If you have a large enough aperture telescope (I believe it must be more than 8") you should consider getting and installing the Telrad device, which is around $40 at any popular astronomy store. Telrad may not be practical on small refractors as it's quite large. Despite that, it's relatively lightweight, even though it's using two AA batteries. The size is dictated by the need for a long enough light path for the collimated reticle image building. There are many DIY Telrad mods instructions and accessories for purchase available online. Most notably:
However, it works for me as is so far.
In addition, there are a couple of different size riser base attachments available on the market for Telrad (see links above). And of course, you can get as many bases as necessary separately to share the Telrad between all of your telescopes.
- The location right below the optical viewfinder may get into the way of using the later or will make the Telrad mirror or collimator lens prone to the fogging from your breath when using the finder.
- Placing it on top - may limit access to it in certain OTA positions as the OTA/EP/Finder will interfere with your body / face placement. Bending over the OTA you may kick it in the process and have the object out of the view when back to the EP.
- Side positions, like mine, below the main focuser may lead to frequent Telrad bumping against your chest or face with shorter eyepieces (that's not the case with my z12, though).
- e.t.c. so try it for yourself during a real observing session.
How to use it properly?
In my experience, Telrad will really start to shine only when accompanied by the right star chart software, which supports it natively (most of the serious astronomy applications do). There are Telrad overlays for paper maps available, but they cannot be adjusted to replicate the real thing well enough.
The secret of indirect pointing success with Telrad is in the dedicated pattern matching technique. You don't need to see the target object or even imagine it's position between the naked eye stars as you do with the direct method using red dot finder or optical finders wit the crosshairs reticle. Here is how I do it:
- Set up your software atlas / digital sky chart so the virtual Telrad circles' sizes it provides are matching the real ones exactly. The best Telrad aware apps allow to fine tune the rings' lines width and gaps orientation. That's crucial for the accuracy so approach it several times using real stars to determine and set the best values matching what you see through the Telrad window, including gaps in the rings. That procedure needed only once in a lifetime of the particular Telrad (or until you re-collimate it).
- Align the Telrad so it points exactly where the scope is (using the alignment screws on Telrad's back and some bright star, preferably Polaris, as it doesn't move much, per the user manual).
- Find and center the object of interest (OI) on the app's screen (the screenshot of the Astromist for PalmPDA app is pictured, but I'm using the DSO Planner on my Android phone since then - much better).
- Turn on the Telrad circles' display option of the app if it's not visible on the screen already.
- Center the OI in the center of the smallest circle on the screen (usually there is a button for that in the app).
- Tune the app to show only easy to see by the naked eye stars (depends on your light pollution condition).
- Note and remember any bright naked-eye stars (reference stars) surrounding the OI and virtual Telrad rings, their positions relative to Telrad circles, AND to their perimeter markers, which are best treated as a regular clock's main divisions (12, 3, 6, 9 o'clock hours) and corresponds to the gaps in the outer rings of the Telrad.
- Find these reference stars on the sky with your naked eye.
- If you cannot see them - try other reference stars farther from the target but no more than 2 outmost ring radiuses away.
- Look through the Telrad and move the scope until its circles are sitting between reference stars exactly as on the simulated image (see image below).
- I have the target object almost always dead center in the eyepiece field of view using just 2 reference stars in close proximity (see notes below).
(Update: It was never addressed, so I made my own app doing that, the DSO Planner, which is better in every regard anyway :) So now I can use just a single star near the circles for precise enough pointing with my Telrad! The app is even taking care of leveling my chart and Telrad overlay with the true horizon, utilizing the gravity sensor).
Another great use for these gap-marks is working with the Dob near Zenith. It's not that easy to point to something within 20 degrees spot around the Zenith because you have the altitude bearings restricting the vertical motion along one direction only. Telrad marks are helping to predict where that direction will be as you rotate the base. When the gap it's properly aligned with the direction to the object, rocking the OTA towards it will bring that object to the center automatically. Again the orientation of marks is crucial for that (or getting used to their orientation relatively to the azimuth axis).
For paper maps' fans, I would suggest finding a laser printer and printing out a Telrad overlay(s) on a transparent film used for over the head projector presentations. Just use a properly scaled drawings for each of your maps' editions. Another cool solution could be found here.
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