Best collimation tools for collimating a f/4

Post by **VinSnr** » Mon Jun 04, 2007 11:27 am

I was tasked to collimate a GSO 8" f/4 last Saturday. Seeing this is a mass produce scope tells me that there quite a few things I need to check. In this post, I will go through some of the steps of what I did as well as some of my impressions on the GSO 8" f/4.

The new GSO single -speed crayford focuser is very good, above my expectations. That thing is really smooth. Infact, I think it is smoother than the MoonLite Single speed. GSO is coming with some really good focusers at bargain prices.

However, it is clear that GSO didn't do a precise job in the area of optical alignment. The secondary mirror is way too near to the primary. The only thing they did well is the squaring of the spider and the focuser, which in all honesty, a pretty good feat by itself. The mirror has a donut ring attached, but the location of that ring need to be validated.

In collimating a f/4 system, a laser is not going to be good enough. Even with a barlow, it will still have some errors. So I decided at that point that the laser method is not good enough, I would need something more precise than that.

Out came Jim Fly's Catseye collimation system. His system consist of a template for checking the center spot for the mirror, a 2" Telecat telescopic tool, a 2" Blackcat Cheshire and a 2" Infinity Autocollimator.

For those who are not familiar with this system, please bear with me as I go trough step by step in using the Catseye system.

First, using the supplied template, I checked the center of the donut ring on the primary mirror. GSO did a good job here, the donut ring was attached exactly at the center of the mirror. I also realised that the mirror is about 7.8" and not exactly 8". Jim Fly's system requires the use of a red triangular sticker on the center of the mirror but seeing that the donut ring is already at the center, I didn't bother to remove it. The triangle would be better, but I can make do with the donut ring.

Putting back the primary mirror cell back into the tube, the next task is to check the secondary mirror. Using the 2" Telecat sight tube, the secondary mirror is carefully adjusted so that a "circle" appears nice below the sight tube. At this stage, it is easier to see the outline of the secondary if you attach a white paper in between the secondary and the primary. The GSO spider is a very simple assembly and it does its job reasonably well. With some effort, the secondary mirror can be adjusted quite nicely below the focuser.

Once this is done, I use the laser to do a rough alignment between the secondary and the primary. Basically just to fine tune the tilt of the secondary to hit the center of the primary. Notice that the laser here is for "rough alignment" and that's all it can be use for. Tells you how much precision is required in this job.

Next is to use the 2" Blackcat Cheshire to adjust the primary. In doing this step, one is required to shine a torchlight onto the primary. The Blackcat cheshire has a sequence of rings drawn on its face (picture HERE). The idea is to get the triangle (in my case it's the donut ring) to be at the center of the cheshire when viewed through its peep hole. You will see the reflection of the cheshire face on the secondary mirror. Adjust the primary mirror until I got the donut ring at the center of the cheshire face.

Once that's done, the next step is the most crucial of them all, and the step that makes this whole system so precise. It is the use of the 2" Infinity Autocollimator. This tool is crazy, it has a mirror surface below it and you will see this mirror surface on the surface of the secondary mirror. It has a peep hole, so you view it through the peep hole, looking down into the secondary. If you think aligning secondary and primary is difficult, now you have another third mirror surface for you to align!

What you see through the autocollimator is a reflection of few "ghost images" of the donut ring. In most scopes, such situation is already quite acceptable for good images but over here it is not good enough.Now, I will have to adjust the secondary so that the few "ghost" images are merge into one solid ring. When this happen, the secondary, primary and the focuser will be in exact optical alignment.

Once this is done (need some effort), it is still not finish yet! have to go back using the Cheshire again and adjust the primary. Then back again using the autocollimator. This process between the Cheshire and the autocollimator will have to be repeated a few times until everything is dead perfect.

It is interesting that once the collimation process is finished, I drop a laser in and I can see that the laser hit the primary dead center and the returning laser also hit the laser face dead center.

The Catseye tool IMO, is the best tool for precise collimation. Although this process sounds long, but it is actually quite easy. And it is quite fun to do. I had fun merging those ghost images together.

Check out the catseye system if you need to do really precise collimation for your newtonion!

kayheem · Post by **kayheem** » Mon Jun 04, 2007 11:43 am

So, what is the view like with a collimated GS 8" F4?

How does the collimation you described compare with using a star test?

KH

Post by **VinSnr** » Mon Jun 04, 2007 11:45 am

kayheem wrote:So, what is the view like with a collimated GS 8" F4?

How does the collimation you described compare with using a star test?

KH

no chance for star test yet, but using a few Pentax eyepieces, I did not notice any coma or whatever on daytime objects. Looked pretty sharp to me.

rlow · Post by **rlow** » Mon Jun 04, 2007 12:59 pm

Good write-up, Vincent.

I have tried the R200SS f/4 with premium eps, and the coma is still very visible. You may want to check it on stars at night as it will be more evident.

Post by **VinSnr** » Mon Jun 04, 2007 1:04 pm

rlow wrote:Good write-up, Vincent.

I have tried the R200SS f/4 with premium eps, and the coma is still very visible. You may want to check it on stars at night as it will be more evident.

yeah probably.

Or my eyes have in-born coma corrector?

Anyway, this scope is for AP, not for visual.

rlow · Post by **rlow** » Mon Jun 04, 2007 1:24 pm

Or my eyes have in-born coma corrector?

Maybe someone should come up with a contact lens with built-in coma-corrector...haha!

Tachyon · Post by **Tachyon** » Mon Jun 04, 2007 1:24 pm

Actually, it sounds kinda scary!!

Post by **VinSnr** » Mon Jun 04, 2007 1:31 pm

Tachyon wrote:Actually, it sounds kinda scary!!

should have contact lens that have the properties of a light pollution filter, coma corrector, and minus violet filter.

That would be freaking cool.

Canopus Lim · Post by **Canopus Lim** » Mon Jun 04, 2007 2:48 pm

What is the 'eye relief' of the sight tube? Is this Catseye system user friendly in pitch darkness?

I do not like to use the cheshire because it is totally not usable with glasses...which defeats the purpose since I cannot even see the donuts and reflections clearly and hence it is impossible to make a good alignment. Not to mention there are parallax errors with the cross as it is not fine enough.

Post by **weixing** » Mon Jun 04, 2007 3:38 pm

Hi,
So you got the GSO 8" F4... it's should be weight around the same as my Celestron 8" F5. Matthew also got one... don't tell it's his??

The new GSO single -speed crayford focuser is very good, above my expectations.

I initially want to get this for my Newtonian, but after some measurement, I don't think it'll reach focus for my DSLR as it's taller than my current focuser...

The secondary mirror is way too near to the primary.

What do you mean by the secondary mirror is too near the primary mirror?? How do you check that?? And if so, I think the focuser hole will also in the wrong position and won't be able adjust it as the secondary mirror need to at the center of the focuser.... unless you cut a new focuser hole at the right position.

Have a nice day.