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2" vs 1.25" diagonal

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#1 eyeoftexas

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Posted 13 March 2020 - 02:33 PM

I'm in the market for a new diagonal for my AT115EDT.  I use both 2" and 1.25" eyepieces.  I was wondering if there is any advantage to having dedicated diagonals for specific sized eyepieces (i.e., a 1.25" diagonal for 1.25" eyepieces), compared to just using a 2" diagonal with an 1.25" adaptor.  I'm not thinking about types or brands here, just the size.



#2 Neptune

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Posted 13 March 2020 - 02:43 PM

No, go with the 2" diagonal. You will more than likely get a better image with the larger size in your 1-1/4" eyepieces. Diagonal quality tends to fall off towards the outer edge.


Edited by Neptune, 13 March 2020 - 04:04 PM.

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#3 sg6

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Posted 13 March 2020 - 03:42 PM

A 2" is a better idea, and looks a lot better on a scope, and your is big enough.

As it is a 115 then I assume enough focuser travel inwards to take up the additional optical path of a 2" diagonal.

 

The Skywatcher 72ED was too close. It was literally rammed focuser into the OTA to just get it focused, and that was targetting a roof line about 200 feet or so away, maybe a little more.

 

Since an infinite target would move the image in more then I took the 2"-1.25" adaptor and had 2.5mm machined off. Should now just manage it a little easier but somewhat uncomfortably close. Think I should have had 3mm.

 

I would expect AT to have a  better specified focuser however.

 

Either Skywatcher are very lucky in that it literally just makes it or someone did a very good job calculating the mechanics. And I suspect luck.


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#4 Spikey131

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Posted 13 March 2020 - 04:38 PM

The only advantage of a 1.25 is that it uses less back focus.  This could be an issue with binoviewers in some scopes.



#5 YAOG

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Posted 13 March 2020 - 05:13 PM

I'm in the market for a new diagonal for my AT115EDT.  I use both 2" and 1.25" eyepieces.  I was wondering if there is any advantage to having dedicated diagonals for specific sized eyepieces (i.e., a 1.25" diagonal for 1.25" eyepieces), compared to just using a 2" diagonal with an 1.25" adaptor.  I'm not thinking about types or brands here, just the size.

Hi!

 

Nope, if you have both 1.25" and 2" eyepieces use a 2" mirror diagonal. As I recall the Astro-Tech 115mm is an f/7 scope so there is not a good chance using a prism will be a benefit to the views as it may cause you some aberrations in your moderately fast scope. A prism might improve the views but you have to try it to be sure, f/7 is the braking point, some scopes it helps and some it hurts. Slower scopes like f/9 or f/10 can be improved and the lower scatter of a prism improves contrast. My advice is to find a friend with a high-end prism and try it out for a night and see if the prism improves or hurts CA and/or astigmatism. Chances are you will get better results from a top shelf mirror diagonal.    


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#6 eyeoftexas

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Posted 13 March 2020 - 06:35 PM

Thanks to everyone's comments and suggestions.  It hadn't dawned on me about back focus issues.  I have an AT 2" diagonal now, and there are no issues with backfocus, so I stick to that size.

 

Clear Skies!


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#7 Kunama

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Posted 13 March 2020 - 09:47 PM

Thanks to everyone's comments and suggestions.  It hadn't dawned on me about back focus issues.  I have an AT 2" diagonal now, and there are no issues with backfocus, so I stick to that size.

 

Clear Skies!

Is there a reason you are changing from the AT diagonal?  What are you hoping to gain from the new one?


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#8 eyeoftexas

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Posted 13 March 2020 - 11:07 PM

The tube holder on my AT diagonal has come lose. When I twist the lock the whole tube rotates. It doesn’t seem to be a twist off piece because it just turns and turns. It takes a contortion to tighten the eyepiece in. It just is annoying. No other reason. Hence, I was just wondering about a replacement and thought about why some are 2” and some 1.25”.
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#9 Kunama

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Posted 14 March 2020 - 12:32 AM

The tube holder on my AT diagonal has come lose. When I twist the lock the whole tube rotates. It doesn’t seem to be a twist off piece because it just turns and turns. It takes a contortion to tighten the eyepiece in. It just is annoying. No other reason. Hence, I was just wondering about a replacement and thought about why some are 2” and some 1.25”.

 

That would annoy me too!

 

I use several versions of Baader diagonals, I have the 2" BBHS mirror, 2" Dielectric Mirror and the T2 Maxbright for binoviewer, I have never had any problems with Baader diagonals.



#10 YAOG

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Posted 14 March 2020 - 01:11 AM

The tube holder on my AT diagonal has come lose. When I twist the lock the whole tube rotates. It doesn’t seem to be a twist off piece because it just turns and turns. It takes a contortion to tighten the eyepiece in. It just is annoying. No other reason. Hence, I was just wondering about a replacement and thought about why some are 2” and some 1.25”.

My advice is to buy a diagonal that matches the quality of the other optics. If this is an older AT115mm FPL-53 triplet as I think it probably is I suggest a top quality replacement if your eyepieces are any good, TeleVue AP and Baader. You already know at least one of the problems budget diagonals have.

 

When you buy top shelf optics with care they will last multiple telescopes, same as buying pro class lenses. My first Canon EF 300mm f/2.8L lasted me 20 years before I had to sell it because Canon CPS could no longer repair them for lack of parts. That was the sharpest, fastest focusing 300mm f/2.8 SLR lens on the planet for 20 years period. What was most impressive after the sharpness and focusing speed was the lack of lateral color across the full 35mm frame. The later IS lenses didn't beat it until the second generation IS lenses came out. I would still be using it if Canon could keep fixing them, that nominally 125mm objective lens was optically an astounding complex lens with both a front Fluorite element and a super-ED len element, hmmm, I wonder where Takahashi got the idea for the TOA lenses from? This lens came out in like 1987 so maybe 15 years before the TOAs were introduced? Heavy but so sharp you could pick out the driver's eyebrows in a CART car at 160MPH on the Long Beach street circuit from 8,000dpi drum scanned chromes. That was the best $4,600 I ever spent on glass. Those were the good old days, I could afford to spend it!

 

Take advantage of some ancient wisdom - buy once cry once. 


Edited by YAOG, 14 March 2020 - 01:24 AM.


#11 eyeoftexas

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Posted 14 March 2020 - 07:21 AM

The AT diagonal I have came with the scope, so it's not like I went for a cheap one.  A Baader BBHS or TV diagonal is definitely at the top of my wish list.  I've just not had the budget yet to move up.



#12 Jon Isaacs

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Posted 14 March 2020 - 08:36 AM

The tube holder on my AT diagonal has come lose. When I twist the lock the whole tube rotates. It doesn’t seem to be a twist off piece because it just turns and turns. It takes a contortion to tighten the eyepiece in. It just is annoying. No other reason. Hence, I was just wondering about a replacement and thought about why some are 2” and some 1.25”.

 

You can probably fix that quite easily.

 

Normally, AT diagonals have plate on either side of the diagonal. Hidden underneath the plates are some small setscrews that keep the eyepiece holder from rotating.

 

Removing the side plates and tightening the set screws should solve the problem.

 

Jon


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#13 eyeoftexas

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Posted 14 March 2020 - 10:23 AM

Jon,

 

Many thanks!  That's exactly what was wrong. bow.gif  Two set screws on either side of the holder, all of which had loosened.  It's now back to normal.  A high-end diagonal remains on my wishlist, but less of a priority now.

 

Clear Skies to all! 


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#14 Jon Isaacs

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Posted 14 March 2020 - 11:03 AM

Jon,

 

Many thanks!  That's exactly what was wrong. bow.gif  Two set screws on either side of the holder, all of which had loosened.  It's now back to normal.  A high-end diagonal remains on my wishlist, but less of a priority now.

 

Clear Skies to all! 

 

Excellent.

 

Your Astro-Tech diagonal is a very good one.

 

I have five 2 inch TeleVue diagonals and when I have had other diagonals like the AT, I could never see a difference. I do like the one piece body of the TV diagonals, the top can never come loose but optically, no difference I could ever see.

 

Jon


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#15 YAOG

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Posted 18 March 2020 - 03:11 PM

Excellent.

 

Your Astro-Tech diagonal is a very good one.

 

I have five 2 inch TeleVue diagonals and when I have had other diagonals like the AT, I could never see a difference. I do like the one piece body of the TV diagonals, the top can never come loose but optically, no difference I could ever see.

 

Jon

The Astro-Tech diagonals are O.K. and some are not O.K.. But I have to ask why you think that eyesoftexas has a very good one when you also say that you could never see any differences between diagonals? 

 

I have tested several different cheap (under $100) moderate (under $200) and premium (over $200) 2" mirror diagonals on the unknowing public at large public star parties around L.A. and somehow they can see differences that show trends and there is a definite preference for the premiums vs the cheaper diagonals. All were checked for alignment and most were pretty close but some like the TPO Quartz, TV, Baader and AP were better than others. All were tested using 100mm class f/9 FPL-53 and Fluorite doublets. I pretty much only own the premium diagonals after the public backed up my experiences with them. 



#16 eyeoftexas

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Posted 18 March 2020 - 06:28 PM

YAOG, I believe you are correct as there have been a few reviews on diagonals that all suggest there are differences, and the ones you mention (TV, Baader, AP) seem to score better.  That's why I mentioned that upgrading is on the wish list.  But, those reviews seem to imply that the differences are only subtle, except when the "cheap" ones are compared to the top-of-the-line models.  Let me ask this, if you could only have one, which would you choose, and why?



#17 Bean614

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Posted 18 March 2020 - 08:43 PM

YOAG wrote "But I have to ask why you think that eyesoftexas has a very good one when you also say that you could never see any differences between diagonals?".

Uh, chip, Jon didn't  say he could not see a difference between ALL  diagonals, but rather between just the AT and TeleVue diagonals! 

  And, at least for the 1.25" versions, I wholeheartedly agree!



#18 YAOG

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Posted 18 March 2020 - 09:24 PM

YOAG wrote "But I have to ask why you think that eyesoftexas has a very good one when you also say that you could never see any differences between diagonals?".

Uh, chip, Jon didn't  say he could not see a difference between ALL  diagonals, but rather between just the AT and TeleVue diagonals! 

  And, at least for the 1.25" versions, I wholeheartedly agree!

Neither did I. Read your own quote of my post, there is no "all" in there. Read what Jon wrote, he said he never saw a difference among the AT and other diagonals like it. 



#19 Jon Isaacs

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Posted 18 March 2020 - 10:14 PM

The Astro-Tech diagonals are O.K. and some are not O.K.. But I have to ask why you think that eyesoftexas has a very good one when you also say that you could never see any differences between diagonals? 

 

I have tested several different cheap (under $100) moderate (under $200) and premium (over $200) 2" mirror diagonals on the unknowing public at large public star parties around L.A. and somehow they can see differences that show trends and there is a definite preference for the premiums vs the cheaper diagonals. All were checked for alignment and most were pretty close but some like the TPO Quartz, TV, Baader and AP were better than others. All were tested using 100mm class f/9 FPL-53 and Fluorite doublets. I pretty much only own the premium diagonals after the public backed up my experiences with them. 

I wasn't there, I don't know what the testing procedures were.

 

 On the occasions when I have compared diagonals, I have just not seen what I consider meaningful differences. 

 

I remember one night in particular.  Excellent seeing, Jupiter and Saturn well positioned in the sky.  I had the 120mm Eon out with it's Orion Dielectric diagonal.  I think I was at 257-360 x range and I decided to see if I could see anything more with one of my Everbrites.  I did some swapping and I couldn't see a significant difference.  

 

The main thing is just get out there and do some observing, whatever you got. Buying more junk is not the key.. 

 

Jon


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#20 YAOG

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Posted 18 March 2020 - 11:21 PM

I wasn't there, I don't know what the testing procedures were.

 

 On the occasions when I have compared diagonals, I have just not seen what I consider meaningful differences. 

 

I remember one night in particular.  Excellent seeing, Jupiter and Saturn well positioned in the sky.  I had the 120mm Eon out with it's Orion Dielectric diagonal.  I think I was at 257-360 x range and I decided to see if I could see anything more with one of my Everbrites.  I did some swapping and I couldn't see a significant difference.  

 

The main thing is just get out there and do some observing, whatever you got. Buying more junk is not the key.. 

 

Jon

The key here is to not buy junk, on this we agree. If you are well dark adapted and you have optics of a certain level the differences can be quite apparent. How great you perceive the differences is a matter of experience and having a baseline optic available to pop into the scope or move to the side by side scope.

 

The comparisons were done with several dozen people on each night, essentially a random smattering of people, from virgins to 40 year visual observing vets. My own opinions are well correlated by the results of these informa surveys. There is a difference and it is obvious to most observers though many novices cannot verbalize very well what exactly they are observing they do note there are differences easily visible. As I have posted before these sessions are informal and the diagonal comparisons were done side by side using Celestron 102mm f/8.8 and Vixen 100mm f/9 FPL-53 doublets and a Takahashi FC-100DL 100mm f/9 Fluorite doublet. They were done during the Mt. Wilson Public Star parties last year on three different dates that my club provided the telescopes and people to share the night sky with the public. The general consensus among the mirrors was the Baader 2" BBHS coated Sitall mirror diagonal was the clear winner over the 2" AP Maxbright and TV Everbrite diagonals which did not seems to be much different. The Baader T2 BBHS coated Zeiss prism showed the most nebulosity over even the Baader BBHS Sitall mirror which surprised me. All eyepieces use for the public sessions were TeleVue Panoptics and TeleVue T6 Naglers.   



#21 Jon Isaacs

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Posted 18 March 2020 - 11:57 PM

The key here is to not buy junk, on this we agree. If you are well dark adapted and you have optics of a certain level the differences can be quite apparent. How great you perceive the differences is a matter of experience and having a baseline optic available to pop into the scope or move to the side by side scope.

 

 

A few comments:

 

- For planetary, your friend Daniel points out that dark adaptation is not a plus, the dark adapted eye is not what you want for resolution.

 

- I will just say this:  I have been observing quite some time and I am confident in my observations.  If I felt the need to get a group of people together to decide whether or not there was a difference between two diagonals, that would indicate to me that the differences are indeed subtle.  If the differences are meaningful, then I should see them all by my lonesome.  

 

- I spend a lot of time at the eyepiece but not a lot of time comparing stuff.  The biggest differences in what people see are the differences in their skills as observers.  I have good equipment, not necessarily the best but very capable.  

 

I think any honest observer has to believe that they themselves are the weak point in the chain, not their equipment, there is always more to be had. That attitude comes from my years as a cyclist.. It's not the bike, it's the rider, you can't buy your way to winning a race, you have to ride your way, hours and hours and hours in the saddle.  The differences in bikes are pretty subtle, the differences in riders are where it's at. 

 

In this hobby, it's hours and hours and hours at the eyepiece.. I am always amazed at what skilled observers can do with some pretty basic equipment.  They are my heroes.. 

 

Jon


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#22 YAOG

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Posted 19 March 2020 - 12:17 AM

A few comments:

 

- For planetary, your friend Daniel points out that dark adaptation is not a plus, the dark adapted eye is not what you want for resolution.

 

- I will just say this:  I have been observing quite some time and I am confident in my observations.  If I felt the need to get a group of people together to decide whether or not there was a difference between two diagonals, that would indicate to me that the differences are indeed subtle.  If the differences are meaningful, then I should see them all by my lonesome.  

 

- I spend a lot of time at the eyepiece but not a lot of time comparing stuff.  The biggest differences in what people see are the differences in their skills as observers.  I have good equipment, not necessarily the best but very capable.  

 

I think any honest observer has to believe that they themselves are the weak point in the chain, not their equipment, there is always more to be had. That attitude comes from my years as a cyclist.. It's not the bike, it's the rider, you can't buy your way to winning a race, you have to ride your way, hours and hours and hours in the saddle.  The differences in bikes are pretty subtle, the differences in riders are where it's at. 

 

In this hobby, it's hours and hours and hours at the eyepiece.. I am always amazed at what skilled observers can do with some pretty basic equipment.  They are my heroes.. 

 

Jon

I think dark adapted is better for DSOs but light pollution is good for planetary observation, this is based on my own experience and I guess Daniel says the same thing. The high magnification planetary use is where critical observation differences between diagonals is easier to see. But at the Mt. Wilson public star parties we had moderate conditions and the main differences everybody noted under those conditions was how large the nebulosity appeared around M45 and M42 and how much deeper they could see into the Trapezium with the scopes used. 

 

It is without a doubt the observer that is the weak link. I mostly have been observing and trying stuff out to see if I spent my money on something helpful or not, the diagonals were a not a surprise so much as I felt there was something going on there but until I spent enough time behind the refractors I didn't see it because the SCTs I had used for so long, even the best of them cannot seem to tease out the differences so obvious with a good apo. 



#23 Jon Isaacs

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Posted 19 March 2020 - 04:35 AM

I think dark adapted is better for DSOs but light pollution is good for planetary observation, this is based on my own experience and I guess Daniel says the same thing. The high magnification planetary use is where critical observation differences between diagonals is easier to see.

 

 

That is why I thought it was very strange that you mentioned dark adaptation.  As I mentioned, in my example I made the comparison at 257x-360x in a 120mm scope on Jupiter and Saturn.

 

I think there's a place for SCTs but the place is not my place. For high power planetary, I definitely get my best views with a Newtonian, refractors are about convenience and under dark skies, complementing Newtonians.

 

Jon


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#24 YAOG

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Posted 19 March 2020 - 12:44 PM

That is why I thought it was very strange that you mentioned dark adaptation.  As I mentioned, in my example I made the comparison at 257x-360x in a 120mm scope on Jupiter and Saturn.

 

I think there's a place for SCTs but the place is not my place. For high power planetary, I definitely get my best views with a Newtonian, refractors are about convenience and under dark skies, complementing Newtonians.

 

Jon

I drove an 8" Newt back when they were not fashionable but cheap, back in late 1976 maybe. Things are hazy then because of the smoke, beer and lack of sleep, I was designing, building and coding old minis and the new hot thing - microprocessor computers, running the family liquor store and observing when I could. I bought it from John Diebel back in the day, what would become the 856 Deluxe complete with shelves and 2-axis joystick corrector and a couple of specially ordered bits for film astrophotography. I also made my own 6" and 8" mirrors in the early '70s and discovered that machines are better for this than I am. I never had a truly large Newtonian, the largest I had was a 10" f/6 but because of the space needed to transport the 10" it mostly sat at home and the 8" got all the use. At the time I owned a German Ford sports coupe called the Capri here in the states. I had that Meade 856 for many years, the mirror was impressively figured, I tested it on the knife edge setup the club used, it put up excellent views against the SCTs of the day which IMO were lousy. Then in the mid-80's these small jewel like Japanese refractors were getting a lot of traction in my group and club, they were were fantastic, portable and so impressively priced I could not buy one. I had other needs, like a smart ASCII CRT terminal and 14" line printer I thought I needed more. But obviously I bought into a refractor much later when cheapish EDs came out from TV, I guess cheap is relative to the Japanese Fluorite doublets. 

 

After that is was the new LX200 in 1992 and the Newts just faded from view for me due to practical concessions of owning small cars and living in small spaces, the LX200 optically and electronically was junk, a nice idea executed on the cheap and and worse, poorly. The LX200 was very pretty and sexy but no match for my trusty old 856 opically so I put the LX200 in the closet and kept using the 856 all the way into the late 90's when I sold the 856 to a friend who always wanted it because my wife said it had to get out of her living room for the baby grand piano, some real furniture and a minivan. I didn't actually use that first LX200 much and only sold it maybe 3-4 years ago for about 60% of what I paid for it. Those were the days. 



#25 Bomber Bob

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Posted 19 March 2020 - 03:51 PM

One night last week I compared 4 dielectric diagonals with my APM 152ED on the Moon at high power.  2" models by GSO, ES, & AT; plus a brand new 1.25" AT.  I didn't see any glaring differences in resolution, BUT I did see a trace of yellow on the brightest tops of crater rims with the ES -- when those were close / at the edge of the field.

 

(Like a lot of observers, I do get better mid to high power views in my F10 & slower refractors with prism diagonals vs. mirrors -- and vice versa for my fast fracs.)




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