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Light path experts here ?

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#1 Disciplus55

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Posted 26 May 2018 - 04:39 AM

Hello,

I have a 16” dobsonian with f/D 4.5 (hand crafted by Dobson Factory in France) and a former microscope Zeiss Binoviewer modified by Denis Levatic, with an optical path of 123mm it seems, and prisms open at 25mm. And with a T2 connection...
I am still debating on what I need: I want to use it with my 16” dobsonian, with lowest magnification possible, with as little vignetting as possible, and 1.25” eyepieces.

 

I tried last weekend the Siebert OCA that has 3 configurations possible on my dobsonian.
- in 1.25x configuration : I can not reach focus.
- in 2x configuration : I can not reach focus.
- in 3.5x configuration : I can reach focus and I have at least 20mm of back focus left.

Now, with other solutions :
- with a 2x barlow, I can reach focus but as said previously, I get crazy high magnification (like 4.5x) and a horrible eye relief with TV Plössl 32mm or E-Pl 10x/25.
- with a Baader GPC (glass path corrector) 2.6x, I can not reach focus, it is supposed to compensate 85mm of optical path...
- with a Explore Scientific Focal Extender 3x, I can reach focus and have maximum 4mm of back focus left.

So, my question is : what solution exists with only one configuration of magnification, that would be the smallest possible magnification, without too much vignetting ? I know it can be done with a 3.5x Siebert OCA and a 3x ES Extender but is it possible to have less magnification and still reach focus ? I have not tried a Powermate 2x with T connection but that would be ideal, but I can’t find any info on what light path I could compensate... massive headache...

Many thanks and clear skies,



#2 Kunama

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Posted 26 May 2018 - 04:48 AM

My plan for binoviewing at native focal length with my current build is to cut the poles 120mm shorter and then make a connector to allow the offcut piece to be screwed back on when not planning to binoview....

This way I can binoview at native without any GPCs etc. They only thing is you must decide before assembling the scope for the night... and you must be sure that your secondary mirror is not a the minimum dimension...


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#3 Jon Isaacs

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Posted 26 May 2018 - 04:52 AM

My plan for binoviewing at native focal length with my current build is to cut the poles 120mm shorter and then make a connector to allow the offcut piece to be screwed back on when not planning to binoview....

This way I can binoview at native without any GPCs etc. They only thing is you must decide before assembling the scope for the night... and you must be sure that your secondary mirror is not a the minimum dimension...

A second set of poles is another option.  I can buy  a set of eight 6 foot long,  1.25 inch diameter black anodized aluminum tubes for about $80 right up the road from me.  Even if another secondary were necessary, it would probably be cheaper than fooling around with correctors and the like.

 

Jon



#4 Disciplus55

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Posted 26 May 2018 - 04:59 AM

Indeed that would be even better, but since it is a craft made dobsonian, the rod connections are a bit odd and not so easy to make... and I don’t dare cutting the original carbon fiber tubes... I was more wondering if a Powermate x2 or a Baader GPC 1.7x for 2” focusers on Newtonian (made for the Mark V) would work, but again, I miss the light path compensation information... I need a light path compensation higher than the Baader GPC 2.6x and similar to the ES Extender 3x, but I have no idea where to find those data’s.



#5 lylver

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Posted 26 May 2018 - 05:40 AM

I have the small lens version : move the inner tube. You can screw any of the lens in front of the smaller tube of OCA or back inside, near the big converging lens. Keep the arrows in the right way.

Myriam

OCA_20170411_150427.jpg



#6 Astrojensen

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Posted 26 May 2018 - 06:39 AM

I have the Baader 1.7x newtonian GPC and it almost fully compensate for the binoviewer light path. You need a little inward focus travel, though. 

 

I rarely use mine, so you can borrow it, if you want to test it. 

 

 

Clear skies!
Thomas, Denmark


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#7 Eddgie

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Posted 26 May 2018 - 07:08 AM

Take Thomas up on his offer.  The Baader 1.7x Newtonian GPC should work.

 

Now, you do need about 20mm of back focus, but if you are a little short, you might get the rest by fully raising your collimation screws.

 

The other option is to go to a lower profile focuser.

Cutting the truss poles to reach focus in native mode will almost always cause aperture loss, but if you just need to trip a few millimeters, that will work fine.  

 

I used the 1.7x GPC though and it worked fine with the Mark V (and your BV should be similar).

 

One last tip.  The eyepeices themselves can somtimes squeek out some in focus.   Some eyepieces have the field stop closer to the end of the barrel than others.  For example some 32mm Plossls have the field stop just inside the end of the barrel.  This can buy you 15mm of back focus, but of course the selection of eyepieces that do this is very limited.

 

On the binocular forum there may be people using Astro-binoculars that can give you ideas for more eyepieces that might save some light path.  This is a common issue with the big astronomy binoculars.  Not all eyepieces will reach focus so APM and others have eyepieces they recommend for their big binoculars.   These might also save you a few millimeters of in focus (if that is all you are missing by).


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#8 Eddgie

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Posted 26 May 2018 - 07:11 AM

Oh, one last thing.  If your problem with some units is that your focuser is running out of outward travel, there is an easy and free solution to this.  Just loosen the eyepiece lock down and slip the unit out.  If you are uncomfortable with the amount needed, just buy some M48 extension tubes and extend the tube in front of the OCA.  

 

This is pretty common to have to do with the Denkmeier BVs.  Often the scopes won't reach focus in high power mode becuase the focuser has to be cranked all the way out.   People just rack the focuser a few millimeters short of being all the way out, then slip the BV out as necessary, and fine focus using the remaining travel.



#9 Disciplus55

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Posted 26 May 2018 - 09:08 AM

Oh, one last thing.  If your problem with some units is that your focuser is running out of outward travel, there is an easy and free solution to this.  Just loosen the eyepiece lock down and slip the unit out.  If you are uncomfortable with the amount needed, just buy some M48 extension tubes and extend the tube in front of the OCA.  

 

This is pretty common to have to do with the Denkmeier BVs.  Often the scopes won't reach focus in high power mode becuase the focuser has to be cranked all the way out.   People just rack the focuser a few millimeters short of being all the way out, then slip the BV out as necessary, and fine focus using the remaining travel.

Nope, not in this direction, I miss some intra focus and my primary mirror is already at its highest position :)



#10 Eddgie

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Posted 26 May 2018 - 09:44 AM

I don't know if the Baader 1.7x Newtonian GPC will work for you or not, but it might. 

 

To use it though, your binoviewer has to have a way to connect.   The Mark V used a dovetail with quick connect ring, and the Maxbright used a T2 interface. 

 

If Thomas is offering though, I would take him up on it, but you need between 32mm and 38mm of back focus and if you don't have this, not much point in trying the 1.7x Newt GPC.  Many Newts don't have this much.  


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#11 wcw

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Posted 26 May 2018 - 09:59 AM

Sylvain, which Siebert OCA did you try? There are a few different models I believe. I use the 1.3x Siebert OCA and it requires very little back focus. It is called "Universal Binoviewer Max Light OCA" and is add #3 on this page: http://www.siebertop...Optics-OCA.html

 

And it can be ordered with T2 threads, as well as a large Clear Aperture lens for fast scopes. I use it on a F4 dob with Televue binoviewer and 24mm Panoptics, and it easily reaches focus and produces beautiful views. I do not notice any vignetting. It should work on your scope.

 

-Bill


Edited by wcw, 26 May 2018 - 10:10 AM.


#12 Disciplus55

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Posted 26 May 2018 - 10:49 AM

Ad #1b , the one with 3 different configurations, 1.25x , 2x and 3.5x : I could only reach focus at 3.5x.

my modified Zeiss bino from Denis is equipped with the Dovetail like a Mark V and T2 connection with the quick connector. I have at least 40mm back focus.



#13 wcw

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Posted 26 May 2018 - 11:07 AM

Then that means that your OCA and my OCA are not the same, right? Because the 1.3x OCA that I have has no problem reaching focus.

 

If it is any help:  I did notice that my 26mm Nagler focuses at almost the exact same focuser location as my binoviewer. The 26mm Nagler has its field stop only 0.05 inches "inside" the eyepiece relative to the shoulder of the eyepiece. So that gives you a reference point.

 

-Bill



#14 Jeff B

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Posted 26 May 2018 - 12:15 PM

This configuration may well work for you.  It does for me.

 

It consists of, from top of the assembly on down, viewers with the Baader male quick change adapter, (just like you have), a Baader female quick changer, Precise Parts adapter to mate the Baader quick changer to a Denk Power Switch, Denk power Switch and finally, the Denk OCS assembly with a Multi-Purpose OCS Cell.  For your scope, you would need the large aperture, A45, OCS cell.

 

This assembly allows three different magnification factors by sliding the switches in and out.  The Lowest magnification factor is ~1.4X, then, ~1.8X, then ~2.8X.  The OCS assembly is adjustable in length to allow you to customize the system to your scope's focuser travel requirements.  

 

But, it's not cheap.  But it is tremendously flexible.  But, but, I doubt it would be more money than a pair of high quality eyepieces.  But, but, but, there's nothing like three different magnifications available from one set of eyepieces with a simple sliding of the switches.

 

Bring on those planet thingies!

 

Jeff.

Attached Thumbnails

  • Assembly.jpg
  • Individual Components.jpg
  • Installed.jpg

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#15 Astrojensen

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Posted 26 May 2018 - 02:27 PM

Ad #1b , the one with 3 different configurations, 1.25x , 2x and 3.5x : I could only reach focus at 3.5x.

my modified Zeiss bino from Denis is equipped with the Dovetail like a Mark V and T2 connection with the quick connector. I have at least 40mm back focus.

If you have 40mm back focus, the Baader 1.7x newtonian GPC will work just fine. On my 12" f/5 it required about 30mm, which I got by changing from a normal GSO crayford to a low profile Baader Steeltrack. 

 

 

Clear skies!
Thomas, Denmark


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#16 noisejammer

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Posted 26 May 2018 - 06:17 PM

You could try an optical relay using a negative-positive pair of achromat lenses. If the negative lens was located at the inboard end of the focuser draw tube, you could get away with about -4 dioptres (I'm doing this in my head.) If you followed that with a positive lens on the binoviewer's nose, you might be able to effectively extend the light cone.

 

I'm sure one of the ATM-ers could do the design if you asked.

 

At f/4.5, you're going to be vignetting over the entire field - you might want to run the system at f/6 or slower anyway.



#17 vkhastro1

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Posted 26 May 2018 - 07:02 PM

I thoroughly discussed the use of the Zeiss "supercharged" binoviewer in various telescope designs with Denis Levatic (prior to my purchases from him).

He highly recommended the dedicated Baader 1.7X Newtonian correctorbvs the Baader 1.25X, 1.7X or 2.6X glass path correctors (best for refractors).

 

I have owned a Denk II/powerswitch/OCS combination & similar Denk Binotron combination in my Dobsonian scopes as well as my refractors.

I totally hated the refocusing necessary with the filter switch.

I prefer no filterswitch (actually shorts the light path) in the refractors.

My preference is no filterswitch and using eyepieces of a single eyepiece line (TV Plossl, Baader Morpheus,etc) that are/or 

nearly parfocal thereby limiting focuser travel. Simply focus with one eyepiece pair and replace with the next pair with minimal focuser fiddling. 


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#18 rob.0919

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Posted 26 May 2018 - 08:01 PM

Hi Disciplus55

 

I'm using an Earthwin system with my 12" F4 and it works really well.

The OCS is 2.4x and the Powerswitch then reduces that to 1.9x or 1.2x

A filtreslide is also included.

 

With my Pan 24s i'm getting about 60x in low power mode which i think for a 12" Newtonian is pretty good going.

I can fit the entire 'Double Cluster' in to the fov and it looks spectacular on a good dark moonless night.

To give me those low power views, i wanted to keep the focal length at 1200mm

I also use a non standard Moonlite focuser, with a long 60mm drawtube, which gives me the flexibility to focus easily on all 3 powerswitch settings.

 

Ok, the 24s may vignette a touch : i don't notice it in the dark though.

Maybe i'm also losing a bit of aperture ? I don't find this a big issue either. I know the binoviewer ( or rather the powerswitch  ) won't take an F4 lightcone.

I've considered the Denk 45mm OCS which may help if its compatible with the Earthwin system which is 40mm

Also there may be a touch of light falloff around the edges of the fov, but its not really noticable. A 12" mirror perhaps compensates.

Coma is not an issue either.

 

I'm not suggesting you 'upgrade' to an Earthwin / Denk system, but just giving you an idea what it can achieve low power wise with a mid size scope.

 

26138259187_1002cda465_z.jpg


Edited by rob.0919, 27 May 2018 - 06:05 AM.

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#19 Disciplus55

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Posted 03 June 2018 - 11:40 AM

Ok, tried the Baader 1.7x Newtonian corrector last night... still can’t reach focus, maybe I still miss 10mm of intra focus... so, so far, the only solution I have found that works, is the Explore Scientific Focal Extender 3x, which gives me a good magnification with TV Plössl 32mm... and of course the Siebert OCA 3.5x. I only wonder now if a TV Powermate x2 would work for me or not...



#20 doug mc

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Posted 04 June 2018 - 08:07 PM

I have a z8 . I found the GSO 2inch ED barlow allows focus with plenty of room to spare. Using the entire barlow you will end up at 3.3x. With the barlow lens removed and screwed into the 2inch to 1.25 inch eyepiece adaptor in my 2 speed focuser, focus is still available with room to spare.  I use a William Optic binoviewers. Using the full barlow I find I need to pull it out a bit, so reaching focus is guaranteed. With the lens in the eyepiece adaptor the power is 2.7x.



#21 Jeff B

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Posted 05 June 2018 - 12:03 PM

Nice set up there Rob. 

 

Regarding vignetting, the OCS might act as a relay system really and may actually increase the illuminated field a bit relative to cyclops viewing.  You can easily check this my simply looking on one side of the viewer without an eyepiece as you switch out slide elements and looking at the reflected image of the primary in the secondary.  If you get more space around the image of the primary and the edges of the secondary relative to the cyclops view, then yes, it's acting like a relay lens.  And that's pretty cool.  Actually, I believe the viewer itself may vignett the Pans as it passes a 26mm diameter light beam through the entrance to the viewer.  

 

Again, nice set up.

 

Jeff


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#22 ryanlu92

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Posted 31 July 2018 - 11:49 PM

 I have both Baader 1.7x Newtonian GPC and 1.3x Siebert OCA on my bino-friendly Teeter STS10in f/5.

I can only have a few mm left with 1.7x and 1.3x Siebert OCA let me use like a normal EP with a lot of range.

So far I found changing filter is a pain for me.  Just FYI.

 

Ryan 




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