A guide to finderscopes
A finderscope is a simple but invaluable accessory that attaches to your telescope. Find out what they are, how to set one up, and whether they are still useful.
A finderscope is a simple but invaluable accessory that attaches to your telescope.
The smaller optical tube provides a wide field of view to help you locate celestial objects before observing them through your main telescope, but it must be aligned accurately to your telescope before use.
There are two types, those with a right-angle finder and those with a straight-through view.
A right-angle finder is more expensive but is more convenient to use.
Remember that you will also need to buy and install a finderscope bracket for the optical tube.
An accurately aligned finderscope makes locating objects much easier, but aligning it at night can be frustrating.
To set up a finderscope, start aligning in the daytime by finding a very distant object using your telescope, centring that object in your telescope eyepiece as accurately as you can.
When doing any sort of observation during the day, make sure not to look directly at the Sun without specially-designed solar filters, or this could seriously damage your eyesight.
How to set up a finderscope
- Locate a distant daytime object (not the Sun!) through your telescope and centre it in the eyepiece
- Now turn to the finderscope and centre the same object on the crosshair
- At night, locate a bright star in the centre of your finderscope
- Look through the main telescope and centre the star in the eyepiece
- Return to the finderscope and adjust until the star is centred on the crosshair
For help with this, read our guide on how to polar align using the Sun.
What finderscope is best for a Newtonian?
There are two main types of optical finder, those with a straight-through view and those with a right-angled view.
Since Newtonian reflectors have the observing eyepiece on the side of the optical tube close to the front of the telescope, a right-angled finder is by far the most comfortable to use.
If you were to use a straight-through finder, you would have to angle your face down the side of the telescope to see through it.
An illuminated crosshair makes it so much easier to centre objects.
Both the Celestron illuminated RACI 9x50mm finderscope or the Altair 10x60mm right-angled illuminated finderscope should be on your shortlist.
Do you need a finderscope if you have GoTo mount?
Once upon a time we were lost without them. The trusty optical finderscope allowed us to get a wide field of view to visually home in on targets, or help us star hop to the next one.
Now don’t get me wrong, in terms of any basic or manual telescope, the finderscope still serves that purpose very well, and is a necessary piece of kit.
But when it comes to GoTo systems, it becomes a little less clear just how useful a finderscope can be.
"What?" I hear you cry, "you still need it for the initial alignment for the GoTo to work accurately don’t you?" Well I think the answer is yes and no to that.
If your initial polar alignment is rough or you have a system that you need to move around a lot, perhaps for public outreach, then I agree that the finderscope remains useful, but only for the alignment phase of setting up.
After that, if your polar alignment and star alignment is accurate then your target should always be in the view of a wide field eyepiece if not bang in the centre.
However, if you are able to conceive a permanent setup for your mount, you can get polar alignment very accurate indeed.
Once done, you will likely find you hardly ever use the finderscope, as the target should be in the centre of the view or imaging system you are using every time.
But today manufacturers have spotted an opportunity amongst all this, and we are now seeing finder/guidescope combinations.
In other words, the fact that many astronomers are now using the finder as a guidescope for deep-sky astrophotography may well mean we haven’t seen the last of the finderscope just yet!
Choosing a finderscope
When choosing which finder is best for your telescope, there are a few things to take into consideration:
Is the finder solidly constructed? Are any parts loose and can the bracket be attached to it easily?
Is the front lens free from aberration and is it sharp across the field of view? Are any of the lenses multicoated and does the front lens have a hood that prevents it dewing up?
Is the finder easy to focus and can it then be locked in position? Is the adjustment front or rear, and is there any play while focusing?
Is the bracket well constructed and does it allow for easy alignment? Are the screws soft-capped to prevent them scratching the finderscope tube?
Are the crosshairs in focus, sharp and centred in the field of view? If illuminated, are they too bright or dim?
8 of the best finderscopes
Right angled finderscopes
Celestron 9x50 RACI
The Celestron 9x50 Right Angle Correct Image (RACI) Illuminated Finder offers 50mm, 9x magnification and a 5°field of view.
It displays celestial targets right-side up and left-to-right correct, which makes it easier to slew to objects and track them as they move across the sky.
It features a double-crosshair reticle etched into the eyepiece that can be focused separately from the finder.
A nice touch is the battery-powered illuminator that brightens the crosshair with a red glow to make it easier to see during dark, moonless nights. This is adjusted with an on/off switch.
This finder also comes with a bracket comprising two rings and a dovetail. Align adjustment is carried out with thumbscrews.
Orion Optics 8x50 right-angled erecting finderscope
Optically, this finder differs very little in terms of quality from its straight-through relations and performed just as well on our deep-sky test objects, including the Crab Nebula.
We found the combination of the right angle and an erect image easy on the eye and better for recognizing objects.
The field of view was just right for fitting in Castor and Pollux, and was excellent across 85%. Orion’s sword was particularly rewarding, while Struve 747 was cleanly split.
During the tests, we also carried out a star count on the Pleiades with the finders, and this was the one finder that picked out the most stars, with stunning views.
Sky-Watcher 9x50 Right-Angled Erecting Finderscope
The Sky-Watcher 9x50 Right-Angled Erecting Finderscope is a quality instrument and would be a good fit for most telescopes.
It boasts a 50mm aperture, which reveals a lot more than the smaller finders that come with many starter telescope kits.
The right-angled diagonal produces an upright and left/right corrected image, making slewing the telescope very intuitive.
Spring-loaded thumbwheels make it easy to centre an object in the field of view, and cross hairs are clear and sharply focused.
This would make a good upgrade or replacement finderscope, as it is easy to set up and use and provides good value for money.
Sky-Watcher 9x50 straight-through finderscope
This straight-through finder fits snugly inside its bracket and, although its plastic screws can become chewed, it was straightforward to align.
It boasts 9x magnification and 5.5° field of view. When trained on Rigel, the field was sharp across at least 75%.
It gave very pleasing views of the Hyades, which just squeezed in.
Several stars in the Hyades also showed their true colours well, and Aldebaran was a wonderful orange colour.
The crosshairs didn’t distract from the display as they were crisply focused and not too thick.
The finder resolved the double Struve 747 and, turning to the deep sky, we found M35 relatively easily, coming across a nice hazy patch of light in the dark sky.
We then glimpsed the faintest object, the Crab Nebula, as a tiny patch, which popped into better view with averted vision.
There was no doubt that this finder performed the task well.
William Optics 7x50 straight-through finderscope
This is a well-made bit of kit that offers a straight-through design with an erect image, a rubber eyecup for the eyepiece and illuminated crosshairs.
We found the latter to be a real bonus as on dark, moonless nights, crosshairs can sometimes be tricky to
During our testing, Castor and Pollux fitted snugly inside the 5° field of view, which was mostly sharp, bar a bit of trailing off towards the edges.
Betelgeuse’s reddish hue stood out well and even with a lower magnification of 7x, we could still resolve Struve 747 and glimpse the Crab Nebula.
Orion Optics 8x50 straight-through finderscope
This is a well-made, robust straight-through finderscope that includes a single mounting bracket with a spring-loaded holder and two plastic screws for adjustment.
The field of view is superb: at least 85% sharp at 5° wide, and with sharply-defined, centred crosshairs.
Despite a lack of eyecup to protect the eyepiece, we nevertheless found a clear and unobstructed view, although this may be an issue in dewy conditions.
We could cleanly split Struve 747 and were also able to centre the Crab Nebula. Indeed, this finder allowed us to locate a range of few deep-sky objects, providing pleasant views of the largest and brightest.
Vixen 7x50 straight-through finderscope
The Vixen’s field of view is wide at 6°, but it sharp and crisp across only 70%.
Despite this, the wider field of view did afford a good look at the Hyades; even the two ‘pointer stars’ of the Great Bear – Dubhe and Merak – just fitted into the field of view.
SV165 Mini Guider Scope
This finderscope offers 30mm aperture and f4 focal ratio and a back focus distance of 45mm. It also boasts a helical focuser and lock ring to help with focusing.
The finder of weighs just 342g, meaning it is easily transportable as an accessory and won't put too much added pressure on a heavy astrophotography setup.
This is a bonus, considering its dovetail guide scope bracket is designed to be attached to most astro imaging setups, featuring a 1/4 thread hole under the dovetail.
The finder is multi-layer coated and comes fitted with nylon-tipped thumbscrews for adjustment.
Paul Money is an experienced astronomer, BBC Sky at Night Magazine's Reviews Editor and author of the annual stargazing guide Nightscenes.