FreeLists / az-observing / [AZ-Observing] Report on the Televue Diagonal

[Stan Gorodenski <stanlep@xxxxxxxxxxxxx>]

I received the Televue diagonal yesterday. I was surprised it came so 
quickly. I feel it may be of interest to others how it compares to the 
Meade 4000 diagonal that came with the 16" LX200R.

First, if you remember, the star images with the Meade diagonal started 
getting distorted and enlarged starting about 2/3 the distance from the 
center of field of view in one direction. This only became apparent 
because of the increased quality of the image with the Panoptic 35mm 
eyepiece I received Monday. With the Televue diagonal the star images 
now stay constant, except for atmospheric effects, all the way to all 
edges of the field. This problem is corrected.

I was expecting to see an obvious difference in the brightness of the 
image between the two diagonals because allegedly only Televue and AP 
use a special process to coat the mirrors that give increased 
reflectivity (the 99% reflectivity stated with some diagonal 
manufacturers is allegedly an optimum value taken at at optimum angle 
and falls off at oblique angles). Unfortunately, I did not see this. I 
swapped back and forth between the Meade diagonal and Televue diagonal 
in looking at the little Dumbbell Nebula (M76), the Pinwheel Galaxy 
(M33), and M42 (the Great Nebula of Orion). I could see no difference 
looking at M76 but it was at a bad angle to observe (which affects 
vision) and so I switched to M33. The brightness of the haze that is the 
galaxy seemed no different between the two diagonals. I thought that 
maybe such a brightness difference may be difficult to detect visually 
when it virtually fills the entire field of view and so within the 
galaxy I picked out a string of 5 faint star in a row and another string 
of 6 of which two were difficult to see. There was virtually no 
difference in being able to see them in either diagonal. Finally, I went 
on M42. I could see no difference in the large body of the nebula but 
because it is so large and bright it is difficult to discern any 
differences, unless they were dramatic, but it was apparent from the 
outset that the two diagonals were not dramatically different. So I set 
my attention on a small patch of nebulosity that borders the large body 
of the nebula and the dark background sky that surrounds it. I could 
detect no difference between the two diagonals. In fact, at times it 
almost seemed like I would see more with the Meade, but this probably 
reflects sensitivity of the image to being well focused and atmospheric 
conditions which says there is no difference between the two diagonals 
with respect to the percent of reflected light by the coatings. Perhaps, 
except for the distortion problem in the diagonal I have which may be an 
uncommon occurrence, the Meade 4000 diagonals are under rated. 
Certainly, for fields of view smaller than with the 35mm Panoptic, the 
Meade diagonal I have is a perfectly good diagonal.
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