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How to Use a Red-dot Finder Scope for Astronomy

Posted by Ryan DeLange on

If you’re having a hard time getting anything to show up in your telescope’s eyepiece, then you probably just need to learn to align and use your red-dot finder scope.

Most new astronomers go through this frustration. Don’t worry, this guide will give you the know-how, and with a little practice, you’ll get the hang of it.

First off, let’s take a look at the instrument. A red dot finder scope has no magnification power. It simply transposes a tiny red dot onto the image you see when you look through the finder scope. Many red dot finders come with a knob that adjusts the brightness of the dot. All red dot finders have two direction adjustment knobs, one for altitude (up and down) and one for altazimuth (right and left).

Here’s how you use it

During daylight hours or at night?

Practicing on terrestrial objects during daylight hours is ideal for a couple of reasons. First, there are plenty of bright, easily identifiable targets for you to practice on. Second, it is easier when you have a visual frame of reference, rather than trying to locate objects separated by light-years of darkness.

If your red dot isn’t bright enough to show up during broad daylight, then wait until dusk - after sunset but while land objects are still visible. Some people suggest practicing with the moon at night, but there is limited variety, and it might not even be up when you want to get started.

Center an object in the telescope’s eyepiece

Start by choosing an object that is fairly large, bright and easily identifiable in the middle distance, like a telephone pole or the point of a roof. Looking through the eyepiece of your telescope, adjust the telescope so that that object is directly in the middle of the image.

Using a low-power eyepiece will make this easier, maybe 20mm or above (the higher the focal length of the eyepiece the lower the power). If the magnification is too high, then it will be more difficult to find objects and tiny adjustments will exaggerate the movement in the eyepiece.

Align the red-dot finder scope

Now, let’s look through the red dot finder scope. The goal is to get the red dot to line up directly on top of the same object. Use the altitude and azimuth adjustment knobs until the red dot is in the right place. Get used to keeping both eyes open, so that one eye is looking through the finder and the other is able to scan the whole sky.

Test it

At this point, your finder scope should be aligned. Now choose another object. This time, look through the red dot finder first and move the telescope so that red dot is on the object. Then look through the telescope’s eyepiece. If everything is aligned properly, that very same object should be visible in the middle of the image.

This skill is one of the most important ones for the astronomer to master. It’s a bit tricky and takes some getting used to. Do you have any questions or tips about using red-dot finder scopes? Your comments below would be helpful.


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Nice Apo and a Real Value

I bought this scope for an industrial client whose needs are industrial, not astronomical, and this Explore Scientific ED127 is intended to be a beam expander in various Interferometric testing setups. For this, optical quality must be high and the carbon fiber tube is a plus for thermal stability. My purchase of the Explore Scientific ED127 F7.5 APO Triplet through Telescope Space was straightforward and pleasant. Ryan made sure everything was smooth.
Upon receipt I tested the scope by looking at a power pole 700 feet away, one that I have used for many such tests. The star test from the insulator glints was good – the focus snap was crisp, the in-focus diffraction blur was round with a faint first ring. I did not look at the through focus balance with a narrow passband filter as it was obvious there was no SA3. Color correction is very good as you would expect with the glasses used. There is no color in focus, and only a hint of subtle color fringing in the just out of focus edges of details. Extended details on the pole were rendered as well as, or better than, with any similar scope I have used on this target – this includes TMB, Astrophysics, and Takahashi scopes.
The build quality of the carbon parts is gorgeous. The metal parts are good quality anodized machined. I would prefer machined tube rings as the cast ones supplied are a little chintzy compared to the rest of the scope. The rack and pinion focuser works well, but the feel is not as good as some stock friction focusers I’ve used. Considering the extra high weight capacity of the focuser the feel may be the price paid. Overall I think the mechanical appearance and function is in line with the price.
I would certainly consider the Explore Scientific ED APO FCD100 triplet scopes for my personal use. My transaction with Telescope Space was great and I will do business with them again.

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