion61

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Posted 30 March 2016 - 06:19 PM

  I thank goodness every day for the one I have, it is the best of three I have owned. I replaced the focuser with a super fine focus correction on a cable. This scope snaps to focus with virtually no SA.

  I am glad a little color doesn't bother me like it does others, I was told this tube was hand picked out of a number of them.

I also own a really decent Sand Cast early C8.

This 6" Achromat beats the C8 on Planetary detail if I use Minus Violet filter and a light blue filter stacked with it. OR if I use My Bino Viewer 15mm Eyepieces W/ 1.5X nose piece, and a Neutral Density Filter. It hardy shows any color at all!

I easily get sub arc second resolution. Ganymede shows a very clean Globe at 1.5 arc seconds. With proper filtering one of it's main Albedo features a white patch was visible ONE TIME! It was a night of amazing seeing and transparency about 4:00 one morning last Spring.

I picked it up for $550.00 WITH a JMI hard case, I got so lucky.

I have always wondered if there were batches of glass that were a bit different than others, By better I mean different types of glass, if suppliers ran low of stock, As many as they have made, that has to have happened. I have looked through a number of these and have witnessed personally a great deal of quality difference in color correction, over all correction.

I am quite jealous of some of you guys and those beautiful Tak, TMB, Lunt, AP etc. You should be very thankful that you are in the position to own Telescopes I can only describe as pieces of Art.

  THEN I see some guy put a scope I'd give my right arm for.....As a finder/guide scope on a big DOB!!!! AARG...



#12 precaud

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Posted 31 March 2016 - 08:58 PM

I bought a CR150 early last year. It was immediately obvious that the scope had been taken apart and played with by at least two others before me; the three sets of alignment marks on the objective lenses bearing witness, only one of them in Chinese. Fortunately they were unsuccessful and so I got it for a song. Last year I put many hours into getting the focuser alignment, lens spacers, and collimation dialed in. Posts by KerryR and Jeff B were invaluable, as well as others here who have nursed and cursed their 6" achro into usable shape. The last task for me was fine-tuning the astiig out by rotating the lenses with respect to each other. I finished this in early Feb this year. The factory marks were off by nearly 120º. Green-filtered star test is now excellent and symmetrical on both sides of focus with no obvious SA or astig.

 

A couple weeks ago, Shane (aka "hottr6") generously offered to lend me his Chromacorr N to check out. After optimizing the spacing, indeed, most of the CA disappeared, 80+% I'd say worse-case. But what surprised me is that, observing Jupiter at 200X in good seeing, there was no more detail to be had with the Chromacorr than without. Color was more neutral, the image was a tad brighter, but there was no more detail to be had with it than without it at full aperture. And there was a slight but noticeable loss of contrast with the CC in. My local astro mentor, a much more experienced observer than I, came over and his comments were the same as mine.

 

And so after all this work, my lowly CR150 is pretty good after all. And I now know that the hassles introduced by using the Chromacorr were not worth the benefits it delivered for me. At say 100-120x and less, it is an excellent scope, period. For high-mag and planetary, either my 8" or 10" dob is much better; no false color, more detail, brighter and more color-saturated images, and comfortable to use at any viewing angle. But the CR150 still good enough at 200-250X that I'll use it for that.

 

This experience really helped me put this scope into perspective and enjoy it for what it is. A good 6" frac is a wonderful thing to have around. Not the best thing, but a wonderful thing.


Edited by precaud, 01 April 2016 - 02:14 PM.

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#13 oldmanastro

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Posted 31 March 2016 - 10:14 PM

I finally got around to work on the lens again, took it out and found that one of the spacers was positioned  off the lens border toward the inside. After repositioning the spacer and making sure that the components were as well centered as possible, the objective went back into the cell. The next clear night the star test revealed no decentering or astigmatism and the best SA correction that this telescope has shown with and without green filter. CA is even better controlled now  and on 2nd magnitude stars I see only one diffraction ring around the Airy disc. This is, I think, as far as this telescope can  go. Looking at Jupiter on that very stable night I was able to go up to 300x with my UO ortho and the image snapped into focus with the moons showing a clear disc. With the minus violet filter finer detail on Jupiter belts was easy to spot and this is something I could not do before at 300x. 

 

From the other comments we could conclude that most of  these 150mm refractors come out of the factory with an average optical adjustment and can be optimized by careful re-spacing, centering and rotational repositioning of the components plus careful collimation. 

 

Clear Skies

 

Guido


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#14 turnerjs085

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Posted 01 April 2016 - 07:30 AM

It may also be the lens cell design letting the elements shift a bit during shipping if the retaining ring isn't tight enough. Does it have centering screws?

Your posts have gotten me curious about my 5in f10 achromatic OG. It is for an atm project, but I think that I'll take it to work and check it out on the opto-alignment machine once work isn't so crazy busy. It is also in a simple cell with no centration adjustments...

Jeremy

#15 KerryR

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Posted 01 April 2016 - 05:25 PM

Inspired by this post, I've decided to re-visit my C6-R. I just reduced the intra-element spacing by a significant amount, hoping that, while I might loose some green correction, I'll pick some back up on the others. I'm sure I went too far, and will have to find something a little thicker, but this will help me begin determining where to go...



#16 oldmanastro

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Posted 01 April 2016 - 05:30 PM

It may also be the lens cell design letting the elements shift a bit during shipping if the retaining ring isn't tight enough. Does it have centering screws?

Your posts have gotten me curious about my 5in f10 achromatic OG. It is for an atm project, but I think that I'll take it to work and check it out on the opto-alignment machine once work isn't so crazy busy. It is also in a simple cell with no centration adjustments...

Jeremy

There are no centering adjustments in the C6R. There was no decentering in my objective before I took it out for re-spacing the elements. The spacers used by the factory provide some binding between the elements. They are basically pieces of tape not unlike duct tape. Your comment is interesting because usually one must provide some looseness in the cell when adjusting the retaining ring. This can always cause some shifting of the crown over the flint element and some measure of decentering. In long focus refractors this shifting is probably not noticeable but in these intermediate focus refractors like the C6R it could be noticed by a keen observer. At least before I took out the lenses decentering was never noticed even though the retaining ring was not tight. An interenting area for study.

 

Clear Skies

 

Guido


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#17 oldmanastro

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Posted 01 April 2016 - 05:38 PM

Inspired by this post, I've decided to re-visit my C6-R. I just reduced the intra-element spacing by a significant amount, hoping that, while I might loose some green correction, I'll pick some back up on the others. I'm sure I went too far, and will have to find something a little thicker, but this will help me begin determining where to go...

I used a gray thin tape used as trimming in cars. It has the same width as the original spacers and it is easy to cut it in length to match the original spacers. My caliper measures the thickness without the backing paper at .06mm. This worked for me. I also went too far with foil and images were bathed in blue - violet. A compromise must be made between SA correction in green light and CA correction.  I wish you the best correction possible.

 

Clear Skies

 

Guido



#18 KerryR

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Posted 01 April 2016 - 08:55 PM

 

Inspired by this post, I've decided to re-visit my C6-R. I just reduced the intra-element spacing by a significant amount, hoping that, while I might loose some green correction, I'll pick some back up on the others. I'm sure I went too far, and will have to find something a little thicker, but this will help me begin determining where to go...

I used a gray thin tape used as trimming in cars. It has the same width as the original spacers and it is easy to cut it in length to match the original spacers. My caliper measures the thickness without the backing paper at .06mm. This worked for me. I also went too far with foil and images were bathed in blue - violet. A compromise must be made between SA correction in green light and CA correction.  I wish you the best correction possible.

 

Clear Skies

 

Guido

 

 

I'll keep that material in mind; nice solution. My spacing appears to need to be thicker, though. Finding thicker spacing material in the past was somewhat challenging. There's no clear sky for me any time soon, so it's going to be a while before I can test the current spacing...



#19 precaud

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Posted 02 April 2016 - 09:13 AM


I'll keep that material in mind; nice solution. My spacing appears to need to be thicker, though. Finding thicker spacing material in the past was somewhat challenging. There's no clear sky for me any time soon, so it's going to be a while before I can test the current spacing...

 

Not that it applies to your situation, but last year I found a formula for optimum spacer thickness for a Fraunhofer doublet, and for a 6" F/8 it came to 0.0039" . So I used .004" tape when respacing mine.


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#20 KerryR

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Posted 02 April 2016 - 12:01 PM

 


I'll keep that material in mind; nice solution. My spacing appears to need to be thicker, though. Finding thicker spacing material in the past was somewhat challenging. There's no clear sky for me any time soon, so it's going to be a while before I can test the current spacing...

 

Not that it applies to your situation, but last year I found a formula for optimum spacer thickness for a Fraunhofer doublet, and for a 6" F/8 it came to 0.0039" . So I used .004" tape when respacing mine.

 

 

It probably does apply, thanks for posting. It's good to know where to start as I begin re-evaluating the spacing.

 

I had a LOT more than that after my last bout of star testing with the Solar Continuum filter (green band-pass), and I think I over did it by a large margin, despite the improvement in green.

 

Knowing that my objective trends towards over-correction in green, I didn't go back to .004". I went back to .006" (electrical tape). I'll lose some ground in pure-green, I think, but I'm hoping white light improves a bit. We'll see... eventually... if the weather ever improves here.



#21 oldmanastro

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Posted 02 April 2016 - 05:52 PM

 


I'll keep that material in mind; nice solution. My spacing appears to need to be thicker, though. Finding thicker spacing material in the past was somewhat challenging. There's no clear sky for me any time soon, so it's going to be a while before I can test the current spacing...

 

Not that it applies to your situation, but last year I found a formula for optimum spacer thickness for a Fraunhofer doublet, and for a 6" F/8 it came to 0.0039" . So I used .004" tape when respacing mine.

 

That is about .095mm according to the formula and .1mm less from what is usually found in the factory spacing. Somehow Synta came up with the .2mm as an average and at this spacing between the elements maybe about 15 or 20% of the C6Rs come out well corrected. 

 

Guido



#22 precaud

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Posted 02 April 2016 - 06:03 PM

Not that it applies to your situation, but last year I found a formula for optimum spacer thickness for a Fraunhofer doublet, and for a 6" F/8 it came to 0.0039" . So I used .004" tape when respacing mine.

 

That is about .095mm according to the formula and .1mm less from what is usually found in the factory spacing. Somehow Synta came up with the .2mm as an average and at this spacing between the elements maybe about 15 or 20% of the C6Rs come out well corrected. 

 

Interesting... It appears you have some inside knowledge on these things... may I ask, how?

 

And, you found best results with .06mm ? That is very thin, .0024".


Edited by precaud, 02 April 2016 - 06:16 PM.


#23 precaud

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Posted 02 April 2016 - 06:53 PM

I poked around a bit, looking for the air space formula, I haven't been able to find it since that first time. I recall it was a function of both objective diameter and focal length.

 

Telescope-optics.net gives this table for Fraunhofer doublets:

 

"Following table gives design specifics for doublets achromats consisting of BK7 crown and F2 flint, with all measures in units of the focal length. The specs are based on 100mm ƒ/10 objectives, but are scalable within the normal range of refractor apertures, up to about 50% faster or any slower objective (by applying the desired focal number ratio to 10 directly to the radii)"

      Type  aplanat  R1   t1    M1    R2   AIR(M2)   R3     t3    M3    R4
   FRAUNHOFER  YES  0.6  0.011  BK7  -0.36  0.001  -0.363  0.007  F2  -1.51

If I understand correctly, for a 150mm F/8:  10 / 8 * .001 * 75mm = .09375mm = .0037 in. spacing. That's about what I used.

 

http://www.telescope...t/achromats.htm


Edited by precaud, 02 April 2016 - 08:38 PM.


#24 oldmanastro

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Posted 02 April 2016 - 06:57 PM

 

Not that it applies to your situation, but last year I found a formula for optimum spacer thickness for a Fraunhofer doublet, and for a 6" F/8 it came to 0.0039" . So I used .004" tape when respacing mine.

 

That is about .095mm according to the formula and .1mm less from what is usually found in the factory spacing. Somehow Synta came up with the .2mm as an average and at this spacing between the elements maybe about 15 or 20% of the C6Rs come out well corrected. 

 

Interesting... It appears you have some inside knowledge on these things... may I ask, how?

 

And, you found best results with .06mm ? That is very thin, .0024".

 

No ... No inside knowledge at all, just learn by doing and reading plus what I can summarize from the thread itself. The .0039" from the formula that you have translates to about .095mm. I have read several other posts in this and other forums where the .2mm original space thickness is mentioned and maybe by coincidence mine were .2mm and at this distance the lenses were undercorrected so the spacing had to be reduced. Yes, .06mm is thin but it worked for me. It was the best compromise between CA and SA. I suppose results will vary. I do wonder if these Fraunhofer designs are all the same or are variations from the original design.




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