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Equipment Discussions >> Refractors

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Mark9473
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5596625 - 12/30/12 05:41 PM

Quote:

the new one's that APM has. In their case, they are a little different still than the IStar's since they both use ED glass as well. The Chinese one uses the older LaF glass, while I think the Russian one is using something like the FPL-53 glass. Because they use ED glass, they are better corrected to start with than an achromat.

However, they apparently are tuned as well to focus on reducing the traditional false color as well. Since they are starting out ahead of the achromat to begin with, they don't have as far to go, so aren't going to be as affected. But it's still not going to be as ideal (again for our Jupiter example) as a normally corrected ED.




Here's a reply from Markus, who as you know is no longer allowed to post in this forum, so he posted this in the vendors forum:

Quote:

A quick note to the discussion on the refractor forum regards correction of our chinese and LZOS 152 doublet ED. both are not designed to reduce purple and increase red spotts. Both are correct designed, optimized for visual observations, tthe classic tradional metthod !!!!!!!!!!!!!




Comments anybody?


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: Mark9473]
      #5596706 - 12/30/12 06:40 PM

Quote:


Here's a reply from Markus, who as you know is no longer allowed to post in this forum, so he posted this in the vendors forum:

Quote:

A quick note to the discussion on the refractor forum regards correction of our chinese and LZOS 152 doublet ED. both are not designed to reduce purple and increase red spotts. Both are correct designed, optimized for visual observations, tthe classic tradional metthod !!!!!!!!!!!!!




Comments anybody?




That's right, Markus isn't allowed to participate here. I'll include a link to his thread over in Vendors so he can respond and we can relay his inputs. It's important to have his input.

APM 6" ED Thread in Vendors Forum

So I have a question for Markus. I went over the the APM site and looked at the detailed plots for these new doublets. The ones I found very interesting were the longitudinal SA color charts.

As I understand it, these show the relative defocus of a particular colour relative to a common reference (usually green as I understand it). If I look at the LZOS double plots, I see most of the lines stick pretty close together near the centre, but the two red ones hang out to the right side. As I understand it, this means there are a bunch of colors that are in-focus (or nearly so) and the red is out of focus. The opposite is that you could focus the red, but other colors would be out of focus. Is this correct?

If I compare it to the LZOS triplet, there is a point where they all meet.

How close am I on this?

Clear skies,


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: Kevin Barker]
      #5596763 - 12/30/12 07:08 PM

Hi Kevin,

Quote:


Recently on the Istar scope club Ales K ( an part owner of istar) has released spot diagrams of 6 different 150 mm doublets and triplets. The red spot sizes are larger but not hugely larger than the blue spot sizes. Although the actual wavelength versus focus sketches are missing.





Your post reminded me to go over to the IStar scope club to look at the details again. You are right, the longitudinal color plots are missing and would help tell a better picture of things, but I did see some interesting stuff in the spot plots.

You are correct, the red does hang out the farthest with best focus in green. The blue is defocused as well, just not as much. The other chart would be able to tell (as I understand it...) if the blue was brought more into focus, would the red get better or worse?

What is really interesting is how it compares to the plot for the 6" achro. The achro has things much more tightly focused than the R30, except for the purple. And, if you look at the area with the most intensity, it's closer in to the Airy disk than in the R30 and they don't extend as far. If you look at the relative diameters of extending colour, the R30's blur seems to be much wider (5 diameters vs. 3) than the achro, however it would appear to be less intense.

The achro basically has much tighter plots in my opinion with the main colours being tight.

However, one interesting thing that I think could explain some of the comments re: the R30 performance. Mike has commented several times on doubles being split very nicely with this lens. If you look at the plots, I can see how that may be the case compared to the achro. Reason I'm guess is that while the color blur is much wider, it is less intensive especially close in to the airy disk. This would make it easier to spot a companion star than in the achro if it were located in the more intense looking blur of the achro. That's a theory I'm thinking right now.

Again, there is no perfect design, so it makes sense if they aren't as good at planetary, they may be very well suited to something else.

Quote:


I chiefly observe in seeing that is typically 6-8/10. The suggestion a scope can show the detail of the 10 inch processed pic continuously put forward surprises me.





I think it depends on the how capable the instrument is, the conditions, and an often overlooked factor of spending enough time at the eyepiece. I know I've seen much of the detail posted in that C11 image in the C14, but it was never all at once, never for more then a brief second (or seconds) and it took hours of observing to get it.

Clear skies,


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johnnyha
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5596797 - 12/30/12 07:31 PM

Ales - any idea where the Istar R30 w/ Raycorr might end up in Gord's ranking?

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Kevin Barker
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Loc: Auckland, NZ
Re: Color correction & Valery's thread on the subject new [Re: ValeryD]
      #5597009 - 12/30/12 10:05 PM

Valery
Was it a well controlled fair test ?

Same observer, night, eyepiece, mag, aperture etc on same object.

Or is what you are talking(comparison of four 5 inch refractors) about just an impression.

Kevin


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ValeryD
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Re: Color correction & Valery's thread on the subject new [Re: Kevin Barker]
      #5597286 - 12/31/12 01:31 AM

Quote:

Valery
Was it a well controlled fair test ?

Same observer, night, eyepiece, mag, aperture etc on same object.

Or is what you are talking(comparison of four 5 inch refractors) about just an impression.

Kevin




Yes. I have had enough eyepieces (SPL design) to keep magnifications as close as possible. the same diagonals and, of course, the same nights, the same time.

But the difference is obvious. It was especially obvious when observing Mars. I always felt the necessity to refocus the scope with shifted to blue correction curve. There was no solid focus - always something wrong. As high magnification was, as lesser defined focus.


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ValeryD
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5597289 - 12/31/12 01:33 AM

Quote:



How close am I on this?






Your understanding of the diagram is correct.


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ISTAR Optical
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Re: Color correction & Valery's thread on the subject new [Re: junomike]
      #5597352 - 12/31/12 03:36 AM

Thanks for sharing this piece of info.

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Gord
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Re: Color correction & Valery's thread on the subject new [Re: junomike]
      #5597660 - 12/31/12 10:17 AM

Quote:


Gord, the Chromacor can be "fiddly", but once adjusted properly It does work!
In my C6R w/o the CC, I would never turn the scope to Jupiter as It's just a purple mess!
With the CC however the purple is gone and the Image is very much Improved, however the AT111EDT is still in a league of It's own. The detail and contrast of the Triplet is just amazing.
I'm not sure If It's the glass used or ??? but the AT111EDT puts up a super rich view (contrast?) of Jupiter that I haven't seen in many other scopes. It's similar to the effect the TV Plossls have on Jupiter as well.

Also, as a side note, This past summer, A friend and I (Tank) were viewing with his RFT 6" F5 Refractor and decided to see how the CC would do with the newer "red shifted glass" (on Vega).
Answer......amazing! w/o the CC, Vega was a mess with the CA extending to about the size of a Full Moon (viewed with naked eye).
With the CC in place (no spacing), The CA was reduced to a size maybe 2X or 3X the size of the Star. An amount similar to what's seen in the older (Semi) Apo Triplets, or ED scopes. And this was a 6" F5 scope, so I'm sure the F5.9 or F6.5 version would fare better.





Mike,

Very interesting feedback. In your opinion, where do you think a Chromacor equipped achro would fit? Or, based on what you were describing in terms of different scopes, do you think it could fit at many levels in the list based on how close of "fit" it is to the scope, and/or it's individual setup in the scope?

I've heard the comments about the need to be precise with the setup before and plenty of negative comments if it is off, and positive ones where it is on. So it seems like it's likely something like the latter I describe. I wonder where it could theoretically be at it's best?

It would be good to hear from more users.

On a more general note, and as I indicated before, the list was just a straw-man, a starting point or hypothesis in the scientific method based on the proposal put forward that different tunings will have a different performance, in this case specifically on Jupiter/Mars. What's left is to refine it based on input, and ultimately test it with real world experimentation.

Clear skies,


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Jeff B
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Re: Color correction & Valery's thread on the subject new [Re: ISTAR Optical]
      #5597665 - 12/31/12 10:19 AM

Quote:

Thanks for sharing this piece of info.




Yes, thank you. I guess I don't understand all the fuss going on here about a designers choice in balancing out the red and blue spot sizes.

Ales has already explained that his lenses use glasses other than the standard BK7 and F2 or F4 to achieve modest reductions in longitudinal CA. What is unclear about that?

That a designer may choose to leverage or redistribute that reduction between the blue and red is entirely up to him/her. I'm no designer but with the leverage, I'd be sorely tempted to leave the blue spot size alone and reduce that of the red. Even though this may shift the best correction slightly away from green, the overall visual correction (as well as the polychromatic correction from C-F) will be improved over that of the classic Fraunhofer lens of equal focal ratio.

My experience is similar to Valery's though using different tools. Using my D&G 11" F12 achromat, selected aperture stops, various Chromacors (O1, N, U1 Chromacor I and a Chromacor II N) with various spacers, I can manipulate the color and spherochromatic correction almost at will. I have to tell you, everytime I favor the correction towards the blue and sacrifice the red, the image suffers. I have always ended up tweaking the system towards a good balance between blue and red or slightly in favor of the red.

BTW Mike, when you used your Chromacor on the 6" F5, you were probably stopping down the aperture slightly with the nose of the Chromacor. Your color correction improvement may have been, in part, due to the stop down.

Jeff


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Jeff B
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5597705 - 12/31/12 10:40 AM

Quote:

In your opinion, where do you think a Chromacor equipped achro would fit? Or, based on what you were describing in terms of different scopes, do you think it could fit at many levels in the list based on how close of "fit" it is to the scope, and/or it's individual setup in the scope?

I've heard the comments about the need to be precise with the setup before and plenty of negative comments if it is off, and positive ones where it is on. So it seems like it's likely something like the latter I describe. I wonder where it could theoretically be at it's best?

It would be good to hear from more users.






I can answer that with a definitive, "It depends".

From my direct experience, I'd say the CR6 with a well matched Chromacor I falls between the old Meade 6" F9 ED and the 6" F8 FPL53 doublet. With a well matched Chromacor II, it's a match for my APM 6" F8 triplet.

On axis.

The trade off when using the Chromacor is an increase in lateral color off axis. The degree and extent of the lateral color depends on the aperture, F stop and optimal spacing but generically, for a given aperture, as the F stop gets slower the spacing of the Chromacor in front of the focal plane increases and lateral color decreases.

But it's not that simple. As you move a given Chromacor forward for a slower F stop for a given lens native correction in green, the system tends to become over corrected in green.

Clear as mud now right?

I do disagree with Ales when he says "results may vary" when using the Raycor for "non-type design" achromats (R30s). Results WILL vary.

But the tweaking is part of the fun of this hobby (just like my audio hobby).

Jeff


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suburbanskies
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Re: Color correction & Valery's thread on the subject new [Re: ValeryD]
      #5597834 - 12/31/12 11:55 AM

Quote:

I can add one important thing: the optical designers in a past knew the optics and human eye work not any bit worser than todays designers. And I think they knew optics better.
All fundamental investigations in this field were done at that old good times. All classical telescopes were designed by masters and be sure, they experimented with different designs and chooses which work best.




I'm sure you are correct. And if you and other opticians keep providing us info about optics, eventually consumers will really understand their scopes

Mark


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: Jeff B]
      #5598019 - 12/31/12 01:49 PM

Quote:

I guess I don't understand all the fuss going on here about a designers choice in balancing out the red and blue spot sizes.




Jeff,

Thanks for weighing in on this as I know you have a lot of experience, and direct experimental hands on experience at that, on this subject. I think you answered the question about what the big deal is in your own post.

If this is a use case where this type of optimization is going to have a negative effect in some cases, it's good to know. It was a surprise to me, but then I didn't understand the interplay between the color blurs and the longitudinal CA before now (as I'm sure was the case for many others).

The examples with the Chromacor were useful too. When used right, it can be up there with the best of them.

On an alternate path on the discussion about blue vs. red color shifting, what cases would it be that the shift to blue is a good thing? I would imagine on something like Uranus/Neptune and we have seen there is an effect on the visibility of the secondary purple blur. What other cases?

Clear skies,


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Denimsky
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5598187 - 12/31/12 03:26 PM

This thread a little bit feels like a vendor bashing toward ISTAR R30 telescopes even though the thread has many useful information.

I don't see much detail about the comparison done by Valery. Also I'm not sure Valery is a owner of ISTAR's competitor.

I don't own any ISTAR telescope but this thread seems to be able to really damage business of ISTAR and we need more data to start this type of thread especially against particular brand of scopes.

I looked at spot diagram of TSA-102.

http://www.takahashi-europe.com/en/TSA-102.optics.spots.htm

TSA-102 has much tighter focus on the blue end but FS-102 is much tighter on the red end.
Does it mean FS-102 is really that much better on Jupiter or Mars?
I both owned FS-102 and TSA-102 but the answer was no for my case even though the view was so close.

I'm not an expert on optics and this thread just feels unfair for ISTAR in particular.


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: Denimsky]
      #5598374 - 12/31/12 05:05 PM

Quote:


I looked at spot diagram of TSA-102.

http://www.takahashi-europe.com/en/TSA-102.optics.spots.htm

TSA-102 has much tighter focus on the blue end but FS-102 is much tighter on the red end.
Does it mean FS-102 is really that much better on Jupiter or Mars?
I both owned FS-102 and TSA-102 but the answer was no for my case even though the view was so close.





Donghun,

Thanks for adding the info about the Takahashi's here. I don't see the longitudinal graphs there, or the airy disk size for reference on the spots. However, it's a comparison against a triplet APO and a Fluorite doublet. The triplet should definitely be better, however the interesting take away is that in the case of the doublet, Takahashi chose to focus the red side more than the blue.

The discussion has been red vs. blue, and this seems to be another data point that the red side of focus is very important. Your experience seems to indicate that both work very well, but the triplet is better. This is what I would expect.

Clear skies,


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junomike
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Re: Color correction & Valery's thread on the subject new [Re: Jeff B]
      #5598460 - 12/31/12 05:52 PM

Gord, I think Jeff answered more elaborately than I ever possibly could have. I would have guessed at the same placement he did for the C6R/CC, but because I didn't have many of the listed scopes, It would have been just a guess.

Jeff, Do you think (from the Diagrams) that Markus's APM 150mm F8 ED (Chinese) Refractor would be better, equal, or worse than a C6R/CC1?

Mike


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junomike
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Re: Color correction & Valery's thread on the subject new [Re: Jeff B]
      #5598476 - 12/31/12 06:00 PM

Quote:

BTW Mike, when you used your Chromacor on the 6" F5, you were probably stopping down the aperture slightly with the nose of the Chromacor. Your color correction improvement may have been, in part, due to the stop down.




Jeff, I do realize that the fast F ratio creates a sudden "stop down" in 6" F5 Refractor, but I minimized the spacing by not using spacers on an AP Maxbright Diagonal (104mm) and also using an eyepiece (5mm XO) with a Field Stop close to the Diagonal Mirror.
I'm sure some Aperture was lost which aided in the reduction of CA, but I was still Impressed.

Mike


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Kevin Barker
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5598599 - 12/31/12 07:12 PM

Gord you say
"Again, there is no perfect design, so it makes sense if they aren't as good at planetary, they may be very well suited to something else."

It seems you have convinced yourself to this end. (the istar R30 design does not perform as well as an C F corrected achromat on planets) based on spot diagrams.

You have however not seen through an R30 scope with your own eyes and in fact are making assumptions based on what a vendor has said before about other scopes.

This (Valery's insinuation directed at the R30 Istar scopes) reminds me a bit of Newton dismissing Cassegrain's scope design a few centuries ago.

I think his thread is unfair in this respect to 2 vendors (Istar and APM). They are being dismissed theoretically without any concrete evidence.

No one has done a comparison with an istar R30 and a similar C F achromat.

I guess at least folks are not still claiming they use the same glass as normal F2 BAK 7 fraunhofer achromats.

Casting dispersions can be unfair when they are not supported by hard facts.



Kevin Barker


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Kunama
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Reged: 10/22/12

Re: Color correction & Valery's thread on the subject new [Re: Kevin Barker]
      #5598635 - 12/31/12 07:41 PM

Quote:

Gord you say
"Again, there is no perfect design, so it makes sense if they aren't as good at planetary, they may be very well suited to something else."

It seems you have convinced yourself to this end. (the istar R30 design does not perform as well as an C F corrected achromat on planets) based on spot diagrams.

You have however not seen through an R30 scope with your own eyes and in fact are making assumptions based on what a vendor has said before about other scopes.

This (Valery's insinuation directed at the R30 Istar scopes) reminds me a bit of Newton dismissing Cassegrain's scope design a few centuries ago.

I think his thread is unfair in this respect to 2 vendors (Istar and APM). They are being dismissed theoretically without any concrete evidence.

No one has done a comparison with an istar R30 and a similar C F achromat.

I guess at least folks are not still claiming they use the same glass as normal F2 BAK 7 fraunhofer achromats.

Casting dispersions can be unfair when they are not supported by hard facts.



Kevin Barker




I think you hit the nail on the head here Kevin. A lot of discussion based on numbers on paper without actually looking through the scope seems like a waste of bandwidth.


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Jeff B
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Re: Color correction & Valery's thread on the subject new [Re: junomike]
      #5598811 - 12/31/12 09:58 PM

Quote:

Jeff, Do you think (from the Diagrams) that Markus's APM 150mm F8 ED (Chinese) Refractor would be better, equal, or worse than a C6R/CC1?

Mike




On axis, perfectly matched and installed, slightly better visually.

As I recall the CR6/Chromacor I combo crossed the .8 Strehl line at ~480 in the blue and ~640 in the red on axis but remember when you get more than a couple of arc minutes off axis, the lateral color spread knocks those values down considerably. To me, the Chromacor really comes into its own for large aperture(8" and above)achromats.

Personally I'm really excited by this APM doublet. I've an old AP 6" F9 "blue tube" triplet that I just love and I paid for it right around what the APM is going for. It performs wonderfully and the differences between it and my APM 6" F8 triplet are rather subtle at focus. If I can get better performance than the old AP in a cheaper, lighter weight, shorter doublet, well....duh.

Jeff


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