Optical Quality Help

[Moonshane]

Hi all

The OOUK mirror in the report below seems to have an excellent figure to my untrained eye. It appears 1/8PV or just better and with a Strehl ratio of 0.984 should give excellent images of all objects? Obviously there will be some coma and a corrector (e.g. Parracorr) might be beneficial but as it's the same focal length as my other scopes it seems ideal as my current eyepieces are all based on this focal length.

Can anyone confirm any possible problems with this mirror based on the report? It's all gobbledegook to me.

It seems a superb mirror and at the price a really good buy.

I have committed to buy it and hopefully you agree with my assumptions

Anyone want to buy a 12" f5.3 OOUK dob?? I'm serious. An ad will follow shortly.

[Stu]

Hi Moonshane

What are the mirror specs on the dob ie is it hi lux, or standard?

What sort of price were you thinking of?

Have always wanted a big OO scope

Cheers

Stu

[Moonshane]

Hi Stu

So I don't get in strife with the mods, I'll send a PM.

[nicnac]

i have only just started looking at mirror testing myself but from what i have read on the matter, "peak to valley" isnt much cop, strehl being the better choice of testing method.

0.984 sounds like a good result based on what i have read so far (in this field)

[johninderby]

It looks like a good OO 1/8PV mirror. It will have Hilux as OO uses it for all mirrors over 1/6PV. The mirror set would be £2,310.00 with VAT from OO.

PV can be useful when it's properly done (as by OO), but some manufactures shall we say inflate the PV a bit or give it as Wavefront to make their mirrors look better.

John

[Moonshane]

cheers John and Nick

I think this settles my mind. I really cannot miss this opportunity so my dob will be up for grabs soon I think.

Best wishes

Shane

[Glasspusher]

John, the wavefront error of a mirror is always half the surface error due to doubling of errors as a result of reflection. So 1/8 wave surface error corresponds to 1/4 on the wavefront. Quoting a mirror as something of a wave is meaningless unless it is qualified as surface or wavefront error. The problem with Peak to Valley ratings alone as that they give no indications of the size of the surface errors. Two mirrors both rated at 1/4 PV (wavefront) could give very different performances in telescopes. Rating a surface in RMS or Strehl is an attempt to overcome this problem but reliable numbers for these values can only really be obtained through interferometry due to the high sampling rate across the mirrors surface. The figures for the mirror shown are excellent.

John

[Moonshane]

Cheers John

Certainly a lot better than if I'd had a go myself and in truth the overall cost will not be that much more than if I had.

[johninderby]

So just a mirror cell, spider and secondary needed and then it's onto bulding the OTA. Truss tube I presume.

John

BTW take a look at this link.

http://www.focuser.com/cgi-bin/dman.cgi?page=category&plugin=dstore.cgi&category=6

[Moonshane]

Indeed John

I really cannot wait to get started. Need to sell some stuff though first. The Moonlite truss gear is lovely.