GSO Optics

[Aster]

Has anyone ever bench tested, Ronchi or Focault, a GSO 305mm F5 Dobsonian Mirror ?

Just lined my steel tube with cork and had to remove the primary for that purpose. While I had it out A friend of mine and myself done a Ronchi and Focault test with mask and found it quite a bit under corrected with a turned edge.

Is this the norm, or am I just one of the unlucky ones getting a bad mirror ?

Aster

[gbeal]

How bad was it?
What did the mirror itself cost?
OK, you have seen through me, it is a pair of loaded questions?
To really cap it off, did you suspect this "lack of performance" prior to the test?
I have a cheap and I mean cheap mirror, presumably Chinese, that absolutely rocks. Maybe I was lucky, but given the amount I paid I expected worse.

[cristian abarca]

About a year ago while at a star party a friend tested a couple of 8 and 10 inch mirrors and they seemed to be pretty good. Regards Cristian

[astropolak (Joe)]

Have a read of this:
http://www.wodenoptics.com/cloudy.html
It may have been the same make of mirror (with similar problems) to what you describe.

Joe

[iceman (Mike)]

I've got a 12" GSO mirror (and rest of tube!) and it appears to be great to me. I haven't tested it with any of those "official" tests, but from the quality of images I can capture using it, I have no issues with the quality.

For the price I paid, it's perfect!

Have you looked through other 12" scopes to see if the view is better or worse?

[janoskiss (Steve H)]

These are inexpensive mass produced mirrors. Good for the price but quality varies from one to the next as you can easily see if you star test a few side by side.

[Eric48]

Believe Bintel had its dob mirrors - which I understand are GSOs - tested in Germany about 2003 using 'null ronchi images' and an 'interfererogram' which indicated an average Strehl ratio of 0.95. I have absolutely no idea what that means - I've simply copied it from a Test Report in Nov/Dec 03 Sky and Space on the Bintel 302 which I got from Bintel and that Report says it would indicate 'very good to excellent quality'. The Report gave the optics a good write up. Can't say whether its still the same but I intend to buy a Bintel 302 dob in next couple of weeks.
Eric

[casstony]

There is some technical testing of one of these mirrors with images at this translated german site; go to 'Tests of Optics', page 9, 'GSO with divisible tube'.
http://translate.google.com/translat...%3Den%26sa%3DG

[Starkler (Geoff)]

Originally Posted by casstony

There is some technical testing of one of these mirrors with images at this translated german site; go to 'Tests of Optics', page 9, 'GSO with divisible tube'.
http://translate.google.com/translat...%3Den%26sa%3DG

I have seen one or two examples of very good gso mirrors, but I wouldn't think the one tested there is a typical sample. Those numbers are typical of the larger "premium" makers, such as OMI or Galaxy.

Funnily enough, my best ever view of Jupiter was through a newt with obvious undercorrection, but I was lucky enough to viewing during one of those rare moments when the atmosphere "froze solid". How sweet they are.

[ballaratdragons (Ken)]

I have looked thru many brands of reflectors in many sizes and must say that the GS mirrrors are very good to terrific. Especially when you pay only several hundred dollars for what you had to pay several thousand dollars for only 10 years ago!

Obviously with mass production there will be the occasional dud, but in general, they are great.

[Aster]

Thanks Fellows, didn't expect such a number of replies to my queery.

Yes, I could tell that there was some fault in the optics due to the diffraction pattern being different inside/outside focus star testing, before I put the mirror on the test bench.

Yes, I have looked through other 300mm Newtonians and the difference is very small. Seeing conditions, turbulance and many other things play a role in this. But it is there.

No, I am not complaining. One gets what one is prepared to pay. Although that doesn't work at times either. I suppose I expected something a bit closer to the advertised 1/16 wave some retailers floating around for GSO Dobsonians. Also with todays techniques, Teflon/Poleurathene laps the quality should be more consistent.

Yes, I have seen your Photos, Mike, I am green with envy. Looking back 30 years ago when we battled with coarse grain films,freezing/backing emulsion, dodging, burning in images whilst spending hours in the darkroom it looks just to easy nowadays, although I am sure it isn't.

Looks like I just got a mirror which is not quite as good as it could be.

Alex

[Aster]

Astropolak

Thanks for the pointer, had a look at the article and have to agree going after what can be seen on the bench with my mirror.

Alex

[Aster]

Originally Posted by iceman

I've got a 12" GSO mirror (and rest of tube!) and it appears to be great to me. I haven't tested it with any of those "official" tests, but from the quality of images I can capture using it, I have no issues with the quality.

For the price I paid, it's perfect!

Have you looked through other 12" scopes to see if the view is better or worse?

So I noticed, after looking at your photos. The only other one I have seen who uses tie down straps to secure the tube to the mount.

How do you attach the tube to the EQ6 ?

I made a standard wooden cradle with a split wooden ring clamped around the tube so I can rotate the whole lot for more comfortable viewing positions.

Alex

[gbeal]

Originally Posted by Aster

I suppose I expected something a bit closer to the advertised 1/16 wave some retailers floating around for GSO Dobsonians.
Alex

Well if it was bought with the 1/16 wave quality assurance, then I would be peeved as well. Take it back. When I bought mine it was advertised at 1/10th wave, and I thought that was a load of cobblers. It is good though, and I am happy.

[Aster]

Still talking about optical quality. I have been out of practical astronomy for quite some time. Things have changed a lot. New terminology has come in describing the accuracy of the mirror for example. In the old days , we called it 1/4 wave ,1/8 wave etc. meaning the total wave front at the focus.

Seeing this is my first comercially made telescope I purchased, all others were home made, I am a bit bamboozled with todays terminology.

So, can someone tell me what 1/16 wave RMS or better means in plain total wave front???

Alex

[casstony]

1/16 wave RMS = roughly 1/4 to 1/5 peak to valley:
http://www.rfroyce.com/standards.htm

[Tannehill]

This is a long shot, you're experienced, and as you say your friend removed the mirror to test it, so that implies an extremely knowledgable person involved (leagues beyond me). But..sometimes the mirrors clips are too tight and pinch the optics. That variable removed before the test?

Scott

[Satchmo]

Originally Posted by Aster

So, can someone tell me what 1/16 wave RMS or better means in plain total wave front???

RMS figure is a better indicator of mirror quality as it factors in the relative surface area of an error , although doesn't tell you necessarily of how well the mirror will perform. If most of the error consists pure spherical aberration then there will be a smooth sinusoidal error of around 1/4 wavelength at the 70% zone. Such a mirror can perform well but planetary contrast will be a little washed out.

If the mirror were pretty good spherical aberration wise but say a rolled up or down zone but only effecting the edge, the mirror may still have 1/16 wave RMS, but give a very poor looking star test, and even worse planetary performance. The golden rule for optics is that errors if any want to be smooth and gentle slope. The only optical claims I've seen on the GSO website say 1/16 RMS ( roughly equivelent to the old 1/4 wave Rayleigh criterion) and seems to be a very reasonable and safe claim on the basis of the handful I've seen.

'Numbers' don't mean a lot without seeing what kind of surface shape is involved. Two different 0.95 Strehl mirrors can have vastly different planetary performance. The optician who understands this and why and how will make consistantly better performing mirrors for planetary observation.

In the case of mass produced mirrors you have no real guarantee about what you get so its a kind of lucky dip that you don't encounter in other hobbies such as Hi Fi for instance. It seems to me that people have much higher expectations of mass produced optics than they used to which is neverthless a good thing.

Mark

[Aster]

Originally Posted by Tannehill

This is a long shot, you're experienced, and as you say your friend removed the mirror to test it, so that implies an extremely knowledgable person involved (leagues beyond me). But..sometimes the mirrors clips are too tight and pinch the optics. That variable removed before the test?

Scott

Thanks for the thought, Mirror was tested hanging in a sling all by itself

Alex

[Aster]

Originally Posted by Satchmo

RMS figure is a better indicator of mirror quality as it factors in the relative surface area of an error , although doesn't tell you necessarily of how well the mirror will perform. If most of the error consists pure spherical aberration then there will be a smooth sinusoidal error of around 1/4 wavelength at the 70% zone. Such a mirror can perform well but planetary contrast will be a little washed out.

If the mirror were pretty good spherical aberration wise but say a rolled up or down zone but only effecting the edge, the mirror may still have 1/16 wave RMS, but give a very poor looking star test, and even worse planetary performance. The golden rule for optics is that errors if any want to be smooth and gentle slope. The only optical claims I've seen on the GSO website say 1/16 RMS ( roughly equivelent to the old 1/4 wave Rayleigh criterion) and seems to be a very reasonable and safe claim on the basis of the handful I've seen.

'Numbers' don't mean a lot without seeing what kind of surface shape is involved. Two different 0.95 Strehl mirrors can have vastly different planetary performance. The optician who understands this and why and how will make consistantly better performing mirrors for planetary observation.

In the case of mass produced mirrors you have no real guarantee about what you get so its a kind of lucky dip that you don't encounter in other hobbies such as Hi Fi for instance. It seems to me that people have much higher expectations of mass produced optics than they used to which is neverthless a good thing.

Mark

That would confirm our readings with a Couder Mask.

6 readings by 2 experienced mirror makers over 5 zones, center to edge, edge to center.

Average, 0, 1.46, 4.32, 5.9, 6.77 mm

Ideal 0, 1.49, 3.071, 4.633, 6.067 mm

I think the figures speak for themselves.

Alex