The Telrad finder sight was designed in the 1970s and the fact that it has changed little since then is testament to the effectiveness of its design. It takes technology developed for deploying bombs in World War II and repurposed it for astronomers.
They retail for around $40 (see today’s price on Amazon), or about £44 in the UK.
In this article we share with you everything you need to know about this classic reflex finderscope, including how it works, how to mount a Telrad on your telescope and how to use it in the field.
As well as all that, we’ll look at the pros and cons of choosing one of these over a red dot finder or magnifying finder.
What is a Telrad Finderscope
The Telrad is what’s known as a reflex sight. It has zero magnification, i.e. 1x power. The glass you look through at the stars is just that: glass. It has no magnification and so it is not at all disruptive to your stargazing.
So, how does it work?
Well, it’s effectively a heads up display for a telescope. A red, 3-ringed reticle, illuminated by a red LED, is beamed onto the glass sheet you look through. This is angled at 45° and the red rings appear to hover in space over the top of your view.
This is a huge benefit because we can keep both eyes open whilst looking through the finder and see the sky that we would normally see only with a superimposed set of three concentric circles. The level of illumination is adjustable, so you can keep them really dim for a dark sky, or brighten them up a bit when you need to.
The three concentric rings are of a set size, from smallest to largest they cover a diameter of 0.5°, 2° and 4°. Using one means always knowing what area of the sky you’re looking at and, as you’ll see below, star hopping is made much simpler.
Many astronomers choose reflex sights over magnifying because there’s no upside down like we get through a magnifying finder, and there is no need to close one eye to use it. Telrads are intuitive and super easy-to-use which makes them perfect for timeworn astronomers and brand-new stargazers alike.
What It Looks Like and How it’s Constructed
One thing that Telrads lack is style and aesthetic appeal.
They look like someone just invented a prototype it in their shed, A-team style. They are blocky, bulky, have lots of void space inside and a fold of foam holding the battery case in place.
It is a cheap looking bulky plastic box with the design quality of a brick. But it’s job is not to be a aesthetically pleasing (thankfully) but to help us find a way round the night sky.
To achieve that it uses very simple technology. Two AA batteries power a red LED. This shines through a mask which creates the reticle. The light then hits an adjustable mirror which bounces it up through a focussing lens and, finally, to the display window we look through.
A lever on the side both switches the LED on and adjusts its brightness, while three adjusting screws on the front move the internal mirror so we can align the reticle with our telescope (see below).
Our video review of the finder shows more ‘inside the box’ details, watch it further down this page.
How to Mount a Telrad on a Telescope
The Telrad is supplied in two parts: the main part which you look through and a separate base which attaches to your telescope.
The mounting base stays permanently attached to your telescope and the finderscope clicks in and out of it. Bases are sold separately so that if you have more than one telescope you can add a base to each and swap a single Telrad between them.
Before you begin, make sure to mount the device the correct way round. The angled glass screen where the reticle shines should be facing you, as should the adjustment screws. The on/off dial, which also varies brightness, should be located on the right as you look through the finder.
Align the finder’s long axis with your telescope’s long axis. The more accurately they are aligned, the better your Telrad will perform. Accuracy is further improved if you install the finder close as possible to the objective end of your telescope, which is the end with the open that lets light in.
This may not be possible or practical if you have a long telescope because you need to think about usability. Just position your Telrad as close as you can to the end of your scope whilst still leaving it easily usable.
The base is attached with sticky foam pads or drilled into place using supplied screws. Both are potentially damaging options, but you can use a hairdryer to slowly melt the glue on the sticky pads to remove the base without damaging the telescope.
We recommend testing the positioning using less permanent methods of holding the finder in place, such as with large zip straps or duct tape. Only secure it once you’ve tested and are happy with the location.
How to Use a Telrad Finder for Astronomy
The first step, as with all finderscopes, is to align it with your telescope. This makes sure that the object you have in the reticle sight is also in your eyepiece.
Alignment is carried out using the three adjustment screws at the rear of the finder. They are set out in a triangle shape with one screw at the top and two at the bottom (see picture).
The top screw moves the reticle in a straight line up and down. The two adjustment screws at the base of the triangle each move the reticle in a diagonal line. The right knob moves the reticle down to the right or up to the left, and the left screw will do the reverse, i.e., down to the left and up to the right.
Alignment itself should be done in the usual way. First, find a bright known object in your telescope’s eyepiece, e.g. Polaris. Secondly, with your telescope locked in place, use the adjustment screws as described above to centre the same object in the smallest ring of the reticle. Finally, double-check the object is still in your eyepiece and make any minor adjustments needed.
For more details on aligning finderscopes click this link for our article.
To look through the make good use of the reticle you don’t need your eyes right up to it, which makes it comfortable to use and, if you have a very long telescope such as a Dob, you won’t need a step to use the finder effectively.
The Telrad finder sight comes into its own for star hopping. The red rings are ideal for moving from a bright, easy to find star to the patch of sky containing the invisible to the naked eye object you are hunting.
Many star maps both online and in print come with Telrad finder rings printed within them. This is the perfect combination for star hopping. The example below is from inside the front cover of Sky and Telescope’s Pocket Sky Atlas (buy a copy on Amazon).
Use tracing paper or acetate to copy the rings. You can then use that trace with the star maps themselves to plan your route across the sky.
It’s also possible to setup and display Telrad circles in astronomy software. The image below shows us doing this in SkySafari 6. The three blue circles are the same diameter as the Telrad’s and centred on Orion’s belt, click the image for a larger version.
Read our detailed guide to star hopping to learn more about using your Telrad to find dim objects.
One final ‘trick’ with this finderscope is to match your eyepiece view to that of one of the rings, e.g. the 1° ring.
When you do that, and you are properly aligned, you can be assured that the small patch of sky you see through the Telrad is exactly what you now see in detail through your eyepiece.
Pros and Cons
In this final section, we take a look at the main benefits and shortcomings of the Telrad.
- You get to keep both eyes open when using it
- Views are exactly as you’re to them, no upside down or over-magnified star fields
- You can use the finder with your eyes some distance from it, making it a good choice for a long-tubed Dob
- Starhopping is made simple and intuitive
- You develop a sense of a star field’s scale using the reticle regularly
- Simple to use, even for a complete beginner
- Separate bases are available, so one finder can service many telescopes
- It is an ugly brute of a design
- They are ungainly/too large for smaller scopes
- Longer focal length scopes may still need a magnifying finder alongside the Telrad because they tend to have a smaller field of view
- Mounting options of sticky pads or drilling your scope are both damaging to your scope. Be sure of its position before fixing it in place, and be conscious of the impact on resale of your scope if you remove the Telrad
- Telrad finders can suffer from condensation, but you can buy a dew shield to mitigate that
Summary and Further Information
Reflex finderscopes have a lot of benefits, and Telrad is the leading model in the field. Its heads up display style superimposes three red rings onto your view of the night sky, making it much simpler to navigate.
They have some downsides, but most astronomers who use them swear by them, as you’ll see in the forum reviews we’ve linked to below.
For a relatively low price, you could be using one yourself next week.
- The easiest way to aim a telescope. The view seen through the window of the Telrad is continuous with the sky around it, not magnified or upside down.
Telrad Video Review
Rother Valley Optics is a renowned UK-based optical retailer. They sell a massive range of astronomy products from filters to telescopes and, with over 12 years’ trading experience, pride themselves on their honest, useful advice.
My thanks to them for the equipment loan. Check them out for yourself here.
*Rother Valley Optics neither sponsor nor have editorial control over this post. We do not receive any commission payments from RVO.
Last update on 2020-08-10 / Affiliate links / Images from Amazon Product Advertising API