Reducing Spherical Aberration In Our Scope Mods

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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Merlin66 »

Walter,
Under corrected mirrors (think Hubble!) can cause SA in reflectors....


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by swisswalter »

Ah Ken

thank you very much , now I'm remembering


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley »

Neo wrote:For Mark, do you have examples of images that suffer from this SA problem and how can you tell if it's not just seeing, tube currents or something else?
Makes me wonder if the proposed SA cure is outweighing the possible side effects that you may be introducing like internal reflections but that's another topic.

Interesting thread, like to see where it takes us.
Here is a classic example in CaK: same filter throughout... (I was still dealing with reflections at this stage with the caK filter - ie the ghost overlain at the bottom of the image. First off 40mm f10
Cak-full-disk-bw-40mm-s.jpg
Next, 50mm f8.4
CaK-full-disk-50mm.jpg
Then finally 70mm f6
Cak-full-disk-70mm.jpg
As the SA increases the contrast decreases and so detail goes.

With the Ha the raw image is quite soft (I think) with the histogram bunched up. Here us a raw unprocessed ha image from a couple of weeks ago...
ss100-f20-flare-ani0006-14-03-16-08-49-14_g3_ap594.jpg
ss100-f20-flare-ani0006-14-03-16-08-49-14_g3_ap594.jpg (116.24 KiB) Viewed 4785 times
Now I know I can sharpen and stretch this up in post processing, but where i'm going with this is would like to improve matters at source...


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Merlin66 »

Mark,
You are assuming that the 50mm and 70mm objectives have more SA?
These could actually be better corrected than the one used in the PST?????


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley »

The 40mm lens is one from a PST. The 50mm is a stopped down from the 70mm (same lens). While great at 540nm and 656nm, the 70mm scope is awful at 393nm.


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley »

Neo wrote:Thank you for the example images Mark. For the Ca images, to be honest I'm not convinced that the loss of contrast is completely because of SA. How do the Ca filters respond to ray patterns that enter at increasing angles? Are the 50 and 70 both the same objective, stopped down with front end diafragm?
Hi Neo, regards the increasing angle of a faster beam on the filter, there will be a shift associated with this, exactly how much I can't categorically say how much. We can get a little from this PDF from the Paris Meudon Observatory where it talks about their CaH filter: in a collimated beam the filter works at 396.85nm, at a 2.5 deg angle this is 396.80nm. There is of course greater thermal loading which also causes a centre wavelength shift - see their CaK PDF with the graph at 3c, 13c & 23c...
HeliographeCaH.pdf
(425.5 KiB) Downloaded 147 times
HeliographeCaK.pdf
(344.61 KiB) Downloaded 163 times


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley »

Neo wrote:The matter of SA is quite new to me but it reminds me of the days when I failed miserably at getting decent images from my new Lunt 1200 Calcium diagonal that I used on a Meade 127 f7,5 apo. From what I remember the Meade, although being a triplet, it suffered from soft images in the blue channel, being probably even worse in the UV regions. I never got to the bottom of it and have no idea if it could be a case of SA or just poor correction in a range that the budget APO was never designed for in the first place.

The Ha image I'm not sure what to make from that, for 100 mm it is lacking in detail sharpness. (even unprocessed as it is)
Your CaK experiences you describe sounds like SA to me. The best scopes for CaK are long focus fraunhofer achromats, the worst triplets, and the faster the f-ratio (generally) the worse they are. To see if it was SA try stopping that meade down and see if matter improve as the effective objective diameter gets smaller.

Yup, the Ha aint sharp is it. Seeing wasn't the best on that day and know I can get sharper shots than that, but I do think there is something going on with it, not sure what though...


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley »

Neo wrote:How is the 100 mm performing in visual spectrum, on it's own without the HA-gear, have you considered doing a startest in red channel?
No IR-leaking through your filter array?
It was taken using a Beloptik Tri Band ERF internally in the tube, i'm removing this because it was giving me reflections in CaK configuration, so, wouldn't surprise me if there wasn't something similar happening in Ha. The Baader D_ERF will be front mounted the next time it is used.


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by thesmiths »

With regards to minimising aberrations near 393 nm, I have heard that lenses from many photo enlargers are very good and also from old fax and photocopy machines (since near uv light was often used with them). However, I have often thought that the venerable RC design should work well here, particular if one also uses a Herschel wedge. There is no lens anywhere in the design and there is plenty of back focus.

I would imagine a properly moded Newtonian would make a good 393 nm scope. I recall seeing a design where the metallization is left off the main mirror so as to act as a built-in Herschel wedge. Some work might need to be done near the focuser to accommodate a stack of filters.


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by DavidG »

It is actually very simple to test a lens for spherical aberration at any wavelength you want ie check for spherochromatism. You setup the lens a double pass autocollimation mode us an optical flat. Since we are taking about small lenses, a good quality flat optics are available for not much money. One can also use a diagonal flat from a large Newtonian. The flat doesn't have to be very flat, just optical smooth so one with couple of waves of power will introduce less then a 1/20 of wave error in the test. It also doesn't need to be coated.
You can either use a white light source and look thru fliters of the wavelengths you wish to test at or use a LED's which produces the wavelengths of interest.
Here is a drawing of how you setup the test using a Ronchi Screen. If the lines are straight the lens has no spherical aberration at the wavelength your testing at. If they bow there are issues. Here is a picture of 4" f/15 lens that I'm testing in green light were it was designed to be perfectly corrected for spherical aberration. As you can see the lens has a problem.

- Dave
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Last edited by DavidG on Thu Apr 03, 2014 1:31 pm, edited 1 time in total.


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley »

Thanks for this information Dave. Yup, looks like there's a bit of trouble with that lens!


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Merlin66 »

Dave, that reminder about the Ronchi was perfect!
You could use the same set-up to "match" different objectives i.e. similar Ronchi results = similar SA


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by swisswalter »

Hi Dave

thank you very much for the hint- I'll give it a go ;-)


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Merlin66 »

OK I asked the question on IIS and got the following reply re measurement of SA.

Record in and out of focus images of a star with a webcam/planetary camera and run the Roddier. If your objective is not an APO, or is a fast APO, use a green filter (I usually use an OIII). Spherical aberration is Zernike 11 (third order, which is the "main" spherical) and Z22 is higher order spherical (zones). It will also tell you everything else you want to know about your lens (astigmatism, coma, even colour correction if you use appropriate filters)

http://www.astrosurf.com/tests/roddier/roddier.htm

(use Google translate).

For example, this is the result of my 102/714 triplet, after collimation and centering.
roddier_results.jpg
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Valery »

Merlin66 wrote:
For example, this is the result of my 102/714 triplet, after collimation and centering.
This tells nothing about performance at the CaK (393nm). SA at 393nm can easily be close to 1L.


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Merlin66 »

Valery,
yes, but...
If we apply a UV filter we should be able to do the same analysis around CaK region.


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Valery »

Merlin66 wrote:Valery,
yes, but...
If we apply a UV filter we should be able to do the same analysis around CaK region.
Take a filter with about 50nm wide and centered at 415 - 425nm and see.

Note, please, that this software is a placebo for telescopes owners and not an interferometer.


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley »

Thanks for the info Ken! :)


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by fjabet »

That is an interesting thread.

Some thoughts :

- refractors are to be optimized for 500-550nm. Actually there are a lot of dispersion in the real world. For example I measured an FSQ85 that was settled in the blue, and mine is near 700nm.

- Some other brand doesn't suffer from slight mechanical uncertainties. AP for example thanks to their oiled spaced design are right on spot, that is a yellow HENE laser line till mid 00', and then green HENE (543nm).

- The less spacing between the lens, and the less dispersion you will have between to samples of the same instrument for a serious manufacturer. Usually variation come from the mechanical variation, optics part are fairly accurate (for lenses, mirrors are a different story...).

- As I mentioned in another thread, SA follows a straight line vs wavelength. When I measure at 473, 543 and 635nm, I can provide the SA for any lambda down to 350 up to 800 nm. I do this for some labs who need to use several different lambda (tomography for example). If you look the report on Airylab website, you have in some of them graph of SA vs lambda. The slope of the line reflect the amount of spherochromatism. Do forget also that focus can partially compensate for SA (approx. one third).

- It is not possible to use Edmund SA plate unless being in a collimated beam. Do not try this on a converging beam : you would have a very strong coma off axis. Even in a collimated beam you must be just behind the collimating lens.

- Correcting SA requires a doublet. That is quite specific, I was thinking of doing that for large SCT in UV applications (Venus) but on SCTs the variation between different samples in quite great.

I'm not sure the PST lens has a lot of SA in the red. It's a 40mm F10 lens, it's spherochromatism would be very very small. I will measure mine some day out of curiosity.

Frederic.


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by swisswalter »

Hi Frédéric

interesting thoughts, thank you very much


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley »

Thanks for the information Frederic :)


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