Straight through observing

[Paz]

I decided to have a quick look at the moon this evening and cut some corners to get out quickly. One time saver was to not bother with a diagonal.

I got excited that maybe the views would be an epic step up.

I didn't notice a huge benefit but I had tube currents to contend with. Anyway ergonomically it was not easy on my neck  and I didn't last long before getting a diagonal out.

Does anyone else do straight through observing sometimes?

[JamesF]

9 hours ago, Paz said:

Does anyone else do straight through observing sometimes?

Occasionally when I'm imaging.

Actually, last night I even did it without an eyepiece :)  I hadn't aligned the mount and when I slewed to the Moon my 127 Mak wasn't pointing in the right place, so I lined it up working from the reflection on the inside of the baffle tube until I could see the Moon directly...

James

[MarsG76]

Only if the object observed is low enough to be comfortable through my SCT... straight through is generally reserved for imaging.

 

[R26 oldtimer]

I've also tried it, in order to tease out more planetary detail. I could see no real benefit, which makes you sceptical about investing in a better or premium diagonal....

[timwetherell]

I've done this once or twice with my refractor when looking for Sirius B in the UK to get the very least scatter possible. I could convince myself there was a tiny difference but to be honest, the difference between good seeing and bad was a million times greater than anything the diagonal introduced. On a good night the pup is pretty clearly visible and on a bad night, it's just not there  It is a literal pain in the neck for higher objects though. And my observatory is quite small so there's only just room for my head between the wall and the eyepiece when focused all the way out like that

 

[StarryEyed]

I was doing some the other day to test out my diagonals. This was on a terestial target. Stright through a TAK FC100 was much better than a 2" William Optics dielectric which was better than an old 1.25" Intes dielectric. I was wondering with the planets so low if I should bother. But it all depends on the seeing. Not a lot of atmosphere between me and one of my neighbours chimneys! 

[DirkSteele]

I think it is quite popular in Japan (hence why Taks come with so many extension tubes!).  I rarely do it as it is not super comfortable, but I have been known to pull out the diagonal if I am going for something super tough (usual a double split) and want as little junk between me and the wavefront coming in.

[RT65CB-SWL]

I did my bit on Friday when viewing Saturn with my 're-modded' ETX105, (c/w 2" SCT e/p adaptor, not shown), and a 2" SkyWatcher 28mm, as it was just above a tree about 200 metres from me, (the tree that is). Quite a pleasant change from viewing with a star diagonal and looking down.

 

Edited August 28, 2019 by Philip R

[boyandorion]

Hehe, last night I was outside for many hours, I didn't count - playing with my humble telescope, to be honest, I was more busy being annoyed by the fact that my scope came with a 45 deg. diagonal, and not a 90 degr. one, than actually thinking about taking it out and see what that did to the view. I'm gonna try it. My scope had me crawling on my knees at times last night .

[RT65CB-SWL]

On 27/08/2019 at 12:33, boyandorion said:

Hehe, last night I was outside for many hours, I didn't count - playing with my humble telescope, to be honest, I was more busy being annoyed by the fact that my scope came with a 45 deg. diagonal, and not a 90 degr. one, than actually thinking about taking it out and see what that did to the view. I'm gonna try it. My scope had me crawling on my knees at times last night .

Depending on what refractor OTA you have, it may not reach focus without a diagonal in place.

Edited August 29, 2019 by Philip R

[Ben the Ignorant]

On ‎25‎/‎08‎/‎2019 at 21:47, Philip R said:

I did my bit on Friday when viewing Saturn with my 're-modded' ETX105

Maksutovs and Schmidt-Cassegrains have a sliding main mirror to adjust focus but when it is a large fraction of a millimeter away from the ideal spot the telescope is no longer diffraction-limited. Removing the diagonal requires displacing the main mirror a lot to reach focus and there lies the problem. The sliding mirror is very convenient, I can use my Celestron 5 as a long-range microscope looking at insects 2.5 meters away and magnifying them 250x like seeing them to naked eye at a distance of 1 cm. The image is still quite sharp but not optimal.

Especially when viewing planets, all the resolution has to be used so moving the mirror away from the optimized position (focus on the infinite) will deny what little you could gain by removing the diagonal. Excellent dielectrics like those from GSO (I bought four of them, all are superb and consistent, same for my StarGuider) are common and not expensive so there is no reason to remove them except as an experiment.

Besides, if optics makers couldn't make diagonals that respect the main mirror's performance every newtonian would be disappointing but that's not the case.

Edited August 29, 2019 by Ben the Ignorant

[Merlin66]

16 minutes ago, Ben the Ignorant said:

but when it is a large fraction of a millimeter away from the ideal spot the telescope is no longer diffraction-limited.

Ben,

I think this is a bit exaggerated .......

[Ben the Ignorant]

3 minutes ago, Merlin66 said:

I think this is a bit exaggerated .......

The formula to calculate the loss of resolving power according to the mirror's displacement was in an old issue of Sky&Telescope, I don't remember which but it was in the late eighties or early ninities, I believe. The margin to keep the scope diffraction-limited was less than half a millimeter on each side of the ideal spot.

[Merlin66]

https://www.cloudynights.com/topic/506609-how-critical-is-the-c11-edge-hd-backfocus/

https://www.innovationsforesight.com/support/celestron-edgehd-back-focus-tolerance/

The performance curves indicate some leaway......

Edited August 29, 2019 by Merlin66

[Ben the Ignorant]

That's a discussion about backfocus, not main mirror position relative to the secondary mirror/plate assembly. The backfocus is behind the main mirror and outside the telescope, but the main mirror's position is inside the telescope. Backfocus is where the focal plane is, but I was talking about where the main mirror is, different things. 

The spacing between optical elements is critical, you can't change the distance between a doublet or a triplet's lenses without damaging the image. Same for catadioptrics, they are designed with a certain spacing between the main mirror and secondary mirror/plate or meniscus. Changing the distance only a little reduces performance.

For example, messing with the spacing in a triplet refractor reduces the Strehl factor by about 1% for every 1/100 of a millimeter of error. Ideally, a Schmidt-Cassegrain or a Maksutov should have fixed optics and an outside rack-and-pinion or Crayford focuser. But then two problems arise: one, the scope becomes less compact and loses the ability to screw accessories firmly onto the rear metal plate. Second, focusing on close objects pulls the eyepiece or camera a long way to the rear and allows flexure and play.

Moreover, the Cloudy Nights discussion is about widefield imaging where losing a little resolution doesn't show. If Mars was in a widefield image it would not look larger than a star because it is not magnified enough.. But magnify Mars as they do in high res and the loss of contrast/resolution shows if the mirrors are not spaced right. Schmidt-Cass and Maks are designed for infinity focus so that doesn't happen but focusing on closer targets lessens performance.

Say the last thing you looked at with your Schmid-Cass was a close tree, and then you switch to an aftermarket external focuser. You can still focus on anything at any distance but because the mirrors were left out of the ideal spacing the scope will underperform all the time.

[Merlin66]

Ben,

The backfocus distance is driven by the separation of the main and secondary mirror..change the spacing alters the backfocus.

Last words:

Telescope Optics - Rutten & Van Venrooij

Section 9.2 (p87) "Close focusing in the SCT"

The graph (Fig 9.8) shows the typical SCT when focused (using the mirror shift) can focus down to 50m before the Image blur exceeds the Airy Disk; this (table 9.2) equates to a mirror movement of 3.354mm.

 

[Ben the Ignorant]

A reminder of the topic is in order. Paz asked about high-power, narrow-field, high resolution visual observing without a diagonal, and all the respondants talked about planetary viewing because they understand that is the subject. Widefield imaging has nothing to do with that. Backfocus was not an issue either because if it was that kind of observing would not be possible to begin with.

I don't have the book you mention so I can't see that for myself, and I'm not sure what the image blur exceeding the Airy disk means. Does it mean the smallest detectable loss of contrast/resolution or does it mean a big loss? I don't know. What I do know, and is recognized by everyone is, taking the traditional meaning of diffraction-limited optics as 80% Strehl factor, that is already too lax.

If a top-brand, say LZOS scope had an 80% Strehl it would be rejected as badly defective, the norm is 95%. Even moderately priced Explore refractors are guaranteed 93%, so a drop to 80% is unacceptable, and vastly worse than any loss of quality a good diagonal could cause. And that was the question. As a general rule, the slightest change in the optimal arrangement of optical elements always lessens contrast and resolution. That's why so many threads are written about collimation, centering and spacing of optics and mechanical components.

No one tells posters to tolerate miscollimation in their newtonian or their Schmidt-Cass. No one wants the slightest decentering in their triplets (or doublets for that matter), and telescopes are returned or sent to the optical bench to be adjusted. Wellenform and Teleskop Austria optimize triplets by respacing the lenses by only a few 1/100ths of a millimeter. Out of specs is never good.

Catadioptrics are also designed with certain accurate specs, they are optimized to focus at infinity with a diagonal in place so it's probably an error to use them when the mirror spacing has changed because the diagonal is removed.

[Captain Scarlet]

On 31/08/2019 at 18:13, Ben the Ignorant said:

...Catadioptrics are also designed with certain accurate specs, they are optimized to focus at infinity with a diagonal in place so it's probably an error to use them when the mirror spacing has changed because the diagonal is removed.

On the basis that the vast majority of people who own such mirror-shift Maks will use that knob at some stage and put their mirrors away from optimal, what strategy would you (or anyone else) use to re-acquire the optimal mirror-spacing and "remember" it? The only thing I can think of for a non-optical-professional is to learn what the star-test should look like, and use it at various positions to home in on the ideal. But I suspect that would be beyond most of us...

Magnus

[Peter Drew]

I've used catadioptric telescopes ever since they have been available to amateurs. Whilst it is true that there is an optimal design inter mirror position, I can't say that I've noticed an obvious image degradation visually despite significant movement to accommodate different focal point requirements. Collimation issues have a much more profound effect.   😀

[Ben the Ignorant]

1 hour ago, Captain Magenta said:

On the basis that the vast majority of people who own such mirror-shift Maks will use that knob at some stage and put their mirrors away from optimal, what strategy would you (or anyone else) use to re-acquire the optimal mirror-spacing and "remember" it? The only thing I can think of for a non-optical-professional is to learn what the star-test should look like, and use it at various positions to home in on the ideal. But I suspect that would be beyond most of us...

Baseless sarcasm. First, the article in the old Sky&Telescope issue was about Schmidt-Cassegrains which work at f/10, a steeper and less forgiving ratio than the f/12 to f/15 beams of maks. Besides, maks have a diifferent optical design that would probably have other tolerances even if the f/ratio was the same. Optical designers obviously make scopes and eyepieces so their focal planes match with minimal mirror motion, only them have to worry about that, not us users. That remembering the star test thing is nonsense.

[Ben the Ignorant]

1 hour ago, Peter Drew said:

I can't say that I've noticed an obvious image degradation visually

Precisely, scope makers know what they're doing so the loss in normal use is negligible.

[Captain Scarlet]

3 minutes ago, Ben the Ignorant said:

Baseless sarcasm. First, the article in the old Sky&Telescope issue was about Schmidt-Cassegrains which work at f/10, a steeper and less forgiving ratio than the f/12 to f/15 beams of maks. Besides, maks have a diifferent optical design that would probably have other tolerances even if the f/ratio was the same. Optical designers obviously make scopes and eyepieces so their focal planes match with minimal mirror motion, only them have to worry about that, not us users. That remembering the star test thing is nonsense.

No sarcasm at all ... I was asking sincerely. These things interest me, as a relative newcomer to this field.

[Ben the Ignorant]

I aimed my Celestron 5 Schmidt-Cass at infinity with the diagonal and basic eyepiece (Explore 24mm/68°, maximal field, minimal power) in place. I switched to all my other eyepieces, getting back to the 24/68 after each eyepiece change. They were Synta Plössl 20/50, Maxvision (Meade clone) 18/82, Hyperion 13/68, Hyperion 10/68, Sky-Watcher Myriad 9/100, Sky-Watcher Panorama 7/82, Sky-Watcher Myriad 5/110, Explore 4.7/82, TS 4/82 and Sky-Watcher Myriad 3.5/110.

The focus knob never moved more than 1/8th turn (and always in the same direction) except for the Myriad 9 that needed a 1/4 turn (in the same counterclockwise direction as the others). I've measured the mirror motion for each turn and it's 1/2mm, so it never moved more than 1/8mm.

Then I removed the diagonal and put the 24/68 directly in the visual back, the refocus was 4.5 turns, that is 2.25mm, eighteen times more. Obviously vastly more than what optical engineers have allowed for normal use with the diagonal's optical path.

19 hours ago, Peter Drew said:

I can't say that I've noticed an obvious image degradation visually despite significant movement to accommodate different focal point requirements.

It's not about accomodating different eyepieces, it's about observing with or without the diagonal's long optical path. In my test the difference is eighteen-fold.

Edited September 3, 2019 by Ben the Ignorant

[Peter Drew]

My range was from star diagonal, presumably the most likely design parameter, increased to a maximum using binoviewers, i.e. several turns. Still produced wonderful planetary and lunar images under good seeing conditions.   The focal plane extension due to decrease in inter mirror distance on a SCT is approximately mirror movement multiplied by the amplification of the secondary squared.   😀