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The postperson delivered my Herschel prism today. Well wrapped, thank goodness, because it had a large boot print and matching tear on the outer padded envelope! The prism came in a white cardboard box and multiple layers of tight bubble wrap inside the padded envelope.
Included
in the package [at extra cost] was a single, Baader, 2" polarizing filter for solar image dimming.
This is fitted to the outlet side, rotating ring. Which has a beveled
undercut so that the three pinch screws cannot lose the ring.No filter must ever be fitted on the inlet [telescope] side of a solar prism! The intensely focused heat from the objective will soon heat a filter to destruction. The moment the filter disintegrates is when your eye is no longer protected. On the outlet side of the prism most of the heat and energy has already been turned away by the prism.
Never use a so-called "Solar Filter" at the focus of any telescope. Cheap telescopes are sometimes provided with these supposed "Sun filters" to screw into the eyepiece. Then, as soon as the filter is destroyed by the focused heat the user is likely to suffer instant blindness! Never risk it.
It should be noted that the polarizing filter is not the main means of energy reduction but merely an eye comforter. The ND3 filter does most of the heavy lifting once the prism has shed all but 8% of the incoming energy. Interestingly [?] the Lacerta solar prisms use the so-called Brewster Angle in their design. Which explains the larger than 90 degree angle of the eyepiece [or output] end.This increased angle [over the more normal 90°] slightly increases the reflected energy. compared with the 90° type which reflects only 6.5%. But, the Brewster Angle has the effect of almost completely polarizing the reflected light. Which means that a single polarizing filter can be rotated to dim the light before it reaches the eye. Hence the rotating collar, with built in filter thread, in the eyepiece adapter.
A
50mm, 2", ND3 neutral density filter is already fitted to the lower
section of the outlet side. The view though the prism is exceedingly
dim. More so than I had imagined until I remembered the ND3 filter was
already installed by Lacerta in this particular model. Note that Lacerta also sell bare prism housings without any input or output adapters. These are for those who already have such fittings and filters so can avoid the extra expense of duplication.Note that the bare prism housing can be fitted with a Baader Click-Lock input adapter to shorten the glass path length. This may save some telescope tubes having to be chopped to reach focus. Most telescope manufacturers have become aware that their products will be used with solar prisms and binoviewers so make the main tube a suitable length.
I hope I have a suitably threaded 1.25" eyepiece holder. It looks like a male T2 thread. WRONG! It is the larger M48 x .75mm male thread. [standard 2" filter thread] I have now had to order an M48 female to T2 male adapter. Plus a T2 female to 1.25"eyepiece adapter with compression band. Will it never end?
Care
will be needed to avoid damage to the installed filters when inserting
eyepieces. I shall have to check the insertion depth carefully if there
is no built-in step to act as a positive stop in the bore of the new EP
holder. Or that the shoulder of the eyepiece hits the top of the holder
before the nose bounces off the filter! The additional depth of my existing, 1.25" Solar Continuum filter must also be taken into account. I just couldn't bring myself to spend the whopping price of a 2" Baader SC filter as well. The piggy bank will have to cool off before The management starts making false accusations of excess frippery in the solar telescope department.
See Part 2:
Click on any image for an enlargement.
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