If a 1.25 prism diagonal has improved SA in an ED refractor, would a two inch prism give the same result or does the extended light path versus a 1.25 inch have an effect?

Spherical abberation correction and diagonal prism size?
#1
Posted 18 July 2016 - 10:38 AM
#2
Posted 18 July 2016 - 11:00 AM
Good question Mark.
I'm helping a buddy of mine construct a bino-friendly OTA for an Istar 8" F6 lens. As part of the design process I needed to know the optical path length of his Orion 2" prism diagonal in an absolute sense and relative to both my Baader T2 prism diagonal and AP 2" mirror diagonal, both of which are my reference standards for prism and mirror diagonals respectively.
It was immediately obvious the Orion 2" prism diagonal was seriously degrading both the SA and color correction of the TEC 200ED I was using to take the measurements (with the graduations in the FT draw tube). There was no such degradation visible via the Baader or AP diagonals.
The obvious physical difference between the Baader diagonal and the 2" Orion is optical length. I don't know what type of glass the Orion uses but I'm sure the combination of glass type and optical path length had a lot to do with the poor results. However, the absolute optical quality of 2" Orion prism is also unknown. I suppose it would have been interesting to stop down the TEC to say 6" and repeat the testing as that would have set the system F stop to 12 or drag out one of my 7" F15 Mak.
But at least in this comparison the degradation was obvious with the TEC 200ED at F9. This came as a complete surprise. Perhaps someone who has modeling software can run a back to back simulation with assumed glasses and optical lengths.
Jeff
#3
Posted 18 July 2016 - 12:07 PM
Good question Mark.
I'm helping a buddy of mine construct a bino-friendly OTA for an Istar 8" F6 lens. As part of the design process I needed to know the optical path length of his Orion 2" prism diagonal in an absolute sense and relative to both my Baader T2 prism diagonal and AP 2" mirror diagonal, both of which are my reference standards for prism and mirror diagonals respectively.
It was immediately obvious the Orion 2" prism diagonal was seriously degrading both the SA and color correction of the TEC 200ED I was using to take the measurements (with the graduations in the FT draw tube). There was no such degradation visible via the Baader or AP diagonals.
The obvious physical difference between the Baader diagonal and the 2" Orion is optical length. I don't know what type of glass the Orion uses but I'm sure the combination of glass type and optical path length had a lot to do with the poor results. However, the absolute optical quality of 2" Orion prism is also unknown. I suppose it would have been interesting to stop down the TEC to say 6" and repeat the testing as that would have set the system F stop to 12 or drag out one of my 7" F15 Mak.
But at least in this comparison the degradation was obvious with the TEC 200ED at F9. This came as a complete surprise. Perhaps someone who has modeling software can run a back to back simulation with assumed glasses and optical lengths.
Jeff
I have the software, do you have the TEC 200ED and Istar 8" optical prescriptions ?
I'd also need dimensional drawings or dimensions for the Orion and Baader prisms .
The AP diagonal obviously has nothing to do with this since it's a flat mirror .
Edited by Rich_B, 18 July 2016 - 12:08 PM.
#4
Posted 18 July 2016 - 03:31 PM
To first see if a difference results in a software run, simply model plane parallel blocks of BK-7 and BaK-4 1.25" thick and 2" thick. Run a couple of aberration-free converging light bundles of differing f/ratio through the glass and see what spherical and chromatic errors obtain.
#5
Posted 19 July 2016 - 08:15 AM
It's everyone's guess, since the effect depends on the initial LA curves placement of the particular instrument. In general due to the relatively small cone width at the prism, spherical aberration is negligible, but changes in spherochromatism, due to the longitudinal shift of colors may not be. It pushes the blue out, while pulling red in vs. green. Here's an illustration that assumed one inch in-glass path and nearly one inch backfocus for 1.25" prism, and 2 inch glass path and nearly two inch back focus for 2" prism. Spherochromatism is about minimized w/o prism, and with a prism it worsens. Change in spherical aberration itself is negligible.
Vla
#6
Posted 19 July 2016 - 09:19 AM
#7
Posted 19 July 2016 - 09:40 AM
Thanks Vla.
I have read over and over that people think that a prism can correct or improve spherical aberration, but my understanding was what is indicated by your plots.. Change in CA is small, but change in spherochromatism can be quite pronounced in very fast scopes.
This confirms what I have told binoviewers for a couple of years now. Prisms, especially in fast telescopes, induce considerable spherochromatism.
While a single prism will not induce the amount that the multiple prisms in a binoviewer will, I still recommend that people use a Barlow or the Baader GPC before a binoviewer or a prism diagonal for planetary or high resolution work.
#8
Posted 19 July 2016 - 01:09 PM
I should clarify what I had in mind saying "spherochromatism" because the nominal spherochromatism, measured at the point of lowest aberration for either red or blue, didn't change significantly, but the amount of spherical aberration in the red and blue *at the location of best green focus* did. The amount of spherical aberration for any given magnitude varies with the point of measurement: it is lowest at the point of minimum aberration, and increases away from that point. What prism does is shifting the point of best red and blue foci with respect to the (best) green focus, thus effectively changing the color error at the best green focus, which could be called the "effective spherochromatism".
As for spherical aberration induced by a prism to any wavelength, it is given as P-V wavefront error (of primary spherical aberration) as W=(n^2-1)T/512n^3F^4, where "n" is the index of refraction, T is the in-glass path, and F is the focal ratio. Za n=1.5, it is W=T/1382F^4. In units of 0.00055mm wavelength, it is W=1.32/F^4; graph below shows it for 1.32" and 2" prism.
Vla
#9
Posted 19 July 2016 - 02:13 PM
Vla,
How about a run once more for the color error, but this time with an aberration-free incoming bundle (as from a perfect parabola)? I ask because this will reveal the "base" impact, which will be clearer to see when not superimposed upon an already aberrated beam (like that of a refractor.) I prefer to see the individual contributions to total effect in isolation. This affords a better appreciation of their magnitude. Thanks!
#10
Posted 20 July 2016 - 12:08 PM
Glenn,
Here it is. Nominal longitudinal shift only depends on the glass thickness, but defocus error, of course, scales inversely to the *square of* (edited) focal ratio. Longitudinal SA changes with the square of f-ratio (in terms of blur size, that means it is eight times larger at f/4 nominally, and 16 times vs. Airy disc, or in inverse proportion to the 4th power of focal ratio), .But the effect is different with refractors, where we have both, overcorrection and undercorrection generated by the objective in unoptimized wavelengths.
Vla
Edited by wh48gs, 20 July 2016 - 04:45 PM.
#11
Posted 20 July 2016 - 01:18 PM
Good stuff Vla and thanks!
This actually lines up well with a study Yuri did on the TEC yahoo group using the F7 TEC 140ED. He first published the design strehl from the C&F wavelengths and a bit beyond. Then, if I remember correctly, he simulated the Baader T2 prism in the light path, which showed only minor strehl effects (slightly better in blue but slightly worse in red) which would be basically invisible visually. Then he lumped in the Baader Mark V bino-viewer on top of that with its prisms/splitter. In a word, yuck, similar to an achromat. Then he also showed the effect on that system of inserting the 1.2X GPC in the nose of the Mark V. Bingo, back to almost design intent. I'm not sure if he simulated just the Mark V by it's self or not with a mirror diagonal.
This lines up with my visual experiences with my TEC 140 ED using the T2 prism and my Denks & Mark V.
Jeff
Edited by Jeff B, 20 July 2016 - 06:27 PM.
#12
Posted 20 July 2016 - 02:25 PM
Thanks, Jeff. Bottom line is one can't predict the effect without knowing the particular chromatic correction of the objective. It could be beneficial, detrimental, or with little effect. But knowing what is the effect of prism alone is still worth something.
Vla
#13
Posted 20 July 2016 - 02:46 PM
Thanks, Vla! Upon such morsels of information does a fuller understanding arrive.
Edited by GlennLeDrew, 20 July 2016 - 02:48 PM.
#14
Posted 20 July 2016 - 03:15 PM
i tried a 2" Zeiss prism in my TEC 140 and did not like the visual results. Compared to a mirror diagonal the view of Jupiter was shifted red and the presentation in general suggested a loss of color correction or some other upset in the general lens correction. I believe my sample of the 140 leans over corrected (?) and that may have been the cause. Anyway, I sold the prism and am very happy with the Baader quick lock mirror diagonal which I believe does not mess with the overall correction in my scope.
#15
Posted 30 July 2016 - 03:27 AM
It's everyone's guess, since the effect depends on the initial LA curves placement of the particular instrument. In general due to the relatively small cone width at the prism, spherical aberration is negligible, but changes in spherochromatism, due to the longitudinal shift of colors may not be. It pushes the blue out, while pulling red in vs. green. Here's an illustration that assumed one inch in-glass path and nearly one inch backfocus for 1.25" prism, and 2 inch glass path and nearly two inch back focus for 2" prism. Spherochromatism is about minimized w/o prism, and with a prism it worsens. Change in spherical aberration itself is negligible.
Vla
Hi wh48gs. very interesting diagram (at last !)
Can you tell us if a prism can improve (a little) an achromat ?
They have more straight lines than curves crossed lines seen with ED refractors.
For my taste it seems that the baader T2 prism have a little (very little) less CA than the dielectric Mirror coated diagonal on my F/6.15 vixen NA 130.
#16
Posted 30 July 2016 - 05:31 AM
I've noticed here on a AR127 f/6.5 that CA is less when using an enhanced aluminum diagonal compared to a dielectric. I've been considering the Baader Zeiss Prism but am hesitant not wanting to spend the funds for no gain. Does anyone have any thoughts/experience on this?
Peace...
#17
Posted 01 August 2016 - 04:16 PM
Hi Jeremiah
May be it's cause of the coatings. The transmission coating for an enhanced diagonal is less in the blue and red than in the green.
Say blue : 90%. green : 97%. red : 90%.
a dielectric have excellent transmission coating 97-99% from 400nm to 700nm. So the enhanced Mirror act like a light filter.
I saw also some old eyepieces with violet multicoating. Looking thru them you see a green color shift.
For you second question i haven't the exact answer but i have a 2" 90°amici Baader prism. I made a lot of tests with focused and defocused stars and planets.
Using my 2" dielectric diagonal i see a violet fringe around Jupiter. Strangely Mars have only a red fringe. Saturn have a slightly violet fringe.
Using the Baader T2 prism seems to give a better view with a little less fringe. On Mars the red fringe is far reduced.
Now using the 90°amici (the light pass thru a big piece of glass) the red and green seem focused together. But the blue is more shifted away. When i see a mild purple fringe thru the dielectric, i see a more wider pale blue fringe thru the amici.
Now : Is a prim shift colors with an achromat : Yes. Is a big piece of glass or a special glass can focus blue, green and red together ?
Probably not. But it could be interesting to test.
wh48gs are you there ?
Edited by jjack's, 02 August 2016 - 06:03 AM.
#18
Posted 01 August 2016 - 05:48 PM
FWIW the 2" Zeiss prism diagonal has a light path of around 4 inches not 2 inches. The T2 prism is just under 2" light path.
for those using the Baader T2 prism diagonal, are you in 2" mode or 1.25" mode? I don't like the way it's halfway between 1.25 and 2 inches in size.
Edited by Scott99, 01 August 2016 - 06:54 PM.
#19
Posted 01 August 2016 - 06:48 PM
When my neck can stand it, I prefer not using a prism diagonal at all. I have seen color correction using my APO Barlow with my vintage achromatic refractors - quite dramatic on Mars & Jupiter - again, without using a diagonal. Doesn't sound like the Baader diagonal is worth the investment for most of my scopes.
#20
Posted 01 August 2016 - 06:55 PM
Hi Jeremiah
May be it's cause of the coatings. The transmission coating for an enhanced diagonal is less in the blue and red than in the green.
Say blue : 90%. green : 97%. red : 90%.
a dielectric have excellent transmission coating 97-99% from 400nm to 700nm. So the enhanced Mirror act like a light filter.
I saw also some hold eyepieces with violet multicoating. Looking thru them you see a green color shift.
For you second question i haven't the exact answer but i have a 2" amici Baader prism. I made a lot of tests with focused and defocused stars and planets.
Using my 2" dielectric diagonal i see a violet fringe around Jupiter. Strangely Mars have a only red fringe. Saturn have a slightly violet fringe.
Using the Baader T2 prism seems to give a better view with a little less fringe. On Mars the red fringe is far reduced.
Now using the 90°amici (the light pass thru a big piece of glass) the red and green seem focused together. But the blue is more shifted away. When i see a mild purple fringe thru the dielectric, i see a more wider pale blue fringe thru the amici.
Now : Is a prim shift colors with an achromat : Yes. Is a big piece of glass or a special glass can focus blue, green and red together ?
Probably not. But it could be interesting to test.
wh48gs are you there ?
Thanks jj
If my eye is on axis Jupiter has very little to no CA (better with enhanced aluminum). Saturn has none at all (either diagonal). Mars has a wee bit of reddish sometimes (enhanced better here). Luna has a blue line on the limb and none anywhere else at all (either diagonal). On Luna the extra brightness of the dielectric is very obvious and the enhanced gives a crisper view, edges on the detail are much better. Overall I cannot complain with the AR127, it gets my attention at what it *can* do despite being a fast achromat. I would like to tame the view a bit if possible on the planets. I made a 4.5" mask and it changed nothing. Next will be a 4" mask but I've been very curious about the prism and what to expect/what might happen. I really don't want to spend the money for the prism and get no gain at all, though it would be a lesson learned. Just costing more than I'm wanting to shell out right now for the lesson learned.
Peace...
#21
Posted 02 August 2016 - 06:07 AM
FWIW the 2" Zeiss prism diagonal has a light path of around 4 inches not 2 inches. The T2 prism is just under 2" light path.
for those using the Baader T2 prism diagonal, are you in 2" mode or 1.25" mode? I don't like the way it's halfway between 1.25 and 2 inches in size.
Hi Scott
I use the T2 Baader diagonal in 2" mode. No vigneting with a 27 panoptic but a little vigneting with an UWAN 28. Stars are seen to the edge but dimmed a lot.
#22
Posted 02 August 2016 - 06:25 AM
Overall I cannot complain with the AR127, it gets my attention at what it *can* do despite being a fast achromat. I would like to tame the view a bit if possible on the planets.
Based on my observations, an APO Barlow would be worth a try - and cheaper than a Baader or Zeiss prism diagonal.
#23
Posted 02 August 2016 - 10:45 AM
FWIW the 2" Zeiss prism diagonal has a light path of around 4 inches not 2 inches. The T2 prism is just under 2" light path.
for those using the Baader T2 prism diagonal, are you in 2" mode or 1.25" mode? I don't like the way it's halfway between 1.25 and 2 inches in size.
Hi Scott
I use the T2 Baader diagonal in 2" mode. No vigneting with a 27 panoptic but a little vigneting with an UWAN 28. Stars are seen to the edge but dimmed a lot.
that's cool. I wonder if it's possible to get the 2" clicklock clamp for my T2 Baader prism. Sounds like it would work OK with an XW 30mm. I also have the Tak 1.25" prism which seems excellent.
btw I did some searching last night - if you search the TEC yahoo group on posts by "Yuri" with message content "prism diagonal" you can see some long discussions on this topic. In the end Yuri advises using the AP Maxbright with the f/7 TEC140, although the color shift from the T2 prism is small it is there (if I'm paraphrasing correctly, too many posts to copy here)
Edited by Scott99, 02 August 2016 - 10:48 AM.
#24
Posted 02 August 2016 - 01:39 PM
Overall I cannot complain with the AR127, it gets my attention at what it *can* do despite being a fast achromat. I would like to tame the view a bit if possible on the planets.
Based on my observations, an APO Barlow would be worth a try - and cheaper than a Baader or Zeiss prism diagonal.
Thank you for reminding me. I have a 2.5x here that I keep forgetting to to try. I think I would rather find a 1.25x but this one will do for a start. It would be nice also to have one that the cell could be removed and screwed into the diagonal. Thanks....
Peace...
#25
Posted 02 August 2016 - 06:12 PM
FWIW the 2" Zeiss prism diagonal has a light path of around 4 inches not 2 inches. The T2 prism is just under 2" light path.
for those using the Baader T2 prism diagonal, are you in 2" mode or 1.25" mode? I don't like the way it's halfway between 1.25 and 2 inches in size.
Hi Scott
I use the T2 Baader diagonal in 2" mode. No vigneting with a 27 panoptic but a little vigneting with an UWAN 28. Stars are seen to the edge but dimmed a lot.
that's cool. I wonder if it's possible to get the 2" clicklock clamp for my T2 Baader prism. Sounds like it would work OK with an XW 30mm. I also have the Tak 1.25" prism which seems excellent.
btw I did some searching last night - if you search the TEC yahoo group on posts by "Yuri" with message content "prism diagonal" you can see some long discussions on this topic. In the end Yuri advises using the AP Maxbright with the f/7 TEC140, although the color shift from the T2 prism is small it is there (if I'm paraphrasing correctly, too many posts to copy here)
Yes, exactly. While the effects of the T2 prism by itself are relatively small, when combined with a typical bino-viewer and its prisms, things get out of hand pretty quickly, degenerating the correction to no better than an achromat and in some ways worse as there is no null in the yellow green part of the spectrum like you get with an achromat. Adding the 1.2X GPC between the T2 and the viewer pretty much returns the system to design intent. The same would also apply to most other APO's as well depending on the design and viewer system. For example, using the Denk OCS system mitigates the chromatic effects as well.
Jeff