Secondary mirror offset is one of the most misunderstood alignment in collimation.
There are two secondary mirror offsets:
1- First offset is towards the primary mirror. This is an easy one. Centering the secondary mirror under the focuser will take care of this offset automatically -- no need to be preoccupied with it.
3- Second offset is away from the focuser. This is the most misunderstood offset. This offset is required only when the reflector comes with a corrective lens located the OTA opening. Manufacturers will take care of mounting the secondary with the proper offset for these scopes -- no need to be preoccupied with it. In addition, this offset "might" be needed when the scope comes with tight baffles and/or tight OTA opening. Again, manufactures take care of this offset for such scopes. The 3rd and last reason why such an offset might be needed is to improve the reading accuracy of DSC though this is a weak reason.
In general, do not think too much about the secondary offset. If it is needed, the manufacture will take care of it.
Quote:
Originally Posted by graham.hobart
Hi Barry, on my Vixen f 4 the instructions for collimation are quite specific about secondary offset. Certainly the collimation changed when I didn't have the offset.
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Hi Graham, can you elaborate? What was specific about their instructions? Is there a web link to their instructions?
Quote:
Originally Posted by Moon
I have a tip that has helped me and might be of some use: cut 4 wooden blocks that fit exactly between the secondary mirror and the tube wall. One for the top, bottom, left and right. This will do 2 things:
1) it will help you make sure the secondary is in the exact center of the tube (left / right) The left/right secondary collimation screws tend to swing the secondary mirror through an arc and will move the secondary mirror away from the center of the tube. Only use those 2 screws to get the secondary into the center of the tube. That will also place it directly under the focuser. Rotate the secondary to align it with the primary and use the remaining secondary screw to get the tilt correct.
2) it's a great way to check the offset is correct. The top block should be 2 x the offset longer than the bottom block.
James
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Hi James, I do not believe these steps will help much. Following the typical collimation steps should suffice.
Quote:
Originally Posted by Shiraz
also, I find this tool to be essential for aligning the secondary, but others do not seem to like it, since what it shows is not immediately obvious - might be worth a look. http://www.iceinspace.com.au/forum/s...ad.php?t=82230
the secondary in my 200 was not offset, so I remade it with proper offset - reduced stray scatter a lot. |
Hi Ray,
In your first post of the link you provided, you stated "it seemed that most Newt alignment procedures get the mirrors and focuser properly harmonised, but largely ignore the alignment of the optical system with the tube itself"
Why do you think it is important to align the optical system to the tube after a successful (or harmonized) alignment between optics?
Interestingly, scopes are designed to get the mechanical parts out of the light path. It does not matter if the OTA is square or round or bend or even irregular. As long as the OTA is out of the light path and as long as the optics are well-aligned with respect to one another then you have achieved good collimation.
Jason