I wasn't too sure where to put this so have plumped for this subject. Originally I posted elsewhere so I hope I'm doing the right thing by moving it.
Given the many comments on this site regarding Cheshire collimators for Newtonians I've saved up a few dollars to buy one.
Finding one though is something different. I've come across this Orion unit from BinTel but, never having seen a Cheshire, I really don't know what I'm looking at. It's simply called a collimating eyepiece.
So we have one from Bintel @ $69 and one from Andrews @ $29.
This is a bit of a price difference. Are they exactly the same thing? Is the Andrews one a combination Cheshire as well or just a Cheshire? Not that I know the difference, I've just heard people say "Make sure it's a combination one".
Yes, 'tis a big price difference. Hmm . . . "now what to do?", thinks he. It seems there is strong recommendation for the dearer 'combo' unit.
So it's a 'combo'; a combination of what? I can understand the cross-hairs being of assistance when collimating a 'scope but I'm not too sure what the other facility is. Please excuse my ignorance. I currently have a laser unit from BinTel - I ordered it with the 'scope - but it's rather sloppy in my focuser/adaptor set up, no matter how tight I secure everything, so I really don't trust it.
I'm probably too late to place an order to have one arrive prior to the weekend so I'm in no rush. Accordingly any further comments and advice will be gratefully accepted.
*edit*
Oh, hello, Rob. Your post popped up whilst I was a-pondering. Thanks.
Hi John, I'm new to reflectors as well, after 30 odd years of using refractors.
I was told by a wiser person than me to buy the combo Orion unit from Bintel.
The reason being, you use the cross hairs at the bottom of the tube to line up your secondary spider. You adjust the secondary collimating screws until the hairs are superimposed onto the spider.
The view beyond the crosshairs is your centre spot from the primary mirror.
You then adjust the primary screws until you centre the spot in the collimation e/p.
I hope I've made this clear enough if not let me know. Cheers, John.
With the distrust of my current collimating tool I've taken to standing back from the focuser and attempting to align the secondary holder by eye within its boundaries. Then I've lined up the centre of the primary mirror with the reflection of my eye's pupil.
Should I then (thinking that I'm close) use the laser to check then I find that the laser reflection doesn't even hit the secondary! I'm a failure, I tell you, a failure.
Hence the need for assistance with a Cheshire. I do not wish to fork out a (to me) significant number of dollars only to find that whatever I purchase is as deficient in capability as is my current unit. The latter has been relegated to use as a pointer when viewing images on a television or computer screen, which purpose it serves quite well (though a bit heavy on batteries).
Well apart from being a simple instrument, it does not require batteries
Here is the instructions that came with the tool, there the clearest I've ever seen
The difference between the Andrews and the Orion one is length.
The "combination" refers to a combined cheshire and sight tube. The Andrews one is too short to be used as a sight tube. The sight tube is used to align the secondary under the focuser.
Thanks, Geoff, I'd sort of worked that out in my own mind but wasn't too sure. It's nice sometimes (comforting even) to have one's own mental machinations confirmed.
Ken, you can download manuals from Orion's website (under customer support -> product instructions). They have downloadable manuals for most of their products. Here is the one for the collimating ep: http://www.telescope.com/text/conten...inst_03640.pdf
Basically, you:
1.align the tilt of your secondary mirror so the dot on the center of your primary mirror lines up perfectly behind the center of the crosshairs. Then,
2.Align the tilt of the primary so the center dot on the mirror falls into the black area in the center of the bright annulus formed by the reflection of the inside of the tool.
When all is aligned, the center dot is in the central black area, centered behind the crosshairs.
This alignment will work fine at f/6 and longer. At f/5 and shorter, I would suggest adding another tool, the autocollimator to improve alignment even further, as the tolerances for misalignment get quite small as the f/ratio shrinks.
As for laser collimators, most inexpensive lasers are, themselves, miscollimated. Unless you have checked the collimation of the laser, I wouldn't trust it. Collimating with a miscollimated collimation tool results in.....miscollimation. Rotate the laser in the focuser. If the center dot wanders, the laser is misaligned.
Oh, one final thing: You always want to collimate the scope to where the eyepiece will be when viewing. Accordingly, always tighten the setscrew on the collimation tool before you collimate.
L.
if not let me know. Cheers, John.
