218 replies to this topic

[KWB]

From this reference, https://refractivein...-FPL&page=FPL53:

 

Hoya FCD100: Abbe number 95.10, refractive index 1.4370

Ohara FPL-53: Abbe number 94.96, refractive index 1.4387

 

So close enough, I guess.

Scott

 

I'd say six one way or a half a dozen the other way.

 

Take your pick. I'd hate to have to live off the difference and also would be reluctant to bet that I could see a difference if the aperture and focal lengths of the telescopes were identical.

[Scott in NC]

I agree that you or I wouldn’t see a difference, Kenny.  I was merely wondering whether an optician can use lenses made from these two glasses interchangeably.  But I guess it looks like the answer is probably yes.

[Adluginb]

Here’s is how Vic discusses it.

 

https://www.stellarv...al-glass-types/

 

From what I read, besides how it’s figured which plays a big deal, he just uses what he can get a better price on.  Meaning to me, there is no difference.  Still it would be nice for someone with way more experience than me (Scott in NC) to review one:)
 

Edited by Adluginb, 03 January 2023 - 11:07 PM.

[Jon Isaacs]

So,not to be persnickety, but if TS Optics uses different glass than Astro-Tech, they are not different versions of the same scope….just similar scopes.

 

These both seem to be the same scope as the StellarVue 80 Access. Originally the 80 mm Access was based on FPL-53 but during production, it was switched to FCD-100.

 

For the owner, I believe it would be possible to distinguish between them. As Echolight pointed out, the TS Optics website may just need an update.

 

Jon

[Jon Isaacs]

I agree that you or I wouldn’t see a difference, Kenny.  I was merely wondering whether an optician can use lenses made from these two glasses interchangeably.  But I guess it looks like the answer is probably yes.

I think the answer is no, the numbers are very similar but not identical..  and I believe these are not the only numbers used in designing an objective.

 

Jon

[SilverLitz]

I would not choose between FCD-100 and FPL-53 based on glass alone, as one is not inherently better than the other.  It comes down to mating elements and optician skill.  The designer will also consider availability, consistency, and easy of machining.

 

Astro-Physics chose FCD-100 for their latest Stowaway and seems to also be using it on the upcoming 110GTX.  Before announcing the 110GTX, Roland posted some designs using FCD-100 and FPL-53, and the FCD-100 design was slightly better.

[GSansoucie]

I’m not interested in parting with my TV85 as it’s going to be carted all over North America for the next few years as we fulltime RV.  However, I really wish I had that rail handle that the AT80 has.  

[Lagrange]

Maybe someone with more knowledge of optics than I have could comment on whether FCD100 and FPL-53 are truly interchangeable, or whether a different mating element would also be required.  Even if the Abbe numbers are nearly the same, I can’t imagine that the refractive indices are the same as well.  And if the refractive indices are different, then it seems as though the curves for the both elements would need to be ground a bit differently.

I was looking for some info about this stuff and found the following interesting quote from Roland Christen on the A-P Google Groups:

 

"There seems to be a never ending discussion about the quality and suitability of these various glasses to make a highly corrected triplet lens. There is a common myth about FPL53 that somehow it has higher performance. That is actually not true. The high performance of each of these SD glasses depends almost entirely on the mating element which must be matched in Partial Dispersion to the ED glass. Ohara has a matching set of crown glasses that produces the highest performance level with S-FPL53, but if you use them with Hoya FCD100, then the performance is not ideal. Hoya has a matching set of crown glasses that produces identical performance as the Ohara S-FPL53 set."

 

Essentially they're not interchangeable and the design would need to be tweaked to accommodate the different properties of the three glass types in each lens.

 

I suspect that either TS-Optics had bought their scopes in a batch that were made before the switch to FCD100, or it's like Echolight suggested that the spec info is out of date.

 

 

I would not choose between FCD-100 and FPL-53 based on glass alone, as one is not inherently better than the other.  It comes down to mating elements and optician skill.  The designer will also consider availability, consistency, and easy of machining.

 

Astro-Physics chose FCD-100 for their latest Stowaway and seems to also be using it on the upcoming 110GTX.  Before announcing the 110GTX, Roland posted some designs using FCD-100 and FPL-53, and the FCD-100 design was slightly better.

Roland seems quite keen on FCD100. The availability of well-matched mating glasses is a big reason as the quote above mentions, but it also sounds like it's a relatively easy glass to work with.

[Scott in NC]

I highly doubt any member here has both these telescopes and can lend an opinion as to the comparison of the optical quality.

Kenny, you were probably correct at the time that you made that statement, but I’ve rectified that deficiency, lol.  Here’s a pic to show what I had fun experimenting with tonight.  More details to come later…

 

[KWB]

I figured you would be the first to step up and do the job. 

 

Any preliminary opinion as to an optical comparison, Scott? 

[Scott in NC]

I don’t have time to do a detailed write-up like I did a few months ago with the AT125EDL vs. FS-128 comparison, but I did get a chance to spend about two hours outside tonight comparing the AT80EDL and the TV85.  The temperature was 42°F, humidity 59%, and seeing was mediocre.  Not bad, not good, just average for our area.  New moon will be tomorrow, so the skies were dark.  I had both scopes set up side by side, with the AT80EDL on a UA Unistar Deluxe mount and UA heavy wooden surveyor’s tripod, and the TV85 on a DM4 and Berlebach Uni-18 tripod.  Eyepieces used were Pentax XWs 7-30mm, and I had AP Maxbright diagonals in both scopes.  Targets viewed were Mars, Jupiter, Rigel, and a number of Messier objects.

 

In summary, the TeleVue 85 has the “look” of a more expensive, heirloom quality scope, yet the AT80EDL still has very nice build quality, and really nothing that I could find fault with.  I really like its textured powder coated finish, similar to that on my former AT125EDL and TEC140. The AT actually has the nicer focuser of the two, with absolutely no detectable backlash or image shift, and has the precision feel of a FeatherTouch.  Don’t ask me how they can sell a scope for this price with such a nice focuser, as a 2.5” FT R&P costs more than this entire scope, once you add the correct adapter and endcap.  While viewing brighter objects, the AT might have had slightly less chromatic aberration than the TV.  I say might because they were really very close, and it was difficult to say with 100% certainty.  Despite this, the TV produced a little better star test IMO, with images inside and outside of focus more equivalent to one another, whereas the AT’s intra- and extra-focal images were more well defined on one side of focus, and a little blurrier on the other side (I forgot which side was which). The TV might possibly have produced slightly sharper views of Mars and Jupiter, and I suspect that that was most likely related to the slightly higher magnification when using the same eyepieces (7% longer focal length with the TV) and slightly more light-gathering ability (13% greater with the TV). Rigel was an easy split with both scopes.  I couldn’t see the E star in the Trapezium in M42 with either scope, but that’s not unexpected with 3”-class refractors.

 

The bottom line is that I like both scopes, and plan to keep them both for a while.  I find the TV more aesthetically pleasing, and maybe very slightly optically sharper despite possibly having very slightly more CA. The AT has the better focuser of the two.  For those of you who can’t justify the cost of a TeleVue 85 and decide to get an AT80EDL instead, you definitely don’t have to feel that you’re settling.  The two scopes are really so close that IMO the AT represents a much better value.  For what you get, the AT is probably underpriced (at its current sale price of $699), and the TV is probably way overpriced.  This is just my personal opinion though, and if others feel differently then I won’t quibble about it, as value determinations are always highly subjective and personal.

 

And for those of you who are wondering why I would have purchased both of these, when they’re so similar, the answer is that I really enjoy trying out new gear and doing such comparison testing.  Thank goodness that we’ve got a thriving used market, because I couldn’t afford to buy all of this stuff new!

 

 

[KWB]

Scott

 

 Bravo on this brief comparison! 

 

I'm so confident I made such a wise choice in choosing the EDL that I'm not going to wait until the Spring to do a comparison to my Orion ED80. That scope is a goner.

 

I'm following in your footsteps as I contemplating seriously in purchasing the regular AT80ED/Fk-61 to replace my ST80 as a low power sweeper and sweep that from my scope inventory. $369 is hard to pass up. My fourth telescope purchase in 6 months.

[Scott in NC]

Kenny, the FK61-based AT80ED isn’t quite as well color corrected as the AT80EDL, but it’s really not bad at all (and still far better than an f/7 achromat would be), and for your intended use it’s quite a bargain at the current sale price of $369.  I think you’d be very pleased by it. I paid even more than that for my former AT80ED when I purchased it used, but that was when the new price was $429. And no, I didn’t think for once that I overpaid, even after the scope went on sale.  It did mean that I took a bit of a hit on the resale value when I ultimately resold it, but no big deal really.  I consider that money to have been a cheap “rental fee” to allow me to try it out for a few months.

 

But I’m curious as to why you want to get an AT80ED to add to the AT80EDL that you already have, unless perhaps there are times when you want to set both scopes up simultaneously.  And I know, I’m not really one to criticize other people’s scope purchase decisions, as I’ve certainly purchased multiple redundant scopes over the years.  So I’m not criticizing, mainly just curious.

[wrvond]

<snip>The AT actually has the nicer focuser of the two, with absolutely no detectable backlash or image shift, and has the precision feel of a FeatherTouch.  Don’t ask me how they can sell a scope for this price with such a nice focuser, as a 2.5” FT R&P costs more than this entire scope, once you add the correct adapter and endcap. <snip>

I suspect the difference is the Feather Touch is milled from a solid block of aluminum while the AT is (I'm guessing) forged aluminum that is then milled to spec.

[KWB]

Scott

 

The EDL has maxed out my Twilight I mount and now I really need an upgraded mount, both for load capacity and smoothness of operation. I haven't decided which direction I'm heading(maybe GOTO?) but I want one of higher quality so that is going to be a significant expense and that will have to wait. I'm not going to be so concerned about ultra portability.

 

I don't believe in a "beater" scope philosophy, but this inexpensive scope is noticeably lighter in weight so my existing mount will work just fine. I will not be so protective about using it and I intend to use it a lot at lower powers. For me this setup is more about grab and go. It is intended to be a direct replacement of my ST80. I have the $369 tonight.

 

You are the perfect person to weigh in as to an opinion on my rationale. Maybe I'm all wet here. This scope will be on sale for a while so maybe I'll come to my senses and not purchase it.

[Scott in NC]

Sounds very reasonable to me, Kenny!  For me, the AT80EDL is my lightweight 80mm scope, and should work fine on all of my mounts.  I’d have to weigh both to be certain, but I’m reasonably sure that it even weighs a little less than my SV Nighthawk 80mm f/6 achromat.

[wrvond]

Scott

 

The EDL has maxed out my Twilight I mount and now I really need an upgraded mount, both for load capacity and smoothness of operation. I haven't decided which direction I'm heading(maybe GOTO?) but I want one of higher quality so that is going to be a significant expense and that will have to wait. I'm not going to be so concerned about ultra portability.

 

I don't believe in a "beater" scope philosophy, but this inexpensive scope is noticeably lighter in weight so my existing mount will work just fine. I will not be so protective about using it and I intend to use it a lot at lower powers. For me this setup is more about grab and go. It is intended to be a direct replacement of my ST80. I have the $369 tonight.

 

You are the perfect person to weigh in as to an opinion on my rationale. Maybe I'm all wet here. This scope will be on sale for a while so maybe I'll come to my senses and not purchase it.

I started with a TW1 under my 80EDL then went to a CG-4 with polar scope and dual axis motors. I would have been perfectly happy with that but a deal came up on a Losmandy G-8s and I couldn’t resist (I tried). 
So if you do buy it, I know where you can get a great mount for it!  

[KWB]

I started with a TW1 under my 80EDL then went to a CG-4 with polar scope and dual axis motors. I would have been perfectly happy with that but a deal came up on a Losmandy G-8s and I couldn’t resist (I tried). 
So if you do buy it, I know where you can get a great mount for it!  

I think my Twilight I mount will be just fine for the 6 pound AT80ED. You were correct, it ain't enough mount for the EDL.

 

I wanted the DSV-2B for the EDL. That isn't panning out. That Losmandy mount is too much mount and too expensive for me. 

 

You are bound and determined to sell me one of your mounts, my friend. 

[RAKing]

I started with a TW1 under my 80EDL then went to a CG-4 with polar scope and dual axis motors. I would have been perfectly happy with that but a deal came up on a Losmandy G-8s and I couldn’t resist (I tried). 
So if you do buy it, I know where you can get a great mount for it!  

 

The Losmandy GM-8 doesn't even know there is a scope on top!  My old GM-8 actually carried a TMB 130SS with no problems.

 

Great review, Scott!  I always wanted to try a TV 85, but never could swallow the sticker price for something I wouldn't use very much.  I am satisfied with my little A-T 80ED (the inexpensive one!).

 

Ron

[Scott in NC]

So what do I do now?  I’ve satisfied myself that the AT80EDL and TV85 are both nice ED doublets, and don’t really want to sell either one.  If I could only keep one, it might be the TV85, given its uniqueness and near heirloom quality.  But the more sensible approach would be to keep the AT80EDL, which is really just as good as an observing tool yet much less expensive.  That extra money in my pocket from the sale of the TV85 could be put toward other astro goodies.  But ultimately I’ll probably solve this one the way I typically solve such dilemmas, and keep both scopes.

[KWB]

So what do I do now?  I’ve satisfied myself that the AT80EDL and TV85 are both nice ED doublets, and don’t really want to sell either one.  If I could only keep one, it might be the TV85, given its uniqueness and near heirloom quality.  But the more sensible approach would be to keep the AT80EDL, which is really just as good as an observing tool yet much less expensive.  That extra money in my pocket from the sale of the TV85 could be put toward other astro goodies.  But ultimately I’ll probably solve this one the way I typically solve such dilemmas, and keep both scopes.

Nothing wrong with keeping both telescopes and that would be my approach. Provided the cost of the EDL will not make or break me, as in the next month grocery bill. Or gas or insurance for my vehicle. Or a house payment that in 2023 that might require the cash from the sale of the TV-85. Real life comes first.

 

Short or none of the above, I keep both telescopes. Maybe I chop some dead weight from elsewhere in my collection. These are 2 quality telescopes I keep to the last.

[KWB]

Scott

 

As a follow up at present I possess 3 80mm telescopes, though I don't have anywhere near the financial commit invested in them as you do with these two. I don't need to sell any of these three, for no more than what any of them would bring to make ends meet. I can afford to sit on them. I don't need 3 80mm telescopes, either.

 

To be totally fair maybe you can't and that is totally understandable. Putting braces on a child's teeth comes first or any other real life priority. If you should have to sell or even want to sell one or the other, I'd never think any negative about you, my friend. 

[lookoutmtn17]

The TV-85 is the only scope I have regretted selling. It is a work of art, built like a tank, and offers great views for an 85mm scope. The AT's are awesome scopes, I own three, but the 85 is a scope you can pass down for generations.

[Adluginb]

Why can’t the AT scope be passed down for generations?  Do they fall apart at a set time down the road?

Edited by Adluginb, 21 January 2023 - 10:06 PM.

[KWB]

Why can’t the AT scope be passed down for generations?  Do they fall apart at a set time down the road?

I'll say this; this 80mm is the finest built telescope I've ever owned. If properly cared for and it will be, it will out live me. The focuser feels like it could last for 50 years easily enough.