Hello Dobson, my Old Friend!

In the town I grew up in India, there was a community building which housed a skating rink, a small library and a science exhibition room for kids. On the 2nd level of this building was a large open balcony, where a tall Newtonian reflector telescope stood on its South-West corner. Every few weeks, or so, there would be some guest speaker - probably a college lecturer or professor - that would be invited to give an astronomy talk to a bunch of kids followed by a viewing session through this telescope. The telescope was huge. If I remember correctly it was probably a 12 inch. We were usually shown moon, Saturn's rings, sometimes Jupiter and Mars and even Andromeda. I believe the limiting factor was the light pollution of the city rather than the optical capabilities of the telescope itself.

A Dobsonian Newtonian type is as simple as a telescope can get. It is an improved version of the Newtonian reflector telescope that was pioneered by Issac Newton in 16th century.

So finally, when it came to owning one, I gravitated towards a Dobsonian type. It is built by Orion and called SkyQuest XT8i.

It is a 8 inch scope and comes with a basic 10mm and 25mm Plossl eyepieces, a 9x50 finder scope, & a focuser.

It also comes with a simple computerized object locator system (that's what the i stands for). Orion boasts that it can locate 14k objects. The process of calibration cannot be more simpler. The locator takes input from the 2 encoders that track the 2 perpendicular axis:

  • The rotation around the vertical axis is called azimuth (or compass bearing), and,
  • The sweep across the sky around the horizontal axis is called altitude (or the angle of elevation).

To calibrate the locator, all that is needed is the encoder readings from these three data points:

  • The vertical position of tube (i.e. to say the angle of elevation needs to be 90 degrees)
  • The location of a known star such as Vega
  • The location of another known star such as Altair

Once these inputs are provided, the locator provides an offset value which is basically a deviation value of the setup from an ideal value that the locator thinks it should be.

sample: w=+0.41; w ∈ [0, 1]

As long as w is less than 0.5, the objects should appear within the eyepiece.

No further inputs are required unless you want to track a planet for which the date input is required.

The prompt walks you through these steps intuitively and it takes less than 2 minutes to get going. This scope does not have any motorized motion so, when I want to find an object from the locator's database, I need to select it and move the tube manually in the direction the locator points to with 2 arrows; one for azimuth and another for altitude. There are encoder counts besides each of this arrow. So you have to drive down these counts to 0. At this point, the object of interest should be in your eyepiece!

This convenience factor is what tempted me to get the locator. A 2 minute setup in the beginning and I don't have to fumble with star charts in dark. That's a good deal and I would highly recommend this accessory to an amateur as well as a seasonal sky watcher.

Collimation is process through which it is ensured that all the optical components of the scope are aligned and on their designed axis. This makes sure that the light entering the scope emerges from the eyepiece and there is no degradation of the visual field of the scope. A scope with perfectly aligned optics would produce an image that is free of color degradation. Such a scope is said to be collimated.

The process of collimation can be daunting for beginners. I would recommend the laser collimation which makes the process much reliable as compared to manual collimation which relies of subjective interpretation of aligned and focused images. Even though the scope comes collimated from factory and needs collimation every 1-2 years on a fair use, you might just want to make sure that the optics haven't moved accidently during your drive to the viewing sit. The laser collimator provides a quick way to do so. I do keep the collimation cap in the bag, just in case the laser collimator dies on me.

Lenses & filters:

  • It comes with a base set 2 Sirius Plossl lenses: 10mm & 25mm.
  • I ordered these additional lenses and filters:
    • 2 Sirius Plossl eyepieces: 7.5mm and 20mm focal lengths
    • 3 color eyepiece filters for planetry viewing: Red, Yellow and Blue (#25, #12 and #80A)
    • 1 shorty Barlow lens with 2x magnification: A convenient way to boost the magnification of any lens. This one comes with a multi coats of anti-reflection coatings.
    • 1 neutral density 1.25" 13% transmission Moon eyepiece filter.

Astro photography:

I got this basic universal adapter for afocal photography. Once it is adjusted for a point & shoot or your phone, it is plug & play convenient to take quick pictures with. The other option is to get a T-ring for the DSLR but since that would also be afocal, it does not make much of a difference unless you want to do lot of post-processing on what you capture.

Other items:

  • The red flash light as a primary light source during viewing.
  • A solar filter for day time sun viewing
  • A lens cleaning kit.
  • Update: Pictures

Object locator


Half moon

Sun spots during 2017 eclipse

References & Further Reading:

Related but unrelated: