Secondary silhouette not round . Please Why?

[pat]

I like to thank everyone who has given me advice on how to collimate my Orion xt10''f4.7 scope.

I'll go through the steps I used.

1) Paper to center the secondary under the focuser.

2) Adjusted secondary so primary and clips fill the secondary.

3) Adjust secondary so crosshairs bisect the center of the donut.

4) Use Orion collimation cap to center dot on primary inside donut.

5) Recheck alignments on secondary and primary.

This is what I see when looking down the peep-hole:

* The secondary silhouette is offset toward the primary.

* The silhouette is not round but oval

* The apex of the oval is toward the primary. With the lower vertical spider vane touching this point.

* This oval proportionately tapers around to the horizonal vanes, and ends there. Looking like a thin "crescent moon".

I'm thinking that the secondary needs to be moved more toward the primary mirror. I'd like to know the reason and if I need to fix it. Pat

[umadog]

The offset may be normal and indicate that you did stuff right, it depends what you mean by it (see images in link below). The oval isn't normal. I'm not 100% certain, but sounds to me like your secondary is tilted too much because it wasn't properly rounded and centred in the focuser. This can happen if you centre the secondary in the focuser before you adjust the tilt. It appears that this is what you've done based on the steps you have listed. Basically, step 2 should come after step 3. You iterate between 1, 2, and 3 until they are right. Once this is done, the alignment in step 1 will need doing only very rarely.

I would check if the secondary set screws are very unevenly adjusted. If so, re-set them and start again. Follow these instructions for the secondary: Telescope Reviews: Useful info about secondary mirror alignment

[pat]

Thanks again, I'll go back and try the steps you listed. Also it's possible that the secondary is not perfectly aligned under the focuser. I tried to follow Astrobabys instructions on this. I'll pick a nice sunny calm day for this. And keep my fingers crossed. Pat

[Jason D]

The secondary silhouette would be circular for a perfectly collimated scope. It is shifted towards the primary mirror because of the offset. That shift gives the illusion that it is oval when in fact it is not. But, it can indeed be oval for less-than-perfectly collimated scope.

Below is the collimation view for my perfectly collimation XT10. You should see something similar to it.

Also refer to the link provided by Umadog.

Jason

[umadog]

Wow! That's oval? It's only very slightly so, no?

[tonel]

I've been attempting to collimate my 200P Dob and think I just about have it (thanks to Astrobaby's guide) but think I might be having a similar problem with the secondary being elliptical or incorrectly positioned. I've managed to take a picture through my Cheshire (racked all the way in). Any advice would be grateful, as my eyes hurt and I think I'm starting to see things now

The crosshairs of the Cheshire are out of focus in the picture but are in line with the spider vanes and intersect the spot in the middle of the secondary. Can only see two of the primary clips in this picture but can see all three when looking through a collimation cap..

[jimmyjamjoejoe]

I've been attempting to collimate my 200P Dob and think I just about have it (thanks to Astrobaby's guide) but think I might be having a similar problem with the secondary being elliptical or incorrectly positioned. I've managed to take a picture through my Cheshire (racked all the way in). Any advice would be grateful, as my eyes hurt and I think I'm starting to see things now

The crosshairs of the Cheshire are out of focus in the picture but are in line with the spider vanes and intersect the spot in the middle of the secondary. Can only see two of the primary clips in this picture but can see all three when looking through a collimation cap..

Looks like the secondary is just a little bit out... Look at the gap between the secondary and the edge of the sight tube in the lower left corner, then the lower right... It is slightly closer to the tube in the lower right. Probable just needs rotating slightly, which will mean readjusting tilt etc...

[Jason D]

Wow! That's oval? It's only very slightly so, no?

A close up

The silhouette is circular

The offset amount is also showed

[umadog]

And the thing on the left is some sort of baffle?

[Jason D]

Correct...

See attachment

[Tim]

Good pics as usual Jason

I will need to reattach a secondary mirror to the stalk. Do you happen to know of any methods for doing this accurately?

Thanks

Tim

[Jason D]

I removed the stock secondary mirror and replaced it with a premium one.

I used the one of the popular methods:

1- I placed the secondary mirror on its face (on a clean soft surface)

2- I cleaned the back of the mirror with alcohol

2- I placed two spacers (needles in my case)

3- I placed 4 dabs of premium Silicone (some recommends 3)

4- I filed the stack surface at the corresponding 4 spots. In my case, my stalk is made from painted aluminum. I basically removed the paint the roughened the surface for better adhesion.

5- I placed the stalk on the top and let it cure over night

I made a paper template to guide me -- see attachment. I chose not to mount the mirror with an offset.

Jason

[Tim]

Thanks