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New refractor OTA. Length - keep options open?

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#26 GlennLeDrew

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Posted 13 February 2013 - 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.

#27 GHarris

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Posted 13 February 2013 - 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.

#28 johnnyha

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Posted 13 February 2013 - 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. :question:

#29 Tamiji Homma

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Posted 13 February 2013 - 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.cloudynig...?Number=4378893

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

Tammy

#30 johnnyha

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Posted 13 February 2013 - 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.

#31 Jeff B

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Posted 13 February 2013 - 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

#32 GlennLeDrew

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Posted 13 February 2013 - 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?

#33 GHarris

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Posted 14 February 2013 - 08:26 AM

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.

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#34 Mark9473

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Posted 14 February 2013 - 09:18 AM

Have you accounted for the length of the focusser body?

#35 GHarris

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Posted 14 February 2013 - 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.

#36 Mark9473

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Posted 14 February 2013 - 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.

#37 johnnyha

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Posted 14 February 2013 - 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.

#38 GHarris

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Posted 15 February 2013 - 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!

#39 Mark9473

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Posted 15 February 2013 - 08:30 AM

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

#40 johnnyha

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Posted 15 February 2013 - 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. :grin: 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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