Using a Newtonian to view the planets - for complete beginners
Using your Newtonian telescope to view the planets is relatively straightforward. However, a little care and attention will pay off with a more rewarding experience.
by Joe Cali (OzEclipse)
This article was inspired by a question that that was posted on a Facebook astronomy forum.
I'm very new in Astronomy and only recently got a telescope. I managed to see Jupiter and Saturn for the first time this weekend. My question is, why do I only see white through my eye pieces? It's a Newtonian reflector 114mm focal length 900mm and I was using a Kellner 9mm eye piece. Thanks
A 900mm focal length and 114mm diameter telescope (top picture) is enough to see a little bit of detail. All the telescopes shown are examples of Newtonian telescopes and this advice applies to any type of Newtonian telescope.
The magnification of any eyepiece is the telescope focal length in mm divided by the eyepiece focal length in mm.
With a 9mm eyepiece, the magnification is 900÷9=100X.
You won't see what you see in pictures taken on large telescopes or spacecraft but on Jupiter, you should at least see the two equatorial belts and a hint of the great red spot. Jupiter's moons and their shadows crossing the planet. On Saturn, you should see the rings, some differentiation and banding in the rings and the shadow on the surface. Surface markings on Saturn are less obvious than Jupiter. Then depending upon the atmospheric stability, the set up(see below), and the accuracy of the optics, you may see more.
The general rule for limiting magnification for a good optic is between 1.5x to 2x per mm of aperture. So for your 114mm scope that is potentially up to 200X. If you have a 200mm scope, the limit is 300 to 400 x.
Before you run out and buy a 4mm or 5mm eyepiece or shorter, ask yourself these questions:-
1. Are you observing the planets in the evening when they are low in the east?
2. Is the telescope well-collimated?
3. Are you leaving the telescope outside and allowing it to cool down to the same temperature as it's surroundings?
4. Atmospheric stability (referred to as "astronomical seeing") is quite variable. Some nights, the atmosphere, even looking high in the sky is too unstable and you can't use high magnifications.
5. Are you sure you are bringing the scope to focus?
1. If you observe the planets when they are low in the sky, the turbulent thick atmosphere you look through will blur out any detail. For tonight, (July 27th, 2021 and in the southern hemisphere, set the telescope outside at 10pm local time to cool and start observing at 11pm. For each week that passes from today, you can start the process 30 mins earlier. Always give a small scope about an hour to cool down before using it. Larger telescopes are equiped with fans to speed up the otherwise more extended cooling process. Observers in the higher latitudes of the northern hemisphere are going through a period of a few years where Jupiter and Saturn are very low in the south and never get very high in the sky at any time of the year. In a few years, these planets shall migrate north and northern hemisphere observers will get better views.
2. The telescope optics need to be well aligned to use higher magnifications. It's a simple process but quite important.
3. If you don't let the telescope cool down before use, convection currents inside the tube may blur the image or make it boil or dance around.
4. Seeing is hit and miss. Sometimes it's good and stable, sometimes, just pack up and go to bed. Some regions, especially those under a strong jet stream influence have more bad seeing. Others have regular good seeing.
5. Some beginners put every accessory in the box into the eyepiece tube before looking. Some telescopes come with an extender tube, and a barlow, which may or may not be required and may prevent you being able to focus the scope. In the illustration above, the telescope has the extenders inserted and despite it coming from a manufacturer's website, probably won't allow that telescope to focus on stars or planets. Try focussing the telescope on a tree or tower on the horizon at a great distance during the day, at least 3-4km away to get an idea of where the focus lies. An out of focus image of Jupiter will appear as a plain large white ball as you have described. An in focus image will appear as a small yellowish white disc with up to 4 moons that look like yellowish white stars all lined up. Some may be hidden behind the planet at any given time. Point the scope to a bright star and make sure you can focus it to a point.
Don't use the aperture mask provided. It might appear to minimise collimation or other problems but it will reduce the detail visible. Resolution gives detail. Bigger aperture gives more resolution. Hence reducing the aperture with the mask will lower the resolution.
Darkening the image with an ND or Moon filter won't make details appear. I view Jupiter through an 18 inch telescope without filters. Very bright but lots of detail visible. Your eyesight can cope and adapt to a bright surface. Our acute vision is our daylight or cone vision, also our colour vision. Our night vision(rods) is less acute but more sensitive to low light. Planet detail is better seen with a brighter image than dark. Certain coloured filters like a Moon filter might increase contrast but again, this isn't your primary problem. Solve the other problems before trying filters. Despite the 18inch having a theoretical maximum magnification of 900x, I rarely observe at greater than 350x due to atmospheric limitations and convenience.
Be patient when observing the planets. The image may boil and dance but if you are patient, may suddenly become still and sharp as a pocket of stable air passes over you so you can see fine details before becoming burry again.
Good luck and enjoy this years planet season.
Above: an 8 inch dobsonian type Newtonian telescope.
Above: A tabletop Dobsonian 5 inch Newtonian scope