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New refractor OTA. Length - keep options open?

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#1 GHarris

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Posted 03 February 2013 - 09:46 AM

I'm in the process of ordering a new 6 inch f/8 ED refractor. I'm having a non-standard OTA made so one option I'm considering is to have a shorter OTA to accommodate binoviewing. I then wouldn't always need barlows or any of the power-adjusting doohickeys that binoviewers often require on "normal" OTA lengths to come to focus. But I have no experience with telescope binoviewers, I just know that I enjoy using binoculars a lot and the idea sounds interesting to me.

I'd normally do the respectful thing - do a heck of a lot of reading on past topics on this forum before asking questions, if I even still need to ask after properly "doing my homework", but I'm not sure how much time I have before I have to confirm the specs of this new OTA for it to be built. So I'd appreciate any advice you can spare the time to give and I hope you'll forgive my ignorance.

I understand that for planetary/lunar viewing I might quite possibly want to use barlows etc. regardless because it's a bad idea to use sub-10mm focal length eyepieces in binoviewers (and I'd want to use, say, 200x magnification), and that this fixes the problem of back focus requirements for high magnification BVing. So if I understand this correctly, the only advantage of the short OTA is to permit binoviewing with lower powers/wider tFOVs. I'm wondering, therefore, how the advantages and disadvantages (of keeping open the option of wide field binoviewing) stack up.

The broad areas of uncertainty for me are:
- How useful is it to have large tFOVs when binoviewing? This is for a 6 inch f/8 refractor, so 24mm Panoptics would give me about a 1.35 degree tFOV, 50x magnification and 3mm exit pupil (minus the brightness drop from binoviewing). That's the limit of 1.25 inch eyepieces. With a 6 inch refractor, is low power binoviewing worth the bother? Maybe there isn't enough light to spare when viewing even the relatively bright DSOs, which are a bit dim in the first place at 6 inches of aperture. A quick, rough calculation suggests it'd be like having the light gathering of one 4.5 inch scope for each eye (though with the resolution of a 6 inch). Perhaps I should only be considering using a binoviewer on this kind of telescope for planetary and lunar views, where there is enough light to spare and I would just use the standard binoviewing tricks of the trade to achieve focus. What do you think?

- As a minor aside, are there credible options for binoviewing with 2 inch eyepieces for wider tFOVs? I can imagine a number of issues - for example 2 inch eyepieces may be too big to fit one's nose between, the whole system needs even more backfocus, etc. And since most telescopes, built the usual way, are unable to accommodate really huge backfocus the market for such things must be small.

- Suppose I end up not using the binoviewers a lot (or not at low powers where the OTA length change is helpful), and mostly stick to mono viewing. Will the design changes I've had made to the telescope feel like a nuisance? Or to put it another way, is there any harm to keeping the option of wider field binoviewing open at this point with a shorter OTA? I know that basically it'll mean I need a lot of focuser travel, plus an extension tube, when monoviewing. Is this likely to shift the balance point of the telescope rather far forward when not binoviewing (with the weight of the focuser closer to the middle)? Is there a plausible risk of there being a bit of flex at the eyepiece end of the scope (I intend to use a good quality focuser, of course)? Is there going to be a lot of faffing about when monoviewing because of the extension tube? Or any aspect I haven't thought of that would be less than ideal?

The answers to most of the questions I'm asking are highly subjective but I'd appreciate your opinions regardless, so that I can make as informed a decision as possible.

One more thing. How do I know how much less OTA length to ask for (than the standard amount for a 6 inch f/8 scope)? This is something for me to talk to the telescope maker about of course but I'd appreciate input from any source. I guess it is slightly affected by which binoviewer I choose (some have longer light paths than others) which is one reason why I'm asking about it here.

#2 Astrojensen

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Posted 03 February 2013 - 10:27 AM

With a 6 inch refractor, is low power binoviewing worth the bother?



Oh yes! At least under dark skies. I have a 150mm f/8 achromat, that I use with Baader Maxbright binoviewers and 25mm Zeiss eyepieces. It is absolutely superb under dark skies. The one degree field is large enough for most objects. If I want to go wider, I go single and use a 30mm ES 82. Both are awesome.

Going 2" binoviewer is extremely expensive.


Clear skies!
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#3 Eddgie

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Posted 03 February 2013 - 10:34 AM

First, and of vital importance...

If you cut down the tube, it is crucial that you also calculate the change in the lenght of the focuser tube, or consider going to a larger focuser tube design.

If you simply cut the tube and use the same focuser, moving the front end of the tube forward may (probably) cause it to push into the light cone with the in-travel required for a binoviewer. This will reduce the apeture of the scope.

You may want to ray trace the system so that you are sure your focuser tube is not going to reduce the aperture.

The problem with a 2" binoviewer is that now you need a bigger diagonal (more light path) and of course the light path though the binoviewer itseld if longer, so now you need to shorten your tube even more, and once again, this means that you are even more likely to have problems with the front of the focuser tube cutting into the light path and reducing aperture.

Only you can decide if this works for you, but you may wind up using 2 or 3 extensions to reach focus when not in bino-mode, and this puts a lot of pressure on the focuser itself because the very long tube racked out of the back for a standard 2" diagonal configuration will present considerable leverage.

If the Telescope maker will ray trace it for you, then you only need to provide the total back focus you require and they can trace that, but you may need to go to a 4" focuser if you want to reduce the number of extensions you need.

As you said, only you can answer the "Is it worth it" kind of question.

In my own experence, keeping the light path as short as possible is of paramount importance for any binoviewer application, but if you are going to customize a telescope, you really need to consider that there is far more to it than chopping the end of the tube off. You don't want to turn an already small aperture into an even smaller aperture, and you may not want to use a configuration that requres a very long extension to use it in mono-mode.

My specific advice.... Pick the system you think you want, which is the binoviewer ane the prism. For example, the baader T2 standard prism has a light path of 38mm. A typical 2" diagonal will have a light path of about 100mm.
A Baader Maxbright would have a light path of about 110mm.

Using a Baader Maxbright/T2, you need about 148mm of inward travel. This configuration might reach focus with no cutting, but limit you to a 22mm field stop eyepiece, or it might take a 1.25x GPC.


Gonig to a 27mm prism, you would add another 15mm or so of infocus (tube shortening).

But if you were to go tot a 2" binoviewer, you might wind up with a light path that is 150mm long in the binoviewer (maybe more.. Ask the vendor how long it is) and another 100mm for the diagonal, for a total of over 200mm. This could mean chopping 4" to 6" off of the tube.

And if the tube has a baffle, you now need to make sure the front of the focuser tube is not going to hit the baffle, so the baffle needs to come out or be opened up.

Dude, you need to do your homework on this. Get as exact a requirement as possible and make sure you have a ray trace to ensure that the focuser tube lenght is adjusted properly to avoid aperture loss and the avoidance of an unusual amount of extension.

As for the 2", I think the 27mm Pans are not bigger than the 24mmm Pans, but this is a rather small increase in true field size. Beyond this, it gets more difficult.

Or, you can do like I have been doing when you need a big exit pupil but don't care about apparent field. I have been using a pair of 40 plossls. They work great when I need the maximum brightness in my C14. The small apparent field does not bother me as much as when monoviewing.

Or you can go somewhere between (32mm Plossl or 35mm Ultrascopic/Parks/Ultima).

Again, do your homework, To ensure you are going to get the full benefit from the tube shortening, it is essential that you know how much light path you require and what adjustments need to be made to the focuser tube lenght or diameter.

#4 EdZ

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Posted 03 February 2013 - 10:54 AM

the irreversible decision to cut the tube shorter too accomodate a binoviewer without need for a higher powered power adjusting doohickey can be avoided by purchased a very low powered power adjusting dohickey from Seibert Optics (or Baader).

A 1.25x power adjustment is almost a negligable change and results in a slight shift in eyepiece choices and maintains original scope tube length.

Frankly, OCA attachments to a binoviewer to accomodate focus is simply not that big a deal. I would never cut a telescope tube to eliminate need for an OCA, unless of course the focuser drawtube was long enough to accomodate all eyepieces used in a normal funtion without any further need.

edz

#5 GHarris

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Posted 03 February 2013 - 11:18 AM

Thanks for the warnings. I will be very careful if I do go ahead with this. I'm acutely aware of my current ignorance!

EdZ, it's good to know that power-adjusting doohickeys are not necessarily all that much of a bother ;)

I was already strongly considering a 3 inch focuser (bit of an off-topic long story as to why) so this would help with avoiding cutting into the light path. Though the focuser travel on it would be about 4 and a half inches which might be rather a lot when a shortened tube is added to that. I will indeed be talking to APM about this so I'll see what they have to say.

Maybe this really isn't worth goofing around with. But then again, a 1/3rd of a degree of extra tFOV (comparing with Astrojensen's example) would provide a nice bit of extra actual viewable area, wouldn't it? I'm getting the impression only 1.25" binoviewers are really at all practical, regardless.

By the way, thanks Astrojensen for reassuring me that low-power binoviewing with this amount of aperture has potential. I guess that although it already affects a 6 inch refractor's arguable weakness for binoviewing, aperture (compared with most other scopes people seem to binoview with), it also plays to a refractor's strength by allowing two eyes to make the most of the good contrast on offer. I wasn't sure where the balance was on that issue.

I absolutely do intend to do a lot of reading, and as I say normally I'd prefer to get myself a lot more prepared before posting to avoid asking particularly silly questions and making daft remarks about doohickeys. Just not sure how much time I have available to think about this before I need to commit to a final spec on the OTA, and I can clearly use your help! I was at least already aware of the additional confounding factor of probably needing to know which particular binoviewer I would want to use. That extra puzzle makes it all look a bit reckless when I have no experience using binoviewers yet, to help me choose one, on top of all the other question marks. If all else fails, and I'm feeling too rushed, I'll fall back on a standard tube length and make do with the choices that leaves me with when it comes to choosing a more patiently researched binoviewer later.

#6 GlennLeDrew

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Posted 03 February 2013 - 12:32 PM

If binoviewers were my thing, I'd most definitely have made/modified a dedicated scope for the purpose. To obtain the widest field, I'd strive for close to the fastest f/ratio the BV by itself (no OCS) could accommodate. This would be about f/5, and perhaps a bit less.

Given your desire for a wide field, why not obtain your fine refractor in the 'standard' length, and look at either a fast achromat or newtonian you could dedicate to wider field observation (of DSOs, I presume)?

#7 Tamiji Homma

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Posted 03 February 2013 - 12:50 PM

I have a 6" f/8 refractor with generous backfocus, about 316mm.

Indeed, it is awkward to use without binoviewer since you need long extension tube for cyclops mode.

I usually add 3.35" long extension tube at end of focuser. Here is a photo taken with 3.35" extension tube in place and 2" mirror diagonal, when it comes focus to 1.5 miles away (near infinity). So this configuration has about 230mm backfocus.

Posted Image

If you have an option of OTA length, I would recommend to have 225-250mm backfocus with at least 100mm focuser drawtube travel distance.

Tammy

#8 GHarris

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Posted 03 February 2013 - 02:53 PM

Glenn, you are talking a lot of sense and postponing this idea, or anything like it, for a more dedicated scope might be the best thing to do. One reason I considered trying to make the most of this 6" is that I don't want too many scopes in the stable, and I'm probably trying too hard to make this one do everything. I'm prone to overthinking purchases like this. I have 16x70 binoculars which I enjoy using and they cover really wide fields well for me. I wasn't sure if the gap in between them and this 6" f/8 was enough to justify any other scopes (in terms of FOV and aperture - I know portability is one advantage of something in-between). I'm also planning on using this scope for single-eyed outreach (I think binoviewers would be too much for a newcomer to understand unless they're particularly patient and enthusiastic), too, so I probably am trying to make this scope wear too many hats.

Tammy, thanks very much for posting with your personal experience of this kind of setup. The picture really shows how extreme it makes things at the focuser end. Can I confirm that I've understood you correctly... You recommended 225-250mm backfocus but at the start of the post you said your refractor has 316mm of backfocus. Can I ask why you chose the larger amount? In the example picture you provided you are using only 230mm (though it looks like a bit more than that - perhaps I'm misjudging it, I guess the distance to the first lens in the eyepiece isn't as far as first impressions suggest).

Would you mind saying a bit more about what's awkward about using it with all the extensions, without the binoviewer?

#9 Mark9473

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Posted 03 February 2013 - 03:48 PM

It doesn't really have to be all that difficult. On my 107 mm f/6.5 I have 180 mm backfocus, and with a 3" focusser having 115mm travel, I can get my Maxbrights to focus without needing to add a corrector lens or magnifier.

I find myself liking low power binoviewing (28x with a pair of 25mm orthos) a whole lot more than I like high power binoviewing, so I wouldn't abandon that idea.

#10 Tamiji Homma

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Posted 03 February 2013 - 04:16 PM

Tammy, thanks very much for posting with your personal experience of this kind of setup. The picture really shows how extreme it makes things at the focuser end. Can I confirm that I've understood you correctly... You recommended 225-250mm backfocus but at the start of the post you said your refractor has 316mm of backfocus. Can I ask why you chose the larger amount? In the example picture you provided you are using only 230mm (though it looks like a bit more than that - perhaps I'm misjudging it, I guess the distance to the first lens in the eyepiece isn't as far as first impressions suggest).

Would you mind saying a bit more about what's awkward about using it with all the extensions, without the binoviewer?


316mm backfocus wasn't really designed to be but it ended up to be that way :)
I replaced stock focuser with Startlight Instruments' FT3545+custom adapter for my scope. I think I can comfortably use 2" binoviewer with much backfocus without OCA.

I have another scope 5" f/7 with backfocus 245mm-ish.
Posted Image

When you use 2" mirror diagonal (lightpath length is typically somewhere 104-110mm), you wouldn't need another extension tube as long as you have 100+mm focuser travel distance.

I often use Baader T2 Prism Star Diagonal (much shorter lightpath), I need to add additional extension tube to come to focus to infinity. That's where awkwardness is coming from.

Here is another example when you use Barlow lens (this case Powermate 4x) in front of diagonal. As you can see, I use 3 extension tubes to come to focus to infinity. This is a lot of length/weight. I wouldn't need that much if OTA was longer.

So 230mm is somewhere I can live with, not too much but there is good chance to come to focus to infinity with inward focus demanding eyepieces (such as Orion Ultrascopic 35mm) when you use binoviewer without OCA.

Posted Image

It would be ideal to have robust extension tube between OTA and focuser.

Tammy

#11 johnnyha

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Posted 03 February 2013 - 07:18 PM

If you get the length of the tube right and you have a long drawtube you may not even need extensions. I have a 6" f8 with a FT3545 focuser with 4.5" drawtube, I can reach focus at 1X to 2.6X in my binos, and comfortably reach focus with all my eyepieces, without the need for any extensions. At 1X binoviewing I have 1/2" of backfocus left, and at 2.6X I have the drawtube extended and 1/2" remaining. It is definitely worth it to figure this out if you can go with a FT3545.

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#12 faackanders2

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Posted 04 February 2013 - 11:06 AM

How useful is it to have wide TFOV when binoviewing?

Wide TFOV is the limit of binoviewing. It is easy to magnify/multiply power, but very difficult to get lower powers.

#13 Eddgie

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Posted 04 February 2013 - 12:29 PM

It is useful for people that like to sweep.

Getting one degree is pretty easy. I can get that out of a C5 with 24mm eyepeices.

And of the 8000 NGC objects out there, 98% of them will easily fit into a one degree field.

But true field is only part of the reason for using low powers. A bigger exit pupil is equally important to some viewers.

I have been using my 40mm Plossls a lot in my C5 and C14 not becuase they give a wider true field than the 24mm Wide Field eyepices I was using. The field is about the same size.

But the binoviewer already robs you of some brightness.

Brightness is mostly a function of exit pupil, and if you want a brighter image from a binoviewer, one way to keep it about as bright as monoviewing is to simply use a mangnification that gives about a 30% bigger exit pupil.

The 40s give a much bigger exit pupil than the 24s, and even though the apparent field is narrower, if it fits into the field, it looks brighter in the 40s.

For clusters, you don't notice it, but for nebula and galaxies, it makes a big difference.

Anyway, two reasons for low power... Bigger true field and bigger exit pupil. Since going to the 40s, I feel that I have a lot of the brightness that binos cost me back. The view is not as immersive, but binoviewing to me is still more immersive than monoviewing, so it is a balance.

#14 GHarris

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Posted 04 February 2013 - 02:27 PM

If you get the length of the tube right and you have a long drawtube you may not even need extensions. I have a 6" f8 with a FT3545 focuser with 4.5" drawtube, I can reach focus at 1X to 2.6X in my binos, and comfortably reach focus with all my eyepieces, without the need for any extensions. At 1X binoviewing I have 1/2" of backfocus left, and at 2.6X I have the drawtube extended and 1/2" remaining. It is definitely worth it to figure this out if you can go with a FT3545.


Which binoviewer are you using? Is it one with a particularly short light path? I'm not sure how else I can understand the disparity between your example and Tammy's. In fact even then the difference is striking. Have you had to pick your eyepieces carefully, with regards to whether they need more or less backfocus, as well? As I mentioned I am considering a 3" APM focuser with a 4.5 inch drawtube.

#15 johnnyha

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Posted 04 February 2013 - 07:25 PM

I am using the MkVs with the T2 prism diagonal to reach focus at 1X, so yes that is about the shortest light path for a binoviewer. On a 6" f8 scope the difference between 1X and 2.6X is about 3.5" of drawtube travel with this setup. As far as eyepieces I use mainly orthos but for 1X usually the 24mm Pans, with no problems reaching focus.

#16 MrGrytt

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Posted 04 February 2013 - 07:28 PM

Most Astro-Physics telescopes are built with somewhere between 160mm to 165mm of backfocus. My particular AP scope has 163.5mm of backfocus. That is just enough to get you by if using the Baader Mark V with the Baader prism diagonal and a 1.25x corrector. Using it with no corrector wouldn't work. Using it with a normal 2" diagonal wouldn't work since the light path of a normal diagonal is a fair amount longer.

If you want to use a normal 2" diagonal you would need about 210mm to 215mm of backfocus built into the scope.

Years ago the original TMB German tubes were built with around 230mm of backfocus and were designed to be used for normal viewing with a 100mm extension in place. When bino-viewing you removed the extension.

If you want to be sure your scope will be bino-friendly I would highly recommend that you plan to use an extension for normal viewing (usually something in the 60mm to 100mm range would be enough). Build the scope with 230mm of backfocus and you'll be in good shape.

It is far better to need to use an extension for normal viewing than it is to need to cut the tube off to come to focus with a bino-viewer.

If the focuser has a lot of travel the extension may not always be needed but it's good to use so the focuser doesn't have to extend all the way out.

Harvey

#17 Jeff B

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Posted 05 February 2013 - 10:17 AM

I would absolutely build the scope "bino-friendly". Just specify how much in-travel you want left when configured for for bino-viewing with a 2" diagonal and viewer of choice. When , building a tube, I typically leave about an inch to inch and a half. That allows me to use my 2" solar wedge, which has a slightly longer light path than my 2" AP diagonal. Prism diagonals may not be the best choice with the new APM doublet but ask Markus for advice. Regardles, specifying the residual in-travel means also picking your diagonal and viewer up front. I use the 2" AP and the Denk II with the power switch with great success. With the power switch, you can get 1X and ~2X (which requires out travel) when not using the OCS. You can only get to focus with the focal reduction mode if you look staight thru (which can be fun for the south summer milkyway actually). You can still use the OCS though but you may need to apply a short extension tube...no problem.

What you end up with is a scope with exceptional versatility.

One look at the double cluster using the binos and a 26-32 MM Plossl pair will completely convince you.

Jeff

#18 GHarris

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Posted 07 February 2013 - 04:27 PM

I think I'm going to chicken out of ordering a shortened OTA. I'll only have the OTA built to be able to accommodate a binoviewer with OCS/power switch (one that focuses at closer to normal mono-viewing positions). A few thoughts on why:

- The scope will be even more front-heavy than a 6 inch refractor generally is, with the focuser moved further up the tube.
- The extension tube malarkey just looks a little bit... extreme. I'm not sure what the right word is, really. I find Tammy's photos of his setups a bit intimidating. I can't help but feel it's going to be a nuisance a lot of the time. I'm not sure if I'd be comfortable with it.
- All I'd gain from it is working at 1x the eyepiece power instead of ~1.3x. I'm not sure if that difference is enough. In viewable area it's certainly something, but I imagine that the brightness of the view might be too low regardless. Some very crude maths, which could be completely wrong (let me know!), tells me that with a 24mm panoptic I'd have the equivalent of a 2.1mm exit pupil when binoviewing with short OTA and the equivalent of 1.65mm when binoviewing with a 1.3x increase in power and a normal-length OTA. I'd probably end up using long focal length plossls for any DSOs that seem suitable.

#19 MrGrytt

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Posted 07 February 2013 - 11:28 PM

The 316mm of backfocus mentioned in Tammy's scope is a HUGE amount and far more than the normal bino-friendly amount of around 170mm. As I mentioned, AP builds around 165mm of backfocus into their scopes.

One other important thing with the optical correctors. They are not only used to increase the magnification but they are also necessary with the Baader Mark V.

From the AP web site: "This optical element eliminates the slight color error and spherical aberration that a prism beam splitter naturally introduces into the light path of all binocular viewers."

For critical viewing you need to use them.

If you go with 170mm of backfocus and have 4.5 inches of drawtube travel you won't need to use an extension for normal viewing. However, at times you may want to use a short extension just to keep from having to rack the focuser out so far.

Harvey

#20 Jeff B

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Posted 08 February 2013 - 04:23 PM

Exactly. Done right, no extension tube is necessary with single eye view, however, the focuser will be racked almost all the way out. A single 1.5" to 3.5" extension tube "fixes" that and you can still pop in the corrector if you want higher power (I use the power switch on my Denks).

Also the difference in tube length is only ~4" and with the extra weight of the binoviewer balance point is not an issue.

Jeff

#21 GHarris

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Posted 11 February 2013 - 08:47 AM

I was a bit overwhelmed by it all when I wrote my last post. I've since managed to cobble together some rough numbers that I understand in my own head, which I'll share with you at the end of the post. Please point out the inevitable mistakes (this is only a rough draft).

I rashly ordered the new BINOTRON-27 system from Denkmeier while it was on sale. I know the Mark V is highly regarded but it seems particularly expensive, I liked the idea of easy collimation adjustment on the BINOTRON should it ever be needed, as well as the ergonomics of the focus adjustment etc, and I imagine I can sell it for a fair percentage of the sale price if I find to my surprise that I don't like binoviewing.

I don't have a number for the lightpath length of the Binotron yet so have used the numbers for a Denk II as a stand in (which I will want to correct as my plans firm up).

The general idea, lifted from Eddgie's old thread, is to use the Baader T2 prism diagonal, with low profile adapters, with the Denk binoviewer when trying to binoview at 1x the normal eyepiece magnification. The T2 prism rig would mitigate the back focus requirements. There's a good chance that I won't implement this setup (for 1x binoviewing, instead of 1.3x+ with OCS/power switch) at first, if ever, but if the numbers hold together I will at least have the choice of making sure that the refractor is configured to give me the option in the future (which is the choice I do have to make now rather than later). Much/most of the time I would still use the binoviewer with the OCS and power switch for convenience and magnification flexibility as opposed to assembling this rig. But it's there if I want it.

A couple of the threads which informed my thinking:

Lightpath length estimates (2nd to last post, by Tammy, in this thread)
Thread on using a Baader T2 prism diagonal with Denkmeier which links to Precise Parts, who'll make a T2 to Denk thread adapter.
2" diagonal lightpath estimates
An adapter I'll need


So. Lightpath estimates as I understand them:

Baader T2 Prism: 76mm with "Clicklock" eyepiece holder (~32mm). Therefore about **44mm** when bare(?).
2" to T2 adapter: 2mm (says link above. I know, strangely tiny)
T2 to Denk adapter: ~8mm (7.something)
Denk II (stand-in): 117mm

44+2+8+117 = 171mm! Less than expected.

The out focus of the system needs to be able to counter this completely to come to focus with normal eyepieces. However the long lightpath of the 2" diagonal I use can help to eat up a lot of this.

Baader 2" Clicklock diagonal: 114mm.

171-114 = 57mm focuser travel needed. Surprisingly accommodating. Too good to be true?

Have I counted anything twice? E.g. is the benefit from the 2" diagonal only the difference between its lightpath and that of the Baader T2 prism diagonal, rather than the 2" diag's full lightpath?

#22 tomharri

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Posted 11 February 2013 - 10:20 AM

Can you afford a Takahashi? They provide enough back focus to go binoing with no barlow. They have these screw on focuser tube extensions that provide a solid, straight connection from focuser knob to eyepiece clamp, bino clamp. But I did have to use a 1.25" diagonal.

#23 GHarris

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Posted 11 February 2013 - 11:35 AM

Of course, yes, sorry, I've been waffling so much my original purpose for this thread has been lost in the noise. I am ordering a refractor whose amount of backfocus I have the option to specify. But I was concerned about the clumsiness and unwieldiness of having to use a lot of extension tubes etc. to get into focus when monoviewing. I wasn't sure how much backfocus I'd need. I have only just begun to get my head around understanding how to know how much I will need. Though I am sure I will have thought something through wrongly in my workings in the previous post so I'd welcome some criticism.

One more thing I forgot to mention. I wondering if I am making the right choice with the Baader 2" Clicklock diagonal I mention in my sums above. I like the idea of it for its "oversize" mirror size as I think I'll want, when monoviewing, to use a max-possible-tFOV eyepiece so I will want a diagonal to be able to serve a large fieldstop. Many options in 2" diagonals limit unvignetted field stops to 42mm or less, I've read.

The William Optics 2" diagonal, which is otherwise a strong candidate because it allows the Denk OCS to be screwed directly into it for optimal placement, may only have 40mm worth of clear aperture. Not sure exactly where this leaves me with the OCS placement and resulting magnification options in the power switch.

As a very minor point, the Baader diagonal has a marginally longer lightpath than most (which as I understand it is a good thing when trying to make up for having quite a lot of backfocus without wanting to use extension tubes where feasible).

#24 GlennLeDrew

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Posted 11 February 2013 - 03:55 PM

Will you have your BV and assorted bits in hand before putting the order on the scope? If yes, do your own measurements; it's easy!

Obtain any simple achromat having a focal length of, say, 300mm. Set it up on a table top. Focus on something across the room, and locate the focal point with a mark. Set up your kit until you see a focused image of the target. Mark the location, and measure the distance between this mark at the focal point you located earlier. This is the required optical path length.

You need not worry about focusing on a target much nearer thaninfinity. All that matters is that you have a defined focus for some object and that the distance between it and the lens does not change.

The lens need not be of high quality; you only need to be able to have an image good enough to ascertain focus reasonably reliably. You could even use a single element lens, stop it down to about f/10 or f/15, and reduce chromatic aberration even further with a colored filter (green perhaps being best.)

#25 GHarris

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Posted 12 February 2013 - 04:19 PM

I did a Fisher Price raytrace earlier today with some drawing software. See attached image. On the left is the 152mm objective. The two large horizontal lines on each OTA are the 1010mm OTA (190mm backfocus). The two smaller horizontal lines within the OTA represent the 3 inch APM focuser with its 114mm drawtube. All to scale.

On the higher of the two OTAs I have placed a 27mm vertical line, on the right hand side, representing a 1.25" field stop at the position of greatest infocus (190mm of backfocus). This is an approximation of my binoviewer setup without an OCS (though I would also use a Baader T2 prism), intended for same-as-monoviewing magnifications with a given eyepiece.

The outer bounds of the light cone are represented by the diagonal lines, from the edges of the line representing the aperture on the left to the edges of the line representing
the field stop on the right.

On the lower of the two OTAs I have placed a vertical line representing a maximum size 2" field stop (46mm) at 1200mm out, the normal focus point. I did this and drew the light cone before moving the focuser tube out in the sketch to a realistic position, and then decided to leave the focuser fully racked into the OTA to provide an unrealistic but harshest-possible test.

The shrinking of the final image for posting here has made this less obvious in the case of the 2" field stop, but in both cases the lightpath is not intruded upon.

I later tried adjusting the top sketch for a 34mm field stop in the same position (the clear aperture of the Baader prism) and found that it made little difference. The diagonal lines were still well clear of the focuser tube ends.

There is still plenty of reading and checking to do but after my earlier confusion it is beginning to look like there's (unvignetted) light at the end of the tunnel.

Attached Files








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