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Are we overlooking the obvious?

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#1 Astrojensen

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Posted 10 June 2019 - 02:12 PM

I know we've discussed this subject before, from time to time, but I've been reading a lot of threads about planetary eyepieces lately and this made me think of it again, since people often see extremely subtle differences between what is often considered very high-end eyepiece lines of excellent quality, yet they rarely mention a crucial part of the light path: 

 

Diagonals. 

 

I'm mostly thinking of the kind used on refractors and cassegrain-type telescopes, but the ones used on newtonians obviously apply as well, though I'll primarily discuss the former type. 

 

I really do wonder, if all the people who discuss high-end planetary eyepieces (or just eyepieces in general) here with such vigor, have all considered how large an impact the diagonal can have on the view. I'm quite sure that some haven't and are just using one, based on the assumption that since it was from a well-known maker, it should be good enough.

 

But is it?

 

My own experience tells me that one can't be sure about that. My first 2" Baader dielectric diagonal certainly wasn't so hot optically, but this only became apparent, when I tested it against another diagonal. And even my 2" Baader Zeiss prism diagonal is not quite as good as my Baader Zeiss T2 diagonal. And it is a fact that even the Baader/Zeiss T2 diagonal isn't invisible. It logically follows that if the diagonal obviously (even if slightly) degrades the view, then this is going to completely overwhelm any subtle differences between eyepieces and will limit what the telescope is ultimately capable of. It doesn't help that even some relatively expensive diagonals have shiny parts in their barrels, are out of collimation, etc. 

 

We're currently seeing almost a kind of war between several companies about who can make the very best planetary eyepieces. Is it time we saw a similar war between who could make the most "invisible" diagonal for planetary observing?

 

 

Clear skies!
Thomas, Denmark


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#2 Ihtegla Sar

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Posted 10 June 2019 - 02:28 PM

For planetary, I use a Baader T2 1.25 prism diagonal, or no diagonal at all.  Right now, the planets are low enough that I don't typically use a diagonal when viewing them.  Just straight through the refractor like Galileo.  I have viewed the planets with both the Baader T2 1.25 inch diagonal and without it and cannot say I noticed an obvious difference but haven't done a really detailed comparison and not sure I would be able to spot subtle differences.  I bought the Baader T2 1.25 inch prism after reading some reviews and analysis that gave it top ratings for planetary work.  But I too would be interested in more in depth analysis of diagonals since you are right that the amount of attention people seem to pay to diagonals seems to pale in comparison to eyepieces but it is an important piece int he light path, and like a chain is only as strong as its weakest link, so is the image at the end of a light path.


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#3 bbqediguana

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Posted 10 June 2019 - 02:57 PM

I would think that for absolutely critical planetary viewing that NO diagonal would be the best choice. Yeah, I know you'll have a crick in your neck from looking up, but that's the price you pay to get all them photons (gotta get 'em all!).

 

On a serious note, I did have a 2.00" Meade dielectric star diagonal, but I found it had a lot of scatter on bright objects like Jupiter and Venus - my "plain Jane" Meade 1.25" prism diagonal looks a lot better to my eyes. Ultimately a Baader T2 Zeiss prism diagonal is what I'd like to get. But that's down the road.

 

I think this is a perfect candidate for Goldratt's "Theory of Constraints" which basically says to get the best performance out of any system, you identify the constraint, exploit it and then move on to the next constraint. This is all related to manufacturing processes, but I think what you are identifying is the same thing in a telescope's optical train.

 

https://en.wikipedia..._of_constraints

 

Cheers!

Rick in Canada (eh!)


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#4 db2005

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Posted 10 June 2019 - 02:58 PM

It's a valid question for sure! waytogo.gif

 

Yes... diagonals sure don't seem to get the same attention as fancy eyepieces do, and maybe one reason is that we can (at least in theory, if we ignore concerns about ergonomics) do perfectly all-right without them, whereas eyepieces are indispensable. I understand it is not uncommon among amateur astronomers in Japan to do their observing "straight through", completely eliminating the diagonal. To most of us, however, the diagonal is a "necessary evil", but it's all too easy to forget that the optical chain isn't better than its weakest link.

 

I have owned a few sub-par or even bad diagonals myself. The flip-mirror diagonal bundled with my Vixen SD81S was severely astigmatic so I tossed it. The flip-mirror diagonal bundled with my Vixen A80M was merely OK but not great. The 2" mirror diagonal bundled with my Skywatcher ED100 was bested on contrast by my WO dielectric. And the 1.25 erecting-prism bundled with another entry-level achromat caused severe vignetting and was just ... how should I phrase this... outright poor.

 

Myself? I always use a 90 degree diagonal, and I always use one of much better quality than those typically bundled with telescopes. But I must admit my observing skills and/or typical seeing conditions are probably not good enough to clearly differentiate between different good quality diagonals. Sure, I can see the difference between an inexpensive diagonal (typically one bundled with a telescope, such as the one bundled with my Skywatcher ED100), and diagonals bought separately, costing maybe 100-200 EUR or more. Last year I dedicated one observing night to comparing views in my Tak FC-100 when observing straight-through, and with my William Optics 2" diagonal, and a 2" Prism diagonal. It is quite possible I was limited by seeing, because I could not detect any difference in the views between straight-through viewing and using the WO diagonal. On the other hand, I did find the 2" prism to introduce noticeable and distracting CA so I quickly quit using it.

 

Most of the time recently I have been using a 2" Baader dielectric diagonal with my SD81S, but I've decided to try using my 1.25" Baader prism for my next few observing sessions instead as I've read many reports that prisms tend to reduce light scatter and enhance planetary contrast somewhat. If using a prism improves the contrast and reduces light scatter noticeably I'd possibly consider looking for a 1.25" Zeiss-spec Baader prism.

 

CS,

Daniel


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#5 TOMDEY

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Posted 10 June 2019 - 03:07 PM

Hi, Thomas! About seven years ago I measured a generous assortment of Star Diagonals for wavefront, angular and lateral alignments, spectral throughput, etc. Here is a copy of the summary. Notable conclusions are that each and every diagonal is different, that premium brands tend to perform the best; far more than half of randomly-selected diagonals suffer notable deficiencies.    Tom

Attached Thumbnails

  • 18 78 Dey Diagonals Paper Summary.jpg

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#6 howardcano

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Posted 10 June 2019 - 03:12 PM

That's the first time I've ever seen any optical component with a 1% Strehl.


Edited by howardcano, 10 June 2019 - 03:14 PM.

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#7 Procyon

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Posted 10 June 2019 - 03:26 PM

That's the first time I've ever seen any optical component with a 1% Strehl.

Lol yea, what happened there?

Before I sold my T2 Zeiss prism I tried it on Jupiter vs a brand new 2" Baader Dielectric diagonal. I sold the T2 Zeiss prism the next day (Really enjoyed it for a few years) and believe a clean diagonal mirror should count a bit also. Lots of other variables though.

 


Edited by Procyon, 10 June 2019 - 06:17 PM.

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#8 howardcano

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Posted 10 June 2019 - 03:28 PM

I wonder if a collimation focuser plug would have a Strehl ratio better than 1%!  smile.gif

 

Reading this thread reminds me that I should finish putting the 2" adapter on the front of one of my long refractors so I can test all of my diagonals.


Edited by howardcano, 10 June 2019 - 03:36 PM.


#9 RadioAstronomer

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Posted 10 June 2019 - 03:29 PM

I totally agree Thomas. I sometimes forget to mention which diagonal I use when discussing eyepieces...

For what it's worth, for critical planetary observation I compared the TV Everbtite 1.25" dielectric, the Baader Clicklock 2" dielectric, the Baader 1.25" prism, and a cheapo 1.25" Celestron prism in several slowish refractors (f/ > 8) and found that the Baader 1.25" prism always provides the sharpest and more contrasty image. It's by far the closest one to straight-through observing. 


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#10 chemisted

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Posted 10 June 2019 - 03:44 PM

I use the Baader BBHS silver in the T-2 configuration.  Bill P. has reviewed the 2" on this site.


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#11 rowdy388

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Posted 10 June 2019 - 03:56 PM

That's the first time I've ever seen any optical component with a 1% Strehl.

Yeah, and look at # 5, 1.25" Celestron diagonal below it that costs only $15. It has a 93 Strehl and

nearly perfect transmission unless I'm reading the graph wrong. scratchhead2.gif 


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#12 Jon Isaacs

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Posted 10 June 2019 - 03:59 PM

A few thoughts:

 

- Refractor and CAT diagonals operate very close to the focal plane so the light that contributes to any point on the focal plane on reflects off only a small portion of the diagonal.  This means the overall flatness is not critical the way it is with a Newtonian and that for higher power observing,  only the very center region of the secondary is used.  A turned edge has no effect on planetary viewing with refractors and cats. 

 

With a 2 inch diagonal , the focal plane is about 2 inches from the diagonal .  At F/8, that means 2"/8 = 0.25" of the diagonal contributes to each point on the focal plane . Any measurement of the quality of the diagonal needs keep this in mind. 

 

- Seeing and the size/optical quality of the objective are the important factors in planetary viewing.  With a Newtonian,  because the light for each point reflects off nearly the entire secondary, it's an important part of "objective."

 

For planetary,  I don't worry too much about the eyepiece .  Seeing and well prepared telescopes are the keys. 

 

Your mileage may vary .

 

Jon


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#13 Spacefreak1974

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Posted 10 June 2019 - 04:01 PM

Thomas

I agree

So much emphasis on eyepieces. So many folks keep the Celestron star diagonal and buy expensive Delos and Ethos eyepieces. My largest improvement to date was switching to a WO 99% Reflective Dielectric diagonal. It is a 2" (we dont need to go down the rabbit hole of 1.25 vs 2"), but just in terms of sharpness the WO diagonal was so much sharper. The Celestron mirror diagonal is horrible. With Highpoint and Agena selling a GSO 99% reflective diag for $69 that should be everyone's first purchase after maybe a dew shield!

 

I now have one of the GSO Highpoint branded 1.25 diagonals too

 

Jon


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#14 Astrojensen

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Posted 10 June 2019 - 04:09 PM

A few thoughts:

 

- Refractor and CAT diagonals operate very close to the focal plane so the light that contributes to any point on the focal plane on reflects off only a small portion of the diagonal.  This means the overall flatness is not critical the way it is with a Newtonian and that for higher power observing,  only the very center region of the secondary is used.  A turned edge has no effect on planetary viewing with refractors and cats. 

 

With a 2 inch diagonal , the focal plane is about 2 inches from the diagonal .  At F/8, that means 2"/8 = 0.25" of the diagonal contributes to each point on the focal plane . Any measurement of the quality of the diagonal needs keep this in mind. 

While this is very true, polish, minimal scattered light and absence of astigmatism is still extremely important. There is absolutely no question that the 2" Baader dielectric diagonal I used was harming planetary performance in the scopes I was using it in and once I discovered this and how serious it was (compared it to straight through and a Baader/Zeiss T2 diagonal and it had WAY more scatter, which clearly lowered the contrast) I stopped using it for planetary observing. It was fine for low-power deep-sky, until I got a 2" Baader/Zeiss prism, which is better.  

 

 

Clear skies!

Thomas, Denmark


Edited by Astrojensen, 10 June 2019 - 04:12 PM.

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#15 Redbetter

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Posted 10 June 2019 - 04:15 PM

The main problem I have seen with 1.25" diagonals is serious misalignment that introduces some massive tilt errors.  I have seen some astigmatism in a few as well, but the #1 killer has been awful build quality for the housing/nosepiece/barrel.  The latter can make it very difficult/nearly impossible to evaluate what is going on with a refractor's objective.  It is even worse for .965".   Mirrors have been far worse than prisms. 

 

I have had good luck so far with 2" diagonals, better aligned, better built.  One of the reasons to use a 2" focuser is to avoid the crapshoot with 1.25" diagonals, particularly if the ratio is too short for a prism.


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#16 Starman1

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Posted 10 June 2019 - 04:16 PM

I had the luck of having about 10 diagonals handy when I got my 4" apo.

(I have since sent 9 of them to new homes).

One really stood out.  It was as close to perfect as I've seen in a mirror diagonal.

Collimation of the scope, followed by insertion of the 90° diagonal, showed no change in collimation whatsoever,

and star images were exemplary in good seeing.

And it was the inexpensive 2" dielectric diagonal provided with the scope in the box!

 

I did not evaluate color rendition, or other issues more strongly influenced by eyepiece choice,

but I did compare using eyepieces I knew were capable of revealing the differences (i.e. TeleVue Delites).

I ditched the adapter provided by the mfr. for a twist-lock type with a long internal collet.

 

But, out of 10 diagonals, this one provided perfect little Airy discs a much higher % of the time on the same night than all the other diagonals.

Mechanically, I would have preferred a more rigidly-machined diagonal if I were using the new behemoth series of eyepieces becoming common today.

But with the small, light, Delite eyepieces, the diagonal is really as close to invisible as I've seen.

 

The reason I don't mention the name is because I don't trust that my experience would be general with this brand of diagonal.

Even cheap optics can sometimes be nearly perfect by accident.  A Bell-shaped curve has a top and a bottom.

But it is interesting that it beat several very high-end star diagonals,

Compare diagonals #9, #14, and #15 in Tom's list.  Then look up the prices.  It's obvious price is not the determining factor.

I think it's the luck of the draw.

Eyepieces vary a tiny amount from sample to sample.  Star diagonals vary more.  Yet who tests star diagonals?


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#17 pao

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Posted 10 June 2019 - 04:38 PM

I know we've discussed this subject before, from time to time, but I've been reading a lot of threads about planetary eyepieces lately and this made me think of it again, since people often see extremely subtle differences between what is often considered very high-end eyepiece lines of excellent quality, yet they rarely mention a crucial part of the light path: 

 

Diagonals. 

 

I'm mostly thinking of the kind used on refractors and cassegrain-type telescopes, but the ones used on newtonians obviously apply as well, though I'll primarily discuss the former type. 

 

I really do wonder, if all the people who discuss high-end planetary eyepieces (or just eyepieces in general) here with such vigor, have all considered how large an impact the diagonal can have on the view. I'm quite sure that some haven't and are just using one, based on the assumption that since it was from a well-known maker, it should be good enough.

 

But is it?

 

My own experience tells me that one can't be sure about that. My first 2" Baader dielectric diagonal certainly wasn't so hot optically, but this only became apparent, when I tested it against another diagonal. And even my 2" Baader Zeiss prism diagonal is not quite as good as my Baader Zeiss T2 diagonal. And it is a fact that even the Baader/Zeiss T2 diagonal isn't invisible. It logically follows that if the diagonal obviously (even if slightly) degrades the view, then this is going to completely overwhelm any subtle differences between eyepieces and will limit what the telescope is ultimately capable of. It doesn't help that even some relatively expensive diagonals have shiny parts in their barrels, are out of collimation, etc. 

 

We're currently seeing almost a kind of war between several companies about who can make the very best planetary eyepieces. Is it time we saw a similar war between who could make the most "invisible" diagonal for planetary observing?

 

 

Clear skies!
Thomas, Denmark

Good question.

For this reason, when possibile (90% of my sessions), I observe only without diagonal (like I use to say, Tak way...). This is the purest way to observe. A good apo refractor, no diagonal. A little more complicated but the views are much more natural especially at high power.

pao


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#18 Ihtegla Sar

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Posted 10 June 2019 - 05:05 PM

Maybe I should spend my money on a taller tripod instead of a better diagonal. I don't mind a cricked neck but when my refractor is pointed up, there really isn't enough room for me to get under it. Or maybe better yet a height adjustable cot, so i could lie down under the telescope and look up.
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#19 Procyon

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Posted 10 June 2019 - 05:14 PM

I use the Baader BBHS silver in the T-2 configuration. Bill P. has reviewed the 2" on this site.


I was looking for the 2" Baader BBHS prism diagonal or the Astrophysics one but ended up buying the 2" Baader Dielectric Clicklock for almost 1/3 the cost. It did not bad I found in Bill's review. Plus it has the full 2" aperture opening. Wouldn't mind trying the Astrophysics one if I can connect it to the Baader 3.25 vb. Has the 2" Zeiss Baader come down in cost? It was insanely priced for a while.


Edited by Procyon, 10 June 2019 - 06:14 PM.


#20 TOMDEY

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Posted 10 June 2019 - 05:18 PM

That's the first time I've ever seen any optical component with a 1% Strehl.

Here is the page on that one and also on a good one. Note that the deficient one there I bought used and it obviously got dropped or something equivalently abbusive. Also, point made by others regarding each field point sees only a small footprint on the fold, so you can generally get away with far worse global wavefront on the diagonal 90% aperture. Ummm... oh yeah, note that the wonderful Astrophysics diag there mainfests a tad of power... but that is entirely unaffective, because the characterization is at the used 90-deg fold, so can just be focused out. That's the good thing about measurings folds at their used config. Frankly, that is the same way I measure regular Newt folding flats. Here is the fringes on my 6.25-inch fold for the 36-inch scope, double-bounce fringes at 90-deg. Dominated by pure power so... actually very good giant fold!    Tom

Attached Thumbnails

  • 20 Dey Diagonals Paper Summary 41.jpg
  • 21 Dey Diagonals Paper Summary 41.jpg
  • 23 Toms 6.25-inch fold 80.jpg

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#21 25585

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Posted 10 June 2019 - 05:46 PM

Another reason slower Newtonians score. Just 2 mirrors and a good eyepiece.....


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#22 russell23

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Posted 10 June 2019 - 06:05 PM

If comparing eyepieces wouldn't all eyepieces be subject to the same errors in the diagonal?

 

I use a Vernonscope diagonal that is 1/26th wave and I also have an AP Maxbright as my backup.

 

For 1.25" I use the Baader prism which I think is excellent. 


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#23 EverlastingSky

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Posted 10 June 2019 - 07:23 PM

Astrojensen Thomas, yes, of course you are correct. The best diagonal is no diagonal. Since I am nimble and fit I find it quite non-demanding to sit on a small stool and view straight through without a diagonal. At 49° North the Planets are lower more of the time so it is even easier. I have the T2 Baader "zeiss" spec prism as well as TV Everbrights in 1.25" and 2" sizes. I only use them when absolutely necessary on Planets.


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#24 CrazyPanda

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Posted 10 June 2019 - 10:19 PM

I had the luck of having about 10 diagonals handy when I got my 4" apo.

(I have since sent 9 of them to new homes).

One really stood out.  It was as close to perfect as I've seen in a mirror diagonal.

Collimation of the scope, followed by insertion of the 90° diagonal, showed no change in collimation whatsoever,

and star images were exemplary in good seeing.

And it was the inexpensive 2" dielectric diagonal provided with the scope in the box!

 

I did not evaluate color rendition, or other issues more strongly influenced by eyepiece choice,

but I did compare using eyepieces I knew were capable of revealing the differences (i.e. TeleVue Delites).

I ditched the adapter provided by the mfr. for a twist-lock type with a long internal collet.

 

But, out of 10 diagonals, this one provided perfect little Airy discs a much higher % of the time on the same night than all the other diagonals.

Mechanically, I would have preferred a more rigidly-machined diagonal if I were using the new behemoth series of eyepieces becoming common today.

But with the small, light, Delite eyepieces, the diagonal is really as close to invisible as I've seen.

 

The reason I don't mention the name is because I don't trust that my experience would be general with this brand of diagonal.

Even cheap optics can sometimes be nearly perfect by accident.  A Bell-shaped curve has a top and a bottom.

But it is interesting that it beat several very high-end star diagonals,

Compare diagonals #9, #14, and #15 in Tom's list.  Then look up the prices.  It's obvious price is not the determining factor.

I think it's the luck of the draw.

Eyepieces vary a tiny amount from sample to sample.  Star diagonals vary more.  Yet who tests star diagonals?

You'd think for the price of these diagonals that they would go through some rigorous QA before being shipped so that they're all more consistent in their quality.



#25 Sarkikos

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Posted 11 June 2019 - 10:02 AM

A few thoughts:

 

- Refractor and CAT diagonals operate very close to the focal plane so the light that contributes to any point on the focal plane on reflects off only a small portion of the diagonal.  This means the overall flatness is not critical the way it is with a Newtonian and that for higher power observing,  only the very center region of the secondary is used.  A turned edge has no effect on planetary viewing with refractors and cats. 

 

With a 2 inch diagonal , the focal plane is about 2 inches from the diagonal .  At F/8, that means 2"/8 = 0.25" of the diagonal contributes to each point on the focal plane . Any measurement of the quality of the diagonal needs keep this in mind. 

 

- Seeing and the size/optical quality of the objective are the important factors in planetary viewing.  With a Newtonian,  because the light for each point reflects off nearly the entire secondary, it's an important part of "objective."

 

For planetary,  I don't worry too much about the eyepiece .  Seeing and well prepared telescopes are the keys. 

 

Your mileage may vary .

 

Jon

I agree, except for worrying about eyepieces for planetary.  Well, I don't worry about them, but I factor them in. 

 

Given a well-prepared telescope with decent optics, and good seeing, I have seen differences among eyepieces for planet, lunar and double star viewing.   The most obvious difference is probably the size of the scatter around a bright planet.  Other differences can be in sharpness, perceived contrast and color tone.

 

Mike 


Edited by Sarkikos, 11 June 2019 - 10:10 AM.

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