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Articles

How to observe the Moon and planets

by Bigzmey

Observing of the Moon and planets is very rewarding. They are not affected by light pollution, easy to find, and even the smallest scopes can provide nice views.

Jupiter, Saturn and Mars are arguably the most enticing astronomical targets. I still remember the awe and wonder of the first view of Saturn I got over 20 years ago in 4.5” Newtonian. Often however the first reports, in particular of Jupiter and Mars, come with a sense of disappointment. “I just see a ball of light no details”, or “I see a small disk I can’t quite get in to focus. Is my scope defective?” Well, like with any other targets there is a learning curve and tips and tricks of trade.

Planets are dots of light in the sky and Moon is large. However, Moon has many fine details and it turns out that to resolve them one need to employ the same strategies as for the planets. There are several important factors you need to address to get the best possible views with your telescope:

1) Magnification

2) Resolution

3) Glare

4) Light scatter

5) Contrast

6) Sharpness

Magnification

Most misleading term. Planets are small so the more power the better? Not really. You need to use magnification optimal for your telescope. The easiest way to find it is to go by exit pupil. Exit pupil is the size of the focused image you see through the eyepiece in your telescope.

Exit pupil = Eyepiece focal length / telescope focal ratio

Telescope focal ratio = Telescope focal length / telescope aperture.

It turns out that for the human eye 1mm exit pupil provides the best resolution for well iluminated targets. Let’s say you have a 90mm refractor with focal length of 900mm and thus the focal ratio of 10. In this case to get the best views of Moon or planets you need to use 10mm EP. For F5 you should use 5mm EP, F8 – 8mm EP and so on. On most nights this will give you the best views.

There are two exceptions: 1) if the seeing is really good (more on the seeing later) and your scope has reasonably good optics you can go down to ~0.5 exit pupil (to see the fine details better). For F10 scope that would be 5mm EP or 10mm EP barlowed with 2x barlow. 2) if the seeing is bad and at 1mm exit pupil the planet is boiling, you need to step down the power and go to 1.5 or 2mm pupil (to see at least some of the main features) – which would correspond to 15mm or 20mm in F10 scope.

Resolution

Resolution depends on two factors: telescope aperture (the more the better) and seeing. Astronomical seeing is a measure of how stable the atmosphere is. If it is steady you will see more details; if there is a lot of turbulence the fine details will be lost. Seeing is the hard stop on how much you can resolve. If the seeing is bad 10” scope will not show you more than 4” scope. In fact, smaller scopes handle bad seeing somewhat better. One can’t control seeing but observing as high in the sky as possible and away from heat sources (like roofs) will help to lessen the effect.

Glare

Moon and most of the planets are very bright. Often fine details are lost in the intense glare at the EP. How to control it? The easiest trick is light pollution. Night vision is counterproductive when it comes to observing the Moon and planets. Turn on your porch light, or better yet observe when the sky is still blue. The best views of Jupiter I had were right before the sunset. If this is not an option or not enough, you can either stop your scope to smaller aperture or use filters. Stopping aperture is quite productive for fast scopes F4-F6. For slower scopes F8-F15 I don’t recommend it since it decreases your resolution. Filters would be more effective (more on filter selection later).

Light scatter

Light scatter happens when bright light from the Moon, planets or stars hits the glass surface in your scope. The effects are similar to glare: loss of contrast and resolution. Unfortunately, you can’t control light scatter with filters. The only way to handle it is to select diagonal, barlow, EP and filter with good light scatter control.

Contrast

High contrast is the goal. You achieve it by controlling the glare and light scatter, and selecting EPs with good contrast. You can also improve the contrast of certain surface features on the Moon and planets using appropriate filters (more on this below). Finally larger scopes will show you more of low contrast features.

Sharpness

Some scopes are sharper than others, but since you likely already have a scope lets focus on EPs and barlows. Many EP designs get soft at higher powers. Ironically, some of them sold as planetary EPs. Orthoscopic EPs (Orthos) are your best friends. Budget barlows can also degrade sharpness. More on this below.

EQUIPMENT

Telescope

ANY scope no matter of size and design can be used. However, if you are shopping specifically for Moon/planetary scope, slow scopes with F8-15 ratios make better planetary scopes. Chromatic Aberration-free design is preferred, since CA degrades resolution at high powers.

From performance vs value perspective, catadioptric telescopes (MAKs, SCTs) in 5”-11” size would be my first choice, following by small 80-120mm slow achro refractors, and small 80-100mm APO/ED refractors.

Larger APO refractors make great planetary scopes but are expensive. Very large DOBs and SCTs could potentially deliver the best planetary views. However, to take advantage of high resolution they provide one needs excellent seeing. This does not happen very frequently, and on average night using smaller scope will be more practical.

Filters

Filters should be your first priority after telescope, and they must be of good quality. Stay away from modern planetary filter sets sold by many vendors. They degrade resolution and add light scatter. For buying brand new I recommend Baader and Lumicon filters. However, the more sensible approach is to look on eBay and classifieds for the older made in Japan color filters branded by Orion, Meade and Celestron. They are of good quality and could be found for a few bucks a piece.

Neutral density and polarizing filters are often recommended for Moon and planets. I used them at the beginning but realized that you get better results using color filters.

Color filters not just cut the glare but also improve the contrast of surface details. Orange #21 is the best filter IMO for the crescent Moon and Saturn and works well for Mars. Light Red #23A and for larger scopes Red #25 are the best filters for Mars. Blue #80A is good for Venus and Mercury, and Green #58 is the best for the full Moon. Jupiter was trickiest to sort it out. I have tried many filters over the years. Among color filters only Blue #80A was somewhat helpful to my eye.

There are a couple of special filters from Baader which I highly recommend for Jupiter, Saturn and Mars (they are too weak for Moon, Venus and Mercury, though). Baader Moon and Sky Glow is the best filter for Jupiter, much better than Blue #80A. For Saturn and Mars, I get better results with Baader Contrast Booster filter. When the planets are very bright (near opposition) I stack Baader Moon and Sky Glow and Baader Contrast Booster filters together and use the stack for all three planets. What I especially like about these two filters is that they cut the glare and enhance the contrast but do not alter significantly the natural colors of the surface features.

Eyepieces

Orthos! No matter what scope you have I highly recommend getting at least one of those for the planets. Orthoscopic EPs combine sharpness, high contrast and excellent light scatter control. Used orthos can be easily find in $40-60 range, and most of them came from one or two factories in Japan, so quality control is typically good. If you prefer to buy new the best value for money are Baader Classic Orthos (BCOs). BCOs also have 50 deg FOV, wider than typical 40 deg orthos, and double as excellent DSO EPs.

The two limitations of orthoscopic design are narrow field of view (40-50 deg) and short eye relief in small focal lengths. You can work around short eye relief by using Ortho with a good quality barlow. For example, 18mm Ortho has a comfortable eye relief of ~14mm. With 2x barlow the effective focal length becomes 9mm for use in the scopes with F8-10 ratios, with 3x barlow effective focal length becomes 6mm for use in scopes with F5-7 ratios.

Over the years I tried many EP lines in the beginner to mid-price range. Some have soft focus at high powers, some low contrast and some terrible light scatter control. Orthos are your best bet for planets. However, if you prefer wider views or long eye relief, based on my experience I can recommend Vixen SLVs, TeleVue Radians and Delites, Explore Scientific 68 and 82 series and Meade 5000 UWAs as good quality Moon/planetary EPs.

Now, someone would say “my EP works just fine on the Moon”, and it does. Moon is very forgiving target. If your view is a bit soft you will still see plenty of larger details. However, testing sharp EP with a stock EP side by side will be a revelation. Like switching from a good analog TV to HD, you don’t realize what you are missing until you see it. :)

Barlows

You don’t need barlows if you have EPs in the right range. Also, budget barlows can degrade the contrast and add light scatter. Good quality barlows however can be beneficial. To get 1mm or less exit pupil in a fast scope you need to use EPs with short focal length. Those EPs could have uncomfortably small eye relief. Using 2x or 3x barlow with EPs of longer FL could be a better option. In addition, barlow increases effective focal length of your scope and as a result you may get steadier planetary views with barlow+EP combo vs EP alone. Based on my experience I would highly recommend Baader Q barlow 2.25x barlow, and in the premium segment TeleVue 2x and 3x barlows.

Diagonal

Often overlooked part in the optical train, diagonal could be responsible for the less than stellar views. Upgrading the stock diagonal should be one of the first priorities. If you telescope has 2” focuser, upgrading to a 2” dielectric diagonal is a sensible choice and will improve the views for both DSOs and planets. I had good experience with mid-range dielectrics from Celestron, Orion, Explore Scientific and GSO.

If you are looking for the best diagonal for Moon and planets, I would go with a good quality prism instead. Prisms scatter less light than dielectric diagonals and more preferable for the Moon and planets. From performance vs price perspective, I would recommend Baader T2 prism.

OBSERVING

Moon

On the Moon the most details are seen in the structures along the Moon terminator. Since the terminator shifts every day with Moon phases, you can return every night and enjoy new features. Even with smallest scopes and binoculars one can see many craters on the Moon. Stepping up in aperture resolves more fine details. With my 8” SCT on average night I can resolve details down to ~1 km and spend the whole session within a single crater studding intricate shape of the walls, central mount, microcraters and other minute details.

Mercury and Venus

They hide for months to eventually appear as the morning or evening star for a short period of time. Mercury is harder to spot since even at the periods of elongation it says close to Sun. Catching Mercury with naked eye is an achievement already. On the rare occasion of calm clear atmosphere near horizon coinciding with Mercury elongation one can resolve Mercury phase even in a small scope.

A rear event – Mercury transit across the Sun disk will occur on November 11, 2019. Very important! – don’t look at the Sun in unprotected binoculars or telescope – you will get blind! However, with proper solar equipment it will be safe to observe.

Venus is easier to see. During the elongation it hangs around for weeks. Even a smallest scope can show Venus phases. In larger scopes with filters one can resolve darker clouds in the Venus atmosphere on occasion.

Mars

During the year Mars travels fairly fast across the zodiac constellations. If it is up in the sky, most of the time you can only resolve a small orange disk with no details. However, once in two years Mars comes to opposition when its apparent size grows considerably. Next opposition is coming in the summer 2020, so get ready! :)

Mars is the hardest planet to observe due to low contrast of the surface details. Good filters and EPs are must. But even with 80mm scope with time and patience one can resolve many surface details during the opposition. The trick is to take your time, stay on the planet and wait for the moments of better seeing, when the surface details shift into focus. This is, by the way, general strategy for observing Jupiter, Mars and Saturn.

Jupiter

Jupiter typically comes out to play for 4-5 months every year. With dynamic quartet of moons and rich surface details, Jupiter is one of the most rewarding targets in astronomy. Even 10x50 binoculars resolve Jupiter disk and the 4 moons. Stepping up in power and aperture one can resolve a couple of major bands with no problem. However, the intense glare of Jupiter needs to be addressed to take it to the next level. With good quality filters and EPs even in 80mm scope the intricate belt system comes to view. You can also observe the transits of the Great Red Spot and shadows of Jupiter moons across the Jupiter disk. Stepping up in aperture to 8” or more will bring up the richness of Jupiter colors, show more of fine details in Jupiter belts and regions (including smaller storms and festoons); and will resolve Jupiter moons into little disks. Observing Jupiter is an acquired skill, you learn to see more with practice.

Saturn

As Jupiter, Saturn comes out for 4-5 months every year. Its apparent size is smaller. 10x50 binoculars show egg like shape and with some practice and sharp optics one can resolve tiny rings around the disk in 15x70 binoculars. The rings are easily spotted even in the modest scope (guarantied Wow moment! :)). Relatively, modest improvement in the equipment will show Cassini division in the rings (no filters required). Saturn clouds system has much lower contrast compared to Jupiter. Filters and stepping up in aperture is required to resolve more details in the Saturn disk and its rings. Saturn’s largest moon Titan is readily visible in a small scope. With larger scopes one can resolve a few more moons.

Uranus and Neptune

They tend to stay in the same constellation for years and autumn has been the best time to observe them for the last few seasons. Both can be spotted as bluish stars in binoculars or small scope. With scopes of 8” and larger one can resolve very small greenish disks with no surface details. In the larger scopes (8” and up) one can spot Triton, Neptune’s moon, and at least three moons of Uranus.

Pluto

Still a planet in my book! :) It has been hanging in Sagittarius for the last few years. At magnitude around 14 at opposition one can only see it as a very faint star in telescopes of 8” or larger.

Planets Marathon

Once every two-three years the planets line up just right to see them all in one night. I have done it in the past – lots of fun! :) I will post ahead when such an opportunity comes by next time.

I could not possible discuss all the nuances of the Moon and planets observing in one short article. Hopefully, I provided just enough info to get you interested or started. Ask away if you have questions!

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