How to observe the Moon and planets

[Bigzmey]

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...
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[JayTee]

An excellent article, thanks, Andrey. It took me years to accumulate all the knowledge that you have spelled out here.

Nicely done,
JT

[Bigzmey]

An excellent article, thanks, Andrey. It took me years to accumulate all the knowledge that you have spelled out here.

Nicely done,
JT

Thanks JT! Yep, took me a few years to.

[helicon]

Great article Andrey - I learned a lot. I have only started to enjoy these views recently as my previous focus was 98.5 percent deep-sky.

[Harshil]

Very nice article with a good explanation! perfect article for the beginner who wants to start a journey in the field of stars.

I still remember, when I was in high school the first time I saw Jupiter through a telescope and I said it looks like a cricket ball with two bands on it. Then I switch to saturn, the planet always considers for their beauty...then venus & mars & more and more...And my journey begins towards the astronomy...

[notFritzArgelander]

Nice article indeed. I especially appreciate the praise of orthoscopics!

[kt4hx]

Though I am not as enamored with planetary or lunar viewing, and I am about 99.99999% a DSO observer, you touched on some fine points. Beginners would do well to heed your advice. I was quite pleased to see you mention the importance of "not" being dark adapted for these objects. Turning on a porch light and actually glancing at it occasionally to prevent pupil dilation is a good thing, in this case.

[notFritzArgelander]

Though I am not as enamored with planetary or lunar viewing, and I am about 99.99999% a DSO observer, you touched on some fine points. Beginners would do well to heed your advice. I was quite pleased to see you mention the importance of "not" being dark adapted for these objects. Turning on a porch light and actually glancing at it occasionally to prevent pupil dilation is a good thing, in this case.

As long as you've got 2mm to play with that's all you need. Aberrations in the eye otherwise?

[kt4hx]

Though I am not as enamored with planetary or lunar viewing, and I am about 99.99999% a DSO observer, you touched on some fine points. Beginners would do well to heed your advice. I was quite pleased to see you mention the importance of "not" being dark adapted for these objects. Turning on a porch light and actually glancing at it occasionally to prevent pupil dilation is a good thing, in this case.

As long as you've got 2mm to play with that's all you need. Aberrations in the eye otherwise?

I am not certain about aberrations. But typically in bright light it is said that normal pupils range from about 2.0 to 4.0mm. I suppose the differences would be due to individual physiology. So I would say a entry pupil of 2.0mm would certainly be easily sufficient for these objects. Of course if one truly wishes to observe the moon, doing so when its full or near full is not the best time anyway. Crescent to first quarter (or last quarter to crescent) provides the more interesting views, at least from my experience, such as it is.

[notFritzArgelander]

Though I am not as enamored with planetary or lunar viewing, and I am about 99.99999% a DSO observer, you touched on some fine points. Beginners would do well to heed your advice. I was quite pleased to see you mention the importance of "not" being dark adapted for these objects. Turning on a porch light and actually glancing at it occasionally to prevent pupil dilation is a good thing, in this case.

As long as you've got 2mm to play with that's all you need. Aberrations in the eye otherwise?

I am not certain about aberrations. But typically in bright light it is said that normal pupils range from about 2.0 to 4.0mm. I suppose the differences would be due to individual physiology. So I would say a entry pupil of 2.0mm would certainly be easily sufficient for these objects. Of course if one truly wishes to observe the moon, doing so when its full or near full is not the best time anyway. Crescent to first quarter (or last quarter to crescent) provides the more interesting views, at least from my experience, such as it is.

My recollection is that the resolution of the eye (no telescope) becomes optimum with a 2mm entrance pupil. With a larger entrance pupil there are aberrations. With a smaller than ~0.5 mm pupil aberrations are still smaller but imperfections in the eye material are a problem. This is the physiological reason that when using a telescope one gets a maximum of resolution at 2mm exit pupil from the telescope and entrance pupil to the eye. The aberrations with fully dilated pupils are inherent because of the eye's inherent design limitations and individual variability.

Anyway, that's what I remember. If there's interest I can look for reference materials. I think I posted something at af.net....

[JayTee]

I was writing an article last year and needed data on the optical quality and characteristics of the human eye but couldn't find what I was looking for.

This is the physiological reason that when using a telescope one gets a maximum of resolution at 2mm exit pupil from the telescope and entrance pupil to the eye. The aberrations with fully dilated pupils are inherent because of the eye's inherent design limitations and individual variability.

This was the information I was looking for. I read many, many medical articles (most of it was gobbildy-gook to me) and did not the info I needed, so I gave up on the article. nFA if you have anything that you could share, that would be nice.

Cheers,
JT

[kt4hx]

As long as you've got 2mm to play with that's all you need. Aberrations in the eye otherwise?

I am not certain about aberrations. But typically in bright light it is said that normal pupils range from about 2.0 to 4.0mm. I suppose the differences would be due to individual physiology. So I would say a entry pupil of 2.0mm would certainly be easily sufficient for these objects. Of course if one truly wishes to observe the moon, doing so when its full or near full is not the best time anyway. Crescent to first quarter (or last quarter to crescent) provides the more interesting views, at least from my experience, such as it is.

My recollection is that the resolution of the eye (no telescope) becomes optimum with a 2mm entrance pupil. With a larger entrance pupil there are aberrations. With a smaller than ~0.5 mm pupil aberrations are still smaller but imperfections in the eye material are a problem. This is the physiological reason that when using a telescope one gets a maximum of resolution at 2mm exit pupil from the telescope and entrance pupil to the eye. The aberrations with fully dilated pupils are inherent because of the eye's inherent design limitations and individual variability.

Anyway, that's what I remember. If there's interest I can look for reference materials. I think I posted something at af.net....

It would seem logical that under bright light conditions, exceeding a 2 to 3mm entrance pupil could lead to degradation and aberrations. I don't have any specific recollections on the subject honestly. I was not aware that the human pupil could constrict as small as 0.5mm. I was wondering how small it would go say when one holds a flashlight up to the eye momentarily. I know that most of us as kids likely have done that! I did read something about as small as 1.5mm, but it was not clear as to whether that was being stated as a constriction limit or a more general reference to bright light conditions.

As an aside, one thread I looked at on CN about whether or not one could damage their eyes if looking at the moon through a large scope I found the following interesting statement:

The sun and moon are both very close to 0.5 degrees in diameter. The sun has a brightness of -26.7. A standard solar filter reduces the brightness of the sun by about 12.6 magnitudes. Viewing the sun through a standard solar filter (optical density 5) effectively reduces it to magnitude -14.1, this is about 1.5 magnitudes (about 4 times) brighter than the moon at it's very brightest.

Of course that was made to allay any fears among those that were worried their eyeballs would be seared by the moon!

Personally, other than momentary discomfort and poor vision, I've never suffered any issues with viewing the moon without a filter. Again, exposing one's eyes to a light source before looking in the eyepiece reduces that initial shock. Then again, I rarely look at any really bright objects, unless its a field star near my real target - something faint and extended.

[notFritzArgelander]

I am not certain about aberrations. But typically in bright light it is said that normal pupils range from about 2.0 to 4.0mm. I suppose the differences would be due to individual physiology. So I would say a entry pupil of 2.0mm would certainly be easily sufficient for these objects. Of course if one truly wishes to observe the moon, doing so when its full or near full is not the best time anyway. Crescent to first quarter (or last quarter to crescent) provides the more interesting views, at least from my experience, such as it is.

My recollection is that the resolution of the eye (no telescope) becomes optimum with a 2mm entrance pupil. With a larger entrance pupil there are aberrations. With a smaller than ~0.5 mm pupil aberrations are still smaller but imperfections in the eye material are a problem. This is the physiological reason that when using a telescope one gets a maximum of resolution at 2mm exit pupil from the telescope and entrance pupil to the eye. The aberrations with fully dilated pupils are inherent because of the eye's inherent design limitations and individual variability.

Anyway, that's what I remember. If there's interest I can look for reference materials. I think I posted something at af.net....

It would seem logical that under bright light conditions, exceeding a 2 to 3mm entrance pupil could lead to degradation and aberrations. I don't have any specific recollections on the subject honestly. I was not aware that the human pupil could constrict as small as 0.5mm. I was wondering how small it would go say when one holds a flashlight up to the eye momentarily. I know that most of us as kids likely have done that! I did read something about as small as 1.5mm, but it was not clear as to whether that was being stated as a constriction limit or a more general reference to bright light conditions.

As an aside, one thread I looked at on CN about whether or not one could damage their eyes if looking at the moon through a large scope I found the following interesting statement:

The sun and moon are both very close to 0.5 degrees in diameter. The sun has a brightness of -26.7. A standard solar filter reduces the brightness of the sun by about 12.6 magnitudes. Viewing the sun through a standard solar filter (optical density 5) effectively reduces it to magnitude -14.1, this is about 1.5 magnitudes (about 4 times) brighter than the moon at it's very brightest.

Of course that was made to allay any fears among those that were worried their eyeballs would be seared by the moon!

Personally, other than momentary discomfort and poor vision, I've never suffered any issues with viewing the moon without a filter. Again, exposing one's eyes to a light source before looking in the eyepiece reduces that initial shock. Then again, I rarely look at any really bright objects, unless its a field star near my real target - something faint and extended.

To clarify, I never intended to imply that the eye's entrance pupil could be restricted by the iris to 0.5mm. But if the eye is looking into an instrument with an exit pupil it is just as if it could.

[kt4hx]

My recollection is that the resolution of the eye (no telescope) becomes optimum with a 2mm entrance pupil. With a larger entrance pupil there are aberrations. With a smaller than ~0.5 mm pupil aberrations are still smaller but imperfections in the eye material are a problem. This is the physiological reason that when using a telescope one gets a maximum of resolution at 2mm exit pupil from the telescope and entrance pupil to the eye. The aberrations with fully dilated pupils are inherent because of the eye's inherent design limitations and individual variability.

Anyway, that's what I remember. If there's interest I can look for reference materials. I think I posted something at af.net....

It would seem logical that under bright light conditions, exceeding a 2 to 3mm entrance pupil could lead to degradation and aberrations. I don't have any specific recollections on the subject honestly. I was not aware that the human pupil could constrict as small as 0.5mm. I was wondering how small it would go say when one holds a flashlight up to the eye momentarily. I know that most of us as kids likely have done that! I did read something about as small as 1.5mm, but it was not clear as to whether that was being stated as a constriction limit or a more general reference to bright light conditions.

As an aside, one thread I looked at on CN about whether or not one could damage their eyes if looking at the moon through a large scope I found the following interesting statement:

The sun and moon are both very close to 0.5 degrees in diameter. The sun has a brightness of -26.7. A standard solar filter reduces the brightness of the sun by about 12.6 magnitudes. Viewing the sun through a standard solar filter (optical density 5) effectively reduces it to magnitude -14.1, this is about 1.5 magnitudes (about 4 times) brighter than the moon at it's very brightest.

Of course that was made to allay any fears among those that were worried their eyeballs would be seared by the moon!

Personally, other than momentary discomfort and poor vision, I've never suffered any issues with viewing the moon without a filter. Again, exposing one's eyes to a light source before looking in the eyepiece reduces that initial shock. Then again, I rarely look at any really bright objects, unless its a field star near my real target - something faint and extended.

To clarify, I never intended to imply that the eye's entrance pupil could be restricted by the iris to 0.5mm. But if the eye is looking into an instrument with an exit pupil it is just as if it could.

I got you now, sorry I misunderstood. I have been working for 12 hours and it is quite late. A little sleep would be helpful. Perhaps (per Ian's post) that is one of the reasons I am pleasingly plump (to put it nicely)!

[notFritzArgelander]

It would seem logical that under bright light conditions, exceeding a 2 to 3mm entrance pupil could lead to degradation and aberrations. I don't have any specific recollections on the subject honestly. I was not aware that the human pupil could constrict as small as 0.5mm. I was wondering how small it would go say when one holds a flashlight up to the eye momentarily. I know that most of us as kids likely have done that! I did read something about as small as 1.5mm, but it was not clear as to whether that was being stated as a constriction limit or a more general reference to bright light conditions.

As an aside, one thread I looked at on CN about whether or not one could damage their eyes if looking at the moon through a large scope I found the following interesting statement:

The sun and moon are both very close to 0.5 degrees in diameter. The sun has a brightness of -26.7. A standard solar filter reduces the brightness of the sun by about 12.6 magnitudes. Viewing the sun through a standard solar filter (optical density 5) effectively reduces it to magnitude -14.1, this is about 1.5 magnitudes (about 4 times) brighter than the moon at it's very brightest.

Of course that was made to allay any fears among those that were worried their eyeballs would be seared by the moon!

Personally, other than momentary discomfort and poor vision, I've never suffered any issues with viewing the moon without a filter. Again, exposing one's eyes to a light source before looking in the eyepiece reduces that initial shock. Then again, I rarely look at any really bright objects, unless its a field star near my real target - something faint and extended.

To clarify, I never intended to imply that the eye's entrance pupil could be restricted by the iris to 0.5mm. But if the eye is looking into an instrument with an exit pupil it is just as if it could.

I got you now, sorry I misunderstood. I have been working for 12 hours and it is quite late. A little sleep would be helpful. Perhaps (per Ian's post) that is one of the reasons I am pleasingly plump (to put it nicely)!

No worry. I'm doing pre travel insomnia so more spheriform than usual myself.

[smp]

Thanks very much, Andrey! Excellent article.

smp

[Mick]

Hi Bigzmey, just found this what an excellent article.
Thank you very much.

[Bigzmey]

Very nice article with a good explanation! perfect article for the beginner who wants to start a journey in the field of stars.

I still remember, when I was in high school the first time I saw Jupiter through a telescope and I said it looks like a cricket ball with two bands on it. Then I switch to saturn, the planet always considers for their beauty...then venus & mars & more and more...And my journey begins towards the astronomy...

Thanks Harshil! I bet Saturn and Jupiter got more people into astronomy than all other targets combined.

Great article Andrey - I learned a lot. I have only started to enjoy these views recently as my previous focus was 98.5 percent deep-sky.

Thank Michael! You certainly got nice tools for that. Have you ever tried your Obsy on any of planets?

Nice article indeed. I especially appreciate the praise of orthoscopics!

Thanks nFA. You and JG got me to try my first Ortho a few years ago. I am just paying it forward.

Thanks very much, Andrey! Excellent article.

smp

Hi Bigzmey, just found this what an excellent article.
Thank you very much.

Thanks Stephen and Mick!

[helicon]

The Obsession is OK, but at f/4.2 is a bit impacted by coloration at the fringes of the images, say, of Jupiter. So I get more aesthetically pleasing views from the 6" frac, f/6.5. Also, the Obsession collimation tends to need to be adjusted before each session, for some reason, I suppose just moving it around on the dolly.

[Bigzmey]

Thanks everyone for the feedback!

Let's re-visit magnification/exit pupil topic. Moon and planets have different requirements than DSOs. For the Moon and planets you want to achieve highest possible magnification without loosing resolution. On the other hand you don't wan't go too high since the empty power will just degrade the contrast and sharpness.

You may heard about 40x to 60x per inch telescope rule. It is also based on the exit pupil and gives you the estimate of highest productive magnification your scope can handle. It does not take into account the seeing, somewhat less intuitive, and takes a few math steps to calculate which EP to use.

This is why I prefer the focal ratio rule. You pick EP with the focal length equal to your scope focal ratio, and on a night with average seeing it will get you close to (or right at) the optimal magnification for planets. If the views are nice and sharp, step up magnification until the image starts to degrade. If the view is boiling, step down the magnification until you can see at least some of the details.

Smaller scopes with sharp optics on average can take smaller exit pupil. With my Onyx 80ED I often end up using 0.4-0.5mm exit pupil. In fast scopes image tend to degrade faster. In my 150ST and 120ST achros I never managed to push below 1 mm exit pupil on planets. In my 8" SCT most of planet observing done in 0.8-1.4mm exit pupil range, but it is mainly controlled by seeing.