62 replies to this topic

[peleuba]

Earlier this week I received a TV76 to complete my collection of refractors up through a 130mm.  For me, anything larger (then 130mm) and it won't get used very often.  Anything smaller (then 76mm) and the views are unfulfilling.  This is a subjective limitation only valid for me - yours may be different.  But really if your using something less then 76mm of aperture, why spend the time setting up a tripod and mount?  Just use binoculars instead!

 

I currently own complete sets of Nagler T4, T5, T6, Delos and Delites a set of TV Plossls and a sprinking of Panoptics.  The only non-TV eyepieces that I own is a full set of Japanese Fujiyama Orthos.  But, I had not owned a TeleVue OTA in at least 12 years.  And, now, all TeleVue OTA's come standard with the 2-speed focuser - except the TV60 and the specialized FLI imaging OTA.  At TeleVue, fine focusing has evolved over the years from a home-brew design to now using the StarLight Instruments pinion mechanism.  TeleVue has done a good job incorporating the StarLight hardware into the look/feel/ruggedness of the OTA.

 

The Televue 76 is a stout observing package with the best build quality in its aperture class.  Optically, I have found TeleVue OTA's to be - overall - very good, but not  great with one single exception - my first NP101 which was phenomenal.

 

To me, the best thing about owning TeleVue is that you've bought into a system where all the accessories are designed to seamlessly (more/less) work with each other.  You never have to venture outside of the TeleVue ecosystem if you don't want to.

 

In any event, here is the TV76 on the bench in double pass autocollimation.  I did perform indoor star tests, too, but still need to stack and process the .avi into single frames.  But, to be clear, the star test will not tell you much more then the DPAC images do.

 

Comments and criticisms welcome.

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Edited by peleuba, 04 February 2020 - 02:01 PM.

[peleuba]

White, Green, Blue, Red

 

 

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[peleuba]

Different null views in white.  Notice the color correction...

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Edited by peleuba, 04 February 2020 - 01:40 PM.

[peleuba]

My impression is this optic is solidly in the "very good" category but is limited by its design -  specifically - being an all spherical doublet at F/6.3. 

 

My hunch is that this uses FPL53 as the ED glass.

 

I do want to point out that the lens surfaces are exceptionally smooth.

 

I intend to use it as an ultra-quick look telescope as well as a spotting scope for birds in my back 40.  

Edited by peleuba, 04 February 2020 - 01:44 PM.

[BCNGreyCat]

About the televue focuser, i have used their early one, starlight retrofit, and the new 2-speed made by starlight. It has vastly improved, but its image shift when adjusting focus seems to be inherited in the design. The drawtube has a bit tiny wobbling in the housing. Comparing to the focuser from AP and starlight, TV focuser is not up to top tier.

Regardless, the televue doublet is a sweet scope. That is the one really built to last and built like a tank. When you hiking in the mountain, you probably can use to fight back a bear and it still functions well.

Congrats to the TV76!

[Spikey131]

Did you get it new or used?

[25585]

You might be interested in this comparison with a Tak FS-78,  http://scopeviews.co.uk/TV76.htm

[Spikey131]

You might be interested in this comparison with a Tak FS-78,  http://scopeviews.co.uk/TV76.htm

In that review, he did not do any testing.  He just pulled some typical graphs of scopes “like” the TV76 and Tak, demonstrating the typical difference between a f/8 Fluorite optic and a faster f/6 non-Fluorite.

 

Interesting review, however.

[peleuba]

You might be interested in this comparison with a Tak FS-78,  http://scopeviews.co.uk/TV76.htm

 

Thank you for the link - I had never seen the website before.  But the quoted Strehl values in the graph are made up.  The author has absolutely no way to validate these.  Even saying "...strehl values of a lens like the TV76..." is irresponsible.  Moreover, the numbers are not even based on real world design parameters because the author does not know the precise prescription (and glass types) of the lens.  You can't make assumptions like this.

 

He's not breaking ANY new ground by saying that a long focus fluorite doublet will be better corrected then a short focal length ED doublet, rather, he is stating the obvious.

 

Posts like my occasional "Bench Test" series offer the reader more information then these types of websites. 

Edited by peleuba, 04 February 2020 - 04:10 PM.

[mikeDnight]

The comparison between the TV76 and the Tak FS78 may be a bit uneven. The FS series had a lower limit of F8 for a reason. Any shorter and it would lose its apochromatic status. Something to do with the Fraunhofer design I believe. So the FS would perform like an ED if it was less than F8, and perhaps much closer to the TV76.

[Tyson M]

Looks like a fantastic sample of the TV76.

The build quality and portability of this scope makes it a true gem I'd imagine, fitting on the smallest of mounts.

Thanks for sharing your results.

[Alan French]

"So the FS would perform like an ED if it was less than F8, and perhaps much closer to the TV76."

 

ED stands for Extra-low Dispersion and is applied to a variety of fluor-crown glasses and fluorite, materials that lie well off the Abbe line and are candidates for fluor-crown/crown doublets and triplets with far better color correction than traditional achromats. The performance of a telescope using ED material, either as a doublet or triplet, depends on the glass choices, the aperture, and the focal ratio.

 

Saying a telescope "performs like an ED" doesn't convey any useful information beyond saying it has better color correction than a classic flint and crown achromat. Even odder is the implication that the ED designation makes it a lesser instrument.

 

Clear skies, Alan 

Edited by Alan French, 04 February 2020 - 06:33 PM.

[Wildetelescope]

My impression is this optic is solidly in the "very good" category but is limited by its design -  specifically - being an all spherical doublet at F/6.3. 

 

My hunch is that this uses FPL53 as the ED glass.

 

I do want to point out that the lens surfaces are exceptionally smooth.

 

I intend to use it as an ultra-quick look telescope as well as a spotting scope for birds in my back 40.  

More or less the same assessment I would give my TV102 based on my experience in the field.  Very good, but an ED doublet.  point it at Sirius, Vega or Venus and you will see color.  Excellent and color free on planets and the moon with excellent contrast and detail.  Gives as nice a view as can be expected of a 4 inch scope for DSO's.  Can be used for imaging with an OSC camera, but you will see a little color on the brightest stars or if you blow the image up.  Great for how I use it and I would not trade it.  I like the original RP focuser on mine. 

 

JMD

[RadioAstronomer]

Thanks for sharing this test. I used to own a TV-76 and was very impressed with the optical (and obviously build) quality. Star test was on par with my current FOA-60. The focal ratio of f/6.3 however was a bit too fast for my taste.

[Jon Isaacs]

About the televue focuser, i have used their early one, starlight retrofit, and the new 2-speed made by starlight. It has vastly improved, but its image shift when adjusting focus seems to be inherited in the design. The drawtube has a bit tiny wobbling in the housing. Comparing to the focuser from AP and starlight, TV focuser is not up to top tier.

Regardless, the televue doublet is a sweet scope. That is the one really built to last and built like a tank. When you hiking in the mountain, you probably can use to fight back a bear and it still functions well.

Congrats to the TV76!

 

I think something's not quite right with that focuser and it needs adjustment. I've had two TV Focusers, neither exhibited an image shift.

 

Jon

[BCNGreyCat]

That’s exactly the same as people say about Tak’s focuser as well. Some says non-tolerate image shift, some says adjusting well then none at all.

I have owned several samples of Tak and TV scopes and dissembled/replaced quite a few of the focusers. They basically same design. In terms of image shift, inconsistency. Some okay, some bad. None of them has starlight or AP focuser quality.

[peleuba]

White, Green, Blue, Red

 

So, overall, this TV76 is smooth with some slight undercorrection at all 4 wavelengths.  Though, I am not sure that "white" qualifies as a wavelength since its a combination of many...  Nevertheless, I am a little perplexed.  Usually the blue and red correction is opposite sign and balanced with a perfect null in green.  

 

I have tested several Synta ED80's which is - most likely - a similar design slightly scaled to 80mm.  Each was perfectly nulled in green with over/under correction in red and blue; with the red and the blue having opposite sign correction - one undercorrected, the other overcorrected.  

 

Its too bad that TeleVue does not entertain technical questions on design/figuring/testing matters as this would be a "teaching moment"    for them and would help me better understand.

 

But, honestly, at only 76mm does it really matter?  This is not a high power planetary refractor.  Its aperture limited and although TeleVue publishes a max magnification of 200x, I don't think that's realistic.  Rather 120x (4mm eyepiece) would be where I would use it mostly.

[Jon Isaacs]

Paul:

 

I use my 80mm's at 160x-200x on the planet's and up to 300x on binary stars.

 

Probably more a sign of crappy eyes rather than excellent optics. 

 

Jon

[gjanke]

.  Its aperture limited and although TeleVue publishes a max magnification of 200x, I don't think that's realistic.  Rather 120x (4mm eyepiece) would be where I would use it mostly.

Paul,

 

First of all thanks for providing the bench test results. I can only speak for myself but I find each and every one you have done to be insightful and provides empirical data, no just opinion. So again thanks.

 

Having owned a number of TV76's and continue to own a TV76, I your find your statement spot on of 120 to 130x. Short of the moon it seems to be empty magnification if you go much higher than that. 

 

Again, you're right it about its limitations, it really doesn't matter cause its a grab and go scope along with being highly portable which makes it a winning design.

 

Gerald

[Erik Bakker]

Very high power observations are sometimes of use on the moon or close doubles. But most of the time, a (much) lower magnification shows more, especially with regard to low contrast or subtle color differences on say Jupiter. Of course, the higher quality a specific optic is, the easier it will shown a good image at high magnifications and more detail than lesser quality similar aperture scopes at lower magnifications. 

 

Even my excellent Astro-Physics 130mm showed the most detail on Jupiter around 110x-160x, less so at 195x. Similarly, a good 3" will show most to a keen eyed observer on low contrast subtle details around 70-100x, with an occasional bump to 150-200x on brighter higher contrast details. 

 

These days, high magnifications seem to be a qualifier for the quality of the optics used, but that is not necessarily the case.

 

If we want to bump up the high magnifications AND see more, we generally need to increase the aperture too.

 

Side-by-side, the quality of the image at a given high magnification IS a great indication when comparing similar aperture instruments, seeing permitting of course.

[Spikey131]

So, overall, this TV76 is smooth with some slight undercorrection at all 4 wavelengths.  Though, I am not sure that "white" qualifies as a wavelength since its a combination of many...  Nevertheless, I am a little perplexed.  Usually the blue and red correction is opposite sign and balanced with a perfect null in green.  

 

I have tested several Synta ED80's which is - most likely - a similar design slightly scaled to 80mm.  Each was perfectly nulled in green with over/under correction in red and blue; with the red and the blue having opposite sign correction - one undercorrected, the other overcorrected.  

 

Its too bad that TeleVue does not entertain technical questions on design/figuring/testing matters as this would be a "teaching moment"    for them and would help me better understand.

 

But, honestly, at only 76mm does it really matter?  This is not a high power planetary refractor.  Its aperture limited and although TeleVue publishes a max magnification of 200x, I don't think that's realistic.  Rather 120x (4mm eyepiece) would be where I would use it mostly.

I use mine @ 160x with the 3-6mm Nagler Zoom frequently.  Useful on doubles and solar system.  Above that, things get dim, but can still be useful on bright tight doubles like eta Orionis and Alrischa.

[peleuba]

I use mine @ 160x with the 3-6mm Nagler Zoom frequently.  Useful on doubles and solar system.  Above that, things get dim, but can still be useful on bright tight doubles like eta Orionis and Alrischa.

 

Thanks for the tip.  I have a few of the TV 3-6 Zooms; that and a 13mm NT6 will stay with the TV76.  Though, I doubt I will use it at 160x very much.  At this magnification I would rather have more aperture.  This will be an ultra-quick look and for daytime. 

[Eddgie]

IT appears to have a slight turned edge?    Nice though and very smooth optics.  

[peleuba]

IT appears to have a slight turned edge?    Nice though and very smooth optics.  

 

No - at least none of any consequence.  The star test shows a VERY smooth (slightly) undercorrected lens with no zones or polishing artifacts.  I think you are seeing the effects of diffraction at the very ends of the bands/edge of the lens.  In the null photos, I was hand holding the camera so there was some slight movement while snapping the pics this lead to an uneven illumination of the edge.  It only takes a fraction of a millimeter of movement, while taking the photos, to make the image look misleadingly bad.  I am confident that the edge is good.

 

See below for a true slightly turned edge in double pass.  The scope was an APM 152ED I owned.  To show you how sensitive DPAC is, the edge issue is less then 1/4 wave.  You can see it in an Inteferogram surface plot below.

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[Jeff B]

A cool thread and great work Paul, as usual.

 

Interesting and unusual result in that the lens has an overall undercorrection across the three colors.   Like you said with faster ED designs I'm more used to seeing undercorrection in blue and a mild overcorrection in red with more of a null in green/yellow.  Boy, it certainly is smooth too.  A nice lens.

 

I was surprised myself at a couple of recent achromats, in particular, an 8" F9 Istar achromat, which displays overcorrection in all three colors with the worst being in blue.  Stopping the lens to 7" provides for a modest improvement, but it is still a bit overcorrected overall.

 

Good stuff and thanks for posting this.  

 

I love this stuff!

 

Jeff