Optical Quality and Effects of Secondary

Interesting read
whichtelescope.com/benchmarks.htm
and
www.peak2valleyinstruments.co.uk/page_1801026.html

Interesting articles there Michael, confirms what I always suspected from looking through SCT's, that they are a distant third behind refractors and Newts given the strehl reducing effects of the secondary mirror size.

Yes, Dave and Michael. The secondary mirror issue was the most vital reason why I bought the 6.2" apochromatic refractor rather than a medium size SCT. And time and time again, it has proven to be the right telescope for me, thanks to the great advice I got from Michael back in 2008 -9.

Aubrey.

Different scopes for different folks,
I'll never forget the best view I've ever had through any scope, it was a home made dob in the trip to the Greek starparty a few years back and we were looking at Saturn, truly photographic !

This is a must read for anyone interested in optics.

Michael a couple of pics as promised after our little chat on Saturday.

1. Orion Optics test report for a 200mm f8 reflector 17% C/O Scope. Ultra grade 1/10 or better.
2. Scope mounted on a HEQ5 and pedestal, this goes against all the rules, but it works well, and yes it acts like a sail when the wind gets up but not too bad.

The above article seems to agree with the test report but I'm open suggestions and opinions.

Thanks for posting
Tony

That test report looks interesting Tony.

I'd be interested as to how they calculated the Strehl and what level of accuracy in in that measurement.
Seems like a very high number for a 1/10th wave P-V mirror if the relationship between Strehl and P-V as outlined on the links in my original post are to be believed.

From looking at the test report, IMHO I think it relates to the primary only and doesn't take account of the surface accuracy or obstruction effect of the secondary. I think they have removed the analysis of the centre of the mirror as it would be blocked by the secondary and so it's performance is not relevant. However, in practice, it was measured, as you can see from the top right diagram.

I think the comparison between P-V and Strehl can be regarded as indicative only. You could have a very smooth mirror with one serious defect which would generate a very poor P-V figure for an otherwise very good mirror. Conversely, you could have lots of defects of similar level giving a moderately rough mirror but none which may be so serious as to give a poor P-V value. The mirrors should however have significantly different Strehl ratios.

The RMS and Strehl figures seem to match up to the tables in the links. I'd be interested to know how many and what points were used to generate these figures.

I see your report also had the RMS.
optical-technologies.info/tag/peak-to-valley-vs-rms/

I reckon they have measured the P-V and RMS/Strehl separately, hence why they don't match the tables.

Another interesting read:
www.telescope-optics.net/Strehl.htm
and
www.astronomycorner.net/notes/strehl.html

If you use the formula in the second link above, the RMS and Strehl correspond up exactly.
Hence IMHO I think they measured the RMS and derived the Strehl ratio using the formula.

As for the mount, it doesn't look as bad as you had led me to believe.