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New to me OTA - Questions

Started by Greg Shaffer, 12/15/2009 07:57AM
Posted 12/15/2009 07:57AM Opening Post
Drove about 2 hrs Sunday to meet the seller and picked up a Parallax 10" F5 Newt with an Ed Stevens primary. I heard good things about his work, supposedly if you ordered one of the smaller Zambuto mirrors 14" and smaller that Ed was who did the figuring. It came with a really nice printed booklet with all the mirror test results and charts in it. Sadly my ignorence of such things leaves me wondering what I am looking at.....Could use some help interpreting what I am looking at.

RTA: .14 (not familar with this term but the booklet calls it "Relative Transverse Aberation")

Strehl Ratio: .999 (I know this is good but I also know there are several methods used to arrive at this with some being more accurate than others. It does describe it as "the wavefront for the entire mirror is averaged using the amplitude error for all of the zones")

Peak to valley wave: Less than 1/20

There are a couple of other specs that I dont grasp either.

P-V Wavefront 1/.. : 75.75

Wavefront RMS 1/.. : 223.46

Of course the weather wont permit me to physically evaluate it as usual when I get a new scope so here I am trying to educate myself on what I am looking at smile

20" MidnighTelescopes f/5
8" f/16 Muffoletto Achro
Meade 6" AR6 f/8.0 Achro
Celestron C11
Parallax PI250 10" F/5 Newt
Vixen FL102S,NA140SS,R200SS
80mm Stellarvue Nighthawk with WO APOupgrade lens, 90mm f/16.67 Parallax Achro
LXD75/LX200, Mini-tower, GP-DX, CG5 ASGT, LXD650, GM-8, G11, GM-100, GM-150EX, GM-200 and a Gemini G40
4" portable AP convertable 44" or 70" pier
Stellarvue M7 Alt-Az, TSL7 Pier/Tripod
12.5" f/5 MidnighTelescopes DOB Swayze optics
Monroe GA
Posted 12/15/2009 05:21PM | Edited 12/15/2009 06:55PM #1

In his book "Star Testing Astronomical Telescopes" Harold Suiter does a great job of describing many of these these terms and how to recognize them through star testing. The book is a great read. See

On the web there are a few resources. For some background information on your questions about RTA, P-V, and Wavefront RMS see:


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Posted 12/15/2009 06:52PM #2
My 12" GSO dob gave very asymetrical star tests for in focus and out focus before Ed Stevens refigured it. Even though two separate tests indicated that the mirror was around 1/7 wave. Star tests are now very similar plus individual observers have noticed the significant difference of before and after for my mirror. Two independent tests showed that the mirror was slightly undercorrected and that my scope showed increased detail on Jupiter using an apodizing screen. That was interpreted to indicate that the apodizing screen was acting as a mask selectively reducing light from the edge of the mirror. After refiguring, the apodizing screen did not increase detail on Jupiter. To make a long story short, regardless of the stats that were provided with my refigured mirror, the proof was in the pudding and I am very satisfied with the results. I thank Ed Stevens for that.


I Yam What I Yam!
Posted 12/18/2009 07:23PM #3
Hi Greg, I also think you will be highly please with Ed's mirror. I used to frequent the Zambuto Yahoo group when Ed was still working there and if I remember correctly an RTA of 1.0 means that the abberition in that dimension is all within the airy disk and an RTA of .14 means that all the photons are focused in a circle .14 the radius of the airy disk. Here is a link that kindof describes it.

They say the claims are unrealistic because the focault test only gives you the data at maybe 4 or 5 radius's on the mirror and the rest are extrapolated and that is true. However Ed also use a Ronchi test which is sensative to the change in slope of a mirror and with it you can visually see all the data points between the 4 or 5 zones measured and tell very accurately if the transition is truely smooth. The machining method that Carl and Ed use has been taught on the Zambuto group and it truely works but still takes a skilled craftsman to master. When you read Texereau or Sam Brown and look at the focault examples that they give they don't have a lot of definition and contrast because they look like an amateurs focault test but Carls and Eds are picture perfect.

Congrats on the acquistion.

Dave H.