I found and fixed a problem with my telescope

[Allan]

I have had my XT12g for almost a year now. In that time I have never achieved a good star test with it. The diffraction rings were always blurry, uneven, rough and were not round. I have a Ronchi eyepiece and the image in that was a bit ragged as well, the only error I could see was an over corrected mirror.

I was getting poor planetary views out of the scope as well. Eventually I got so frustrated with it that I put in an order with Carl Zambuto for a new mirror. Now, I don't have the mirror yet, but I thought I should at least spend some money on the secondary mirror to compliment my new primary. So I ordered a 1/30 wave secondary from Antares.

The original secondary was attached by double sided tape, which is not ideal. I installed the new mirror in the accepted way, with 3 drops of silicon, and used wooden skewers to keep an air gap between the mirror and the stalk while the silicon cured.

So last night I used the new secondary for the first time and the improvement it has made to the telescope is like night and day.

The star test looked totally different. The diffraction rings are now sharp and round. The lines in the Ronchi eyepiece are now razor sharp. The pattern was very easy to read and clearly showed a turned down edge.

But importantly, I turned the scope to Jupiter and it showed a world of detail like I always thought I should see in a 12" but haven't until now.

Obviously something was not right with either the original secondary, or the way it was attached to the stalk. The cost of the new secondary now seems like a small price to pay for the huge improvement it has made. I am still going to fit the Zambuto when it is ready. I'm sure it will show a nice improvement over the original primary, considering as well that I can see it has a turned down edge and is slightly over corrected.

I pass this on out of interest and also for those who have similiar scopes that may be experiencing the same problems that I have had.

[Shiraz (Ray)]

interesting and thanks for posting.

I just replaced the secondary on an imaging GSO because it had gone potato crisp. (GSO mirrors in the Orion I think).

[Inmykombi (Geoff)]

Well done Allan.
I cant wait for a Squiz through the new optics.

Geoffro.

[Allan]

Yes Geoff, you can have a look when you grace us with your presence at Chaffey Dam.

Seriously though, I can't wait to put the Zambuto mirror in it and see how that performs.

[2stroke (Jay)]

Great to hear your solved your issue though and the views should be kill with the new zambuto primary.

[Allan]

Thanks Jay. Carl Zambuto said the best way to deal with the long wait time for the mirror, was to "go away and think about something else". Easier said than done though.

[glend (Glen)]

Haha, must be nice to be able to tell your customers that. I have to check out those Antares secondaries.

[AG Hybrid (Adrian)]

That's very interesting. The secondary was introducing astigmatism. What sized secondary did you get?
Congrats on the incoming CZO mirror too.

[Allan]

I stayed with the original size of 2.6". The Antares mirror was about 5mm thicker than the original and flawless to look at from the back and sides, at least compared to the original which had a rough opaque appearance.

[AG Hybrid (Adrian)]

Cool. I think ill upgrade mine too. Don't think I'll need a 1/30 wave mirror though. 1/20 will probably do me. I can understand it for you however. You don't want to bottleneck the quality of your CZO mirror with an average secondary.

I just looked at my secondary mirror and its attached by double sided tape as well. Do you have a link with a guide to attaching it like you did?

[Allan]

http://www.protostar.biz/ftp/instman.pdf

Protostar make good secondaries as well, this link is from their website and includes mounting instructions.

A few extra things I picked up along the way.

For best results 3 blobs of silicon in a triangle are best. More than this and there is the risk that if the blobs cure unevenly then they will place an uneven force on the back of the mirror and introduce astigmatism.

Leave the silicon and mirror in the room together for a day to temperature equalise before sticking together.

I used bamboo skewers as the spacer between the mirror and stalk. After a day or so I pulled them out and that left a nice gap.

I had to trim my mirror stalk back a bit to take into account the extra thickness of the Antares mirror.