I brought a new Astromaster 130 EQ and collimated it using a laser. However, whenever I try to observe moon I get a perfect picture but with other planets I see only a bright glowing ball which increases and decreases in size as I adjust the focusing knob. Also, I see secondary mirror shadow in between that bright glowing ball. Kindly help me where I have gone wrong. I am using all kinds of eyepieces from 20mm to 06mm. I have a Barlow lens 2x also.

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    $\begingroup$ "I see secondary mirror shadow in between that bright glowing ball" this sounds like you're not in focus. The shadow should disappear when you reach focus. When you turn the focuser and the image decreases in size then keep going in that direction until you reach focus. Although, you say you can focus on the moon...have you tried doing that first and then moving to look at a planet without adjusting the focus? $\endgroup$ – Aaron F Oct 7 '20 at 11:05
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    $\begingroup$ When you adjust the focus knob far from best focus while looking at a bright object, you definitely should see the secondary mirror shadow, that's 100% normal. I think the problem is that planets are pretty darn small and you may simply be not noticing surface detail on the spots when you are at best focus. Slow down, watch carefully. Saturn and Jupiter are large and bright, as you reach minimum size while focusing, look for thin line of Saturns rings (now almost edge-on) and a few Galilean moons near Jupiter. Just take it slow! $\endgroup$ – uhoh Oct 7 '20 at 15:36
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    $\begingroup$ Also, when collimating with a laser, before tightening the laser collimator in the focuser, try rotating it and observing the point of light it creates. That point should remain still while you rotate the laser. If it draws a circle then your laser collimator needs collimating itself! (it probably has three tiny hex screws that let you adjust it) If you use a badly collimated laser to collimate your telescope then you end up decollimating it. Remember that you can use a defocused bright star to double-check your collimation. $\endgroup$ – Aaron F Oct 8 '20 at 12:02

Subject Size

In angular dimensions, planets are tiny compared to the Moon. The Moon is roughly 30 arc-minutes (about 1/2° ... varying slightly from apogee to perigee). In real life they are, of course, MUCH larger than the Moon... but they are also MUCH farther away so they appear much smaller.

In comparison to the Moon's approximately 30 arc-minutes size, Mars is nearing it's closest approach (making it appear larger than usual) but is a still a mere 23 arc seconds. Saturn is 17 arc-seconds across.

This means you are trying to resolve fine detail on an extremely tiny area and this is pushing the limits of what your telescope handle.


Other factors such as optical collimation (alignment of the optical axis of the primary and secondary mirrors). Portable Newtonian reflectors tend to need somewhat frequent collimation. Collimation is easy to perform and will improve optical quality. Also, if the temperature of the telescope mirror has not had time to acclimate to the ambient temperature, the mirror itself will reduce the quality of what you can see (e.g. if a telescope were stored in a warm house but taken outside on a cool evening). Smaller telescopes will acclimate faster because the mirror has less mass.

Seeing Conditions

Seeing conditions will also have a large factor. Weather fronts such as cold fronts, warm fronts, or the presence of the jet stream will impact seeing quality. There is little you can do about this. (Large telescopes can use 'Adaptive Optics' but this isn't a common thing in amateur astronomy.)

The best analogy I've come across to describe seeing conditions is to imagine placing a coin in the bottom of a pool or fountain. If the surface of the water is perfectly flat -- no waves -- then you can easily see the coin. With magnification you can see details on the coin. But if a second person starts tapping the water surface to create waves... you can see the coin ... but not details. The coin is optically distorted by the waves. The atmosphere does the same thing when we use telescopes. It need not be physical "waves" but can be things such as mixing warm & cool air in the upper atmosphere.

Infinity Focus

Everything in space is effectively focused to "infinity" for your telescope. If anything is focused, then everything is focused. This means if it makes things easier for you, focus your instrument on the moon and achieve the sharpest focus possible. Then re-point the telescope to a planet and should not need to re-focus the telescope. You will likely want to do this using your 6mm eyepiece (because changing eyepieces usually requires re-focusing the instrument. There is such a thing as "parfocal" eyepieces -- which all share the same focus and you can swap eyepieces without refocusing. Most eyepieces are not parfocal with other eyepieces.

Maximum useful magnification

There are some formulas for working out the maximum useful magnification of an instrument.

One such (rather simplistic) approximation is that the maximum magnification is 2x the aperture when measured in millimeters. As you have a 130mm aperture, this would imply that 260x is the maximum magnification. HOWEVER (huge caveats) this assumes flawless optics in flawless seeing conditions -- which almost never happens. A more practical limit is roughly 1x of your aperture. This becomes subjective as to when you decide that the magnification is making things worse instead of better.

It turns out mathematically... that the focal length of an eyepiece needed to provide magnification that matches 1x of your telescope's focal length in millimeters... will always be the same as the focal ratio of the telescope.

Since you have an f/5 telescope, a 5mm eyepiece will provide 130x magnification.

Both your 20mm and 6mm are fine and should not over-magnify the image ... you can use a 2x barlow with the 20mm, but you will not likely enjoy the views of the 6mm eyepiece combined with the 2x barlow.

There are more technical ways to discuss maximum focus such as the very simple Dawes' Limit or using Rayleigh criterion. These methods discuss maximum resolving power of an instrument based on aperture and wavelength of the light. But even these methods have a low-bar. That is to say... their standard is whether two points of light with some amount of angular separation could be resolved as two points of light and not just one merged point of light. They wont look sharp ... they'll just be blurry points that have noticeable separation.

One conclusion that can easily be drawn by looking at the formulae for maximum resolving power of an instrument... is that the bigger the aperture of the instrument, the greater the detail it will be able to resolve (which is why professional astronomers want to strive for larger and larger telescopes).

On a personal note, my first telescope had a 3.5" aperture. I could see that Saturn had "rings" and I could see that Jupiter had "bands". Then I got a 5" telescope... and on nights with better seeing, I could just barely make out that the ring system on Saturn had a dark band in it (the Cassini division) and that Jupiters belts sometimes had texture. Over the years I would eventually get larger telescopes and, unsurprisingly, larger aperture telescopes allowed for viewing greater amounts of detail. Have a reasonable expectation as to what you should be able to resolve with a 130mm (roughly 5") instrument.

Atmospheric Dispersion

When looking at anything low in the sky you may notice some "color fringing". If a planet has a slightly blue edge on one side and a slight red edge on the opposite site, then this is typically a result of the atmosphere acting like a prism ... it's separation the light spectrum because the object is very low in the sky. I live at fairly northern latitudes and this is true of observing outer planets during the summer months (when Earth's northern pole is tilted away from the outer solar system and toward the Sun ... from my latitude. But southern hemisphere observer's are tilted toward the outer solar system, so they see these same planets very high in the sky where atmospheric dispersion doesn't impact them as much. Basically outer planets appear high in the sky when observed during your hemisphere's "winter" season and low in the sky when observed during your hemisphere's "summer" season.

This dispersion will tend to blur the surface details of any planet. It isn't really the fault of your instrument (there is a device called an Atmospheric Dispersion Corrector which is basically an adjustable prism and it is used to reverse the dispersion created by the atmosphere).


Actually you only think the Moon appears to be perfect in your eyepiece. Take a good look at Lunar images made from above the atmosphere.

Campbell gave you an very good explanation for what amounts to eyepiece viewing with any telescope is disappointing. This will of course get all the traditionalists up in arms but you have the ability to make up your own mind.

You have done nothing wrong, you have only discovered a seldom discussed truth about amateur astronomy. To me the view is all that matters. I have dumped all my traditional eyepieces into a junk box and exclusively view with electronic eyepieces.

No doubt someone will cry foul and say this answer has nothing to do with the question but your question indicates you want to see better with your telescope and that is what I am responding to.

By the way I started viewing with an eyepiece over sixty years ago and acquired all the skills many claim make them the best observers. I just never let the experianse get to my head. I used some very good equipment and but did not like the view because I always knew some sort of technology would come along and give me the view I wanted, and it has.

Now watch the fur fly. If you find any humor in the responses to my post, do a Google on Vogon :)

  • $\begingroup$ Perhaps you should explain what you mean by an "electronic eyepiece". That's open to interpretation as it stands. $\endgroup$ – StephenG Oct 8 '20 at 5:18
  • $\begingroup$ When viewing planets I find my eyes do a much better job than my camera does: they're higher resolution and can see details that my camera isn't able to. When viewing faint objects I find my camera does a much better job than my eyes do: it's much more sensitive and can see details that my eyes aren't able to. $\endgroup$ – Aaron F Oct 8 '20 at 10:31
  • $\begingroup$ Also, you say "this answer has nothing to do with the question", which is true, but then qualify it with "you want to see better with your telescope and that is what I am responding to" - but you haven't actually described how they can see better with their telescope using electronic eyepieces. Which electronic eyepieces do you use, with which telescope(s) do you use them, and how do they make planets look better than they do when looking through a standard eyepiece? $\endgroup$ – Aaron F Oct 8 '20 at 11:52

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