GSO Deluxe Dob - worth the extra?

[Luke]

I must get around to checking if the centre dot on my GSO mirror is central. If it's slightly out, that might explain why my Paracorr improves the view so much (the Paracorr has a slight barlow effect, which would give a bit more margin for error as far as I know).

That's interesting, oldfruit, that you could tell no difference between the SW and GSO. For me there's no question my SW's were a little bit better than my GSO (though as above, I wonder if the dot on my GSO is slightly out!).

I had two F5 12 inch SW dobs at the same time for a while (one was a solid tube, the other was a flextube). I could not tell the sligthest difference between those two.

[prairie_stars]

There are various reasons why a dob might underperform, inaccurate center-spotting, or poor mounting of the primary or secondary. Check those primary mirror clips that they're not pressing down hard on the mirror.

Long story short, I have a dob with GSO mirrors, and after sorting out the various scope issues, the mirror turned out to be nearly perfect. It's a humdinger as they say. GSO has been called on to make a lot of the quartz mirrors for expensive R-C scopes that are gaining in popularity, so my guess is they can make a great mirror with some consistency. 

[oldfruit]

I must get around to checking if the centre dot on my GSO mirror is central. If it's slightly out, that might explain why my Paracorr improves the view so much (the Paracorr has a slight barlow effect, which would give a bit more margin for error as far as I know).

That's interesting, oldfruit, that you could tell no difference between the SW and GSO. For me there's no question my SW's were a little bit better than my GSO (though as above, I wonder if the dot on my GSO is slightly out!).

I had two F5 12 inch SW dobs at the same time for a while (one was a solid tube, the other was a flextube). I could not tell the sligthest difference between those two.

I did say "we could see no difference to our untrained eyes".   What may be acceptable optically for me may not be for someone more experienced. Sometimes optical defects are slight and only noticeable when someone in the know points them out to someone like myself.

[Ags]

I must get around to checking if the centre dot on my GSO mirror is central. If it's slightly out, that might explain why my Paracorr improves the view so much (the Paracorr has a slight barlow effect, which would give a bit more margin for error as far as I know).

That's interesting, oldfruit, that you could tell no difference between the SW and GSO. For me there's no question my SW's were a little bit better than my GSO (though as above, I wonder if the dot on my GSO is slightly out!).

I had two F5 12 inch SW dobs at the same time for a while (one was a solid tube, the other was a flextube). I could not tell the sligthest difference between those two.

Wouldn't a star test show if the center spot was out?

[Luke]

Thanks for the idea I have no idea if that would do it, I've never done a star test, just collimated   I'll look into it!

it would be great if there's a way for me to check without having to remove the mirror, especially as it's okay when using the coma corrector

Wouldn't a star test show if the center spot was out?

[Sneezy24]

Got my Revelation 8" Dob yesterday, hoping for first light tonight!

Very impressed with the quality, hope the optics are sufficiently good for old eyes.....

[emadmoussa]

enjoy!!

[Ags]

Tell us how it works out :-)

[Ags]

Just noticed this:

http://www.gs-telescope.com/content.asp?id=116

" 8" Parabolic primary mirror 1/12wave "

Seriously? Orion Optics UK premium scopes offer 1/10 wave at maximum.

[Sneezy24]

Got first light last night! Just a short play when I got home.

Stunning views of the moon, great to see the mountains and craters, particularly close to the terminator..

Then had a look at Jupiter, using the 9mm EP and 2x Barlow got a wondeful view, I could see the bands, and also all four moons.. (checked the current S@N to make sure I was seeing the right things!).

Awesome!!!!

OPtics look good, no apparent chromatic aberation, focus slightly softer away from centre but really had to look for it (and might possibly be caused by my eyes - I had cataracts and had implants, focus is softer towards the outer eye).

But NO complaints! Went back inside with an ear-to-ear grin!

[emadmoussa]

The softer edges are most likely related to the eyepiece, not the telescope.

[AlexB67]

Glad you enjoyed it, hope you have many pleasurable hours with it.

You'll not have to worry about chromatic aberration in your Dob, you can buy plenty cheap refractors for that , I'll put my coat on and run since I'll have the frac mob beating me up soon

[John]

Just noticed this:

http://www.gs-telescope.com/content.asp?id=116

" 8" Parabolic primary mirror 1/12wave "

Seriously? Orion Optics UK premium scopes offer 1/10 wave at maximum.

Misleading advertising IMHO or at the very least it needs to be qualified. That figure is 1/12 RMS which equates to 1/4 wave PV. 

Telescope House use the same general spiel.

[AlexB67]

If I recall correctly 1/4 PV equates to about 1/13.9  RMS, so 1/12 RMS ( root mean square, and average measure of error over the whole mirror ) comes out slightly worse than 1/4 PV. I agree it is misleading, really they should say PV or RMS or it may as well counting chickens. Initially I fell into that trap too when I was trying to get my facts straight on these scopes.  The price of the scope and the value of 1/12 are enough of a give away that RMS is what is meant.

A useful bookmark I have, you can check in there, IIRC it goes through the RMS PV comparison debate without getting too involved.

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

[Ags]

That makes more sense, sub 1/4 PV doesn't sound so good. Their ATM mirrors are '1/16 wave' according to the site, sounds a bit better.

[prairie_stars]

Misleading advertising IMHO or at the very least it needs to be qualified. That figure is 1/12 RMS which equates to 1/4 wave PV. 

Telescope House use the same general spiel.

It's not misleading, and it's not RMS. They are referring to the error in the surface of the mirror. PV and strehl measure the resultant wavefront coming from a mirror which means the waveform interacts with the mirror surface twice: coming and going. I have a secondary mirror that is tested as 1/30 wave. It's a common way for an optician referring to the surface accuracy of the mirror.  Of course telescope users are more interested in PV and strehl, especially of the entire optical system. A 1/12 wave mirror might roughly give you a 1/6 wavefront PV.

[John]

It's not misleading, and it's not RMS. They are referring to the error in the surface of the mirror. PV and strehl measure the resultant wavefront coming from a mirror which means the waveform interacts with the mirror surface twice: coming and going. I have a secondary mirror that is tested as 1/30 wave. It's a common way for an optician referring to the surface accuracy of the mirror.  Of course telescope users are more interested in PV and strehl, especially of the entire optical system. A 1/12 wave mirror might roughly give you a 1/6 wavefront PV.

They don't actually specify that though do they ?

I spoke to the UK vendor about this and they conceded that the Revelation branded GSO scopes primary mirrors were around 1/4 wave PV and the 1/12th figure was RMS.

[Sneezy24]

Is there a danger that we are getting into territory similar to hi-fi fanatics who listen to their equipment, not the music??

How much actual difference does the mirror 'gooness' make? How can these numbers be equated to actual visual differences?

[Ags]

I think on DSOs it would be impossible to tell the difference. But I think it would make a difference on planets, which are my main interest.

TS can do an optical test for me, it would cost 100 euros.

Prairie Stars, I hope you are right :-)

[AlexB67]

Is there a danger that we are getting into territory similar to hi-fi fanatics who listen to their equipment, not the music??

How much actual difference does the mirror 'gooness' make? How can these numbers be equated to actual visual differences?

The 1/4 PV is considered a sort of sensible limit. See that link I posted earlier and if you want and http://www.telescope-optics.net/ , while not to get overly hung up about you would see differences for example when splitting doubles, planetary details ( but not massive differences as if to say 1/4 PV is like going from horrible to say 1/8 PV massively better)  .

I'll add that I've never looked through a 1/10PV or anything better than my own SW, but from what I read in reviews and various articles, and also John will perhaps tell you If I am not mistaken his OO optics mirrors he can push things that bit more on planets and so on, doubles. Incremental PV upgrades will offer a bit better detail and cleaner views on the whole is the trend, just as more costly eyepieces can help too. Often though you may not even notice it in this beautiful UK weather these days mind you. 

I agree there is a lot to be said for the argument if you are happy with what you see, there is no problem.  Problem is with the weather as it is we have too much time to think and not enough time to observe :0)

[todd8137]

From the one that came to are group night it performed great and the views were brill not sure if that was the scope or the naglers

Pat

[Sneezy24]

I'm very much a beginner, but have a science/engineering background so can understand some of the tecky stuff.

But, for visual observing, what is more important, the mirror(s) or the eyepiece??

I will almost probably buy new EPs at the Leamiongton show, I realise that the two suppplied with my Rev are 'basic', but I am really after good views, not necessarily 'perfect' views. I may be missing the point, but the heavens are wonderful, if a star is slightly fuzzy, does it matter??

[Sneezy24]

Oh, and on the music front, I had a long e-mail string with an old friend, talking about vinyl vs. CD vs. MP3. So I did an experiment.

He chose three pieces of music, he played each one on vinyl, CD and MP3 (at 320), he stated vinyl was the best.

Repeated the test, with me playing the music and him not knowing the source....

For two pieces he said MP3 was best, the other CD was best!

Not very scientific, but interesting!

[Ags]

Having done some more reading, I'm inclined to agree with prairie_stars.

[John]

Having done some more reading, I'm inclined to agree with prairie_stars.

I'd be interested in learning more - can you post a link or pointer ?

I'm aware that the subject is a complex one and there is far more to it than a few bits of data.