77 replies to this topic

[peleuba]

Over the last couple of weeks I've had the privilege of testing a few scopes from CN members and elsewhere.    

 

Here is a really nice, well corrected, very smooth TeleVue 85.  I think, over the last  decade, the TeleVue doublets have taken a back seat as China now owns this part of the market.   I've  tested, perhaps, two dozen TeleVue OTA's and I've never seen a dog.  The quality ranges from good to superb.  Early on, the quality of the imports varied widely, but now they're very consistent.  Great news if your interested in offerings at this end of aperture ladder.   

 

At the same time, I also had an NP101 in for testing.  And, for me, I really like the build quality of the TeleVue OTA's -  simple, understated, robust.  You'll never see red (or yellow) anodized trim on a TeleVue.

 

I tested at three wavelengths plus white.  I also did a high power star test.  There is not a whole lot to comment on, just another great TV OTA that should become part of the repository of tests here on CN.

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Edited by peleuba, 01 October 2024 - 11:11 AM.

[peleuba]

Blue

 

 

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[peleuba]

Red

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[peleuba]

White and Star test

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[25585]

My TV fracs are keepers, a green and a brass 85, a brass 102 and ivory Genesis.  shame about their eyepieces.

[Highburymark]

Over the last couple of weeks I've had the privilege of testing a few scopes from CN members and elsewhere.

Here is a really nice, well corrected, very smooth TeleVue 85. I think, over the last decade, the TeleVue doublets have taken a back seat as China now owns this part of the market. I've tested, perhaps, two dozen TeleVue OTA's and I've never seen a dog. The quality ranges from good to superb. Early on, the quality of the imports varied widely, but now they're very consistent. Great news if your interested in offerings at this end of aperture ladder.

At the same time, I also had an NP101 in for testing. And, for me, I really like the build quality of the TeleVue OTA's - simple, understated, robust. You'll never see red (or yellow) anodized trim on a TeleVue.

I tested at three wavelengths plus white. I also did a high power star test. There is not a whole lot to comment on, just another great TV OTA that should become part of the repository of tests here on CN.

Nice work - I suspect your tested example is almost identical optically to my 85. And in-focus CA is not an issue.

[peleuba]

Nice work - I suspect your tested example is almost identical optically to my 85. And in-focus CA is not an issue.

 

Thanks.  Yes, it's likely to be similar.  And, yes, the color correction is fine in focus.

 

The only "flaw" in most well made ED doublets like this is they are aperture limited.

Edited by peleuba, 01 October 2024 - 02:20 PM.

[Bill Barlow]

The picture of the star test, why is the in focal fresnel rings image mostly white while the out focal fresnel rings image is mostly purple?

 

Bill

[Jeff B]

Excellent spherical and color corrections.

 

Perhaps a bit of astigmatism?

 

Jeff

[alnitak22]

Thanks Paul…it looks very good and I’d wager my 23 yr old sample would test very similarly. I’m still delighted with mine and as you note, its only limitation is aperture. But it will certainly show everything well that an 85mm scope can show. 

[peleuba]

Excellent spherical and color corrections.

 

Perhaps a bit of astigmatism?

 

 

Yes a bit.  But, oddly its not apparent outside.  I think it may have to do with the horizontal nature of my test bench, the vertical orientation of the lens and the rather fragile (from a collimation standpoint) lens cell.   A TV76 I tested showed the same.  But this at a very low level.  After modeling in Aberrator, my sense is its between ⅛ and a ⅒ wave.  This was at ~240x using a 2.5mm NT6 eyepiece on an optical collimator.

[Erik Bakker]

The overall correction of this lens looks very nice, even with a tiny bit of astigmatism. Was a bit surprised at the level of CA in the images of the star test.

[peleuba]

The picture of the star test, why is the in focal fresnel rings image mostly white while the out focal fresnel rings image is mostly purple?

 

 

Its not mostly white.   There is a ton of color that contributes to the haziness within the confines of the outer ring.  The violet is noticeable because its visible well away from the Fresnal pattern.  Using a green filter cleans it up a bit.   

 

I wonder if the glass types have changed over the years in these TV doublets?  TeleVue never announces changes to stuff like this.  As an example the coating on their eyepieces - like the NT6's - have evolved over the years.  Anyway, I used to think it was FPL53, but now I am not so sure...   That ED glass is not currently available in large enough blanks for telescope objectives.     A-P has changed the glass type in the 130GT vs the 130GTX so its not without precedent.  Glass companies - especially Ohara - don't care about us and our scopes.  The semiconductor, camera, and healthcare industries drive their production.

Edited by peleuba, 01 October 2024 - 04:08 PM.

[Polyphemos]

Hey Paul. In the past common wisdom has been that the defocused image in a star test should be consistent for both in and out defocus. More recently it’s been noted by Roland and others that this isn’t the case for ED refractors, and this phenomenon is illustrated by the the in-out defocused star test images in your #4 post.

 

Can you comment on why ED refractors behave differently, and how someone could use the in-out defocus images from a star test to evaluate the objectives quality and performance?

[peleuba]

Can you comment on why ED refractors behave differently, and how someone could use the in-out defocus images from a star test to evaluate the objectives quality and performance?

 

One word - spherochromatism.

 

To be fair, I have rarely seen a star test in a refractor that is precisely the same on either side of focus primarily due to the chromatic effects inherent in a lens based telescope.  The problem is particularly acute in ED doublets faster then F/10.  Optimal glass combos help, but they do not solve the problem.  Even the mighty Fluorite will have some residual chromatism.

[Scott in NC]

…just another great TV OTA that should become part of the repository of tests here on CN.

Nice job, Paul!

 

I added it to the list.

[Polyphemos]

One word - spherochromatism.

 

To be fair, I have rarely seen a star test in a refractor that is precisely the same on either side of focus primarily due to the chromatic effects inherent in a lens based telescope.  The problem is particularly acute in ED doublets faster then F/10.  Optimal glass combos help, but they do not solve the problem.  Even the mighty Fluorite will have some residual chromatism.

Thanks Paul, now I understand. I hadn’t made the connection between ED scopes being generally much faster than their relatively long crown and flint progenitors, and how that would make a marked difference in spherochromatism. Now it makes perfect sense.

[hyia]

Hey Paul. In the past common wisdom has been that the defocused image in a star test should be consistent for both in and out defocus. More recently it’s been noted by Roland and others that this isn’t the case for ED refractors, and this phenomenon is illustrated by the the in-out defocused star test images in your #4 post.

 

Can you comment on why ED refractors behave differently, and how someone could use the in-out defocus images from a star test to evaluate the objectives quality and performance?

What Paul said.

 

Suiter does have a section in the 2001 version of his book addressing this.  There are two items I specifically recall.  Roughly, one, he says that all the images in the book were calculated for monochromatic light.  i.e.  It works really well for mirrors.  Two, he recommends using filters.  I know that Eddgie has made a lot of posts on this as well, and he often recommends doing a variation with a 33% obstruction.

[GOLGO13]

Hey Paul. In the past common wisdom has been that the defocused image in a star test should be consistent for both in and out defocus. More recently it’s been noted by Roland and others that this isn’t the case for ED refractors, and this phenomenon is illustrated by the the in-out defocused star test images in your #4 post.

 

Can you comment on why ED refractors behave differently, and how someone could use the in-out defocus images from a star test to evaluate the objectives quality and performance?

The in-focus image is the most important when it comes to doublets. Sometimes the difference between inside and outside focus is on purpose with a doublet. It's hard to judge apo doublets in what I've seen. The in-focus image is the only thing that you can count on for assessing the quality.

[Highburymark]

One of interesting aspects of the TV85 is that the two objective lenses are quite far apart. A quick laser test shows this. And - of the star tests I’ve seen over the years - the rings in the defocused images are a little better defined in the 85 than most Chinese ED doublets.

[alnitak22]

The in-focus image is the most important when it comes to doublets. Sometimes the difference between inside and outside focus is on purpose with a doublet. It's hard to judge apo doublets in what I've seen. The in-focus image is the only thing that you can count on for assessing the quality.

Yes. Visually and in focus, the only star I see slight color on at 70x per inch in my TV85 is Sirius. Comparing over a number of years with a friend’s Tak FS78, the color seen in both scopes while in focus at high power on Sirius was very similar. OUT of focus, the Tak showed slightly less color. 

[peleuba]

One of interesting aspects of the TV85 is that the two objective lenses are quite far apart. A quick laser test shows this. And - of the star tests I’ve seen over the years - the rings in the defocused images are a little better defined in the 85 than most Chinese ED doublets.

 

Interesting.  I had not paid any attention to the lens spacing and I've been looking at (and through) different TV85's for two decades.  I will do this at some point this week.

 

Adjusting the spacing will adjust the spherical correction of the telescope.  But this is not without unintended consequences as the color correction will be affected, too.

 

I would love to see the design of these TV85's as the glass type has always intrigued me.  What was available for use 25 years ago is not available today.  

[Scott in NC]

 

I would love to see the design of these TV85's as the glass type has always intrigued me.  What was available for use 25 years ago is not available today.  

And this is a good reason why some manufacturers like TeleVue don’t typically disclose their glass type. When you’ve had a successful scope model for 20+ years it’s nice to be able to slightly tweak the lens prescription based upon available ED glasses without having to explain to the public why you’re switching from FPL-whatever to a different FPL-whatever, to FCD100, etc., when the lens performance is for all practical purposes unchanged.

[alnitak22]

And this is a good reason why some manufacturers like TeleVue don’t typically disclose their glass type. When you’ve had a successful scope model for 20+ years it’s nice to be able to slightly tweak the lens prescription based upon available ED glasses without having to explain to the public why you’re switching from FPL-whatever to a different FPL-whatever, to FCD100, etc., when the lens performance is for all practical purposes unchanged.

Agree. And btw, this year is the 25th anniversary of the introduction of the TV85. Pretty successful scope in the world of short shelf life gear!

[Jon Isaacs]

 

 

I would love to see the design of these TV85's as the glass type has always intrigued me.  What was available for use 25 years ago is not available today. 

 

 

Is FPL-53 not available at all? O'Hara still has the FPL-53 page available with what I assume to be a date of this year. 

 

https://oharacorp.com/glass/s-fpl53/

 

Jon