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Effective Aperture of C5 with Mark Vs

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#1 Eddgie

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Posted 11 December 2012 - 01:58 PM

A PM with a CN member promted me to measure the effective aperture of the C5/T2/Mark V combo and I thought I would share the results.

I used a pair of 15mm Plossls for this test, so the field stop of that eyepiece is in the lens housing but recessed a bit from the bottom so that it is almost exaclty in the sampe plane as the top of the Binoviewer eyepiece holder. A longer focal lenght eyepeice with the stop down in the barrel will have about 10mm less light path, so this represents the worse case I think (most shorter focal lenght eyepecies have the stop right at the junction between the barrel and the eyepecie housing).

Anyway, the light path is measured like this..

I have the Televue SCT to 2" adapter. As measured internally from the back flat surface of the SCT port to the end of the SCT adapter, it is 1.6" (40mm).

The light path though the T2 standard prism (not the Zeiss prism equipped unit but the regular Baader T2 prism) is listed at 42mm.

The Mark V is listed at 120mm I believe.

So, total back focus was 202mm.

I was not able to focus quite at infinity, but used a very distant tree, so I could be a tiny bit off, but I think it was less than a small tweak of the focuser. This would make any effective aperture loss appear a bit worse than if I measured at true infinity, but again, I don't think I was off all that much.

I brought the scope in and used a green laser to project the aperture on the wall and marked carefully and used a caliper to measure.

Effective aperture was measured at 4.83" or about 122.6mm.

I also did the same measurement using a standard 2" diagonal with the 2" nose connected directly to the bino body. The effective aperture measured this was was a bit less than 4.3" or about 109mm, which is a painful hit. Would not recommend using this configuration for binoviewing (2" diagonal... I noticed dramatic dimming using a 2" bino based system in my C14!).

Anyway, it appears that aperture cutoff in the C5 occurs slgihtly before 200mm of back focus. I had worked up some rough estimates a while back that suggested 180mm or so before aperture loss occurs, so it may be a bit more than this, or perhaps it gets more agressive as the secondary baffle startst to cut in.

But up to about 200mm of back focus is likely to be fine in the C5 for binoviewing, keeping in mind that when aperture reduction starts, it can become very agressive very quickly.

I am hoping we can get more measurements from more forum members so people can get a better picture of the tradeoffs.

I plan on going to the Televue SCT Shorty to 2" adapter for my C5 in hopes of getting full aperture back. This will shave about 10mm of back focus off.

Will report when I have had a chance to aquire and test.

#2 Astrojensen

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Posted 11 December 2012 - 02:18 PM

Hi Eddgie

Try the Baader SCT/T2 adapter for shortest possible light path. It is only 15mm long. The 32mm Baader T2 Maxbright prism has an optical thickness of 35mm, according to Baader's homepage. Not 42mm, as you write. I don't know how Baader measures back focus on the Mark V. I've also seen figures as low as 111mm. I also need to precisely measure the Maxbrights. Figures differ between 100mm and 110mm.


Clear skies!
Thomas, Denmark

#3 GlennLeDrew

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Posted 11 December 2012 - 04:14 PM

When dealing with the optical thickness of prisms, it must be borne in mind that the glass moves the focus rearward by an amount roughly equal to 1/3 (depending on the index of refraction) the optical path length through glass. In effect, then, a prism is optically thinner than it is physically, the equivalent air path being about 0.67 that of the glass oath length. We can see that in this application prisms offer the benefit of squeezing some extra back focus into the train.

#4 Astrojensen

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Posted 11 December 2012 - 04:46 PM

When dealing with the optical thickness of prisms, it must be borne in mind that the glass moves the focus rearward by an amount roughly equal to 1/3 (depending on the index of refraction) the optical path length through glass. In effect, then, a prism is optically thinner than it is physically, the equivalent air path being about 0.67 that of the glass oath length. We can see that in this application prisms offer the benefit of squeezing some extra back focus into the train.


I know. That's why I wanted to measure the Maxbrights myself. That's the only way to get reliable figures. I'll do it with my one of my old Zeiss telescopes, which have graduated focuser drawtubes. I don't know why I haven't done it before. Guess I didn't think it was all that important. I do suspect I'll find a number closer to 100mm than to 110mm, since they reach focus on so many scopes with room to spare.

The only way to find the optical thickness of the binoviewer is to focus them on a distant target, then remove them and focus the same eyepiece on the same target and find the difference.


Clear skies!
Thomas, Denmark

#5 Eddgie

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Posted 11 December 2012 - 04:56 PM

The clearance problem is going to be the top edge of the Mark V dovetail connector on the Mark V. This rimg extends out so that it is even with the plane of the front face of the diagonal box. When rotated, this ring describes an arc that passes directly over the inside edge of the focuser knob. To short, and when rotated to the right, the connector ring will hit the focuser knob.

If someone knows exactly how thick this is can can send me the dimension, I can measure to see if there would be sufficient clearance.

I am not sure where I got the 42mm figure from. Perhaps it was for the T2 unit with the Zeiss Prism..

That would make my light path less than 200mm though, so loosing aperture even with a light path of 198mm.


And I asked Santa for a 3.25" to 2" Clicklock for the C14, and a standard SCT Clicklock for the EdgeHD, so with the Shorty back, I think I am good.

But maybe the SCT adapter would work for the C5. Again, my concern is simply that it will intefere with the focuser.

On the other hand, I am considering mounting the C5 Alt-az for binoviewing, and in this cobfuguration, the focuser knob would be under the prism and it would never interfere.

And I know that most people would not be bothered by it, but I want my scopes to work at full aperture. Allowing the already big central obstruction to gain even more relative size bites me.

Anyway, thanks for the correction. It doesn't change the fact that the setup reduces aperture, but helps refine the model to suggest that aperture loss starts at less than 200mm back focus in a C5 (which I had already suspected).

#6 Eddgie

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Posted 11 December 2012 - 05:12 PM

After finishing the note, I decided to look closer at the situation. I put the binovieres on the C5 and put it on the mount, and there was only one orientation where I think I would get inteference, and it is nto an angle where I think the Bino would ever get into.

So, will go with the Baader SCT adapter. Thanks for reminding me about this component and making me look at it again.

I still want to go with the Clicklock on the other scopes because I am sure there will be times when I want to use long focal length 2" eyepeices for wider field viewing, so dropping the bino out and plugging in a 2" diagonal will be quick and easy, and since these live on GEM mounts, there is often a desire to rotate the bino. The clicklock looks beneficial for these scopes.

And if I want to use it on those scopes, I will have it....

Thanks again for getting me to look again..






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