12" GSO secondary

[SkySearcher (Daniel)]

Hi Guys

I have been having some issues trying to collimate my 12" GSO dob. I have had it for about 8months now and have been trying with varying degrees of success to get it collimated. I had never looked through such a scope prior to my purchase which doesn't help the situation much. I have constructed a basic tool using a film canister with a 1.5mm hole drilled in the exact centre of one end. I then painted a very small circle around the inside of the small hole and am able to illuminate it using an LED attached to another small hole on the side.

I can easily line up the primary centre spot with this small hole in the filem canister and can do a star alignment accurately using a magnifications upto 333x.

However, my question is, how do I know the secondary is lined up and will a star test show an incorrectly collimated secondary?
When looking through the film canister there are many positions the secondary can be in and still show the full primary reflection and of course I can then adjust the primary to place the centre spot directly in the centre of the draw tube.

I lined it all up and looked at 47tuc. At 15mm it was just ok but at 9mm it was a blurry blob with barely any resolution. The start test still shows a 'perfect' donut.

I just have the standard eyepieces supplied with the Scope, but I'm sure it can do better.

Any ideas?

Thanks in advance

[iceman (Mike)]

You should look at getting a cheshire/sight-tube from Bintel or somewhere. They're only $50 or so and are more accurate than a film cannister (which I also used in the beginning).

You can then shine a red light on a reflective surface and light it up at night.

For the secondary,
1. Make sure the secondary mirror appears round and centered in the middle of the focuser (you don't need anything in the focuser for this). Just make sure your head is aligned.

2. Now use the 3 screws on the secondary to adjust the tilt so that the primary mirror is centered within the view of the secondary. I use the mirror clips as a guide - that is, ensuring the mirror clips of the primary touch the edge of the view in the secondary, equally all around.

You can use a sight-tube for this.

[erick (Eric)]

Hi Daniel

I have a Cheshire tool and a laser tool and a barlow to use the barlowed laser technique. These appear to do a good job on my 8" GSO. I don't believe that the 12" GSO is significantly more difficult to collimate.

I'm happy to bring them round some time so you can experiment with them and see what would work best for you, before you spend money on one or more collimation tools.

PM me if you are interested.

Eric

[Starkler (Geoff)]

Originally Posted by SkySearcher

[FONT=Arial][SIZE=2]I
I lined it all up and looked at 47tuc. At 15mm it was just ok but at 9mm it was a blurry blob with barely any resolution. The start test still shows a 'perfect' donut.

At 15mm you are viewing at 100x, with a 9mm at 166x.
If your scope isnt properly cooled or the seeing is mediocre this is what you will might see regardless of how well collimated your scope is.

[SkySearcher (Daniel)]

Thanks for your replies
Eric, Thanks for the offer! I have sent a PM

Thanks Mike the whole ofset thing is quite confusing, and most articles I read say not to warry if the secondary is not centered.

So, if the secondary mirror is not lined up will that show in a star test?

[GrahamL]

have a look at this site Daniel
http://www.piscescs.com/astro/collimat/notools2.html

Its much like mike mentions ..with no tools to start I found it easier to seperate the two to get your head around what your seeing
particularly if the primary tilt is a way off.

[Tannehill]

If the secondary is not positioned correctly under the focuser, but still correctly reflects the center of the focuser axis to the center of the primary mirror, you will not notice image quality problems, but only slight (probably undetectable) reduction in brightness, since you are just vignetting the light cone.

If the secondary mirror is not accurately reflecting the center of the focuser axis to the center of the primary mirror, you could notice true image collimation issues (blurry stars than don't want to focus, hazy images of the planets, etc). So, yes, secondary misalignment in this sense will matter.

An f5 scope like you have will be touchier w/r to collimation than an f6 or slower scope. Seeing will often limit the issue, as stated, but if you are keen to look at double stars, the moon, the planets, then you should invest in more precise collimation tools: Either a laser collimator with a barlow attachment (which will let you align teh secondary and primary), or chesire/sight-tube set.

The primary (non-barlowed) beam of the laser - assuming the laser itself is well collimated - is a fast and accurate way to align things, but it isn't a 'must have' tool. you can achieve equally accurate outcomes with the chesire/sight-tube combo. If you want super-accurate collimation, invest also in an autocollimator.

There are many great internet resources, but to be honest 15 min with a collimation veteran - who can see what tools you have to use and adjust the collimation algorithm to your tool set - is probably invaluable in getting you squared away.

S

[skies2clear]

Does your GSO Dob have a primary mirror cooling fan? If so, and after getting your collimation right, give the cooling fan a go,

CS
Nick

[SkySearcher (Daniel)]

Thanks for the tips all.

The seeing is shocking where I now live, but I have used the scope at a number of locations (none ideal) and always allow it to cool. I have fluked it a few times and seen good views though

Erick has offered to come over and show me his collimation tools, which I think will be very helpful!