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Mark V Light path lenght?

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#1 Eddgie

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Posted 16 March 2013 - 09:56 AM

I think I have seen conflicting numbers for this. Seems like I have heard both 120mm and 130mm.

Does anyone have a published number for this?

I was working under the assumption that the light path lengh was 120mm, but I am feeling that it must be longer.

I was recently surprised to see that my EdgeHD 8" lost more apeture using the Mark V/T2 prism than my C5 does.

The C5 is reduced from 127mm to 122mm. The EdgeHD 8" is reduced from 203 to 190mm.

I believe that the EdgeHD 8" does not have the same back focus of the standard C8 before aperture reduction occurs, but if my light path calculation is off, this could account for some of the differnce.

Thanks for helping.

#2 tomcody

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Posted 16 March 2013 - 10:31 AM

Eddgie,
As posted on the Baader web site, the MarkV is listed at 123mm including the quick change coupling. As baader lists the quick change as 11mm long in the T2 chart on the Alpine Astro web site, that means the MarkV is 112mm (although as you must use the quick change with it , you are back to 123mm).
Rex

#3 Eddgie

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Posted 16 March 2013 - 11:37 AM

Thanks.

I thought that this was about right, the the Mark V was about 120mm. That is the figure I was using. Of course I was off a bit but not enoght to account for the aperture loss in the EdgeHD 8".

My guess is that the EdgeHD 8" design makes it cut off quicker than the standard C8. The standard C8 would start cutting off at about 180mm, but you had to have 240mm of back focus before reducing the aperture to 7.5".

It would appear that substantial cutoff is happening with the EdgeHD 8" at 198mm of back focus.

My configuration is Baader 2" clicklock (37mm measured with electronic dial), T2 Prism (38mm according to Baader figures) and Mark V, (123mm).

So, surprised to see that I am loosing so much aperture with only 198mm of light path.

I am going to try the Televe Shorty 2" SCT adapter. This unit is only 23mm, so that will save about 14mm. I doubt that this will recover all of the aperture, but hopefully will get me closer to 200mm.

I don't think I would like the Baader SCT connector because it does not allow for easy rotating of the diagonal, but for someone with Alt-Az scope, this may be the right choice. I think the light path length on that unit is only 15mm.

Thanks for the info though.

#4 tomcody

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Posted 16 March 2013 - 12:56 PM

Eddgie,
If you are giving up rotating the diagonal? condsider a sct to T-thread adapter, it should only add about 5mm to the light path, ( you can use the baader t- thread spacer washer set available at alpine astro to set the position that the diagonal tightens at) that should get you down to 161 mm
and if I rember correctly from the Celestron site the Edge is optimized at either 145 or 148 mm. One more thing if you use rings instead of the dovetail plate to mount the scope, you could loosen the rings and rotate the scope to position the binos.
P.S. If it were me, I might try going straight through , no diagonal, and get to 145mm to optimize the system, use it like a big mounted binocular.
Just a thought!
Rex






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