My telescope is equipped with a very good 8 x 50 finder scope, that produces a 7-degree true field of view. The lowest possible magnification with my telescope (focal length 2000mm) is 62.5x which I achieve using a 32mm Televue plossl eyepiece. It produces a field of view of 46.5 arc minutes. Using both the finder scope and the 32mm eyepiece I usually can locate most of the deepsky objects.
However, I wanted a tool to point my telescope quickly at the moon, the planets and naked eye stars, before switching to the finder-scope. Therefore I ordered the Telrad, a zero power (no magnification) pointing device that helps you point your telescope exactly in the direction you want it to point.
The Telrads weight is 11 ounces (about 300 grams) including the base. The dimensions of the Telrad are:
Length 21.3 cm
Width: 4.7 cm
Height: 12.7cm (on Base)
When the Telrad arrived, it took me only a minute to mount it on my TAL telescope. The Telrad is composed out of two main components only, the Telrad pointer itself and the base on which the pointer is mounted. On the Telrad base you find some double-sided adhesive foam you can use to attach the base to the telescopes tube. You can also drill some holes into the tube and mount the base using the provided mounting screws. I chose to glue the base to the tube of the telescope using the adhesive foam. (The foam can be removed from the tube by heating it with a hair-dryer and peeling it off slowly)
Next step was the alignment. Through the reticle on the Telrad you see three lighted (LED) circles. The different circles represent a field of view of 0,5 (inner circle) 2 middle circle) and 4 degrees (outer circle). You can adjust the brightness of these projected circles by using the reticle brightness control . Point your telescope with the help of your finder scope (that you already have aligned with your telescope) on a bright star. Adjust the Telrad pointer, using the three small knobs on the back, until the star is centered in the inner circle.
Once the Telrad had been aligned it did exactly what I expected it to do. I pointed my telescope on several stars and planets that were visible to the naked eye, and they where right in the center of my finder scope and my 32mm eyepiece. I never had to align it again.
The use of the Telrad during the observing session
When looking through the Telrad you see the sky just as it is, without any magnification. There are no upside-down or mirrored views. You see three red circles that seem to hover in front of the stars or objects that you are looking at. If the object you want to observe is visible with the naked eye, just point the Telrad directly at the object. If the Telrad is properly aligned it should be in the centre of the eyepiece. If the object is not visible, then look for a naked eye star which is nearest to the object. Centre this star in your field of view by pointing the Telrad at it. Then switch to the finder scope or the eyepiece to Star-hop to the object you are looking for.
A great help to locate faint stars or deep-sky objects with the Telrad, are books and star atlases that use the Telrad circles on their finder charts. I use the Telrad in combination with the SkyAtlas 2000 (laminated field edition) from Tirion. With this star atlas comes an overlay that shows the three circles of the Telrad on the right scale. This overlay makes it very easy to locate objects. A few examples:
- How to locate M57
First point the Telrad on Vega (Alpha Lyrae). Then move the telescope in Southeastern direction to the two bright stars Beta and Gamma Lyrae. Put the two degree circle on these two stars, and M57 should be more or less centred in the field of view
- How to find Hubbles variable nebula, NGC 2261
Just centre your Telrad on 15 Monocerotis the magnitude 4.6 star at the base of the Christmas Tree in NGC 2264, the Christmas tree cluster in Monoceros. Hubbles variable nebula is just outside the 2 degree middle circle in the south-southwest direction. Move 15 Monocerotis in the opposite direction, north-north east, until this star lies just outside the two degree circle. Hubbles variable nebula should be in the field of view (eyepiece) now.
Besides the SkyAtlas 2000, Harvard Penningtons The Year Round Messier Marathon also uses finder charts for Zero Power finders like the Telrad in his book for locating all the Messier Objects (see the picture above). Then there are many planetarium and other software programs for making star-maps, that have field of view indicators like the Telrad, that you can project on the screen and print on your maps. I work with SkyTools2. This program has the option to use a reflex sight (like the Zero Power Telrad) as a finding device projected onto your star map. The three circles are set on 0.5, 2 and 4 degrees. They can be changed if necessary (for use with a different reflex sight).
After the observing session
At the end of the observing session, you remove the Telrad from its base by loosening two side screws. The base stays attached to the tube. When you mount the Telrad the next time, you just have to put it on the base and tighten the two side screws again. No aligning necessary. You can buy extra mounting bases if you want to use the Telrad on different telescopes.
I can recommend the Telrad wholeheartedly to all of you who want to star-hop trough the night-sky. It is an inexpensive addition to your finder instruments that you use to locate objects. I use the Telrad to locate the faintest visible naked-eye star near the object I am looking for. Then I switch to my finder-scope or the eyepiece to finish my search. Through the whole process I use the star-hopping technique.
There are some fine accessories for the Telrad, like a Telrad dew-shield and a Telrad Pulser. For more info on these accessories have a look at the site of Scopetronix