Noob question - Off Axis Guiding

[wimvb]

5 hours ago, ollypenrice said:

"I was pondering on this the other day.  For reflectors with movable mirrors clearly it's better to guide on the same light cone.  But what about newts and mak-newts for instance.  Why should they suffer from mirror movement any more than a refractor suffers from lens movement?

James"

I was just mulling over the same question! We know that mirrors are usually left slightly loose, or at least not firmly gripped, to avoid pinching - and yet refractor objectives are also made of glass and are held firmly enough not to move. Could it simply be the size of the glass elements, the larger sizes of mirrors being more prone to expansion? I really don't know but I know a man who does. I'll ask him.

Olly

If you hold a lense in front of your eye, and move it slightly, the image doesn't move much. Thank the gods for that, because otherwise us spectacle wearers would have constant headaches.

Otoh, if you move/wiggle a mirror slightly, the reflection wiggles double the amount, because angle of reflection = angle of incidence, and image wiggle = sum of incidence and reflection. 

Edited May 7, 2020 by wimvb

[ollypenrice]

40 minutes ago, wimvb said:

If you hold a lense in front of your eye, and move it slightly, the image doesn't move much. Thank the gods for that, because otherwise us spectacle wearers would have constant headaches.

Otoh, if you move/wiggle a mirror slightly, the reflection wiggles double the amount, because angle of reflection = angle of incidence, and image wiggle = sum of incidence and reflection. 

Ah yes! Good thinking.

Olly

[alacant]

1 hour ago, spillage said:

scope rings clamped down as tight as possible

If you're using a separate guide telescope [1], I'd recommend securing the main mirror with silicone; the pH neutral  type which doesn't harden completely. A blob on each of the basal supports and the lateral felt lining is all that's needed and could save you the odd light frame or two.

HTH

Or not. It benefits all Newtonians;)

[ollypenrice]

This is what my optical guru said in reply:

Yes, in general, a mirror is much more sensitive to any kind of unwanted pressure from the mount.

A reflective surface gives more ray deviation under a certain pressure than a refractive element does.

Thats also why a lens performs better than a mirror when surface imperfections (rms and P/V values) are the same.

[alacant]

1 hour ago, wimvb said:

spectacle wearers

Well, that's me convinced. It just has to be an 8" f4 refractor for me!

[wimvb]

26 minutes ago, alacant said:

Well, that's me convinced. It just has to be an 8" f4 refractor for me!

I think I have the best of both worlds, a 7.5" apo-wannabe,  a MN190.  (Or is that the worst?) 😋

Edited May 7, 2020 by wimvb

[ollypenrice]

47 minutes ago, wimvb said:

I think I have the best of both worlds, a 7.5" apo-wannabe,  a MN190.  (Or is that the worst?) 😋

I always wonder why the MN190 has gone quiet. I tried to buy one years ago but the rather odd UK dealer wouldn't send abroad so I went for a refractor and stayed there. The design surely has vast potential. Steve Loughran, on here, used to post great stuff from his.

Olly

[JamesF]

1 minute ago, ollypenrice said:

I always wonder why the MN190 has gone quiet. I tried to buy one years ago but the rather odd UK dealer wouldn't send abroad so I went for a refractor and stayed there. The design surely has vast potential. Steve Loughran, on here, used to post great stuff from his.

Olly

I have one that will be going back on the NEQ6 that I've just started belt-modding.  My reading suggests that they may be a bit of faff to get set up properly, but once sorted can be very good.  Shame I didn't have it sorted earlier really, but there's only so much free time.  Then I'll need to find a suitable camera for it.  Gina also has one as far as I'm aware, but she's doing much more very wide field stuff at the moment.

But I agree, you don't see too many people posting images from them.  Yet they're not that common on the second hand market as far as I can tell.  Makes you wonder what has happened to them all...

James

[spillage]

21 minutes ago, JamesF said:

what has happened to them all...

Maybe they are such a pig to set up correctly that most have ended up going into landfill..Or the back of a cupboard..😮

Edited May 7, 2020 by spillage

[JamesF]

39 minutes ago, spillage said:

Maybe they are such a pig to set up correctly that most have ended up going into landfill..Or the back of a cupboard..😮

Well, I'm happy to accept donations

James

[wimvb]

55 minutes ago, spillage said:

Maybe they are such a pig to set up correctly that most have ended up going into landfill..Or the back of a cupboard..😮

 

1 hour ago, JamesF said:

I have one that will be going back on the NEQ6 that I've just started belt-modding.  My reading suggests that they may be a bit of faff to get set up properly, but once sorted can be very good.  Shame I didn't have it sorted earlier really, but there's only so much free time.  Then I'll need to find a suitable camera for it.  Gina also has one as far as I'm aware, but she's doing much more very wide field stuff at the moment.

But I agree, you don't see too many people posting images from them.  Yet they're not that common on the second hand market as far as I can tell.  Makes you wonder what has happened to them all...

James

 

1 hour ago, ollypenrice said:

I always wonder why the MN190 has gone quiet. I tried to buy one years ago but the rather odd UK dealer wouldn't send abroad so I went for a refractor and stayed there. The design surely has vast potential. Steve Loughran, on here, used to post great stuff from his.

Olly

I love mine. They had a bad reputation as being heavy (but aren't compared to a sizeable refractor), having a poor focuser (skywatcher have fixed that) and being difficult to collimate (not if you leave the secondary where it is, and they hold collimation well). They actually have a lot that speaks for them; a large aperture for their focal length, a focal length that is just in the sweet spot for todays cameras with small pixels, no faffing with flatteners or coma correctors, no critical spacings. If I had the funds, I could imagine buying an eq8 (or a mesu) and another MN190 for a dual rig.

[alacant]

11 hours ago, wimvb said:

MN190

Hi

Well, in 4 years of astro clubbing and having seen myriad distinct telescopes and designs, I must admit to having to so a Google for this one.

It seems to be a Newtonian Reflector with a lens at the front. @wimvb do you have a photo? I'd love to check I've understood this correctly!

Cheers

[JamesF]

2 hours ago, alacant said:

Hi

Well, in 4 years of astro clubbing and having seen myriad distinct telescopes and designs, I must admit to having to so a Google for this one.

It seems to be a Newtonian Reflector with a lens at the front. @wimvb do you have a photo? I'd love to check I've understood this correctly!

Cheers

It's kind of a cross between a mak and a newt.  It has a corrector similar to a mak, and what would be the secondary mirror of the mak (that focuses light from the primary down the baffle tube to the back of the OTA) has been replaced by an angled secondary (like a newt) so the focal plane is to the side near the "top" of the OTA.

James

[wimvb]

2 hours ago, alacant said:

Hi

Well, in 4 years of astro clubbing and having seen myriad distinct telescopes and designs, I must admit to having to so a Google for this one.

It seems to be a Newtonian Reflector with a lens at the front. @wimvb do you have a photo? I'd love to check I've understood this correctly!

Cheers

Not atm. But there is one at flo. The "lens" in front is a corrector plate which holds a slanted mirror. Since there are no spider vanes, there are no star spikes. Because of the mirror, chromatic abberation is almost nonexistent. MN stands for Maksutov Newtonian. 

[alacant]

1 hour ago, wimvb said:

MN stands for Maksutov Newtonian

Ah OK, so a comet hunter?

 

[wimvb]

3 hours ago, alacant said:

Ah OK, so a comet hunter?

 

Galaxy hunter is more like it, for me at least. But yes, the Explore Scientific 6" is labeled as such.

Here's the MN190 with the 150pds, which is about the size of the ES comet hunter.

[rickwayne]

Per using the same rig on multiple optical tube assemblies: If I had more than one, I'd say "sure!".

ZWO OAG: That's what I just acquired. The only tricky bit is using spacers to get the two distances close enough that the difference can be taken up by adjusting the guide camera on the OAG stalk. Here, for example, is my rig. The spacer between the OAG and the scope is spurious; it just happened that the only spare adapter I had between 42mm threads (the 1.25" nosepiece) and 48mm threads (the OAG) was a 20mm long spacer. The filter wheel is 20mm thick, the spacers between the wheel and the imaging camera soak up the equivalent distance to the 120MC up top. Theoretically I could eliminate one ring each -- the ones directly attached to each camera -- but when I have the flattener/reducer installed, I need the extra backfocus. (The OAG screws directly onto the back of the FF/R in that case, eliminating the 20mm spacer and the nosepiece attached to it.)

Bit of a Rube Goldberg contrivance, to be sure. But it's the hardware I happened to have.

There are a pair of small thumbscrews at right angles to the OAG's optical path. One lets you first adjust the prism "stalk" to dip to the correct depth into the main optical path, so it's lit without shadowing the main sensor. Once you've secured that nice and tight, there's another that lets the guide camera slide up and down on the stalk. You set up the scope and focus the imaging sensor on an object -- helps to do it in daylight -- and then use the second thumbscrew to slide the guide cam up and down until it's in sharp focus too. Tighten everything down, and you should be done unless you bash something hard enough to displace it.

 

[wimvb]

If you have zwo cameras, zwo filter wheel, and zwo off axis guider, you don't need any other spacers than those supplied. Zwo have done the calculations for you.

https://astronomy-imaging-camera.com/product/zwo-oag

[rickwayne]

If you think about it, in order for the guide and imaging sensors to be in focus simultaneously, they have to be at the same distance from the objective. So they're the same distance from any point behind the objective too, including the front of whatever assembly you're moving from scope to scope.

As I alluded to before, the fact that the pickoff prism is a few millimeters from the optical axis makes this not EXACTLY equivalent between all focal lengths. If you had a super-duper-short F/L a line drawn from the objective's center to the pickoff prism would be a broader angle, and so the optical path to the prism would be longer than that from the center to the point below the prism on the optical axis. But for telescope focal lengths we're talking pretty acute angles and hence pretty teeny differences.

Although, when I do the math, if you took it off a 400mm scope and put it on a 2000mm one, the discrepancy ought to be 280 microns if the pickoff is 15mm from the axis. Hey wait a minute, that's enough to make a noticeable difference in focus! Critical focus for a system in the F/5 range is maybe 10 microns thick.

However this is assuming a spherical field of focus. If you've got a field flattener in the optical path, I think all these bets are off and moving the system between scopes will maintain critical focus.

[rickwayne]

On 08/05/2020 at 12:53, wimvb said:

If you have zwo cameras, zwo filter wheel, and zwo off axis guider, you don't need any other spacers than those supplied. Zwo have done the calculations for you.

https://astronomy-imaging-camera.com/product/zwo-oag

You will note that their diagram does not include 20mm of filter wheel between the OAG and the imaging camera, though. Sure, you could put the wheel in front but then you're trying to guide through your filters; since I do a lot of narrowband, I want all those sweet, sweet photons getting to my guide camera.

[wimvb]

6 hours ago, rickwayne said:

You will note that their diagram does not include 20mm of filter wheel between the OAG and the imaging camera, though. Sure, you could put the wheel in front but then you're trying to guide through your filters; since I do a lot of narrowband, I want all those sweet, sweet photons getting to my guide camera.

 

[rickwayne]

Oh, well, a 1.25" nosepiece <sniffs>...

😁

I originally used that when I set up the OAG but I was having problems with rigidity. So I went all-screw-threads intead.

[jimjam11]

I have a love-hate relationship with my ZWO OAG and this thread shows one of the issues I found with it:

I setup my OAG on my ZS73 which can illuminate a full frame sensor and is only F5.9. When I used it on my 200PDS @ F4.5 the light cone must be steeper because the prism shadowed the main sensor. As a result I needed to move the prism even further out to the point where it isnt always well illuminated:

 

However, despite this the result is measurably smaller, rounder stars with my 200PDS (compared to a 50mm guidescope). It makes no difference with my ZS73 because the image scale is much higher (0.86" /px vs 1.77" /px).

The other difference I have noticed is variability in guiding based on seeing. It is not uncommon for my guiding to vary between 0.3" and 0.7" (like the above) on a resonable night. The 50mm guidescope just reports 0.5-0.6" RMS because it isnt sampled well enough to guide down at these levels...

 

 

[ollypenrice]

43 minutes ago, jimjam11 said:

I have a love-hate relationship with my ZWO OAG and this thread shows one of the issues I found with it:

I setup my OAG on my ZS73 which can illuminate a full frame sensor and is only F5.9. When I used it on my 200PDS @ F4.5 the light cone must be steeper because the prism shadowed the main sensor. As a result I needed to move the prism even further out to the point where it isnt always well illuminated:

 

However, despite this the result is measurably smaller, rounder stars with my 200PDS (compared to a 50mm guidescope). It makes no difference with my ZS73 because the image scale is much higher (0.86" /px vs 1.77" /px).

The other difference I have noticed is variability in guiding based on seeing. It is not uncommon for my guiding to vary between 0.3" and 0.7" (like the above) on a resonable night. The 50mm guidescope just reports 0.5-0.6" RMS because it isnt sampled well enough to guide down at these levels...

 

 

Yes, your experience with the prism in two systems is similar to ours. I don't think you can assume that an OAG/camera can always be swapped between scopes without adjustment.

Olly