Thursday, July 21, 2011


Not so long ago I've got acquainted with the Telrad object finder. This is definitely the king of all zero magnification finders, and I believe of many optical finders as well. Just remember that it is not just a red dot finder, which you are most likely familiar with from experience with some cheap telescopes. You need to learn how to use it properly in order to achieve unbelievable accuracy and pointing time under 10 seconds for any kind of celestial objects.

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.

The model, which I have purchased at GSSP star party have a simple proprietary 2 clamping screws base plate mount with double sticky foam strips at the front and back edges. So the installation is very easy but might seem kind of permanent. It provides two small holes for mounting screws as well, so if you don't like the reliability of the sticky tаpe mount, or have reconsidered the installation location - you can always cut/rip the foam pads, clean the surfaces with gasoline (or citrus cleaner), and re-install the base using screws in the new location.

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.

Anyhow, it is wise to plan the installation first. You can easily try and consider different locations for your Telrad without sticking it to the OTA permanently using a piece of duct tape over the middle of the base mount for several prospective mounting locations while testing. The location must be convenient for your particular telescope setup (including its mount range capabilities) and for your observing habits (are you tall, short, using the ladder or stool, e.t.c.). For example:
  • 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.
My choice is - on the side, below the EP, as pictured above (the image is rotated, the optical finder is actually straight at the top of the OTA). I call it the "Bazooka targeting style". That location helps a lot when working near the zenith as you have good azimuth leverage being at the base and can orient yourself with dimensions of the mount's freedom naturally. Also, I like changing my standing at the eyepiece position to sitting on my heels with knees on the closed cell foam pad for working with Telrad periodically. That gives my body some rest and stretch, but most importantly allows to have the Telrad view and the digital star chart screen in front of me side by side for direct star pattern matching (holding the phone in the left hand while the right hand is on the Dob's ball handle). That's the key for the indirect pointing with Telrad technique success.

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:

  1. 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).
  2. 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).
Observing session:
  1. 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).
  2. Turn on the Telrad circles' display option of the app if it's not visible on the screen already.
  3. Center the OI in the center of the smallest circle on the screen (usually there is a button for that in the app).
  4. Tune the app to show only easy to see by the naked eye stars (depends on your light pollution condition).
  5.  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.
  6. Find these reference stars on the sky with your naked eye.
  7. If you cannot see them - try other reference stars farther from the target but no more than 2 outmost ring radiuses away.
  8. 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).
  9. 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).
Note: The perimeter markers of Telrad (gaps in the circles) are often not matching markers implemented by a software application developer (usually a cross over the circles). That depends on the OTA position of the Telrad bracket used, telescope mount type, and perhaps manufacturing and/or assembling process of the collimation mask of the Telrad itself. So it is possible to have some angle between real and virtual (simulated) markers. I don't know of any planetarium application so far providing adjustments for that angle, neither of a Telrads mechanism to rotate the illuminated pattern. Software developers are usually aligning the cross with one of the coordinate grids (polar or alt-azimuthal). Therefore, at least 2 reference stars for the reliable pointing are necessary. I hope my request for such a feature (changing marks angle) in my favorite Astromist application will be implemented soon so I can use just a single reference star for pointing one day.

(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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