Is this a dud mirror?

[doppler]

Hi all, I have this 8" f4 unbranded reflector but I can't seem to get a good focus. There seems to be a "glow" around bright objects, bad visually but worse with a camera attatched. Anyone have an answer?
Here is a 10 sec sub of the lagoon nebula.

[DavidU]

It looks like it's badly out of collimation and also some OTA obstruction.

[brian nordstrom]

:) Hi Doppler , any chance of a few photos of the whole scope , a look down the tube and a photo looking down the eyepiece tube without an eyepiece fitted .
These will help in negating a few basic problems like Dave say's , bad collimation and perhaps a spider vein in the light path ? .

I look forward to helping as it looks like your scope night have a built in barlow , but only going off the photos ( not to bad by the way :thumbsup: ) as you have 'Double Coma'? ( yes I know its un-scientific , but ) and in my opinion only a lens will produce this , not the primary mirror but I have seen this once and it turned out to be the secondary , it was a bathroom mirror type , eg. second surface not first surface like as real secondary should be :question::question: .

Brian.

[doppler]

It looks the same as this one. http://www.iceinspace.com.au/forum/showthread.php?p=1101887#post110188 7 .

It has really thick cast spiders and a oversized secondary, I think it might be badly designed, it looks like it might be a skywatcher clone, but they have thinner spider vanes and baffles in the tube. Saturn looks sharp but has a really anoying/distracting glow around it. I did not pay much for it and am wondering if it is worth persevering with or just striping for parts.

[raymo]

The scope and mount look to be the one that National Geographic shops
sell.
raymo

[doppler]

The mount and tripod that came with the scope are great (eq3 clone ?) The scope is dissapointing though.

[brian nordstrom]

:question:Photos please old mate , I am sure that between Raymo , Dave , myself and anyone else that wants to help , we will diagnose and sort your problems , but we do need some photos .

Some of the National Geographic scopes are quite good , but their QA is not up to the SW standards so its a bit of a lucky dip , that's why people who buy SW scopes tend to like and keep them , they also have a good return policy .

Does yours have any sticker to give its aperture and focal length , like D200 f800 or the likes ? maybe D150 f750 ? this will help immensely .

Brian.
Brian.

[doppler]

Thanks guys, I'll take some pics tonight after work. There are no stickers at all on the tube. The primary is 8" and the secondary is 3" and overall the tube length is 550mm. I thought it was an f4 but that would make the focal length 800mm, but the tube is too short for that. Could it be an f3?

cheers Rick

[torana68]

If its a Foco they are cheap rubbish the views may get better when colimated but it has 99c quality optics. The mount isn't bad if its free I had a 4 1/2 on one and it was ok.

[cometcatcher]

If you want to, bring it over and we could put the Ronchi tester on it and see what sort of curvature it has.

[raymo]

With those dimensions it would be around f/2.75. Is there by any chance,
as Brian mentioned, a lens, [or a place where one could go] at the bottom of the focuser tube that would extend it to f/4 or 5 ?
raymo

[Shiraz]

Yep, it will almost certainly have a lens (probably a single lens) to partially correct for a spherical primary. http://www.telescope-optics.net/sub_aperture_corrector.htm

In principle, this type of design can work OK, although the only one I have seen (branded Bushnell) was pretty bad.

First thing to do (as Roger says) is make sure it is properly aligned and that the correcting lens is clean - there is probably not much tolerance for mis-collimation.

[doppler]

Hi guys, I have taken some pics and measurements. The distance from the primary to secondary is 500mm and from the secondary to the eyepiece around 300mm, so it would be an f4 mirror. The secondary is 76mm and with the camera attached the focuser drawtube creates a large obstruction in the telescope tube. There are no lenses in the focuser. I think that the size of the secondary coupled with the focuser drawtube obstruction and 3mm thick spider veins are the cause of the aberations.Thanks Kevin I might take you up on the offer for a Ronchi test, the scope might be worth modifing if the mirror is ok.

Rick

[cometcatcher]

As large as the secondary is, I don't think it is the source of those aberrations. GSO scopes of that speed have huge secondaries also, as do their RC line, which is even larger at 50% diameter.

[doppler]

Here are some close ups of the aberrations. Ithink you can see where the focuser is blocking the glow at the top of the stars.

[cometcatcher]

To me it looks like spherical aberration. I wonder if the mirror is even parabolised?

[raymo]

After a good look at the pics I am now pretty sure it is the 8" sold by
NG. Give one of their stores a call. They should be able to tell you
the make and model. You might even be able to get some documentation for it.
raymo

[torana68]

It's a Foco, Chinese made, they make rubbish but also an improved version, I think if you find their web page you may be able to find out if this is the cheapie if the better one. Either way if it doesn't respond to collimation don't spend any money on it :)

[doppler]

Thanks for the input, I found this review on what looks to be the same scope. Here is the link http://www.vcastro.com/reviews/helios/. The primary is crap (spherical), pity as the rest of the assembly is quite solid. Luckily its worth what I paid for it in parts.

Rick

[cometcatcher]

So it's as I suspected. That's a pity, a spherical mirror would need a re-polish to a parabola and then be re-coated. Unless there are front aperture corrector plates for such a thing available? That would turn it into a Schmidt Newtonian. But then I read the flat is no good either so probably not worth fixing.

[multiweb]

Buy a quality primary and secondary and replace them in the existing hardware and you'll have a pretty sweet newt. It looks solid.

[DavidU]

Ah, it's a Bird Jones Newtonian.

[Shiraz]

Probably was David, but it seems that there is no longer any corrector lens...yikes.

[doppler]

The sad part is there are probably thousands of these scopes in use and the users think they are good. Galileo might have been impressed but you have to feel sorry for the rest. The views of the moon are ok but if you have a look through a "proper" 8" scope its chalk and cheese. If the manufacturer would have put an other 10% in the mirror it would have been a good scope.

[GrahamL]

A friends family had one of these things and I foolishly suggested we take it out back for a spin , poor views, weird mount, and collimation didn't look to far off , was looking at the pool close by and seriously considering giving it a nudge when nones looking and heading back inside ;)

I'd start again ,(hate to say it ),than try to make a silk purse out of that sows ear or parts like you say.

[bkm2304]

The odd looking into your star images is due to the MASSIVE intrusion of the focuser into the light path. No amount of collimation will fix that. I have to agree with the other posters, you need to euthanise this sad scope - to end your own suffering as much as the scopes. :(:(:(:(

Richard.

[Shiraz]

maybe try it with a 2x Barlow before you consign it to the bin - that might help a bit.

[brian nordstrom]

:) I think that link answered the main problem with this scope , the secondary , its made from an 'ordinary hand mirror' eg. a second surface mirror like I mentioned earlier , a telescope's secondary should be 'first surface' that is aluminised on the front ( first surface) not behind like a normal bathroom mirror , the reflected image has to pass thru normal ( not optical ) glass twice before the image hits the eyepiece , bad practice !!! , this and a spherical primary :shrug: well .....

Brian.

[doppler]

I just pulled the secondary out and it is a front surface coated mirror. I think the problem is still mainly with the curve of the primary, which can't focus the light properly.



Visually the focuser tube is not blocking the light it is only there when the camera is attached. I could move the primary forward to increase the back focus.

Rick