GSO Dob optics - Page 2

Starkler (Geoff) · #21 09-05-2006, 05:40 PM

The rings are less sharp inside focus as the scope focusing at some point in the atmosphere, rather than at infinity. To get sharp rings at inside focus, the seeing needs to be near perfect, whereas outside focus any night of medium seeing will show well defined rings.

In assessing optics by star test, you are looking for identical distribution of light between the inside/outside focus images. For example, if your outermost ring is dimmer than the rest on one side of focus, an brighter on the other, than you have an issue with the mirrors figure.

janoskiss (Steve H) · #22 09-05-2006, 06:05 PM

Thanks Geoff. I am probably just seeing the seeing then.

I will have a look at the relative intensities of the rings next time.

Starkler (Geoff) · #23 09-05-2006, 06:29 PM

Some light reading about star tests

mickoking · #24 09-05-2006, 08:39 PM

Quote:

Originally Posted by janoskiss

I've been getting spectacular detail on Jupiter with it, but I am still waiting for some good clear, dark and moonless skies to really give it a go on DSOs, esp galaxies.

It is great to randomly look for galaxies listed in Uranometria. 300mm will show you a lot of universe.

mickoking · #25 09-05-2006, 08:44 PM

Quote:

Originally Posted by ballaratdragons

My GS12" gives great star test rings either side of focus. (when collimated)

Micko, I am more than pleased with my GS12" optics. Pity I can't say the same about my EP's but they do a pretty good job for their price.

Star testing does indicate very good optics. I also agree that the supplied eyepieces are not too good. The GSO thrives on quality eyepieces.

dugnsuz (Doug) · #26 09-05-2006, 11:02 PM

Hi,
If you've been following this thread...
http://www.iceinspace.com.au/forum/s...ead.php?t=9744
...you'll know that I've yet to experience the beauty of my 12" optics.
I have owned the 10" GSO Dob in the past and I can only agree with the rest of you as to their quality.
Going off topic a bit but, I had a 12.5" "Darkstar" dob when I lived in the UK around 1993 - this had a top quality mirror by David Hinds, but the quality of the finish,mount and finder left a lot to be desired.This was "the" dob to have at that time and was around 700-800 UK pounds around $1900 by today's standards.
The 10" dob I bought from Bintel around 3 years ago was $1099 + freight.
You can get the same model today from Andrews for $599 + freight.
And if you're lucky like me you may even pick up 2nd hand bargains.
These scopes are phenomenal for these prices when compared with what used to be around for similar prices - Tasco department store stuff.
GSO really have nailed the mass production of large budget mirrors.
But, as for the EPs

Cheers
Doug

ballaratdragons (Ken) · #27 09-05-2006, 11:10 PM

It is peculiar that a company such as GS can make such high grade mirrors, but at the same time make such absolutely cruddy EP's!

Makes no sense at all. Only thing I can think of is that GS don't make the EP's. Maybe they buy them in from somewhere else. It's not uncommon!

janoskiss (Steve H) · #28 09-05-2006, 11:49 PM

I think you guys are being unfair on the GS EPs, especially the plossls. They do a very good job! Fully multicoated full 50 degree FOV plossls, with barrels properly blackened, for $25 a pop atm from Andrews is amazing value! Nothing comes close at twice the price! I am rediscovering the GS 25mm in the binoviewer, and they do an excellent job. There is really not much between them and top Japanese plossls like my Antares Elites, which cost over $100 ea.

dugnsuz (Doug) · #29 10-05-2006, 07:56 AM

The shorter focal lengths are the ones I dont rate.
The 25mm up to 15mm EPs that come with these scopes are useable.
Cheers
Doug

janoskiss (Steve H) · #30 10-05-2006, 08:39 AM

Yes, one's better off barlowing the 15 and 20mm than using the shorter focal length ones.

Roger Davis · #31 10-05-2006, 07:50 PM

Distribution of the rings, should be the same inside and outside of focus. The ring pattern in an ideal "perfect" telescope should be the dead set same. I have never seen one that fits this criteria, and I have not yet seen a GS or Synta even approach this criteria. The scopes that I have star tested show very well corrected optics (see Star Testing Astronomical Telescopes by Richard Suiter), but never perfect. There was a great article on telescopes and their performance with steps of 1/8 wave deformation done by Roger Ceravolo in an ATM Journal which was duplicated in an S&T magazine a few years back. Check it out for interests sake.

Merlin66 (Ken) · #32 11-05-2006, 07:22 AM

Guys,
I note in this thread no one has mentioned a very easy quick test which will show the overall figure of the mirror, and identify Spherical abb ( much easier than the star test) collimation and tube currents....
The Rochi test using a 80 or 100 line per inch grating ( photographic copies available) when placed very close to the focus ( assuming you're looking at a reasonably bright star) should show perfectly straight and equal spacing; any distortion you in the lines see is coming from the optical system/ tube and atmospherics. Easy to photograph the outcome for comparison between scopes!!

BTW it also is a great focussing aid; when positioned exactly at the focal point the lines disappear and a uniform grey is seen.

Roger Davis · #33 11-05-2006, 08:03 AM

Wasn't avoiding it just talking on the same vein as previous posts.
You can also Ronchi with the mirror in the scope during the day, but the secondary gets in the way a bit.
Best way I have found is to find focus on a star with an eyepiece in place, then remove the eyepiece and place a knife edge where the eyepiece was. You can then perform a Foucault test in the null position. That is, you will have straight lines when the mirror is a perfect parabola. Any deviation from straight lines indicates a problem. Same for focusing, back in the dim dark past (circa 1978) that's how we used to focus our cameras (film that is, archaic now) by placing a knife edge at the film plane. Many devices were made for this purpose, they could be used now to test your optics.
I have one good tool that I carry with me, it's an old 15mm eyepiece with the lenses removed and a piece of 100lpi Ronchi screen glued where the exit lens was. Great for checking scopes on the field of a night!

Dennis · #34 11-05-2006, 10:25 AM

Quote:

Originally Posted by Roger Davis

>snip.
Best way I have found is to find focus on a star with an eyepiece in place, then remove the eyepiece and place a knife edge where the eyepiece was. You can then perform a Foucault test in the null position!
>snip

I still carry a scar on my nose from a Gillette razor blade when Foucault knife edge testing an 8" mirror in the 70's. My pinpoint light source was a tin can with a small hole, itself covered by a piece of aluminium foil with a pinprick acting as the light source.

In the tin can was a 12W light bulb. My cheek accidentally touched the hot tin can and when the auto-response system kicked in, my head jerked up and the adjacent razor blade slashed my nose!

Be careful, be very careful!

Cheers

Dennis

Satchmo · #35 11-05-2006, 10:40 AM

Quote:

Originally Posted by Merlin66

Guys,
I note in this thread no one has mentioned a very easy quick test which will show the overall figure of the mirror, and identify Spherical abb ( much easier than the star test) collimation and tube currents....
The Rochi test using a 80 or 100 line per inch grating ( photographic copies available) when placed very close to the focus ( assuming you're looking at a reasonably bright star) should show perfectly straight and equal spacing; any distortion you in the lines see is coming from the optical system/ tube and atmospherics. Easy to photograph the outcome for comparison between scopes!!

I'd need to add that this test has fairly poor sensitivity with an F5 scope and needs to be used with caution in declaring optics good or not. You need very good seeing conditions and view with no more than a few bands visible at most . The slightest bending of the bands in ward or outward , particularly out near the edge of the mirror can indicate significant spherical abearration to affect the airy disc at high power.

Most mirrors pass this test , looking superficially straight to the layman , but particularly with larger mirrors, the bands need to be knife edge straight right to the very edge. For example on a 20 " mirror , a 'kink' in the last 15 mm of a band at the mirrors edge edge , could mean you have the light of a 4 " mirror being thrown outside the Airy pattern ..not a good look at high power, Defects like this, masked superficially by the fuzzing of the seeing are often endlessly blamed on poor seeing.

Mark

mickoking · #36 11-05-2006, 10:41 PM

Quote:

Originally Posted by Roger Davis

Distribution of the rings, should be the same inside and outside of focus. The ring pattern in an ideal "perfect" telescope should be the dead set same. I have never seen one that fits this criteria, and I have not yet seen a GS or Synta even approach this criteria.

My GSO is not perfect but in all honesty its not all that far from it. For an inexpensive and fast (f5) newtonian Dob I can't complain

mickoking · #37 11-05-2006, 10:50 PM

Quote:

Originally Posted by janoskiss

I think you guys are being unfair on the GS EPs, especially the plossls. They do a very good job! Fully multicoated full 50 degree FOV plossls, with barrels properly blackened, for $25 a pop atm from Andrews is amazing value! Nothing comes close at twice the price! I am rediscovering the GS 25mm in the binoviewer, and they do an excellent job. There is really not much between them and top Japanese plossls like my Antares Elites, which cost over $100 ea.

My main dissapointment is the 32mm monster supplied with the 300mm Dob. Even thru my f8.3 (120mm) refractor about one third of the field has coma and this of course is worse in the Dob. fortunately the 27mm Pan does a brilliant job in the Dob.