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Equipment Discussions >> Refractors

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Gord
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Reged: 01/06/04

Loc: Toronto, ON, Canada
Re: Color correction & Valery's thread on the subject new [Re: Jeff B]
      #5598878 - 12/31/12 10:59 PM

Quote:


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, you aren't the only one who's interested in the APM Chinese ED. I was very close to breaking out the plastic, the heck with the CFO's wrath!

I've been digging around doing some reading and found some interesting numbers to help put things in context a little. I think it is important to point out how much better these APM doublets both are as compared to the achromat's. These are from the longitudinal SA numbers, and again, just to put things in context.


  • Good APO triplet(ex. APM LZOS triplet) has focus shift in the range of 0.1mm.
  • These new APM doublets appear to be in the range of 0.3-0.4mm.
  • A 6" F15 Fraunhofer his a shift of about 1.13mm according to what I was reading about at the link below by Bob Royce. I couldn't find a 6" F8 to make a good comparison, but it's my understanding that the F8 will be worse still. Maybe someone here can give the value.


The APM's are much closer to the true APO's than they are the achro's. I'm just how much they could be giving up with this correction design?

As I mentioned above, I found a very interesting read about achro color correction that Robert Royce has put together. Have a look here:

Robert Royce on achromat objectives

It gives a really good intro into a bunch of the things being talked about in this thread. An interesting factoid he mentions about a large achro (the 36" Lick) is that the logitudinal shift between the main colors is not millimeters like above, it's inches! 1.5" he states. Wow, can you imagine having to rack the focuser 1.5" to hit the different color focus points?!

Clear skies,


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: Kevin Barker]
      #5598901 - 12/31/12 11:22 PM

Quote:


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




I agree, you are right. It's something that would be very good to see. I suggested it to Ales earlier in the thread.

Quote:


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




I don't think anyone has claimed the R30 were the same as F2/BK7 Fraunhofers. I stated at one point that they are achromats (modified ones) and you know, I'll stand by that. Here's why (although this is a poor horse that has been dug up and beaten to death so many times...).

Achromat does not mean a Fraunhofer. Achromat is just a general label regarding color correction performance. There can be Fraunhofer's, Steinheil's, Littrow's, Clarks, etc.

Not all achromats are using F2 and BK7. There are other ways to do it (see the Bob Royce link above for an example). And it's unlikely they are using ED glasses otherwise the cost would be higher and when ED glasses are used, it's almost certainly mentioned for marketing purposes.

Given the color correction of the R30's (they're still colorful), and that achromat's can come in various flavors of glass combinations, they're achromats. And they have shifted color correction to De-emphasize the traditional purple halo.

Clear skies,


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Kunama
professor emeritus


Reged: 10/22/12

Re: Color correction & Valery's thread on the subject *DELETED* *DELETED* new [Re: Gord]
      #5599089 - 01/01/13 03:34 AM

... looking forward to your comments once you have seen through the R30 Scope ....

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Gord
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Re: Color correction & Valery's thread on the subject new [Re: Kunama]
      #5599374 - 01/01/13 11:00 AM

Happy New Year all! Here's to a great 2013!

Well, it seems my integrity is now being questioned, but this does bring up an important point. I am no longer in a position to offer my personal experience in trying to verify this theory as if I report anything other than a positive result to the the product, my results will be considered biased.

I have made an additional suggestion to Ales at IStar that he have one of the 6" F8 R30's reviewed by an experienced observer. I have suggested Jeff (JeffB) would be an ideal candidate because of his extensive knowledge of the theory, observing, testing, and experimentation. He is very respected and his knowledge outstrips most here. He is also one of the earliest IStar customers and has the 6" F10 like mine which would be a good candidate to test against for the 30% improvement benefit of the R30's. I believe he also has access to other 6" F8 achromats and has (or has had) many other scopes of this nature.

At this point we need to see some real use results reported. Looks like James Ling and his friends are set to do some testing with the new APM, and I'm sure there are many looking forward to hearing their reports.

Clear skies!


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Jared
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Re: Color correction & Valery's thread on the subject new [Re: Denimsky]
      #5599443 - 01/01/13 11:51 AM

Quote:

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.




Keep in mind that the Airy disk size for a 4" f/8 scope is just under 11 microns. The "boxes" on the Takahashi website are 100 microns across. So, any spots that are 1/9th the width of the box or less are essentially perfect. So, while it may appear that the FS is much better corrected in the red, for all practical purposes the triplet is also perfect (at least on paper--these are theoretical spots, not actual measurements). The TSA spot size stays smaller than the Airy disk size up to roughly 650nm.


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junomike
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5599639 - 01/01/13 01:33 PM

Gord, I think this is a fantastic thread and It does answer some question while creating others. For me though, It's timing is perfect as I'm looking to upgrade to a larger Apo in the very near future. I just haven't decided on how or what yet?

So If the newer APM 6" F8 is going to perform (CA wise) similar to mt C6R/CC, then I see no point. If It's going to be similar to the color correction of the Synta 120ED's them I'm in and will order the lens cell replacement!

Inevitably this thread (and James Ling's as well) will most likely help me decide, but I think I'll be looking for a Triplet 130mm FPL 53 or TEC 140.

Mike


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johnnyha
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Re: Color correction & Valery's thread on the subject new [Re: junomike]
      #5599861 - 01/01/13 03:48 PM

My guess is and especially after reading James Ling's latest update, that the new $4K APM 6" APO doublet will *handily* beat the C6R/CC image.

Happy New Year Everybody!


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Kunama
professor emeritus


Reged: 10/22/12

Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5600134 - 01/01/13 06:48 PM

Quote:

....... I am no longer in a position to offer my personal experience in trying to verify this theory as if I report anything other than a positive result to the the product, my results will be considered biased.........




On the contrary Gord, I for one would love to hear your opinion of the R30 performance once you have had the opportunity to use it and compare it, whether your conclusions and opinions of the R30 compared to the standard achro are positive or negative matters not, but I would prefer to hear them once you have used the scope.
You may well be correct in what you advocate.


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Kevin Barker
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Reged: 04/22/09

Loc: Auckland, NZ
Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5600189 - 01/01/13 07:26 PM

Gord
I am well aware of what an achromatic doublet is but thank you for the explanation.

My comment comes from reading the link in the material you originally posted which insinuated that normal achromats have been blue shifted to create R 30 R 50 etc

I enjoyed last night great seeing and using an old 5 inch f-6.5 lens(ex C5R??) and a home made alt-az set up and surprise surprise Jupiter had a wealth of detail I was away camping and did not have room for my Zeiss APQ130/1000 but reckon it would have been even better. Very qualitative comment really most folks just enjoy using a scope and observing the night sky. I have no idea if the 5 inch f-6.5 is CF corrected??

It is amazing what a cheap lens can deliver.

Kevin Barker


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: Kevin Barker]
      #5602040 - 01/02/13 10:04 PM

Matt, Kevin,

I do want to say thank you for your contributions in the thread. Points, counter-points, and different voices are important to a healthy discussion. If I am provided with the opportunity to view through or test these telescopes (any telescope!), of course I take the opportunity.

You have to remember that for many of us here (at this time of year...), there aren't many chances to even see a clear sky. It's a good chance to do some reading, research, and discussion.

And speaking of simple scopes, I have a nice Synta 4" F10 achromat that turns out surprisingly nice images and a very nice amount of detail on Jupiter. I believe I commented on it in the Jupiter comparison thread above.

Clear skies,


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5602095 - 01/02/13 10:49 PM

So, the latest test information posted by James and Richard on the new APM in the other thread seems to match Valery's predictions quite well, right down to the red halo and softness. I've been doing some more reading on the APM doublets and some older discussions regarding them that have occurred leading up to now. There were some color correction discussions in this thread (page 6):

Interesting new APM 6" F8

Here, Vla (wh48gs) has posted some LA graphs for a design using OK4/OF1 vs. a fluorite doublet, both with balanced red/blue correction. I believe the OK4/OF1 on the left is supposed to represent the new LZOS 6" F8 doublet, or something like this design could be.

From the diagram, I see the de-focus distance between green and blue/red appears to be about 0.2mm (since blue/red are together). From the LA graph we mentioned above for the actual new APM doublets, the de-focus covering green/blue/red was about 0.3-0.4mm.

So, would the following be true:

1. By pulling the blue line in as in the APM design vs. the one shown by Vla above, the red line ends up pushed out more so the overall amount of de-focus is worse
2. In the balanced design shown above, there will be a slight traditional purple halo (where the red/blue overlap)
3. The larger de-focus design will have a softer image
4. The less de-focus design will be sharper, but display a bit of traditional false color

I just want to know if I'm interpreting and understanding the designs correctly.

Thanks!


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Kevin Barker
super member


Reged: 04/22/09

Loc: Auckland, NZ
Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5602311 - 01/03/13 02:44 AM

Gord
The forum's name is after all "cloudy nights", I think you could do well to venture to some clear skies and look through some scopes. Perhaps venture to the Southern hemisphere or close to the tropics!!!!

In the Istar proboard forum you have mentioned wavelength versus longitudinal focus graphs. I am not surprised they are not displayed. I have rarely seen these from manufacturers.

This information would likely be more commercially sensitive and could well be misrepresented. Although it is an easy way to see what is going on with respect to which wavelengths focus at the same point as you have stated.

I am also not surprised Istar does not send an expensive scope to your mate Jeff to check out either as they are a relatively young company and sales may not be that high to date.

I like the idea of their flexibility with respect to providing large lenses for amateurs at relatively good prices.

I also question why another vendor is sniping at another vendor.

Surely he is above this sort of caper. It is not a good look and one wonders why bother.

I am not convinced that there will be any appreciable difference between a scope with a slightly blue leaning correction as opposed to a C F correction. But maybe there is a slight difference??

I suspect any difference might be overruled by other factors.

I do not think there is a spot diagram anomaly either, there will be variation in the stated f ratio, I calculated green airy disc sizes similar to what was stated but for example I reckon the 180 mm spot diagram is for an f 8.45 scope. ( from the degree measurements in mm)

Clear skies and good seeing (hopefully through a nice refractor)

Kevin


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: Kevin Barker]
      #5603346 - 01/03/13 04:42 PM

Hi Kevin,

On the blue shift question, it's obvious from the testing so far with the new APM that this small amount of extra red de-focus is visible at the eyepiece (the notes about the red halos). But as you say, the real question is how much of a difference would there be to the FeC correction.

Looking back over the thread, Valery made an interesting comment that would make for a good test. He said a prism diagonal would improve the red correction, but worsen the blue. That would seem to be opposite of what this FeD corrected design is and should be closer to the normal FeC. So a good test would be to use a prism diagonal and see what effect if has on the image (better/worse/same).

On the IStar spot diagrams, I agree the calculated airy disk sizes look right to me as well. What doesn't look right is that they are displayed differently in each diagram, and the color blurs are very large, and bigger in the R30 than the achromat! The 6" F8 R30 is supposed to have a color blur equivalent to a 6" F10.4. The F10.4 is already going to have a larger airy disk than the F8, so the blur is going to have to be much smaller on the R30 relative to the F8 achromat.

But as I indicated in the IStar forum thread, it now occurs to me that it's not clear how the 30% value is measured. The sample spots shown just don't illustrate to me as clearly what's going on as the ones on Robert Royce's page I linked to above do.

Clear skies,


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fred1871
scholastic sledgehammer


Reged: 03/22/09

Loc: Australia
Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5603583 - 01/03/13 07:35 PM

This is a fascinating thread and I'm hoping we'll get some genuine light, and not just heat, from it, despite notions about makers trying to sabotage their rivals. Usefully, one would address the ideas presented, as Gord and some others are doing, rather than engage in ad hominem comments. The issue is different types of colour correction, not "who" said something.

That said - and addressing the matter of looking through telescopes - Gord has already pointed to some observing experience by others that has been described on another thread here.

I'd add the discussion a couple of years ago, when Clive Gibbons detailed his experience of getting superior performance with his apo refractor by using a prism diagonal. He had previously suggested this, and provided some further details on the improvement it made. Fits Gord's latest comment re Valery's suggestion in this regard.

This was unsurprising to some of us because we'd heard details of colour correction in a lot of apo refractors being optimised for CCD imaging, not for visual use. Which meant there was a difference. So it was interesting to see Dick Suiter, in the 2nd edition of his "Star Testing" book, describing different forms of colour correction in section 12.4. THere he briefly describes,and gives "color focal-shift curves" in a diagram, for a number of 6-inch f/10 instruments.

As he remarks, there are differences to the range of wavelengths within the apochromatic range, according to design. And preferences for which design will work best will vary with the intended use.

Locally, in the Southern Hemisphere, I spend a fair bit of time looking through telescopes. Regrettably, the only Istar R30 I know of in my region is not yet on a mounting (that's being built). I'll look forward to seeing through it when it is set up, and discover whether it's better for doubles and less good for Jupiter, or great for both but with reduced blue halo, or needs a chromacorr (or equivalent) to give of its best.

Meanwhile, I think Gord's most recent note raises some good questions, which should be treated as questions, not answers (the latter approach leads to dismissive comments). I'll now put on my flame-proof jacket.


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: fred1871]
      #5603643 - 01/03/13 08:13 PM

Thanks Fred! I had forgot about Clive's investigations. I'll have to do some searching, I now do recall a lot of discussions about prisms.

I really need to get that Suiter book...

Clear skies,


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ISTAR Optical
Vendor- IStar Optical
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Reged: 08/09/09

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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5606033 - 01/05/13 05:07 AM

Dear Gord,
size of the Classic Achro blur is approx. 100 while size of the R30 is approx. 40. Im no longer sure if we are looking at same diagrams here. Im leaving Europe next week, very busy, but once I have more time upon my arrival from US and Asia I will post some detailed info on this subject. Thank you for your patience,
cheers,
Ales


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: ISTAR Optical]
      #5606346 - 01/05/13 11:11 AM

Quote:

Dear Gord,
size of the Classic Achro blur is approx. 100 while size of the R30 is approx. 40. Im no longer sure if we are looking at same diagrams here. Im leaving Europe next week, very busy, but once I have more time upon my arrival from US and Asia I will post some detailed info on this subject. Thank you for your patience,




Hi Ales,

Thank you for taking the time to answer questions here. I've gone back and looked at the images some more and I think I understand the numbers you are referring to (micron size), but they don't seem to agree with the diagrams. I wanted to compare the regular achromat to the R30, and the only one I could find where both were shown is the 6" F8's shown here:

IStar forum 6" F8 spot comparisons

So, there are a few things that don't match between the diagrams and the numbers you quote above. For example, the airy disk size shown on each graph is different, but the scale ruler is shown as 40micron on each graph. The airy disks will be the same for each scope since they are both 6" F8, so one of the above numbers is incorrect (should be either 50 for the achro, or 80 for the R30. Or both could be off and the ruler just inaccurate.

Either way though, the airy disk on the R30 diagram is half the size of the one for the achro, so the scale of that blur diagram is actually twice the size of what it is showing relative to the achro diagram. Both have airy disks of around 10 micron.

The other part that I don't understand is the diagram for the achro. It appears to be showing the red extremely tightly focused and the blue extremely defocused. In fact, the red is as tight at the green! According the the discussion we have been having, if the red/blue focus points are left to be that far apart, then the blue is being let go farther than it should be relative to red. In this case, the overall blue defocus is what is making up the blur you are quoting as the blur size for the achro.

If I compare it to a more balanced spot diagram as shown in the link earlier by Robert Royce, I see the spot diagram is much tighter for the achro he is showing, and actually about the same amount as you show for the R30.

It appears you are considering the blur size measurement to be the maximum defocus value in microns for either the red or blue defocus, whichever is worse. Is that correct?

I think it's a good measure from what I have seen based on the discussions here.

Looking forward to your clarifications.

Thanks,

Edited by Gord (01/05/13 11:18 AM)


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5606360 - 01/05/13 11:16 AM

Hi Ales,

Forgot to add; the easiest way to show what you are trying to describe is the LA graphs and matrix spot diagrams for the corresponding designs similar to what Markus has done for this APM offerings. These show exactly the performance differences between the designs.

Could you provide this information?

Thanks!


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Napersky
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5615659 - 01/10/13 02:41 PM

Gord,

Roland says it best:

http://geogdata.csun.edu/~voltaire/roland/color.html

Fast 6"F8 Cde achromat: 550 - 650 nm
Long 6"F15 CeF achomat: 480 - 650 nm
Fast 6"F9 ED doublet: 450 - 650 nm
Fast 6" fluorite doublet: 420 - 1000 nm
Fast 6" FPL52/53 triplet: 380 - 1000 nm
Fast 6" fluorite triplet: 360 - 1000nm

It would be interesting then to divide the cost of each lens by its
useful wavelength range. For instance, a 6"F8 Cde achromat selling
for around $800 today would come in at $10 per nanometer. (our 6"
EDFS at $4900 comes in at $7.90 per nanometer). Interestingly, an 8"
SCT selling for around $900 comes in at $3.81 per nanometer. No fair
asking how a Newtonian would fare!

Seriously, why would you need correction well into the blue-violet
past 480nm? With black and white emulsions, this was necessary
because they have considerable sensitivity down to 380nm. Today's new
blue sensitive CCD cameras also need good correction in the violet.
Also, CCD cameras pick up lots of IR light below 650nm, so correction
to 1000nm is a distinct advantage. For pure visual use, it would be
quite sufficient if the useable range extended only from 440 to 650
nm. So, check the above table for your particular needs and make your
choice.


I noticed your list. Find a way to test the telescopes and find their spectrum.Then you can pigeon hole it in Roland's list.

2 Cents


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fred1871
scholastic sledgehammer


Reged: 03/22/09

Loc: Australia
Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5657060 - 02/01/13 08:03 PM

Gord, did we ever get a reply/response from Ales about the particulars of the Istar optical designs, that would match the kind of information you mention Markus providing for APM designs?

I and I'm sure a lot of others would like to know more about the Istar scopes and how their optical design differs/compares with other offerings. I don't think I've bought my last refractor yet.


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