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# What does "RTA" Tell me about a Primary

3 replies to this topic

### #1 gaz-in

gaz-in

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Posted 20 February 2013 - 08:19 PM

Hi All,

I was looking at a primary that had an RTA of 0.23 . What does this tell me about the quality of the mirror?

Thanks In advance for the inputs

### #2 Jason D

Jason D

Fly Me to the Moon

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Posted 20 February 2013 - 08:24 PM

Hi All,

I was looking at a primary that had an RTA of 0.23 . What does this tell me about the quality of the mirror?

Thanks In advance for the inputs

RTA stands for Relative Transverse Aberration. Basically, it means that all focused light rays off your mirror will be confined to a diameter of 23% the length of the airy disc diameter. RTA of 0.23 represents an exceptional mirror.

Jason

### #3 gaz-in

gaz-in

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Posted 20 February 2013 - 08:31 PM

Thanks, It is an Ed Stevens Mirror with a Strehl of .991, RTA of 0.23 and a peek to valey wave of <1/20. Another mirror I am looking at is Strehl 0.993, RTA of 0.73, and a peak to valley wave of <1/20. Is there realistically a difference between the two (assuming you can believe the ratings).

### #4 Jason D

Jason D

Fly Me to the Moon

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• Joined: 21 Oct 2006
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Posted 20 February 2013 - 08:38 PM

Thanks, It is an Ed Stevens Mirror with a Strehl of .991, RTA of 0.23 and wavefront of <1/20. Another mirror I am looking at is Strehl 0.993, RTA of 0.73, and wavefront of <1/20. Is there realistically a difference between the two (assuming you can believe the ratings).

Mine is also Ed Stevens with 0.992 Strehl and 0.26 RTA. Do not read too much into these numbers. These numbers are based on measurement samples. I sould say all these mirrors are more or less the same in terms of Strehl/RTA (good quality mirrors). Base your decision on other factors.
Jason

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