GSO Deluxe Dob - worth the extra?

[Ags]

I was just reading AlexB67's link (which was slightly broken, so here it is fixed):

http://www.nicholoptical.co.uk/Optical%20Standards.pdf

The terminology it uses seems to be consistent with prairie_stars reading:

A telescope mirror may be described has having ¼ wa

ve PV wavefront error. This means

that the distance between the highest and lowest po

int on the mirrors surface is 1/8 wave.

Remember that the surface error is halved at the wa

vefront due to reflection. On the face

of it this mirror meets the established criterion f

or a good mirror. Consider this, we have

two mirrors both ¼ wave PV (wavefront) 1/8 wave on

the surface, one has a small bump

¼ wave high, the other has several bumps ¼ wave hig

h. Both meet the established

criteria but the first mirror will out perform the

second mirror. Clearly, a PV rating is not

giving the full picture it fails to take into accou

nt the size of the surface defects.

I think the least tortured beading of the GSO page is that they mean surface error, not wavefront error or RMS. I have asked TS for a clarification.

[John]

I think the least tortured beading of the GSO page is that they mean surface error, not wavefront error or RMS. I have asked TS for a clarification.

I thought that the strehl ratio was now being used in an effort to bring more clarity to this:

http://www.rfroyce.com/standards.htm

I agree that neither PV or RMS are much of a guide to the potential performance of a mirror.

It would be helpful if GSO and their vendors were more specific about what it is they are actually quoting rather than leaving us to try and deduce it !

To be fair to Telescope Services I notice they are now simply claiming "diffraction limited or better" for their GSO made dobs. Telescope House state "....Parabolic primary mirror 1/12wave.." for the Revelations so no clarification.

[Ags]

I had a prompt reply from the vendor. They state that 1/12 wave = 1/6 PV. Which is fine and dandy in my books :-)

Still might get an optical test done though.

[AlexB67]

it would interesting to see that report brings if you are willing to share it and see if it can lift some of the mist surrounding the topic should you do decide to do it  . 

Just a thought though.  Would the test be really worth it for the cost ?, suppose it comes out worse than expected, you could demand another one, but to test that would they charge again ? I suppose if you re going to invest in it, it would in the hope that you get what they say you should get, even though I am still not very clear what sort of mirror you would get, ( strehl would be more useful and they haven't stated so they may say it is within tolerance) 

If you do get  report all good and it may give you some piece of mind, that money would buy a nice eyepiece too

Personally I'd have more fun trying out your scope at a party or club and pitch it against a couple of others like skywatcher if you should ever get the opportunity, using a range of eyepieces, pick a couple of tricky targets like doubles and various stars that will push the optics to the limit and see what that brings, I think I'd gain more confidence from that than a sheet of paper.   

[Mike73]

Still might get an optical test done though.

Huygens tested my 16" GSO.

I think you could drive yourself mad analysing and testing eyepieces and scopes until it reaches the point that your learning more about equipment than whats actually up there!

[Mike73]

Still might get an optical test done though.

Huygens tested my 16" GSO.

I think you could drive yourself mad analysing and testing eyepieces and scopes until it reaches the point that your learning more about equipment than whats actually up there!

[John]

Thats a good test for a big mirror Mike 

[Mike73]

Thats a good test for a big mirror Mike 

It serves its purpose John.

[Ags]

GSO mirrors good. Case closed. I won't hear another word! :-)