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Orion 150mm Mak, clear aperture/CO

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#1 moynihan

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Posted 06 July 2005 - 07:23 PM

In case anyone is interested:


I called Orion tech support the other day to find out what the true clear aperture was, and the diameter of the central obstruction was, on the new 150mm Maksutov. Got an email response this evening, quote:

"We spoke earlier this week about the clear aperture and secondary obstruction of the new 150mm Maksutov-Cassegrain. Here they are:

The secondary obstruction including the baffle is 47mm.

The clear unobstructed aperture is a true 150mm."

#2 UWastronomer

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Posted 06 July 2005 - 07:42 PM

so, they could call it a 197mm Mak? Hmm... That's a 7.75" aperature... ?????? Why does that not sound right to me? Dunno, but I did a calculation... And it's right! :D

I took the area of the objective that has light grasp, and converted the area to an equivalent refractor. It'd be a 5.9" refractor... which is 150mm of objective. So yes, this is in reality, a 7.75" Mak, or about 8-9" in overall tube diameter. Sounds like a good deal! :jump:

Now, I checked my 127mm Mak, and it seems they fudged the light grasp on it. I reverse calculated, and the light grasp they claim is bigger than it actually has. I actually measured it, and it's right at 127mm. I wonder if they fudged on this new one too... :john:

#3 moynihan

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Posted 06 July 2005 - 07:58 PM

so, they could call it a 197mm Mak? Hmm... That's a 7.75" aperture... ?????? Why does that not sound right to me?


I think they mean the reflective surface of the primary is 150mm (not counting the primary baffle and hole, and the mechanical blockage of anything hold ing the mirror. If i recall correctly, their other maks are overstated (127 is actually about 122, etc).
So the actually "clear" aperture (allowing for the secondary/spot & baffle) would be:

((75x75)x pi)-((23.5x23.5)x pi)= 15,928 square mm (or, a light grasp surface of 142.44mm, or 5.6 inchs).

By diameter, the CO would also be 31.3% (9.8% by area).

Actually the obstruction would probably not block as much light as apparent above, in a CAT, since the CO is "blocking" some dead zone, i.e., the primary's baffle and hole.

??

#4 UWastronomer

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Posted 06 July 2005 - 08:01 PM

ok. Makes sense.

#5 Sky Captain

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Posted 07 July 2005 - 12:20 AM

Yeah, the numbers look right and add up but...I will bet that if you call Orion back and talk to a differant tech. you will get a differant answer. Thats just my experience anyway with Orion, on brand new stuff anyway.
:penny: :penny:

#6 Clive Gibbons

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Posted 07 July 2005 - 08:31 AM

so, they could call it a 197mm Mak? Hmm... That's a 7.75" aperture... ?????? Why does that not sound right to me?


I think they mean the reflective surface of the primary is 150mm (not counting the primary baffle and hole, and the mechanical blockage of anything hold ing the mirror. If i recall correctly, their other maks are overstated (127 is actually about 122, etc).
So the actually "clear" aperture (allowing for the secondary/spot & baffle) would be:

((75x75)x pi)-((23.5x23.5)x pi)= 15,928 square mm (or, a light grasp surface of 142.44mm, or 5.6 inchs).

By diameter, the CO would also be 31.3% (9.8% by area).

Actually the obstruction would probably not block as much light as apparent above, in a CAT, since the CO is "blocking" some dead zone, i.e., the primary's baffle and hole.

??


Hopefully, to clarify the clear aperture discussion... ;)

When a manufacturer states "clear aperture", they mean the useable diameter of the lens or mirror, regardless of the central obstruction. In the case of the 150mm Mak, it's the corrector lens which has 150mm of useable aperture. The primary mirror should be a little bigger than this, to catch all the light which is diverging slightly from the corrector lens (which is a weak negative meniscus). If the primary mirror is 150mm aperture, then the effective aperture of the scope would be slightly less than 150mm. Maybe 145mm, but that's a guestimate.

It's good we're spelling "aperture" correctly. :grin:

Yours bafflingly,

Clive.

#7 Ken603a

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Posted 07 July 2005 - 10:10 AM

It is tempting. If I didnt already own two scopes, I would probably buy this one.

#8 Starman1

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Posted 07 July 2005 - 03:12 PM

When a manufacturer states "clear aperture", they mean the useable diameter of the lens or mirror, regardless of the central obstruction. In the case of the 150mm Mak, it's the corrector lens which has 150mm of useable aperture. The primary mirror should be a little bigger than this, to catch all the light which is diverging slightly from the corrector lens (which is a weak negative meniscus). If the primary mirror is 150mm aperture, then the effective aperture of the scope would be slightly less than 150mm. Maybe 145mm, but that's a guestimate.

It's good we're spelling "aperture" correctly. :grin:

Yours bafflingly,

Clive.

The Sky & Telescope test of the 127mm Mak pointed out the actual aperture of the light-gathering portion, due to the spread by the corrector, was 120mm. That should make the 150mm about 140mm effectively.

#9 Clive Gibbons

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Posted 08 July 2005 - 09:36 AM

The Sky & Telescope test of the 127mm Mak pointed out the actual aperture of the light-gathering portion, due to the spread by the corrector, was 120mm. That should make the 150mm about 140mm effectively.



Ahhhhhhhh, verrrrrry interesting!
Thanks for the info, Starman1.
This might explain something a buddy of mine noticed with his Synta 150mm Mak. Compared to a 6" f/5, using the same magnification, the Newtonian showed a brighter image. If the Mak has an effective aperture of 140mm, that could be the reason.

Thanks again,


Clive.

#10 ATY

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Posted 11 July 2005 - 02:26 PM

And that's why Meade uses over-sized mirrors in their SC and MC scopes to account for the slight divergence of light reaching the primary mirror. So when Meade stated an 8" aperture the primary mirror is actually 8.25" and you got a full 8" useable aperture. Alex

#11 Clive Gibbons

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Posted 11 July 2005 - 02:36 PM

And that's why Meade uses over-sized mirrors in their SC and MC scopes to account for the slight divergence of light reaching the primary mirror. So when Meade stated an 8" aperture the primary mirror is actually 8.25" and you got a full 8" useable aperture. Alex


Hi Alex.
WRT the Meade 8" SCT, in order to get 8" of clear aperture, the primary mirror would only need to be about 8.1" diameter. This is because the Schmidt corrector plate doesn't diverge light as much as a Maksutov corrector.
Meade used an 8.25" primary mirror, to provide better off-axis illumination. This would have worked better, if the baffle tube system didn't vignette as much as it does.
All that being said, Meade SCT's and MCT's do have primary mirrors large enough to fulfill their stated clear apertures.

Clear skies,


Clive.


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