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Optical Quality and Effects of Secondary

  • michaeloconnell
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Last edit: 7 years 1 month ago by michaeloconnell.
7 years 1 month ago #101274

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Replied by dave_lillis on topic Optical Quality and Effects of Secondary

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.
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7 years 1 month ago #101279

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Replied by flt158 on topic Optical Quality and Effects of Secondary

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.
7 years 1 month ago #101281

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Replied by dave_lillis on topic Optical Quality and Effects of Secondary

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 !
Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor
7 years 1 month ago #101288

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Replied by tony h on topic Optical Quality and Effects of Secondary

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




7 years 1 month ago #101333

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Replied by michaeloconnell on topic Optical Quality and Effects of Secondary

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.
Last edit: 7 years 1 month ago by michaeloconnell.
7 years 1 month ago #101355

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Replied by dave_lillis on topic Optical Quality and Effects of Secondary

Dead right there Michael, the p-v value is not any guarantee, if you had 2 mirrors both one sixth wave but one had a wave front diagram that looked like it had been blasted with a shot gun while the second mirror had only a small area of high deviation, then those 2 mirrors are not going to perform equally
Dave L. on facebook , See my images in flickr
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Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor
7 years 1 month ago #101357

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Replied by michaeloconnell on topic Optical Quality and Effects of Secondary

it had been blasted with a shot gun

That would make for an interesting mirror Dave...
For the record, IFAS wishes to denounce all violence carried out on mirrors, OTAs and all other astronomical-related equipment, including their owners...
:-)
7 years 1 month ago #101359

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Replied by tony h on topic Optical Quality and Effects of Secondary

I Just like to say I’m no expert on optics and have no formal training just an interest in telescope optics as part of this hobby so anything I say take with a huge grain of salt.
The telescope in question was purchased second hand and cost me no more than the price of a good/premium eyepiece, but it had a test report and that’s something my other scopes did not have.

OO.UK, state on paraboloids they electronically remove the central section of the mirror on the final test
The first test is full aperture with no central section removed (as indicated on top right panel)

So yes it looks like primary mirror only test, I assume that the secondary is of equal quality but you know what they say about assume, I suppose I could check with them.

From what I can gather the RMS figure is of multi point, if not the whole mirror, which as you say is more accurate than P-V. Were a single or couple of points tested could give an unfair mirror reading either way good or bad.

I think its worth checking out Orion Optics Web site as it explains it much better than I can.
www.orionoptics.co.uk/OPTICS/optics.html

Looking again at the first link posted StrehI Ratio Table
The table states a mirror of .992 strehl and RMS 0.014 is equal to 1/20 Wave. Orion Optics only states that the mirror is equal to 1/10 or better.

What I found very interesting in the first link posted is the relationship of secondary quality and size when taken into consideration gives a more accurate and overall rating. When I put my figures into this formula I get a value of just over 1/9 wave for the complete scope.

You might be interested in this site were you can run your own test report.
I posted my results here a couple of years ago on a Celestron 100mm f9 ED refractor, it’s a bit finicky to use but worth the effort, and its fun, I can post results again if you wish.

New WinRoddier Version 3 User Manual - Compubuild

As someone once said at the end of the day it’s a hobby and most people can’t discern ¼wave from 1/8 wave when at the eyepiece. Having said that, I think I would rather have 1/8 wave scope. ;)

Tony
7 years 1 month ago #101360

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Replied by michaeloconnell on topic Optical Quality and Effects of Secondary

I Just like to say I’m no expert on optics and have no formal training just an interest in telescope optics as part of this hobby so anything I say take with a huge grain of salt.

Ah no, too late for that disclaimer now Tony, start as you mean to continue...
;-)

So yes it looks like primary mirror only test, I assume that the secondary is of equal quality but you know what they say about assume, I suppose I could check with them.

The secondary is usually of a higher grade.
It's small and flat so easier to polish accurately.
Often 1/20th wave.

From what I can gather the RMS figure is of multi point, if not the whole mirror, which as you say is more accurate than P-V. Were a single or couple of points tested could give an unfair mirror reading either way good or bad.

AFAIK, it should be quite a few points spread right across the mirror.
Of course, if one wanted a good result, I'm sure one could be selective in what points are used...
However, I don't think OO would do such a thing as they would be found out quite easily and it damage their reputation. I have heard only good things about their mirrors.

Worth downloading the program Aberrator:
aberrator.astronomy.net/html/mdibeta.html

MIchael.
Last edit: 7 years 1 month ago by michaeloconnell.
7 years 1 month ago #101362

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Replied by dave_lillis on topic Optical Quality and Effects of Secondary

Hey,
the RMS value if done properly is usually very indicative of the mirror quality, I can remember that the number of points for my own mirror was a 4 figure value (I cant find the diagram), and that the rms value was very low, much lower then the p-p value, indicating a mainly smooth mirror with small areas of deviation.
and tbh I wouldnt get too hung up on this, most people under most conditions wouldn't notice any visible improvements beyond 1/6th wave accuracy, possibly 1/4th wave.
There are far bigger things to worry about in a dob such as tube currents. My scope is a truss scope so it doesn't have a tube, but I can put on a shroud which has the effects of a tube.
For planetary viewing, putting on the shroud usually has a very obvious deleterious effect on the view, so when I use the shroud I have about 2-3 feet above the mirror unshrouded when viewing the planets..
This is one of the reason the SAC 12" dob has a hatch just above the primary mirror, allowing warm air rising off the mirror to escape the tube, works a charm.
Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor
Last edit: 7 years 1 month ago by dave_lillis.
7 years 1 month ago #101363

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Replied by Marto on topic Optical Quality and Effects of Secondary

What Dave said about tube currents....I have big problems with them on my SCT, it can last hours.
OTOH I have the F6 version of Tony H's scope and am nowhere near as plaqued with currents. As long as it's within an asses roar of ambient it will get down to it rapidly enough. That's also a 1/10 wave scope.
In the war between SCT and Newt, collimation is the achilles heel of the newt, still, easily overcome with a god quality collimation kit.
7 years 3 weeks ago #101538

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Replied by dave_lillis on topic Optical Quality and Effects of Secondary

Hi Martin,
Thermals in a SCT are a killer, the bigger the scope the worse it gets, I often found that with my own 12" SCT that it could take 3 hours or more to settle down, the central baffle that the primary mirror rides on is a big culprit for this. The plum off it would absolutely ruin the images and make the optics look like theyre uncollimated.
what is the newt you refer to with the 1/10 wave optics?
My own dobs mirror is about ~1/8th wave, but again, if the mirror is a few degree above ambient it mushes the planetary views, I have a digital thermometer on the back of it and a cooling fan, , just need to be carefull I dont cool it to the dew point, that can be an issue,
Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor
Last edit: 7 years 3 weeks ago by dave_lillis.
7 years 3 weeks ago #101542

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Replied by Marto on topic Optical Quality and Effects of Secondary

It's an ooptics f6 newt. Had it for many years, been thrilled with it for most of that! When I lived in rabat I used to leave it under the air con, or when not not too hot just let the evening breeze blow through it. Either option gave great views for whole observing session. I'm considering buying an sct cooler, it protrudes in through the baffle and blows Ambient air around, with exhaust vents at the back. Ivediy'd one with limited success.
7 years 3 weeks ago #101547

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Replied by dave_lillis on topic Optical Quality and Effects of Secondary

Yea, a number of years ago I was going to get one aswell, dont need it now as its in an observatory these days so its always ambient.
Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor
7 years 3 weeks ago #101548

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Replied by michaeloconnell on topic Optical Quality and Effects of Secondary

Damian Peach has an interesting discussion on this topic on his DVD. Basically, as I understand his view on this matter, atmospheric turbulence has a far greater impact on image quality than optical quality and central obstruction.
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7 years 2 weeks ago #101641

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