bargain achromats

[slice of heaven]

Now you've done it Geoff
Seems confusion reigns supreme in this thread.

[Starkler (Geoff)]

Originally Posted by GrampianStars

the ED80 kills my 8" SCT on planets

I also have an ED80 but no way can I resolve the same detail with that as I can in my 10 inch dob and I wouldnt expect to with 80mm of aperture.
It really sounds to me that there is something wrong with the sct.
Dave47tuc used to own a 10inch lx200 which gave good planetary views (sold to mojo I hope he is using it, its too nice a scope to be collecting dust).

Im not bagging refractors, I just dont believe that a 4 inch achromat will beat an 8 inch gs dob (properly set up and collimated) on planets. Theres two factors working against the achromat, aperture and colour.

[slice of heaven]

Its not only the sct Geoff, there seems to be a few scopes in trouble in this thread.
Especially if 60mm and 80mm refractors are knocking off 8",10" and 12" newts.
Properly setup the big ap wins handsdown.

[Starkler (Geoff)]

Originally Posted by slice of heaven

Its not only the sct Geoff, there seems to be a few scopes in trouble in this thread.
Especially if 60mm and 80mm refractors are knocking off 8",10" and 12" newts.
Properly setup the big ap wins handsdown.

When we had our Vic meet a few months ago, there were three other GS dobs there, two that I got to look through. Both of them had obvious issues.
If at least two out of three from this small sample had issues, I wondered how many others out there were like this. Thats what prompted me to put together the GS optics how-to http://www.iceinspace.com.au/index.p...7862c1ea9321d8

In the case of dave47tucs scope, his primary was clamped in too tightly.
I havent heard back about Migs scope yet.

[slice of heaven]

Maybe that link should be thrown up more often to keep people aware of the issue.
I'll remember I said that.
As you've stated its a common problem that needs to be addressed.

[[1ponders] (Paul)]

I have the HD-102EQ f/10 Celestron/Synta Achromatic refractor on an EQ3/4 mount with the diabolical aluminium legs. It has way to much lever arm movement for that size mount but should work well if they really are EQ5 quality mounts. Pitch the legs of the tripod in the bin and put some solid wooden legs on it.

If the refractors from you link are the same as mine, and I'm betting they are, its not a bad little planetary scope, but it has an abysmal cheese grater focuser. Though once you clean the Synta snot grease out of it and regrease it the improvement is quite dramatic. I've even used mine to image through. (see here http://www.iceinspace.com.au/forum/s...ead.php?t=2304 ) Naturally the CA is much worse when imaging, but it does give a pretty good sharp image for a cheap refractor.

After seeing the amount of CA in my f/10 I'd imagine in the faster focal length scope it would be pretty abysmal. It would probably make a good finderscope or guidescope, but too colourful as a regular visual scope.

[ving (David)]

Originally Posted by seeker372011

you can get a 0.963 to 1.25 convertor for $10 from BATSC in melbourne

I used one to convert a $50 GoLo scope -a 60 mm refractor into a really light weight guide scope that can be piggy backed on my 8 inch newt

good idea..
thanks narayan

[ausastronomer (John Bambury)]

Originally Posted by ving

focal length ken, the 900mm will give more magnification but like adding a brlow to your scope it makes the image dimmer and narrower too. the 600mm will give a brighter image and wider, but the trade off is less magnification.

David,

Importantly the shorter scope will have SIGNIFICANTLY more false colour on bright objects than the longer scope. Basically the short scope is best suited for low power wide field deep sky work and the longer scope better suited to lunar/planetary double stars.

CS-John B

[ausastronomer (John Bambury)]

Originally Posted by Starkler

Dont get me wrong here!
I mentioned it for the benefit of those who might be hoping that a 4 inch achromat is going to somehow deliver superior planetary views to their GS dobs. I fear these people will be dissapointed unless their dobs arent set up and collimated properly. Getting that right is cheaper and easier than shelling out for a new scope to do something the dob should be superior at anyway.

Geoff is 100% correct here, no 4" refractor is going to compete with an 8" dob in terms of "visible lunar/planetary detail" a properly set up 10" dob will outperform or compete with the best 6" refractors. A 5" and smaller refractor is not even a contest to a 10" dob.

I will post something on the GS dob optics thread about what to check for optically with your GS dobs.

http://www.iceinspace.com.au/forum/s...ead.php?t=3370

CS-John B

[dugnsuz (Doug)]

Hi All,
New to this excellent forum - first post.
I've recently contacted Dan at AOE re this package.
I was worried about the flimsy aluminium tripod.
Dan said he'd upgrade to the tubular steel version for an additional $40.
That's a spicy meatball!

Cheers

Doug Robertson.

[ving (David)]

hey thanks doug, and welcome to the forum