Pointing Telescope on a Non-GOTO Mount

Once you acquire a telescope, it is important to know how to point the telescope to a celestial object such as a nebula or galaxy so that you may be able to see the object and photograph it. If you have a GOTO mount, you enter or select name of the object such as Orion Nebula or Andromeda galaxy and the telescope slews and points it towards the object. The GOTO mounts are costlier than the those without the functionality. In case you have a non-GOTO mount, how do you do it? In any case, you need a tracking mount  – the mount moves automatically to compensate for the rotation of Earth so that the object in the viewing field of the telescope doesn’t go out of the field – to see or image for minutes. Non-tracking mount might be good enough only for observing and imaging planets.

Pointing Telescope Without Camera

This case is fairly simple. You directly see the object through eyepiece of the telescope by moving your telescope appropriately using manual controls provided with the mount. The way you do it is slightly different for reflector and refractor telescopes.

Reflector Telescope

Reflector telescopes inevitably have eyepiece perpendicular to the telescope tube (OTA – Optical Telescope Assembly) as shown below:

This is because they have a mirror at the one end of the tube and the other end is open for collecting light. Pointing open-end of the OTA toward the desired object by looking at it perpendicular to the tube through eyepiece is a bit difficult. Hence, you need a finder scope or red dot finder that is fixed to the OTA so that you can directly look at the sky for an object and adjust the telescope to point at the object for viewing it later through the eyepiece.  An OTA with the finder scope is as following:

As the finder scope or red dot finder is fixed over the OTA and is parallel to the main tube, it is easy to point OTA toward the desired object by looking through the finder scope or red dot finder. Once the tube is positioned, you can look through the eyepiece for a more clear and magnified  view of the object.

Refractor Telescopes

Eyepiece of a refractor telescope is at the one end of its OTA, whereas the other end is open and is pointed to the desired celestial object. An example refractor OTA is as following:

But a 90 degree prism can be attached at the end of eyepiece holder (not shown in the above picture) to avoid crouching and craning neck particularly when one has to look for celestial objects at or near zenith. In this case, a finder scope or red dot finder is needed to easily point the OTA to a desired object.

Pointing Telescope with attached Camera

In this case, a finder scope or red dot finder is essential to look at and point the OTA toward the desired object, as camera – regular or dedicated – has to be attached to the eyepiece holder of the OTA. An example of reflector telescope with a camera is as following:

The camera attached is a Canon mirrorless camera. You can use DSLR or mirrorless cameras. Some such cameras are explicitly built for astrophotography and they cost a bomb! You need to have appropriate T-mount to fix the camera to the eyepiece holder. Note that you need to remove the usual lens of the camera before attaching it to the OTA as the telescope works as a lens for the camera. However, you can use some lenses of camera without telescope OTA to photograph some celestial objects.

A refractor with dedicated camera is as following:

Here the dedicated camera is shown attached to the end of the refractor. The camera is in turn connected to a laptop using a USB cable to see the object the OTA points to.

Do OTA and Finder scope or Red dot finder point to the same Object?

Not exactly. But they are very close. There are multiple reasons for the difference. First, optical axis of the main OTA and and that of finder scope or red dot finder are different. When you fix them to the OTA, they may not be exactly parallel to each other. In case of red-dot finder, its base may not be at the center of the dovetail saddle it is mounted on. RDF shows a red dot in the sky. You can move your OTA such that dot is very close to a star. But the screen of camera or laptop shows a big field of sky with many stars in it. Now you don’t know which one of the stars the RDF is pointing to! You need to know this for you to center the desired object in the screen of attached laptop or camera. It is important to practice RDF or finder scope to point to well known star such as the ones in the Orion belt so that you place red dot and/or cross-hair at the right position in the sky for you to see the desired object  in the center of the screen. After this, it is better not to change position of the RDF in its saddle! However, you may not be able to see some objects such as the Flame nebula either in the sky or on the screen. In this case, you should know the star near by the nebula and point telescope towards the star. Once you process all your photographs, you will be able to see the nebula or galaxy. Further, your processing of the photographs yields desired result only if you used right ISO or Gain.

A refractor with RDF mounted on it is shown in the following photographs:

In the first photograph, the part in the black color is the red dot finder (RDF). It is fixed over saddle base which is in red color. The saddle base itself is attached to the OTA, which is also in red color. The OTA is hardly visible in the first photograph but it is clearly visible in the second one. The RDF is fixed in the saddle using a few screws. The RDF may be aligned to left edge, right edge or somewhere in the middle of the dovetail saddle base. Every time there is a change in the exact position of the RDF in the saddle, what the OTA sees and what you see through the RDF may be slightly different. As a result, you may get doubt whether what you see through the RDF is the same as the one you see through the OTA, on the camera or laptop. The problem is pronounced more when your camera sensor size is very small. So, it is advised that you always fix the RDF aligned with some edge so that you know exactly where to point the red dot in the sky for the desired object to be shown in the camera or on the laptop at appropriate position on the screen.