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Equipment Discussions >> Binoviewers

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GHarris
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Reged: 06/06/09

New refractor OTA. Length - keep options open?
      #5659620 - 02/03/13 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.


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Astrojensen
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5659676 - 02/03/13 10:27 AM

Quote:

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!
Thomas, Denmark


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Eddgie
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5659690 - 02/03/13 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.


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EdZ
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5659722 - 02/03/13 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


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GHarris
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Reged: 06/06/09

Re: New refractor OTA. Length - keep options open? new [Re: EdZ]
      #5659759 - 02/03/13 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.

Edited by GHarris (02/03/13 11:24 AM)


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GlennLeDrew
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5659901 - 02/03/13 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)?


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Tamiji Homma
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5659931 - 02/03/13 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.



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


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GHarris
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Reged: 06/06/09

Re: New refractor OTA. Length - keep options open? new [Re: Tamiji Homma]
      #5660142 - 02/03/13 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?


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Mark9473
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5660229 - 02/03/13 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.


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Tamiji Homma
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5660278 - 02/03/13 04:16 PM

Quote:

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.


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.



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

Tammy


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johnnyha
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Re: New refractor OTA. Length - keep options open? new [Re: Tamiji Homma]
      #5660601 - 02/03/13 07:18 PM Attachment (25 downloads)

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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faackanders2
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5661534 - 02/04/13 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.


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Eddgie
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Re: New refractor OTA. Length - keep options open? new [Re: faackanders2]
      #5661667 - 02/04/13 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.


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GHarris
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Re: New refractor OTA. Length - keep options open? new [Re: Eddgie]
      #5661900 - 02/04/13 02:27 PM

Quote:

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.


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johnnyha
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5662343 - 02/04/13 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.

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MrGrytt
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5662346 - 02/04/13 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


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Jeff B
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5663257 - 02/05/13 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


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GHarris
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Re: New refractor OTA. Length - keep options open? new [Re: Jeff B]
      #5667430 - 02/07/13 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.


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MrGrytt
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5668108 - 02/07/13 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


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Jeff B
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Re: New refractor OTA. Length - keep options open? new [Re: MrGrytt]
      #5669349 - 02/08/13 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


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GHarris
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Re: New refractor OTA. Length - keep options open? new [Re: Jeff B]
      #5673636 - 02/11/13 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?


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tomharri
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5673766 - 02/11/13 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.

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GHarris
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Re: New refractor OTA. Length - keep options open? new [Re: tomharri]
      #5673889 - 02/11/13 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).


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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5674300 - 02/11/13 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.)


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GHarris
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Re: New refractor OTA. Length - keep options open? new [Re: GlennLeDrew]
      #5676189 - 02/12/13 04:19 PM Attachment (18 downloads)

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.

Edited by GHarris (02/12/13 04:23 PM)


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GlennLeDrew
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5676941 - 02/13/13 01:53 AM

Why the different distances for the termini of the focus in each diagram? The appearance is of a shorter focal length in the upper diagram. If both diagrams are to the same scale and for the same focal length, the ray bundles in both cases must terminate at the same focal surface distance behind the objective.

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GHarris
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Re: New refractor OTA. Length - keep options open? new [Re: GlennLeDrew]
      #5676973 - 02/13/13 02:51 AM

I knew I'd have done at least one thing wrong. That's why I posted it up! Don't be under any illusions that I know what I'm doing (I'm sure you weren't). I can see now how the higher up picture is thoroughly daft. It was the one I did first, and then I started to adapt the lower image from a copy of it. Might as well have just deleted the original.

Is the lower picture more sensible? The focuser is fully racked in, the 46mm field stop is 1200mm away from the objective and the light is not obstructed. Let me know what else is fundamentally wrong, I'm sure there will be something.

I totally agree with what you said in your previous post, by the way. It would be better to measure the focus distances myself. But the way delivery times for the Binotron are likely to stack up relative to the "deadline" for agreeing on the final spec to build the refractor OTA I'm not sure I will have a chance to. If I can, I will, but meanwhile I can only speculate.


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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5677198 - 02/13/13 08:25 AM

I see what you are going for. In an f8 scope the light cone will shrink 1" for every 8" of travel. I would draw the cone as just that - a cone that terminates into a point at 6X8=48". I may be wrong too but that is what makes sense to me.

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Tamiji Homma
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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5677347 - 02/13/13 10:12 AM

Here are a few examples of what you can expect with 215mm backfocus refractor with DenkII binoviewer without OCA.

2mm AP MaxBright 2" diagonal -> 2" Denk nosepiece -> DenkII -> Pano 24
6mm Denk R1 diagonal with PowerSwitch -> Denk Dovetail adapter (DC) -> DenkII -> Pano 24
39mm Baader T2 Prism Star diagonal -> Starlight Instruments Baader Quick Changer-DC -> DenkII -> Pano 24

The first column is how many millimeters left on focuser to go when you focus to infinity.

As you can see, if you are planning to use 2" diagonal (you are planning to use Baader 2" Clicklock?) with
binoviewer, you may run out inward focus distance without OCA if you wish to go low power and wider view. Astro Physics MaxBright diagonal is one of the shortest light path 2" mirror diagonal I have.

You might want to take a look at this thread in Eyepiece Forum. Toward end of thread, Brandon posted
Excel spread sheet to show you how much focus distance is required (compare to other eyepiece)
Master Eyepiece Focus Distance Chart

Or you can directly download the spread sheet from
http://www.cloudynights.com/ubbthreads/download.php?Number=4378893

When inward focus distance is about to run out, every millimeter counts

Tammy


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Re: New refractor OTA. Length - keep options open? new [Re: Tamiji Homma]
      #5677409 - 02/13/13 10:48 AM

The most important measure is the actual length of the focuser you intend to use when fully racked in, from where it meets the tube to the exact end of the drawtube. If you can give me that I can give you a pretty good ballpark of how long the tube should be.

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Jeff B
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Re: New refractor OTA. Length - keep options open? new [Re: Tamiji Homma]
      #5677483 - 02/13/13 11:21 AM

Tammy, nice postings. From my experience the Lumicon 2" diagonals have ~4-6 mm shorter light path than the AP diagonal. My "standard", however, is the AP diagonal.

My only concern using a prism diagonal with an F8 refractor is possible lateral color, especially since the bino viewer uses prisms as well. However, the proof would be in the viewing.

BTW, of use to us ATM'ers, the end of the AP 2.7" focuser draw tube will actually retract about an inch into the focusers body when you remove the 2" adapter. If I install their older 1.00" extension (which will retract into the body) and their thin 2" adapter, I gain an extra inch focuser travel (giving it ~5.5"), which is enough to accommodate my Denk II and older style power switch without need of an extension tube when using monovision.

This thread is also useful for those of us who have Cass's that my have back focus restrictions as well.

Good stuff

Jeff


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GlennLeDrew
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Re: New refractor OTA. Length - keep options open? new [Re: Jeff B]
      #5678038 - 02/13/13 05:25 PM

As Johnny pointed out, it suffices for the purpose of analyzing binoviewers on scopes to consider only the ray bundle for on-axis light. That is, just the single light cone coming to focus at the field center.

In such cases as this, if your illumination is 100% at the field center only, that's quite good enough. And this way you won't get unnecessarily freaked out by the *inevitable* clipping of edge-of-field rays by the BV's own front aperture. Inevitable. So why bother introducing the extra complication in the first place?


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GHarris
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Re: New refractor OTA. Length - keep options open? new [Re: GlennLeDrew]
      #5678943 - 02/14/13 08:26 AM Attachment (13 downloads)

I have attached a new version of my slapdash and simplistic raytrace. This time the light path is represented by the red lines and these two lines converge to a point, on the right hand side, 1200mm away from the objective. In the top sketch I left a 27mm vertical line at the point of 190mm of backfocus, but otherwise the top and bottom sketches are the same.

Everything is to scale - 152mm objective on the left, 1200mm focal length, OTA length 1010mm (to take the 190mm backfocus into account), focuser drawtube 114mm long and 3 inches wide. The focuser drawtube is fully racked in.

There seems to be oodles of spare space between the light cone and the edges of the focuser drawtube. I'm sure this is all still a naive oversimplification in some ways (I haven't taken into account the extra millimetre or two, here or there, added by 3" to 2" eyepiece adapters etc).

By the way, thank you very, very much Tammy for all your work providing data on focus positions of eyepieces, diagonals and binoviewers in other threads. It really helped me to begin to understand what I need. I know it would still be better if I could actually measure the combination I'm hoping to use, in action, but it looks like I won't have a chance to do that.

I am planning on using 190mm of backfocus because (as described in an earlier post in this thread) I plan on using a Binotron 27 (I've since been told this has a lightpath of about 118mm), a Baader T2 prism diagonal (in the mid-40mm range of lightpath, or less), adapters for Denk to T2 (~8mm) and 2" to T2 (less than 5mm). This seemed to add up to a lightpath less than 175mm. I added 1.5cm of tolerance and decided I'd only need 190mm of backfocus, rather than 220mm.

For the no-OCS/powerswitch configuration that needs all this infocus I'll use 24mm Panoptics.

My intention would be to only use the two inch Baader Clicklock diagonal in the more conventional situations where I'm either monoviewing or binoviewing with OCS and power X switch. It has a slightly longer lightpath (~114mm) than the "average" 2 inch diagonal which would be welcome for the purpose of providing me with as much out focus as I might want. With 190mm of backfocus, when I use the large Baader diagonal, I'll need 76mm of my 114mm focuser drawtube's travel just to get to the same focus point with any given eyepiece. I'm thinking that probably the estimated inch and a half of leftover outfocus range on the focuser tube, if my numbers below hold together, should be as much as I need. A small extension tube is the worst that could happen if 38mm isn't enough, which I'll live with. It would be nice not to have to use an extension tube where possible. But if anyone can see a hole in these numbers I'd be grateful to know. It looks like I will have to confirm the final specification of the OTA shortly so I'd appreciate any extra certainty.

Mentioning the T2 prism diagonal reminded me... should I get the Zeiss one or the cheaper one? Think I've seen these two separately advertised on a shop site. Do they have the same lightpath? I latched onto the Zeiss version for some reason, might be that I've already discovered that it's the right one and then forgotten again, but I'll look up the answer myself if I can.

Edited by GHarris (02/14/13 08:51 AM)


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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5679000 - 02/14/13 09:18 AM

Have you accounted for the length of the focusser body?

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GHarris
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Re: New refractor OTA. Length - keep options open? new [Re: Mark9473]
      #5679335 - 02/14/13 12:17 PM

In one of my emails with the dealer I'm talking to, the following was said:
"One thing to remember about the back focus distance is we measure it from the end of the 2" clamp on the focuser. Therefore we take the 1.25" out."

So it sounds like it's taken into account. I've also been told, referring to the 3" APM focuser I am planning on using, that "The clamp on the deluxe clamp unscews to reveal a T2 thread". So that might actually give me an extra millimetre or three of additional usable backfocus, when I use the Baader T2 prism rig. And that small bonus won't be at the expense of out-focus when monoviewing because I'll have the 2" deluxe clamp back in place for that.


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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5679373 - 02/14/13 12:32 PM

No what I meant is: 1200 mm focal length minus 190mm back focus minus focusser body length equals something smaller than the 1010 mm tube length you've drawn. I think.

One further comment: think about putting a Baader 2" clicklock clamp at the end of your focusser.


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johnnyha
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Re: New refractor OTA. Length - keep options open? new [Re: Mark9473]
      #5679503 - 02/14/13 01:37 PM

Yes you need to subtract the focuser length, from where it meets the end of the tube to the end of the drawtube visual back. The point where the tube meets focuser can be tricky as some adapters have a lip or overhang.

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GHarris
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Re: New refractor OTA. Length - keep options open? new [Re: johnnyha]
      #5680813 - 02/15/13 08:13 AM

Thank you for raising these points. I may be misunderstanding you both again, but I think this isn't a problem. My drawings are crude and their purpose was to estimate whether the focuser drawtube was going to cause vignetting if the scope was configured for a certain amount of backfocus (I'm settling on 190mm). So for convenience I just drew a 1010mm long OTA and have not taken into account the exact shape and configuration of the end of the tube. More realistically, for example, the tube might be 1000mm long with part of the focuser body sticking out 10mm (or whatever). But the position of backfocus, based on where the 2" adapter comes out, and hence the position of the focuser drawtube is the same regardless. So I can use this simplification to see that there will be no vignetting caused by the focuser drawtube.

There is also quite a lot of freedom for even more backfocus, from the point of view of avoiding drawtube vignetting. It's a bit less obvious on the shrunk version of the drawing (the original is larger than the one I've posted) but the focuser tube could go much further into the OTA before it cuts into the lightpath as drawn.

Apologies if I'm being monumentally dull and missing your point completely!


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Re: New refractor OTA. Length - keep options open? new [Re: GHarris]
      #5680835 - 02/15/13 08:30 AM

The only point is: make sure you don't order a 1010 mm tube length!

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Re: New refractor OTA. Length - keep options open? new [Re: Mark9473]
      #5681502 - 02/15/13 01:28 PM

I think you just about got it GHarris. Also keep in mind you want the focal point to hit in the middle of the drawtube travel. Since we both are using 6" f8 scopes with 4.5" drawtubes, I can make a ballpark figure.

With the drawtube racked in completely, my focuser is exactly 6" long from where the focuser adapter meets the end of the tube, to the end of the 2" visual back. The tube itself is EXACTLY 33" long. So including the length of the racked-in focuser that's 39" total. Mine is NOT a bino-friendly tube but I have installed a slightly shorter focuser than the original, and I use the Baader 1.25" prism T2 system which has the shortest possible lightpath - so I reach focus at 1X with 1/2" to spare.

Again, what you actually want is for the focus point to hit right in the middle of your drawtube travel. I suggest making your tube a few inches shorter than mine if you are using anything other than the Baader Ts 1.25" prism and Mark Vs. So if your focuser is also exactly 6" long, I would make your tube around 31" or 787mm. If your focuser is 7" long then I would make the tube 30" long. So, 37" minus your focuser length.

If you ARE using the shortest lightpath system with the 1.25" T2 prism diagonal and Mark Vs or Maxbrights, and your focuser is also 6" long, then you can make it closer to 33" as mine is and you should just barely reach focus at 1X.

This is "ballpark" because the focal length may be a little different between my lens and yours - my lens has a 1216mm focal point. So if your is 1200mm then subtract another 1/2" from the lengths above. Or maybe your lens cell is attached a little differently in front so maybe 1/4" difference there. Yes it gets a little complicated. So please don't hold me to these figures. The only true way to know is to actually measure the whole setup together at the eyepiece before cutting the length but you don't have that luxury. But my tube is 33" if that helps.

Hopefully this will help more than confuse.


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