Chuck wrote:
Kurt, it seems to me that you can't use the Blug on a closed-tube Newtonian, since you can't see it. Am I off the mark here?
As others have noted, the blug can be pointed down tube as shown here - http://www.greatredspot.com/1037161-1037161-BlugImage.jpg or the blug can be pointed up tube on a closed Newt, as shown here on my scope - Blug with light cone http://gallery.utahastronomy.com/main.php?g2_itemId=11118 Comparing the preceding image and the following image, illustrates what I mean by a clear unambiguous signal of collimation on a fast Newt - Blug without light cone http://gallery.utahastronomy.com/main.php?g2_itemId=11121 The point of collimation is to get a truncated cone of light centered up focuser. In a fast Newt, it is possible to center the reflection of the primary mirror circle mark on the laser collimator - as nearly shown in the image labeled "Blug without light cone" and as illustrated in Carlin's 2003 article. In the "Blug without light cone" image, collimation is not co-planar between the secondary and the primary. Only the center point of the secondary is collimated with the primary. In the image labeled "Blug with light cone", the light cone coming off the primary is now unambiguously travelling up the focuser tube. The secondary and primary are collimated "holographically" in Nils Olaf Carlin's terms or as I like to call it - they are co-planar collimated instead of co-point collimated. The adjustment differences between the two images is quiet small - again an effect probably limited to a fast Newt. There was no detectable visual difference in the alignment staring down the open focuser tube or by using a Cheshire between the two images. The adjustment difference between the two images was only a two or three of twists on two knobs on the primary mirror. Between the two collimations, there was a detectable difference in image brightness but not in resolution (based on rough planetary tests that I did last season). But another primary benefit - again probably limited to GEM-GOTO mounted fast-Newts - is there is a visually detectable pointing accuracy difference between the two settings. The image labeled "Blug without light cone" actually points several arcminutes to one side of the position of the scope's polar axis. This may be a case of a better tool being more sensitive to alignment errors that really have no corresponding improvement in visually preceived details. In that case, it's extra work without a gain as compared to collimating with a Chesire, cap or with the eyeball down the tube. Whether high-end laser collimation makes any difference on other scopes, like an SCT - which are inherently more optically centered than a Newt - I couldn't say from experience. I don't own an SCT, although Glatter's instructions explain how to align an SCT using his laser collimator. IMHO the main benefit is being able to collimate a fast Newt quickly and accurately without any fuss. It really was worth it - to me - to shell out the $190 in extra gear as compared to the no-cost method of punching a nail hole in a used film canister. But again, that's me and not everyone's cup-o-tea or price point. I've lived both methods and both collimation approaches are equally valid. The whole collimation-alignment routine makes me think it would have been better to go with an SCT as opposed to the Newt that I ended up selecting. - Kurt P.S. - As Don noted, it is natural to be concerned about the blug falling out of the focuser and ruining the mirror. Mine is pretty secure, but I still generally collimate with the tube horizontal to be on the safe side. I don't really worry about the blug falling out. Up thread there was a link to a new design that incorporates the blug target into the laser collimator. No danger of anything falling on to the mirror with that design. ____________________________________________________________________________________ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ