"Blug™ is a charming contraction of ‘Barlowed collimation plug’ and it does something remarkable to the already acutely useful laser light being emitted from the collimator. After the laser beam passes through its anti-reflection negative lens, this neat little devil (placed at the base of the focuser tube), casts a returned shadow of the collimation target on its angled face, making adjustments of the primary mirror clearer and therefore more accurate. There can be no sweeter sense of achievement than when you can see that an optical system is ‘closed’, with all elements in-line and on-axis — an achievement that is readily apparent in use, too. After all, once your responsibility of arranging the mechanics has been met, you can leave physics to do the rest." Steve Ringwood writing for Astronomy Now
The Blug™, short for Barlowed collimation plug, is a new collimation accessory used together with a regular laser collimator to perform Barlowed laser primary mirror alignment.
The Blug does not attach to the collimator like a self-Barlow attachment. It is inserted in the inner end of the focuser drawtube, inside the telescope tube or upper cage, and it is retained by an O-ring in the base of the Blug. It has an anti-reflection coated negative lens mounted in an axial hole, and a white 45 degree screen face. Blugs are made in two sizes to fit 1¼" and 2" drawtubes.
On a truss tube dob, the 45 degree face of the Blug is easily visible from primary adjustment position. You observe the shadow of the primary mark on the Blug face as you turn the primary adjustment screws to center it.. With a solid tube scope, the Blug can be turned 180 degrees to face the front, and the target shadow seen easily from the front.
The Blug extends 3/8" into the back end of the drawtube, so the laser collimator used with it can not extend past this or it will interfere with the Blug and not allow it to seat properly. A long collimator can often be drawn back in the eyepiece holder and clamped so it will not interfere with the Blug.
I supply a shim kit with the Blug for use if the drawtube inner end is larger than the eyepiece nominal size. The plastic shim strips can be inserted under the o-ring, and in this way drawtubes as much as 1/10" oversize can be accommodated.
The current Feathertouch focuser has a 2" filter thread at the inner end of the drawtube, with a large relief diameter past the thread. In order to fit this focuser, I supply all 2" Blugs with an adapter o-ring which fits on the end of the Blug and acts as a "spacer" so that the Feathertouch focuser threads will engage the Blug retention o-ring. However, for those who will be using the Blug primarily with a Feathertouch focuser I now produce a Blug that will fit the Feathertouch directly without using an adapter. It is called the Flug (tm). The only difference between the Blug and Flug is that the retention o-ring on the Flug is right up against the Flug shoulder. The Flug will also fit fine in most regular 2" drawtubes.
Adjust the secondary mirror angular alignment as usual, to place the laser spot in the center of the collimation target on the primary. If an aperture stop was used in the collimator, remove it for Barlowed collimation because you want as much light as possible through the Barlow lens to produce the highest shadow contrast.. Insert the Blug into the inner end of the drawtube and make sure it seats firmly against the drawtube end so that there is no danger of it falling out. Adjust the primary mirror to center the collimation target shadow on the Blug face. Remove the collimator and Blug and you are done.