11 replies to this topic

[shams42]

Hi all,

 

I am very happily using my Binotron-27 in my 10" f/4.8 dob with outstanding results. I am enjoying visual astronomy more than ever with this setup.

 

Looking to the future, I am planning to buy a 16-18" scope in the next few years. Before the Binotron, a paracorr-II permanently resided in the focuser of my scope. I am pretty sensitive to off-axis aberrations and really dislike coma. I have noticed that the OCS-A45 does a pretty good job of coma correction in the Binotron -- I see little if any coma in in-focus star images.

 

I will be considering focal ratios from f/4.5 down to the f/3.7ish range. I am well acquainted with the issues of vignetting that may occur with excessively fast focal ratios. However, I do not know how effective the A45 will be at correcting coma in faster focal ratios. How fast can I go? 

 

If you have used a Binotron-27 in a large, fast dob, I would love to hear your experience regarding the effectiveness of the coma correction. 

 

[Eddgie]

I don't think that OCS really corrects coma.  It is a cemented doublet.    

 

Mostly, you are just not seeing it because of the reduced true field you get when using the OCS.

 

At least if it did correct coma, one would think if you used it in a coma free scope, it would induce aberrations, and in my 6" Apo, stars were very pinpoint at the edge of the field with the OCS in place. If it were really a coma corrector, I should have seen coma when used in the refractor, and I don't know how an all spherical cemented doublet could be anything other than an amplifier.  

[shams42]

Thanks Eddgie. I am confused, though, because I thought coma was not a property of true field. Holding AFOV constant, as magnification increases, true field decreases. The decrease in field would tend to diminish coma, but the increase in magnification would tend to reveal coma. These two factors perfectly balance one another, so coma should be just as visible at high magnification. And the primary way that binoviewers reduce the true field is by increasing magnification via the OCS. Practically speaking, at some point the blur induced by atmospheric seeing would tend to obscure the coma.

 

Many in this forum have commented on the apparent lack of coma observed while binoviewing. Its a curious phenomenon that I do not understand. I am really interested in any reports or feedback from the folks who have tried the Binotron-27 in fast scopes. Should I limit myself to a 16" f/4.5? Should I consider a 16" or 18" f/4? What about an 18" f/3.8? 

[RobTeeter]

I spent a couple nights observing with Russ Lederman at the Rockland Astronomy Club's Summer Star Party (Massachusetts) last week. We spent hours using my 20" F/3.5 Truss-Dob with my *original* Denkmeier binoviewer outfitted with a new (to me) A45 OCS (upgraded from my old A37) and the new LOA 3-D eyepieces.  I removed my built-in Televue Paracorr Type II (SIPS) coma corrector so that the OCS had room to come to focus, which it did at all three power settings (most likely 1.3x, 1.7x, 2.3x based upon the older power switch unit I have and the spacers I'm using with the new OCS to reach focus).

 

With all of that said, Russ and I both remarked how the views were sharp right to the edge of the FOV in the LOA eyepieces. Whether Eddgie is right or not about the reduced true field being the main reason why the coma was reduced, the important part was that it was reduced (for all intents and purposes eliminated, actually) even at the low power setting of 1.3x.  The views were incredible. At no point did we wish we could somehow get the Paracorr reintroduced to the lightpath. And we looked at objects that spanned the entire field of view like M31, the Double Cluster in Perseus, M8 and some smaller objects like M17, M11, M15, M71, etc. that are in rich star fields so your eyes were looking at the object, yes, but also everything framing the object and all stars were sharp. 

 

So I'm sold!   I wouldn't worry about binoviewing with a faster focal ratio scope. It was a great experience for us under some really dark skies.

 

Good luck,

[Eddgie]

Thanks Eddgie. I am confused, though, because I thought coma was not a property of true field. Holding AFOV constant, as magnification increases, true field decreases. The decrease in field would tend to diminish coma, but the increase in magnification would tend to reveal coma. These two factors perfectly balance one another, so coma should be just as visible at high magnification. And the primary way that binoviewers reduce the true field is by increasing magnification via the OCS. Practically speaking, at some point the blur induced by atmospheric seeing would tend to obscure the coma.

 

Many in this forum have commented on the apparent lack of coma observed while binoviewing. Its a curious phenomenon that I do not understand. I am really interested in any reports or feedback from the folks who have tried the Binotron-27 in fast scopes. Should I limit myself to a 16" f/4.5? Should I consider a 16" or 18" f/4? What about an 18" f/3.8? 

Coma increases as you move further and further off axis.  

 

There is more coma for a star 15mm off axis than for a star 12mm off axis and it is progressive.  The further you go from the center of the field, the worse the coma gets.

With an OCS, you are limited to a narrower image circle than you can get with a native eyepeice.

 

Also, the shallower light cone means that any image degradtion caused by the eyepeice itself is also reduced.

 

It is impossible for a cemented doublet to reduce coma.  

 

I agree that is is hard to see coma when using an OCS, but it is also hard to get much of a true field.

 

In my 12" f/4.9 dob, the edge is very sharp with D21s, but the low power true field is about .67 degrees.  

With the same eyepiece used native, the field would be .94 degrees.  The field then over one quarter of a degree larger with just the eyepiece.  There is a lot more coma in that extra 1/4th degree true field.

 

Compare that to using a 27mm Panoptic with a field stop that is 30.5mm.  That would give a true field that is 1.16 degrees.    This would be an image circle that is 73%  larger than low power mode with D21s.

 

Now, I am not saying that stars are not sharp when using the OCS, but I am saying that the reason you don't see much coma is because you are using a tiny fraction of the mirror. 

 

Essentially, only the center 50%.   Since most of the coma is outside of that circle, you can't see it.

A cemented doublet cannot correct coma, so the only logical explanation is you are simply not seeing far enough away from the central axis to see the coma in the mirror.  You're just using the middle. 

[shams42]

Rob,

 

That's awesome feedback. In fact I was just looking at your 20" f/3.5 this morning. Nowhere near having the cash together to place an order yet, but I sure am dreaming!

 

I've been trying to ray-trace candidate optical systems with the binoviewer in place. It's complicated since there are three different light cones to consider - mirror to OCS, OCS to power switch reducer/barlow, and power switch to eyepiece. 

 

If my numbers are correct, it looks like I might be able to get away with f/3.5 with no aperture reduction! I'm not sure how the off-axis illumination will be affected though. It will certainly be reduced, but by how much? And will it be noticable?

 

Did you notice much dimming of the field with the binoviewer in place?

[Eddgie]

It is not really necessary to ray trace.

 

Is all you need to know is how far your focal plane falls outside of the OTA, and how far the end of the OCS tube extends away from the focal plane.

 

For exmple, my focal plan sits about 4" outside of the tube.

 

When the tube is inserted and the focuser is racked in all the way in low power mode (I can reach focus without extending the OCS tube) then you need to know how far in front of the focal plane the OCS sits.

 

In my scope, the OCS is about 2" inside of the tube when I am in low power mode and the focal plane for the scope is about 4" outside.

This means that I have a 45mm retriction that is about 152mm in front of the focal plane of the scope (where the light cone would normally converge to a point). 

 

IF I divide the 152mm by 45mm, I get 3.37, so that is how fast a light cone I could take with that configuration.

 

So, you really don't need to ray trace.  Is all you need to know is how far in front of the normal focal plane the restriction is, and then you divide that distance by the diameter of the restriction.

 

This is my own calculation for my own scope, but it should be somewhat similar for you I would think.

 

Now, this does however mean that only the very center of the field will be fully illuminated, and off axis, the falloff is going to be extremely high.   Figure 75% illumination falloff at the edge of a 25mm image circle.

Even at f/4.9, my falloff is on the order of 60% in low power mode.  Was perhaps 70% with 24mm wide field eyepeices.  The outside of the field was quite noticeably darker at the outside under my somewhat bright skies.

 

Under dark skies, it is a lot easier to tolerate off axis illumination falloff because the field still looks mostly black.  When the sky is brighter though, the sky glow in the center of the field will appear much brighter than at the outside, you you get this kind of doughnut appearance.

[shams42]

Thanks Eddgie, this is helpful. And yes, mostly what I was trying to go was to determine where the OCS will be located with respect to the converging light cone. It's complicated because this depends on whether you use the low-, medium-, or high-power arm of the power switch.

 

I have read that fast focal ratio systems tend to have much slower light falloff toward the periphery of the field than do slow systems. I think that's a general principle of Newtonian design, and I do not know if it holds when you place an obstruction that may choke off some of the off-axis rays far ahead of the focal plane.

 

I assume that the issue you are talking about is only a problem with large field stop eyepieces in low-power mode, correct?

 

I am happily using a pair of Pan 24s in lower power mode with my B27. I can barely see the vignetting -- in terms of the sky background darkening -- all even under my suburban skies. I can see that dim stars disappear near the edge of the field, and if I defocus a bit I can see how the aperture is being cut off. Surprisingly, though, I find these eyepieces very satisfying to use with the binoviewer. I just wish they had a few more mm of eye relief for my eyeglasses.

 

I will also say that I used a Denk I binoviewer with a power switch diagonal in my C11 a few years ago. I could not stand to use reducer mode with the Pan 24s in that setup because the vignetting was severe. So I do have a threshold where vignetting really bothers me, but using the B-27 at f/4.8 with Pan 24s has not reached it. That's why I'd love to spend a few minutes with by binoviewer in a nice f/3.5 scope so I could see for myself.

Edited by shams42, 27 August 2015 - 12:29 PM.

[Eddgie]

The only time there is a concern is when the low power arm is in.

 

This will require considerable in-focus travel.   In many cases, the in-travel requirement will be enough that the front of the OCS will extend into the light path of incoming light, though that hurts nothing in iteself.  Only explaining how it behaves.   Once you go to medium power, the focuser will have to be racked out considerably.

 

For example, in my scope, with low power mode, my focuser is racked in so far that I only have about 2-3mm of travel left.  In this configuration, the front of the OCS tube extends maybe 1" in front of the primary mirror and is about 6" in front of the focal plane (and again, you don't need to worry about ray tracing with all different configurations.   The only thing that matters is how far in front of the focal plane the 45mm restriction is).

When I go into medium power, I have to rack the focuser outabout 1.75".  Now, the nose of the OCS is just about even to the inside wall of the tube.  Completely out of the light Path and less than 4" in front of the focal plane of the mirror, so now, the light cone is almost fully unvignetted.  

When I go to high power, the amount of focus travel is far less than what is required to go from reducer mode to mid power mode.    Now, I only have to rack out an additional half inch or so.

 

My focuser travel is only about 2.3 inches, and I only just barely reach focus at both ends without extending the OCS tube (which puts it closer to the light cone) or slipping the binoviewer out of the focuser to reach focus in high power mode.

Anyway, There should not be any concern in medium and high power mode.   The OCS is much further away from the light cone in these modes than in reducer mode. It is only reducer mode that poses a potential problem.

 

It is the same in SCTs.  It is only the extreme in-focus requirement when using the reducer that becomes an issue even on SCTs.   In medium and high power, the mirror moves far enough back to focus that any damage done in low power mode is undone in medium and high power modes.

 

You should be OK though down to about f/3.4 or maybe a bit faster than that.

 

How a Newtonian behaves, and how a Newtonian behaves with a binoviewer and OCS may be two very different things.   In general, I think you are right, that field illumination improves with aperture, but that is without a 45mm restriction in the light path.    

 

The Geometry of it suggests that there will be considerable off axis illumination falloff in low power mode, but that is not as serious as it sounds.  It can look objectionable under bright skies, but under dark sky, it can go unnoticed.

[faackanders2]

I have never used a paracorr II with my binoviewer in 17.5" f4.1, but I do use a paracorr II with single eyepieces.

[shams42]

Thanks all. I will consider a 20" f/3.5, 18" f/3.75, and 16" f/4. It's going to be quite some time before I am ready to buy, but man have I got the fever!

[faackanders2]

Denk II id 17.5" f4.1 Dob and like it.  LoA 21 3D may have brought me back to using the binoviewers again; but I had a long haiatus with the advent of 100-120 AFOV (ultra wide TFOV) single eyepieces.  Neither will be going.