Diagonals - expensive vs mid range

[heliumstar]

I have to say the lens is really good. I was observing the Saturn the other day. Could properly see the bands which I can't remember I saw with a scope that small apart with Mak 127 years ago when the seeing was really good (hint to myself: take notes).

Only had a few sessions with 'lowly' diagonal and eyepieces but first impressions are very good. XWs now ready and Tak prism ordered. I had issues focusing so used barlow without the lens as an extender. Just ordered an extender from FLO as well as I suspect I'll have an issue focusing with Tak prism as well (as with Celestron prism) due to short light path.

[hal9550]

I have 5 diagonals:

• Fairly bog standard mirror 1.25 - came with a scope - still works, rarely user it - iv convinced myself its age means it will have lost come reflectivity - not dielectric
• Brand-less Prism - came with some celestron gear i bought on ebay - perfect condition - no problems, uses once or twice - 
• Meade 1.25" Prism - perfect condition, easy to clean gently with ISA - was my main diagonal for many years
• Omegon 2" Mirror Diagonal - bog standard 2" - my most used 2" until i got (below)
• Moonfish 2" 99% Dielectric - its lovely - i preferentially use it now when 'two inching'

I would like to get a 2" prism - eventually - but my wish list is very long. Id like because i have more faith in prisms, due to lack of deterioration - and easy when it comes to cleaning - never really had any problem with mirrors though - just a preference, perhaps i used my Meade Prism for so long, it chromatically changed my thinking 

[Saganite]

20 hours ago, Mr Spock said:

So... has anyone used a prism as a secondary on a Newtonian?

Well, yes, but so long ago that I couldn't say anything meaningful.  My homemade 6" reflector of circa 1974 had a piece of screwed rod sticking through the square tube,on the end of which was a plate and the prism, which I still have somewhere was glued to that.  I knew a great deal less about the physics of telescopes then, practically nothing, but it worked and gave me my first ever view of Saturn as well as memorable views of the Moon.  Collimation has improved somewhat; I simply put my hand down the tube grabbed hold of the prism and rotated it whilst looking through the eyepiece until I could see something.  Ah ignorance was bliss.......

Edited September 6 by Saganite

[Highburymark]

On 05/09/2024 at 11:18, JTEC said:

Has anyone experimented with viewing straight through?  I believe the optical design of some scopes - by AstroPhysics?- took account of the fact that a diagonal would be used, but presumably that doesn’t apply to most.  When I’ve tried straight through viewing with the TEC and, most recently with one of the Stellalyra triplets, it’s been my impression that it gave a slight edge to image quality.  After all, the diagonal is only there to save our necks and, except for the situation that Don mentions where a prism can correct optical weaknesses in the scope, a diagonal’s contribution, especially if dirty, can only be in the direction of image degradation.  So, getting the best possible  image quality from your refractor might mean taking up yoga. 

I have - but only with a mirror binoviewer and short 4mm eyepieces (so I could reach focus without having to barlow the power even higher than it already was). Views were lovely, though it takes a bit of planning - and I didn’t do any conclusive comparisons with diagonal views. 

[Elp]

Before I bought any visual stuff all the forum posts trended towards getting a dielectric mirror diagonal above a prism, so that's what I did.

I did however recently acquire a WO erect prism (for a finder) and was kind of "dreading" that the views would be worse, they're actually quite good and comparable. Haven't tested side by side yet.

[hal9550]

On 07/09/2024 at 16:22, Elp said:

Before I bought any visual stuff all the forum posts trended towards getting a dielectric mirror diagonal above a prism, so that's what I did.

I did however recently acquire a WO erect prism (for a finder) and was kind of "dreading" that the views would be worse, they're actually quite good and comparable. Haven't tested side by side yet.

Yea i have gone down the rabbit whole a few times, and read the various threads on Prism V Mirror - theres many, both here and on CN

I think i am always sitting on the fence when it comes to Diagonals. Mirrors seem to be recommended most places. And yet i always find myself worrying about possibly needing to clean them. I was pricing up some 2" prisms recently and the price difference between Prisms and mirrors was stark. i dont think i can land a 2" prism in Ireland for less than 250 euro

 

Theres also lots of discussion on Amici prisms adding distortion - again i dont know, im just an enthusiastic amateur

Has anyone bought a reasonably good value 2" prism recently? Id be curious about the price. Iv some eyepieces on the way, and i am always tempted to allow mission creep kick in, and get a 2" prism. 

There is also some decent value Dielectric 2" Mirrors on Aliexpress - curious to know, if anyone has pulled the trigger on these!

[Highburymark]

I’ve had several dielectrics - WO (two examples), SW, and the pricier Baader white 2”, and couldn’t separate them - they all performed identically to my eyes. 
I use prisms for planets, including the Baader Zeiss T2 and Takahashi turret - the Tak turret uses the same prism as the cheap 1.25” Takahashi prism, which is a real bargain at around £110 UK price, if you can manage with 1.25” eyepieces. Unfortunately the best 2” prisms are more expensive.

[hal9550]

2 minutes ago, Highburymark said:

I’ve had several dielectrics - WO (two examples), SW, and the pricier Baader white 2”, and couldn’t separate them - they all performed identically to my eyes. 
I use prisms for planets, including the Baader Zeiss T2 and Takahashi turret - the Tak turret uses the same prism as the cheap 1.25” Takahashi prism, which is a real bargain at around £110 UK price, if you can manage with 1.25” eyepieces. Unfortunately the best 2” prisms are more expensive.

Yes i think the cheapest 2" Prism i have seen is an Omegon Amici - it clocks in around 250 euro give or take - 

I trawled Aliexpress numerous times, and did find a 2" 45 degree Erecting Prism, which was very reasonably priced - aprox 70 euro - most places i have read recommend AGAINST these for astronomical use though

First Light Optics have some good prisms, but the EU UK trading agreement limits my options there so - im gonna keep my eye on this thread

The crux of it is - i am overhauling my kit atm - and ordering from many places - and i could possibly squeeze in a 2" prism, for the right price. Iv sold a lot of stuff here in Ireland, and had to let go my backup 2" Mirror,  - when it comes to diagonals, i always like to have AT LEAST that one back up so -  

[Louis D]

6 hours ago, hal9550 said:

There is also some decent value Dielectric 2" Mirrors on Aliexpress - curious to know, if anyone has pulled the trigger on these!

I picked up one of these off of Amazon about 6 years ago for $50.  While there's nothing wrong with the mirror or housing, the compression ring is too close to the end.  For eyepieces with undercuts, it tends to push them up at angle off the lip of the receiver tube.  As a result, I never use it.  I ended up paying $70 to $75 for used copies of GSO made 2" dielectrics.

5 hours ago, hal9550 said:

I trawled Aliexpress numerous times, and did find a 2" 45 degree Erecting Prism, which was very reasonably priced - aprox 70 euro - most places i have read recommend AGAINST these for astronomical use though

I'm looking at picking up one of these to use with my 127 Mak when in spotting scope mode, so I can get a larger, unvignetted FOV when using widest field 1.25" eyepieces or mid-focal length 2" eyepieces like my 20mm Founder Optics Marvel 80°.  Despite being a 2" diagonal, it only has 31mm of clear aperture.  However, this is a major improvement over the 1.25" 45 degree Amici diagonals that have only 17mm of clear aperture.

[hal9550]

5 minutes ago, Louis D said:

I picked up one of these off of Amazon about 6 years ago for $50.  While there's nothing wrong with the mirror or housing, the compression ring is too close to the end.  For eyepieces with undercuts, it tends to push them up at angle off the lip of the receiver tube.  As a result, I never use it.  I ended up paying $70 to $75 for used copies of GSO made 2" dielectrics.

I'm looking at picking up one of these to use with my 127 Mak when in spotting scope mode, so I can get a larger, unvignetted FOV when using widest field 1.25" eyepieces or mid-focal length 2" eyepieces like my 20mm Founder Optics Marvel 80°.  Despite being a 2" diagonal, it only has 31mm of clear aperture.  However, this is a major improvement over the 1.25" 45 degree Amici diagonals that have only 17mm of clear aperture.

Interesting - id love to know your thoughts on AMICI Prisms, versus regular prisms - i kinda think i would leave the 45 2" prism in that case, as i would be using it for larger EPs with larger fields - so probably wouldnt gain much

In terms of buying something, i still MIGHT consider an ALIX 2" mirror - if im not upgrading to a prism, i would still like a backup - and while the deficiency you spelled out makes perfect sense to me, its just going to be my backup

[John]

Quite a few diagonals use a rather thick ring inside the eyepiece tube where it joins the body of the diagonal. This ring can cause quite noticeable vignetting at the field edges when eyepieces with the max field stop size. It's annoying because generally these diagonals are otherwise well made and of good optical quality. 

Although they are quite expensive I do really like the "machined out of one piece of alloy" approach that Tele Vue use for their Everbright diagonals in both the 1.25 inch and 2 inch fittings. No internal rings with those and no chance of either the eyepiece or scope end barrels unscrewing under the weight of a heavy load either.

I recently picked up a pre-owned TV 1.25 inch Everbright enhanced aluminium diagonal from an SGL member and it's a lovely little thing 🙂

 

 

[hal9550]

37 minutes ago, John said:

Quite a few diagonals use a rather thick ring inside the eyepiece tube where it joins the body of the diagonal. This ring can cause quite noticeable vignetting at the field edges when eyepieces with the max field stop size. It's annoying because generally these diagonals are otherwise well made and of good optical quality. 

Although they are quite expensive I do really like the "machined out of one piece of alloy" approach that Tele Vue use for their Everbright diagonals in both the 1.25 inch and 2 inch fittings. No internal rings with those and no chance of either the eyepiece or scope end barrels unscrewing under the weight of a heavy load either.

I recently picked up a pre-owned TV 1.25 inch Everbright enhanced aluminium diagonal from an SGL member and it's a lovely little thing 🙂

 

 

Yea i have been reading various articles for years - I probably end up in what someone on CN referred to as 'analysis-paralysis'!! 

I would google various questions  - Prism v Mirror, Amici Prism v Prism, and so on - and i could probably average out the responses with the words 'that depends'

This is definitely a bit of mission creep. What happened was i sold a Telescope at the weekend. and ended up bundling some eyepieces with it for a combined deal. Again its all part of my upgrade project to get my 82deg Eyepieces. When we had the deal concluded i noticed he now had a long refractor, some decent 2" EPs, and no 2" diagonal - so we renegotiated to include one of my 2" Diagonals. 

I suppose the trouble is, i hadn't checked the price on a replacement for myself. And when it comes to diagonals, i prefer to have that second one, just in case - the backup!

I was simply going to buy another one - but then decided to look at upgrading the diagonal too - and i always wanted a prism! (more mission creep)

i suppose if someone could point me at a good value reasonable quality prism, 2" - or a 2" decent value/quality mirror

https://www.astroshop.eu/diagonal-mirrors/bresser-star-diagonal-99-percent-reflectivity-2-/p,54719

If i swerve and avoid the prism, has anyone any experience with the above? I dont rate Bresser as Top Tier, but they tend to be reliable

 

On the subject of Tele Vue - Most of the time, when asking about anything Eyepiece, or Diagonal, related - i get the response the TV version is better, and its true! I believe it for sure; At this stage, i would say TV set the benchmark and everyone else competes! The trouble is i cannot justify the expense - im sure its the best, having checked out the price. It is more expensive to buy a TV 2" Diagonal, than it is to buy the 5" Maksutiv iv been eyeing up. And for me i couldnt swallow that. 400 euro for a Diagonal 2" is too much - and they have a higher 'everbright' version for nearly 600 euro. I cant justify it regardless of the quality, which im sure is impeccable 

Maybe there is one that could be recommended as a quality value version. Im not fussy on brand so

Edited September 18 by hal9550

[John]

26 minutes ago, hal9550 said:

....On the subject of Tele Vue - Most of the time, when asking about anything Eyepiece, or Diagonal, related - i get the response the TV version is better, and its true! I believe it for sure; At this stage, i would say TV set the benchmark and everyone else competes! The trouble is i cannot justify the expense - im sure its the best, having checked out the price. It is more expensive to buy a TV 2" Diagonal, than it is to buy the 5" Maksutiv iv been eyeing up. And for me i couldnt swallow that. 400 euro for a Diagonal 2" is too much - and they have a higher 'everbright' version for nearly 600 euro. I cant justify it regardless of the quality, which im sure is impeccable 

Maybe there is one that could be recommended as a quality value version. Im not fussy on brand so

I nearly always buy used so I've managed to (mostly) avoid Tele Vue's high retail pricing.

Thinking back on of the best "value priced" 2 inch diagonals that I owned was made by GSO. It has been marketed under a lot of brandings over the years including Revelation, TS (Telescope Services) and many more. This is what they look like branded by Rother Valley Optics:

RVO Horizon 2" Quartz Dielectric Mirror Diagonal - Rother Valley Optics Ltd

 

 

[Zermelo]

25 minutes ago, John said:

I nearly always buy used so I've managed to (mostly) avoid Tele Vue's high retail pricing.

Thinking back on of the best "value priced" 2 inch diagonals that I owned was made by GSO. It has been marketed under a lot of brandings over the years including Revelation, TS (Telescope Services) and many more. This is what they look like branded by Rother Valley Optics:

RVO Horizon 2" Quartz Dielectric Mirror Diagonal - Rother Valley Optics Ltd

 

 

My 2" is a Revelation-branded GSO, and I really like it - very secure. I replaced the 1.25" adapter with a Baader Clicklock 2">1.25" reducer, which makes it a pleasure to use.

[bosun21]

While I appreciate the slightly restricted field due to the safety ring on some diagonals they don't bother me in the slightest. I like the WO dielectric 2" diagonals. They have a 1/12th wave lambda mirror and 99% reflectivity. As I don't own any eyepieces with a field stop large enough to vignette the safety ring doesn't bother me. I use the 30mm UFF as my wide field eyepiece and it works just fine. The diagonal wouldn't be suitable for owners of the Vixen LVW 42mm for instance.

[John]

20 minutes ago, bosun21 said:

While I appreciate the slightly restricted field due to the safety ring on some diagonals they don't bother me in the slightest. I like the WO dielectric 2" diagonals. They have a 1/12th wave lambda mirror and 99% reflectivity. As I don't own any eyepieces with a field stop large enough to vignette the safety ring doesn't bother me. I use the 30mm UFF as my wide field eyepiece and it works just fine. The diagonal wouldn't be suitable for owners of the Vixen LVW 42mm for instance.

Generally I agree on the quality side but I'm one of those weird individuals who does like to get the widest true field possible from an eyepiece barrel size from time to time so it's helpful if the diagonal does not get in the way of that. It also seems slightly odd that WO's largest field stop eyepieces can't be used to their full potential in the companies own diagonals.

Probably minor niggle rather than a show stopper though.

 

 

[bosun21]

4 minutes ago, John said:

Generally I agree on the quality side but I'm one of those weird individuals who does like to get the widest true field possible from an eyepiece barrel size from time to time so it's helpful if the diagonal does not get in the way of that. It also seems slightly odd that WO's largest field stop eyepieces can't be used to their full potential in the companies own diagonals.

Probably minor niggle rather than a show stopper though.

 

 

Yes I agree. Not being entirely suitable for their own branded eyepieces just doesn't make any sense. I don't think that these types of decisions are taken by people with the relevant astronomy experience though.

[Don Pensack]

12 hours ago, hal9550 said:

Yeah, I have gone down the rabbit hole a few times, and read the various threads on Prism v. Mirror - there're many, both here and on CN

I think I am always sitting on the fence when it comes to Diagonals. Mirrors seem to be recommended most places. And yet I always find myself worrying about possibly needing to clean them. I was pricing up some 2" prisms recently and the price difference between prisms and mirrors was stark. I don't think I can land a 2" prism in Ireland for less than 250 euros.

 

There're also a lot of discussions about Amici prisms adding distortion - again I don't know, I'm just an enthusiastic amateur.

Has anyone bought a reasonably good value 2" prism recently?  I'd be curious about the price.  I've some eyepieces on the way, and I am always tempted to allow mission creep kick in, and get a 2" prism. 

There are also some decent value Dielectric 2" Mirrors on Ali Express - curious to know, if anyone has pulled the trigger on these!

Prisms need cleaning periodically, too, and dielectric mirror coatings are fairly hard, and cleaning is easy.  Cleaning is not a difference to pay attention to.

But chromatic aberration is.  You don't want to add to the problem with your choice of diagonal.

Many have pointed out that a prism diagonal (the 90° type) seems to be OK in an f/7 triplet refractor, but I wouldn't recommend one with a doublet until f/9 or so.

Prisms also seem to do fine with SCTs, MCTs and CCs.

I would point out that Stellarvue, Tele Vue, and Astro-Physics, all high-end refractor makers, sell only mirror diagonals.

 

[Mr Spock]

One prism I won't recommend is the Antares 2" 45° erecting prism. I bought this to use in the 60mm for birdwatching. CA is so severe it's unusable. I've since managed to get hold of a used Celestron 1.25" 45° erecting prism and that's just fine.

[hal9550]

11 hours ago, John said:

Generally I agree on the quality side but I'm one of those weird individuals who does like to get the widest true field possible from an eyepiece barrel size from time to time so it's helpful if the diagonal does not get in the way of that. It also seems slightly odd that WO's largest field stop eyepieces can't be used to their full potential in the companies own diagonals.

Probably minor niggle rather than a show stopper though.

 

 

Learning a lot here - May i ask a question that probably sounds silly!

At what point does the Barrel size/fieldstop/diagonal ring, clash with the AFOV of an eyepiece?

My fairly amateur understanding of field stops, AFOVs, and barrel size, is that there is a cut off at 82 degrees, and aprox 16mm - above which needs to be two inches - if we drop to 70 degrees, i think i have seen some 1.25" pieces - and there is the 24mm 68 degree eyepiece too

Im just wondering if this topic is something i should be concerned about RE getting my 82 Degree EPs - Currently at 1.25, i will be pitching my flag up to and including 14mm at 82, and 17mm and 70 degrees, respectively. Then im jumping to 2" for 22mm it seems - and i already own some 70 degrees beyond that

 

Im sorry if the wording of the question is baffling, im just trying to work out where there could be a vignetting (which i amateurishly referred to as a crop), with the set up i will soon have!

 

EDIT: Im asking this question in relation to Diagonals - and in reference top the fact i am not really trying to MAX OUT the AFOV with eyepieces - not looking at 92 or 100 degree EPS

Edited September 19 by hal9550

[Louis D]

In my experience, the closer the clear aperture constriction is to the eyepiece, the more obvious and severe the vigetting or outright cutoff will be.

I did some experiments recently and am working up a report on this subject.  Suffice it to say, someone should market a 2" diagonal with a 1.25" nose piece.  That moves the constriction far enough away from the eyepiece that all but the widest TFOV eyepieces will see only modest vignetting.  I realized this after having used widest field 2" eyepieces with my 127mm Mak.

If you just put the 1.25" adapter on the 2" filter threads of the eyepiece, you will see severe vignetting and outer field cutoff in many 2" eyepieces well before you get the widest field ones.

The effect is also true of the WO 1.25" dielectric diagonal and its other-branded brethren.  I can see quite severe vignetting when using it with my 32mm Plossls, making it rather maddening to use it while trying to find faint fuzzies.  These diagonals have a stop ring with maybe a 22mm clear aperture at the bottom of the eyepiece receiver barrel.  I can conceive of no reason to have it there.  A small lip would be sufficient to prevent a long eyepiece barrel from contacting the mirror.

[hal9550]

Its a side step away from the topic of Mid Range V High End Diagonals - but i wanted to ask a question RE Filter Use with Diagonals

Is there any issue with attaching a Filter to the Diagonal, instead of the eyepiece? Im thinking of those nights where i am observing the moon, for long periods. In real amateur hour style, i used to screw a lunar filter to each eyepiece i was using - and would inevitably have to keep swapping the filters between those Eyepieces i was going to move to. 

Then i began just screwing the filters into a diagonal, which saved a lot of messing around. 

• Is there any issue with doing this - placing the filter on the diagonal, and only removing if im changing observation target?
• Is there any issues with doing this, relating to Prism v Mirror? Is it better to use filters with Mirror Diagonals, as opposed to Prisms for example!?

Sorry for the side step, it just occurred to me, and im learning a lot, so i had to ask 🙂

 

[Don Pensack]

4 hours ago, hal9550 said:

Its a side step away from the topic of Mid Range V High End Diagonals - but i wanted to ask a question RE Filter Use with Diagonals

Is there any issue with attaching a Filter to the Diagonal, instead of the eyepiece? Im thinking of those nights where i am observing the moon, for long periods. In real amateur hour style, i used to screw a lunar filter to each eyepiece i was using - and would inevitably have to keep swapping the filters between those Eyepieces i was going to move to. 

Then i began just screwing the filters into a diagonal, which saved a lot of messing around. 

• Is there any issue with doing this - placing the filter on the diagonal, and only removing if im changing observation target?
• Is there any issues with doing this, relating to Prism v Mirror? Is it better to use filters with Mirror Diagonals, as opposed to Prisms for example!?

Sorry for the side step, it just occurred to me, and im learning a lot, so i had to ask 🙂

 

No, there is no problem with doing this.

However, if a 25% transmission neutral density filter is OK at 50x, at 100x, the moon is 1/4 as bright, and you will want a 50% transmission neutral density filter.

At 200x, you won't need a filter at all.

 

So, the best option is for a variable polarizing filter, where the two halves are separable into two single polarizing filters.  You thread 1/2 of the dual filter to the front of the diagonal,

and the other half to the bottom of the eyepiece.

Then, you dial in exactly the brightness reduction you want by merely rotating the eyepiece in the diagonal.

 

Even in a large aperture telescope, however, the Moon is only too bright at low powers, and you can't see much on the moon at low powers.

In my 4" refractor, Moon viewing begins around 100x and goes up, and the Moon never needs a filter.

It's only too bright for comfortable viewing at very low powers.

 

[Don Pensack]

On 19/09/2024 at 05:16, Louis D said:

In my experience, the closer the clear aperture constriction is to the eyepiece, the more obvious and severe the vigetting or outright cutoff will be.

I did some experiments recently and am working up a report on this subject.  Suffice it to say, someone should market a 2" diagonal with a 1.25" nose piece.  That moves the constriction far enough away from the eyepiece that all but the widest TFOV eyepieces will see only modest vignetting.  I realized this after having used widest field 2" eyepieces with my 127mm Mak.

If you just put the 1.25" adapter on the 2" filter threads of the eyepiece, you will see severe vignetting and outer field cutoff in many 2" eyepieces well before you get the widest field ones.

The effect is also true of the WO 1.25" dielectric diagonal and its other-branded brethren.  I can see quite severe vignetting when using it with my 32mm Plossls, making it rather maddening to use it while trying to find faint fuzzies.  These diagonals have a stop ring with maybe a 22mm clear aperture at the bottom of the eyepiece receiver barrel.  I can conceive of no reason to have it there.  A small lip would be sufficient to prevent a long eyepiece barrel from contacting the mirror.

Louis,

I believe you can put a 1.25" nosepiece on a Baader 2" diagonal, though I think you need a 2" SCT tread to T-thread reducer to do so.

[Paz]

On 19/09/2024 at 07:48, hal9550 said:

Learning a lot here - May i ask a question that probably sounds silly!

At what point does the Barrel size/fieldstop/diagonal ring, clash with the AFOV of an eyepiece?

My fairly amateur understanding of field stops, AFOVs, and barrel size, is that there is a cut off at 82 degrees, and aprox 16mm - above which needs to be two inches - if we drop to 70 degrees, i think i have seen some 1.25" pieces - and there is the 24mm 68 degree eyepiece too

Im just wondering if this topic is something i should be concerned about RE getting my 82 Degree EPs - Currently at 1.25, i will be pitching my flag up to and including 14mm at 82, and 17mm and 70 degrees, respectively. Then im jumping to 2" for 22mm it seems - and i already own some 70 degrees beyond that

 

Im sorry if the wording of the question is baffling, im just trying to work out where there could be a vignetting (which i amateurishly referred to as a crop), with the set up i will soon have!

 

EDIT: Im asking this question in relation to Diagonals - and in reference top the fact i am not really trying to MAX OUT the AFOV with eyepieces - not looking at 92 or 100 degree EPS

Something to consider with diagonals and vignetting is the light cone and how wide it is as its path goes through the diagonal (or any particular thing you might have in the optical train). There will be a point somewhere in the light path where the light cone comes to a focus and has no width, and either side of that the light cone will expand at a rate determined by the focal ratio of the scope. So depending on the light cone you can put things upstream of an eyepiece that may be smaller than the field stop of the eyepiece and get away with it.