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#1151 davidc135

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Posted 19 February 2021 - 07:39 AM

I found it online and I think it was right here on Cloudy Nights.

I was trying to find the size of the obstruction in a ETX-125.

And I must have spent the entire evening (raining anyway) on the subject because 

the baffle around the secondary mirror is causing confusion, it flairs out larger then

the secondary spot it's self.

Also the primary is actually 5.5" in diameter, adding to the confusion.

Some are saying if we say 5" for the primary and INCLUDE the secondary baffle,

40% central obstruction.

And I can say there is no way the ETX-125 has a 40% obstruction, it's too good.

If we use 5" aperture and the actual secondary spot diameter 1.55" = 31%, that's better

but what about the true size of the primary 5.5" and the true size of the spot?

28% obstruction. I feel this is more inline with what I see in the eyepiece but it

probably isn't correct.confused1.gif

 

Robert

It could be 40%. I tested my 2045 with pieces of masking tape around the co and at the edge and marked the limits when I placed my eye at the focus without an ep. Easier with an sct but maybe stretch masking tape across the whole aperture

I found the aperture was 99mm and the co 45.5mm giving a huge 46% but the image quality is still impressive although it would be put in it's place by a good 3'' refractor. The DX-8 seems to have 35/36%.

 

I wonder how much difference it would make to a Questar 31/2 if a 40% paper disc was stuck on. I expect it could be seen.

David


Edited by davidc135, 19 February 2021 - 07:41 AM.

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#1152 ccwemyss

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Posted 24 February 2021 - 11:31 AM

The AP users group just got permission to post Roland's original 1981 Sky and Telescope article about his NASA glass triplet. It's an interesting read, and the ads are also pretty nostalgic. I notice that one is from a retailer for the AP drive corrector. 

 

https://s3-us-west-1...ve article.pdf"

 

Chip W. 



#1153 pbealo

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Posted 24 February 2021 - 03:49 PM

Link doesn't work



#1154 DAVIDG

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Posted 24 February 2021 - 04:15 PM

The AP users group just got permission to post Roland's original 1981 Sky and Telescope article about his NASA glass triplet. It's an interesting read, and the ads are also pretty nostalgic. I notice that one is from a retailer for the AP drive corrector. 

 

https://s3-us-west-1...ve article.pdf"

 

Chip W. 

 There was a correction to this article published I believe a month later in Sky and Tel.  Also  Roland had an article in Telescope Making describing his lens and how it came to be. The  original idea was from an Applied Optics article  published many years earlier that showed how to choose the three types of glass. Then Mike Simmons who was one of the co-inventor  of the Astroscan, inventor of the Simak scope and also developed the GOTO systems for Meade and Celestron wrote a program that looked at all the different  three combination of glass that would work with R2=R3  and R4=R5 so those surfaces could be oiled together.

 

                    - Dave 


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