Planet Observing

Observing the planets of our solar system with a telescope is an enjoyable part of amateur astronomy. And unlike other forms of observing, such as deep-sky, it can be done from the center of a city as well as out in the country. No travel required. If it is clear, we can quickly set up one of our telescopes on the balcony and start observing. So here are a few tips on viewing our neighbors in the Solar System.

Telescope

Any decent quality telescope over 60 millimeters (about 2.5 inches) will show some detail on Jupiter, the phases of Venus and Mercury and the rings of Saturn. If we just want a quick look I will go with my 90mm APO refractor.

But better viewing can be had with 150mm (6 inch) and larger instruments. This is because the planets are small and to tweak out fine detail we will need to use high powers. And since at best you can use 50X per inch of aperture, you can see the need for size.

The actual telescope type is of lesser importance, but I personally like to use either reflectors or apochromatic (color free) refractors. Some excellent choices for a planetary scope are:

• a Maksutov Cassegrain of 150mm or greater
• a Newtonian Reflector of 150mm or greater
• a Schmidt-Cassegrain of 200mm or greater (this is what I use at the moment)
• a Apochromatic Refractor of 130mm or greater (expensive though)

But don’t despair if your scope isn’t listed here. Remember the best telescope is the one you use, so please use what you have. There are ways to squeeze more performance out of any optics.

Mount

One word is all that is really needed here, STEADY. As long as it is stable, and you don’t mind pushing it along, a Dobsonian mount, or a non-motorized alt-az with no slow motion controls will work fine. Of course it is irritating to wait for the image to stabilize again after you nudge the scope along. That is why a well built, steady, motorized mount is much nicer since the planet will remain in your field of view for as long as you want to view it. But again using what you have is more important.

Eyepieces

Since the planets are small, a lot of magnification is required to observe fine detail. But you can’t go overboard either. On the best nights, with the steadiest skies, the maximum usable magnification you can use is 50X per inch of aperture (8 inch scope, 400X). But those nights are far and few between. More often than not you will find your maximum magnification will be about 25X per inch. On my 8 inch I my most used eyepiece for planetary work is a 9mm Orthoscopic, which offers about 222X on that scope. This combination has afforded me outstanding views of Jupiter, Saturn and Mars.

Keep in mind that eyepieces are expensive. You can but some good all-round eyepieces and a quality Barlow and those can offer you years of viewing pleasure. But if high contrast, high detailed views are what you want I suggest a few Orthoscopic eyepieces. They can be had for around $100 each. Highly recommended.

A quick word on magnification. To figure out the magnification a given eyepiece will provide on a given telescope use the following formula.

Magnification = focal length of telescope divided by the focal length of the eyepiece. So my 8 inch Schmidt Cassegrain has a focal length of 2000mm. If I use my 9mm Ortho we find, 2000/9 = 222.22X.

If I use the same eyepiece with my 91mm refractor what do we get? It has an f ratio of f6.6. So 6.6 x 91mm = 600 so a focal length of 600. Then 600mm divided by 9mm = 66.66X.

Eyepiece Filters

Although optional, a set of color filters can really help pull out different details on the planets. Here are some filters I can suggest with their Wratten numbers.

• #12 Yellow – Deepens blues and brings out reds and yellows on Jupiter and Saturn, also improves surface detail on Mars. Best on 150mm or greater scopes.
• #80A Blue – This filter can really pull the detail out on Jupiter’s cloud belt and Great Red Spot. Also useful on Saturn, enhancing contrast.
• #58 Green – Adds contrast and detail to the belt structure on Jupiter and Saturn. Also useful on the Martian polar caps. Best on 200mm or greater scopes.
• #25A Red – Fantastic filter for increased contrast on Jupiter. Really makes the belts “pop out”. Also very nice on Mars. Only recommended for scopes greater than 200mm. OR
• #21 Orange – Does somewhat the same as the #25 Red, just not as well, but can be used with smaller scopes.

Get Comfortable

Being comfortable makes any type of observing far better. I have 2 different chairs that I use for observing. Also be sure to dress right. Remember you will be standing still so you will need warmer clothes than you think.

I hope you enjoy your travels through our solar system with your telescope. As your experience grows you may be interested in learning to sketch what you see or even get into planetary imaging. The sky truly is the limit.