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Some Handy Measurements

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#1 Jeff B

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Posted 24 January 2020 - 03:51 PM

I though you all might be interested in this info as many of us have the AP and Baader BBHS and Zeiss spec prism diagonals (with the T2 quick changer system).  

 

Using my DPAC test set up, I directly measured some interesting stuff.

 

1. AP diagonal has a 105mm light path from the front face to the top of the diagonal.

 

2. The Baader BBHS silver diagonal has a light path of 51mm from the front face to the diagonal to the top of the T2 quick changer

 

3. The  Baader Zeiss spec prism diagonal has a light path of 39mm from the front face of the diagonal to the top of the T2 quick changer.

 

4. With the 1.25X  and 1.7x GPCs screwed into the bottom of my Zeiss 30mm CA viewer, I needed to rack the focuser out an additional 21.2mm and 38.8mm respectively, on average, for all three diagonals.

 

Regarding number 4, when you reverse the GPC elements and put them into the quick changers rather than into the bottom of the viewer as I did,  those differences in focuser rack out will be slightly larger, maybe a couple of mm's,  as they are slightly further away from the focal plane in that configuration, giving them slightly more magnification.

 

Jeff


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#2 Eddgie

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Posted 24 January 2020 - 06:19 PM

Very nice work.

 

These figures are pretty close to what Baader provides in the Maxbright Owners Manual and Alpine (for the diagonal light path measurements).

 

Baader is usually pretty good about this stuff, but the big question mark is why the 1.7x and repeatedly been reported to only be giving about 1.5x.  Now most people would be happy if they can still reach focus, but again, Baader is usually pretty close on specs for light path lengths, so surprised about the miss on this other kind of important spec for the .1.7x..

 

Nicely done though.  Actual measurements are always good.


Edited by Eddgie, 24 January 2020 - 06:25 PM.

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#3 Jeff B

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Posted 24 January 2020 - 07:06 PM

Very nice work.

 

These figures are pretty close to what Baader provides in the Maxbright Owners Manual and Alpine (for the diagonal light path measurements).

 

Baader is usually pretty good about this stuff, but the big question mark is why the 1.7x and repeatedly been reported to only be giving about 1.5x.  Now most people would be happy if they can still reach focus, but again, Baader is usually pretty close on specs for light path lengths, so surprised about the miss on this other kind of important spec for the .1.7x..

 

Nicely done though.  Actual measurements are always good.

Thanks Eddgie.

 

Yeah, I've noticed the short fall in the 1.7X GPC's magnification too.  I measure ~1.55 to 1.6X -ish, depending on where it is in the light path, the viewer used and the focal ratio of the objective.  Coupled with its modest back focus extension, it's my favorite GPC to use with my CFF 160 F6.5 triplet.  

 

Actually, I've not bothered to measure the 1.25X GPC though.  I wonder how accurate that number is too.

 

Jeff


Edited by Jeff B, 24 January 2020 - 07:07 PM.


#4 Jeff B

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Posted 31 January 2020 - 03:41 PM

Some more handy, direct measurements, again, taken in my DPAC set up but this time of viewer light path lengths. 

 

All viewers use a Baader style T2 Quick changer adapter (custome for the Denk II) and measurements were made with the Ronchi grating sitting on the top of a viewer eyepiece collet while focusing to a null in DPAC.  The viewers were attached to the same diagonal with the same T2 dovetail.

 

My usual suspects (with the results) are from left to right in the line up photo:

 

1. Zeiss "Sharpest" (now with "APO" designation too), with Baader Click-lock eyepiece collets from Denis Levatic:  137mm

 

2. Zeiss Sharpest like above but with "blue" eyepiece collets, again from Denis:  127mm

 

3. Good old Denk II:  118mm (114mm w/o the T2 dovetail adapter)

 

4. Zeiss 30mm with blue collets from Denis: 118mm

 

5. Good old Baader MK V, super charged by Denis for 30mm clear aperture:  112mm

 

These measurements should be of help.  I really need them for my refractor ATM projects.

 

Enjoy.

 

Jeff

Attached Thumbnails

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Edited by Jeff B, 31 January 2020 - 05:41 PM.

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#5 bcarter1234

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Posted 31 January 2020 - 05:36 PM

Hey, I have that exact same dryer!

 

A buddy and I had just picked up his brand new sprint car from the fresh from the paint shop. We drive to another guys house to drop off some tools. When we pull up the guy and comes out and says "Wow! Nice trailer!" He did not have a racing bone in his body. ;-)

 

Thanks for posting this information, helpful even for a reflector user. Hmm, Zeiss 30mm.....

 

Take care,

Brent



#6 Eddgie

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Posted 31 January 2020 - 07:31 PM

It is incredible that they can keep the Mark V light path 112mm.  That is shorter than the new Maxbright II.

 

The Denk is longer than other reports (116mm) but I would imagine that this is due the light path length of the T2 adapter. 

 

137mm is pretty tall but many of the new refractors are binoviewer ready, but this might be a bit too much even for some of them. With a T2 diagonal, that is 175mm of light path. 

 

 

For future reference, the Binotron is 127mm with the focusers all the way down. 

 

Good data.  Thanks for providing. 



#7 denis0007dl

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Posted 01 February 2020 - 01:49 PM

Some more handy, direct measurements, again, taken in my DPAC set up but this time of viewer light path lengths. 

 

All viewers use a Baader style T2 Quick changer adapter (custome for the Denk II) and measurements were made with the Ronchi grating sitting on the top of a viewer eyepiece collet while focusing to a null in DPAC.  The viewers were attached to the same diagonal with the same T2 dovetail.

 

My usual suspects (with the results) are from left to right in the line up photo:

 

1. Zeiss "Sharpest" (now with "APO" designation too), with Baader Click-lock eyepiece collets from Denis Levatic:  137mm

 

2. Zeiss Sharpest like above but with "blue" eyepiece collets, again from Denis:  127mm

 

3. Good old Denk II:  118mm (114mm w/o the T2 dovetail adapter)

 

4. Zeiss 30mm with blue collets from Denis: 118mm

 

5. Good old Baader MK V, super charged by Denis for 30mm clear aperture:  112mm

 

These measurements should be of help.  I really need them for my refractor ATM projects.

 

Enjoy.

 

Jeff

Very nice collection Jeff,

 

didnt know you have so many binoviewers from me bugeyes.gif



#8 AtmosFearIC

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Posted 01 February 2020 - 09:59 PM

This is really helpful as I’ve been thinking of changing from the Baader MkV. Never realised that it was one of the shortest on the market. Maybe binoviewing with the Mewlon 250CRS is not going to be practical with the corrector installed.

#9 Jeff B

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Posted 22 March 2020 - 11:47 PM

Someone here on CN asked me a question that prompted me to investigate something I have suspected for quite sometime and that is the amount of back focus gain the Baader GPCs provide, may depend on their distance from the focal plane, or, in other words, the optical path length of the viewer/GPC combination.

 

Basically, I figured that, like any barlow element, the further you place the GPC from the focal plane, the greater its magnification and with the greater magnification, comes a greater amount of back focus.  As the viewers I use all have very different optical path lengths, I was curious so I ran some additional back focus measurements using the Baader 1.7X GPC with the viewers you see below with the exception of the Denk II.  

 

And indeed, I was right. 

 

For example, I measured the back focus gain with my Zeiss Sharpest with the Baader Click Lock eyepiece holders (first viewer on the left in the group photo below) with the Baader 1.7X GPC installed facing down into the top of the quick changer (it does not screw into the bottom of the viewer the way it does with the Baader MK V or Zeiss 30mm viewer), and with the element oriented properly.  This configuration, with this viewer, gives the greatest optical distance between the GPC element and the focal plane of all of my viewer/GPC combinations.  Well, I gained an additional back focus of.....54.5mm. 

 

Now the shortest optical distance between the 1.7X GPC element and the focal plane is with my Baader MK V (first viewer from the right) with the GPC screwed into the bottom of the viewer (again, with the GPC element properly oriented).  This configuration gained an additional back focus of 35mm.  A very large difference of 19.5mm!

 

So,as you can see, the back focus gain when using the Baader 1.7X GPC depends very much on the optical path length of the specific viewer and the placement of the 1.7X GPC element (either up inside the viewer, or down inside the top of the quick changer).  The longer the combined path length, the greater the gain in back focus...just like a barlow, which is what it is.  And each configuration will have a unique magnification associated with it too.

 

No wonder people have reported different magnification levels for the 1.7X GPC!  

 

Here is the data from left to right in the picture (with the Denk II excluded) with the Baader 1.7X GPC mounted down in the top of the Baader Quick Changer (except as noted):

 

1. Zeiss "Sharpest" with Baader Click-lock eyepiece collets from Denis Levatic:  137mm viewer optical path length, 54.5mm BF gain

 

2. Zeiss Sharpest like above but with "blue" eyepiece collets, again from Denis:  127mm viewer optical path length, 48.5mm BF gain

 

3. Zeiss 30mm with blue collets from Denis:                                                    118mm viewer optical path length, 44.0mm BF gain, 39mm with GPC threaded into the viewer

 

4. Good old Baader MK V, super charged by Denis for 30mm clear aperture:       112mm viewer optical path length, 40.5mm BF gain, 35mm with GPC threaded into the viewer

 

I will have to repeat these measurements using the 1.25X Baader GPC.

 

Stay well and safe.

 

Jeff

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Edited by Jeff B, 22 March 2020 - 11:54 PM.

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#10 Eddgie

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Posted 23 March 2020 - 01:10 PM

Here is the thing about the Baader GPCs and why if you are not going to use them correctly you might as well use any old Barlow.

 

The GPCs were designed by Roland Christen and the design goal was to have the GPC correct for the spherochromatism that is induced into the wavefront by the prisms in the binoviewer and they were designed to go into the Mark V housing, or into the T2 top of the prism diagonal for the Maxbright (and any other T2 ready binoviewer).

 

I could have told you that they will indeed change the magnification if the spacing has changed.  I thought everyone knew that because it has been reported many times that the 1.25x used in front of the diagonal gives about 1.7x.  That is pretty old news.

 

Again, the point of the GPC is not just to reduce light path, but to also eliminate the spherochromatism.

 

Nice work, and a nice report, but this data has been out there a long time.  

 

If one wants an increase in back space and is thinking of buying a GPC and putting it at some meaningfully greater distance than specified, my advice would be to just save the money and use a Barlow.  

 

As a side note, the Televue 2x Bino Vue amplifier, which was designed by Al Nagler and also was designed to correct spherochromatism, is also T2 friendly. 

 

Now whether the change in spacing is great enough to meaningfully degrade the view is of course open for debate (Solar white light is a great place to test but the scope has to be a very high quality Apo without color of its own) but the point is that even if it is not that much different, then why would someone spend the money on one if they could get there with a cheap Barlow lens?


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#11 Eddgie

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Posted 23 March 2020 - 01:16 PM

And there is also the aperture constraint of the GPC.  The small clear aperture means that it is best positioned as close to the entry point of the BV as possible.  For every multiple of the focal ratio you move it forward, the fully illuminted field is reduced by 1mm. Putting it in front of a 1.25" diagonal on a fast telescope (f/6 for example) would mean that even before the light cone reached the front aperture of the BV, it would already be vignetted by the GPC. 

 

I mean none of this should stop you or anyone else from putting it wherever you like, but "System" components are typically only designed to give their absolute best performance when they are configured as shown in the system manuals and diagrams.  People though routinely accept compromises to performance to get something the works well enough for them, and that of course is an individual thing.  While I can say how a system will behave, I cannot say that it is bad or incorrect to do it. That is a personal judgement. 


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#12 Mattimac

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Posted 23 March 2020 - 04:41 PM

So since the GPC are designed to also correct for spherochromaticism, and the Zeiss "sharpest" use mirrors rather than only prisms, is it technically better to use the Zeiss with a Barlow (or Powermate) only?



#13 denis0007dl

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Posted 23 March 2020 - 06:01 PM

So since the GPC are designed to also correct for spherochromaticism, and the Zeiss "sharpest" use mirrors rather than only prisms, is it technically better to use the Zeiss with a Barlow (or Powermate) only?

Zeiss APO will also work great with GPC as well!

GPC there wont negatively affect image quality in any way!



#14 fate187

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Posted 24 March 2020 - 05:20 AM

So since the GPC are designed to also correct for spherochromaticism, and the Zeiss "sharpest" use mirrors rather than only prisms, is it technically better to use the Zeiss with a Barlow (or Powermate) only?

Zeiss APO will also work great with GPC as well!

GPC there wont negatively affect image quality in any way!

That is in fact one concern I have been thinking about. Jeff B did some extensive testing. Remarkably, he found, that a CFF 160 with Zeiss sharpest and the Baader GPC gave very sharp and color free views. That was also one reason why I got the Zeiss apo viewer and GPC from Baader.

I also thought, that a no prism bino would give rise to a worse view with the GPCs, but I wasn't able to see it either. Granted I have not too much viewing experience and conditions are not ideal for me at the moment.

 

Maybe someone could explain, why views are still excellent in the Zeiss mirror viewer? Does this viewer really only use mirrors without ANY prism in the path?


Edited by fate187, 24 March 2020 - 05:21 AM.


#15 denis0007dl

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Posted 24 March 2020 - 05:40 AM

Zeiss APO use 3 mirrors and one cube beamsplitter made by two cemented prisms!

Zeiss achieve perfectness with that design, especially new one.
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#16 Jeff B

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Posted 24 March 2020 - 11:16 AM

Eddge, thanks as always for your clear perspective and experience.

 

While it has been written about many times, there have been no real published actual measurements that I know of concerning the back focus gains using the Baader GPCs and these popular higher end viewers.  I find such measurements very useful for my ATM work as well as figuring out which viewer to use with which scope and which GPC.  And, well, that's because I have many viewers. 

 

All of these BF measurements were made in green light, in DPAC, to the tops of the eyepiece collets.  They seem independent of the refractor in front of the viewer with very similar results from F7 to F12.  

 

With regards to just using a "regular", off the shelf,  barlow, it's not that simple.  I've tried doing just that with several barlows such as the AP Barcon, TV 2.5x barlow, an older Celestron Silver Top 2.0X barlow and one other 1.8X unit.    In all cases the spherical performance (in green) of the system in DPAC was seriously degraded and huge amounts of back focus were required.  Which makes sense as all of these barlows were operating way off of their design distances to the tops of their eyepiece holders.  So just using a simple barlow does not really work well.  The barlow elements need to be specifically designed for bino-viewer use due to the long optical path lengths involved.    

 

So it comes as no surprise that I found that the Denk OCS/Power Switch and Baader GPC systems do indeed work extremely well with most viewers,  meaning reasonable back focus gains and no disturbances (or even subtle improvements) to the SA content as seen in green light in DPAC.  I'd like to try the TV corrector too but I don't have one.  As barlows go, these are very mild in strength compared to conventional barlows.

 

However, I still rely on actual star testing and solar system observations (yes, including white light solar, especially the degree of granulation seen, as you say, a great test) to custom match a viewer system to a specific scope, for example the CFF 160 F6.5 with the Zeiss Sharpest and 1.25X & 1.7X GPCs clearly excels for me. 

 

But one thing that did stand out in the context of the binos and refractors I use was the Zeiss Sharpest.  It clearly has the least amount of "native" color disturbance, compared to some of the other viewers, especially with the faster scopes, say F8 and faster,

 

Jeff


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#17 Kent10

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Posted 24 March 2020 - 11:31 AM

Hi Jeff,

 

Perhaps you mentioned this already.  Do you screw the GPC directly into your Zeiss Sharpest?  How do you use it?  Did you reverse the GPC lenses compared to using it with the Mark V?

 

Thanks, Kent



#18 Jeff B

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Posted 24 March 2020 - 12:20 PM

Hi Jeff,

 

Perhaps you mentioned this already.  Do you screw the GPC directly into your Zeiss Sharpest?  How do you use it?  Did you reverse the GPC lenses compared to using it with the Mark V?

 

Thanks, Kent

Kent, no, the GPC cannot screw into the bottom of the Sharpest.  Like you said, to use them with the Sharpest, I have to reverse the element and then mount the GPC face down into the top of the 2" adapter or Baader diagonals, however, with the Baader diagonals I can screw the GPCs into the diagonal's T2 threads, which centers them better.  

 

Actually, I'd really like to see Baader do a Denk style power switch with the GPCs.  That way, I don't have to fumble around in the dark with individual GPCs, multiple diagonals or reversing GPCs.  I really love the Denk power switch system.

 

Jeff


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#19 Jeff B

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Posted 24 March 2020 - 01:26 PM

So I repeated these tests with the Baader 1.25X GPC and I included the Denk II as well.

 

Here is the data from left to right in the picture with the Baader 1.25X GPC mounted down in the top of the Baader Quick Changer (except as noted):

 

1. Zeiss "Sharpest" with Baader Click-lock eyepiece collets from Denis Levatic:  137mm viewer optical path length, 30.5mm BF gain

 

2. Zeiss Sharpest like above but with "blue" eyepiece collets, again from Denis:  127mm viewer optical path length, 26.5mm BF gain

 

3. Denk II with custome BAAder quick change adapter:                                    118mm viewer optical Path length, 23.5mm BF gain

 

3. Zeiss 30mm with blue collets from Denis:                                                    118mm viewer optical path length, 23.5mm BF gain, 21.0mm with GPC threaded into the viewer

 

4. Good old Baader MK V, super charged by Denis for 30mm clear aperture:       112mm viewer optical path length, 21.5mm BF gain, 18.0mm with GPC threaded into the viewer

 

Stay well and safe.

 

Jeff


Edited by Jeff B, 25 March 2020 - 01:20 PM.

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#20 fate187

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Posted 24 March 2020 - 02:49 PM

Jeff do you have the 2.6 GPC as well? Could you make measurements on the magnification factor? I tried to do it myself. But I do not know how much obstruction the Zeiss sharpest introduces with my eyepieces due to its smaller clear aperture compared to a larger MK V...



#21 Jeff B

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Posted 24 March 2020 - 04:40 PM

Sorry Michael, no I don't have the 2.6X GPC.

 

I'd certainly like to make magnification measurements and correlate them to optical path length for each of the 1.25 and 1.7X GPCs but I do not have facility to do that in doors.  I would need to take measurements outside with star drift times or, better yet, some sort of distant measure, like bricks on a wall or shingles on a roof.  I do not have a good reticle eyepiece either.

 

Anyone have any ideas?

 

Jeff



#22 daquad

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Posted 02 April 2020 - 02:22 PM

From the Agena Astro site, the added back focus for each of the three GPC's:

 

1.25X  -30 mm

1.7X    -65 mm

2.6X    -120 mm

 

https://agenaastro.c...s3-2456317.html

 

Dom Q.



#23 daquad

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Posted 02 April 2020 - 02:25 PM

Also of interest.

 

https://www.baader-p...maxbright).html

 

Dom Q.



#24 Jeff B

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Posted 02 April 2020 - 10:51 PM

Thanks Dom.  Trouble is the added back focus depends on what viewer is used and how its used, basically the optical path length from the element to the focal plane.  

 

Jeff




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