33 replies to this topic

[Jond105]

This summer and into now, I’ve really grown into Lunar observing as well. I love snapping photos of itwoth my cell phone, and observing the craters. 

 

That said, what becomes more important with Luna observing, aperture or Focal length to really start diving into the craters. This may be a silly question, but I look at shots done with maks, refractors, big Newts. Does any of it really matter in the end?

[petert913]

Some of the sharpest images I’ve ever viewed of the Moon were through a 60mm f/16 Tasco. 

[james7ca]

Well, the seeing conditions are the most important factor, as those can pretty much invalidate any changes in either aperture or focal length. However, if you could eliminate the problems with seeing then aperture would definitely rule (with well-corrected optics and when viewing on axis). But, focal length would come into play when working off axis and when selecting eyepieces.

[scngc7317]

My Moon Scope's, 6" F-15 and a 40mm F-21

 

 

 

Edited by scngc7317, 09 October 2019 - 12:04 AM.

[db2005]

I'd say optical quality is very important regardless of which telescope design you choose. The bright moon can be a good test of stray light suppression, light scatter and CA control (if it's a refractor) in a scope. Optical artifacts/problems not typically seen on fainter targets may suddenly seem to pop up on the bright moon (although the moon is IME not the most challenging target). And you'll want excellent correction of SA, otherwise the moon will look "flat" and with reduced contrast and "pop".

 

I tend to get my prettiest, highest contrast views of the moon with my 3"-class APOs, but I believe this might at least in part be due to considerably fewer seeing issues compared with larger apertures. However, the most detailed views I've ever had of the moon have been in my 8" SCT. 8 inches of aperture simply shows more detail than 3". But the larger SCT is more sensitive to seeing conditions and generally doesn't provide the same level of rich contrast as a high-quality APO. I find the moon frequently "pops out" with an almost 3D-like appearance in a good APO due to its insanely rich contrast, a visually stunning effect I have never seen in any other telescope design I've tried.

 

I am guessing that, for photography, larger aperture invariably wins because aperture allows you to resolve finer details, while the lower contrast in an SCT can always be enhanced digitally in post-processing. But for visual use, at least my eyes prefer the richer contrast in a small aperture, high quality APO. But it's not like one is better than the other, it's just a question of different horses for different courses. And, a 3" APO is an incredibly comfortable grab-and-go scope in a way that a much larger scope isn't. On the other hand (and to put things into perspective), a high-quality 3" APO can easily cost more than an 8" SCT including mount and accessories. I like to be able to combine the best of both options, so I keep an 8" SCT in my stable to complement my smaller APOs. It's a match made in heaven (pun possibly intended).

[nicknacknock]

Aperture, quality optics, seeing conditions. This image was taken @ 260x through the eyepiece of a Tak FS-128 with an android phone.

 

CN compression also applies to admins, so believe me when I say the actual photo is oh-so-much-better.

 

I am also partial to Maks for lunar viewing and imaging, but you MUST let them cool sufficiently to perform at their best.

Attached Thumbnails

• 

[sg6]

Presuming all is specified reasonably well and performs well I would have said focal length over aperture.

Simply the moon has a habit of being fairly bright so you do not need aperture.

 

Go lunar observing and half the recommendations are get a moon filter.

 

So buying greater aperture to then add a moon filter to reduce the light gather is a bit pointless when you think about it.

[Jond105]

Presuming all is specified reasonably well and performs well I would have said focal length over aperture.

Simply the moon has a habit of being fairly bright so you do not need aperture.

 

Go lunar observing and half the recommendations are get a moon filter.

 

So buying greater aperture to then add a moon filter to reduce the light gather is a bit pointless when you think about it.

It’s interesting you say that too. Earlier this year I sold off or gave away all my moon filters as I didn’t enjoy them anyways. I was thinking Focal length would have been majority answer as well. My 120ED puts up great views, but it got me thinking tonight, would a 102 mak with 1400FL give me more of up close and personal touch. 

[nicknacknock]

Filters: The more you increase magnification, the smaller the exit pupil becomes, the less brightness you get. No need for filters IMHO

 

120ED vs 102MAK: It is a matter of combining appropriate eyepieces / barlows to reach same magnification, but with added resolution of the 120ED due to larger diameter and no central obstruction Vs the 102mm Mak. But the Mak is cute and tiny and very capable. Easier to mount, easier to move about. Does need a bit of cooling, but not that much. Been there and stupidly sold it...

[happylimpet]

Aperture. Obviously of a decent quality, but aperture is what reveals detail.

 

Small telescopes deliver a low magnification sharp looking view, but the fine detail doesnt exist. Its sharp because the magnification is low.

 

Double the aperture, double the resolution, simple as that, provided the atmosphere obliges! Which it does more often than some people would maintain.

[MikeTahtib]

If you're going to use a reflector, mirror quality is very important.  I learned this when I had my 2001ish vintage Nova mirror refigured by Mike Lockwood this year.  One of the biggest differences I noted was the moon.  Before, I could see features on the moon, but the smaller ones could not really be seen, or made sense of, when examined closely.  It's a hard thing to describe, but it was something I noted often, and found frustrating.  Detail in the refigured mirror is much clearer in this respect, probably more refractor-like.  

If refractors give a more tightly-controlled image than a similar-quality reflector, then that would be the way to go for lunar.  

I wonder how much the resolution advantage of a large aperture reflector is lost due to diffraction, coma, viewing through a wider expanse of air and a filter, compared to a more modestly sized apo.  Maybe you really need to compare apos to apos  .

[happylimpet]

I wonder how much the resolution advantage of a large aperture reflector is lost due to diffraction, coma, viewing through a wider expanse of air and a filter, compared to a more modestly sized apo.  Maybe you really need to compare apos to apos  .

Diffraction - essentially none.

Coma - none if using a coma corrector, very little otherwise (depending on f ratio)

Viewing through a wide expanse of air - none, assuming you know how to get to thermal equilibrium (clue - use fans, its easy)

A filter? none also

 

How much extra money stays in your pocket? Vast.

 

Reflectors rule.

[Jon Isaacs]

Presuming all is specified reasonably well and performs well I would have said focal length over aperture.

Simply the moon has a habit of being fairly bright so you do not need aperture.

 

Go lunar observing and half the recommendations are get a moon filter.

 

So buying greater aperture to then add a moon filter to reduce the light gather is a bit pointless when you think about it.

 

I agree that moon filters are pointless, the eye adapts to the brightness of the moon.  In lunar viewing, what aperture buys is resolution, if you want to see the finest details, it requires aperture.  And it does buy brightness, useful at high magnifications.  

 

Jon

[SeaBee1]

Jon, my experience has been thus - with my Celestron Omni 102mm f/10 (now retired), the moon looked very good, detail was good, contrast as good as could be expected for an achromatic scope, CA was well controlled, but still present. With my SW 120 ED, more detail stood out, I was beginning to see an almost 3D view of things, especially along the terminator. I could also bring the magnification up a bit more than the 102mm, but the seeing conditions had more impact. And CA? What CA?

 

With my 10 inch reflector, it is an OMG experience... I had it out Monday night and it quickly reminded me why I love this scope. The detail and contrast that is visible is like being in a Lunar lander on approach... words simply can't describe the view. The 3D appearance was eye popping. It was a decent night, not great, with the seeing like a 3/5, so at higher mags, there was a bit of waviness at times, but mostly good. Keep in mind that my 10 inch reflector is an ATM scope, so a lot of attention was put into getting top performance.

 

This has just served to remind me that a good refractor is no slouch... but has some limitations. A good reflector with some aperture is magical.

 

Good hunting!

 

CB

[james7ca]

Presuming all is specified reasonably well and performs well I would have said focal length over aperture.

Simply the moon has a habit of being fairly bright so you do not need aperture.

 

Go lunar observing and half the recommendations are get a moon filter.

 

So buying greater aperture to then add a moon filter to reduce the light gather is a bit pointless when you think about it.

But, remember that a telescope can't make an object appear any brighter than as seen with the naked eye. Telescopes only magnify, and given that magnification the surface brightness of an object will either appear to be the same as seen with the naked eye (nearly, ignoring any transmission loss in the optics) or dimmer (but never "brighter," regardless of aperture).

 

Furthermore, the perceived brightness of an object is limited by the size of your eye's dark-adapted pupil and all a telescope can do is fill that pupil by some amount as determined by the size of the exit pupil from the optic system. So, while aperture does play a role in determining the size of the exit pupil (as a function of magnification) that doesn't allow a generalization to the effect that a larger aperture will produce a "brighter" image (under all situations). It really comes down to the size of the exit pupil (determined by aperture and magnification), your eye's dark adaption, and the apparent field of view of the eyepiece (the latter a consideration if the object/moon does or does not fill the entire field of view).

 

Having said that, it's generally true that given any particular magnification and eyepiece a larger aperture will produce a "brighter" looking image. But, if you change magnification or apparent field of view with a change of eyepiece or have an exit pupil that exceeds the size of your eye'e pupil then the determination becomes more complicated. In fact, once the size of the exit pupil from the eyepiece equals the dark-adapted size of your eye's own pupil the surface brightness of an object won't change with increased aperture (since the light from that increased aperture won't be able to enter your eye).

 

Also, for completeness I should add that all of the above applies specifically to the viewing of extended objects (like the moon), since point-source stars do appear brightened with increased aperture.

Edited by james7ca, 09 October 2019 - 11:15 PM.

[Jond105]

Appreciate everyone’s input. 

[Ken Watts]

I read your original post and gave it some thought.  Today I read up on seeing and according to what I understand, seeing is limited to about 1 second of arc due to atmospheric turbulence.  That being said, I believe a longer focal length is the better choice.  With a longer focal length, longer focal length eyepieces can yield higher magnifications.   I have and 8-in f/6 scope and top out at about 250x before turbulence becomes too much for me.

 

Clear and steady skies!

 

Ken W

[elwaine]

Some of the sharpest images I’ve ever viewed of the Moon were through a 60mm f/16 Tasco. 

Sharpness and detail are two entirely different things. Larger apertures are required to see smaller details. "Featurless" regions seen through a 60mm refractor suddenly become populated with countless small craters, shadows, mounds, riles, and other details when viewed through apertures of 150mm - 300mm, etc., even when the views are not super sharp. The trade off with larger apertures is mainly related to issues ergonomics and the need for better seeing conditions.

[revans]

Aperture, quality optics, seeing conditions. This image was taken @ 260x through the eyepiece of a Tak FS-128 with an android phone.

 

CN compression also applies to admins, so believe me when I say the actual photo is oh-so-much-better.

 

I am also partial to Maks for lunar viewing and imaging, but you MUST let them cool sufficiently to perform at their best.

I've been away from lunar imaging for a long time.  Does everyone take photos at the eyepiece with smart phones these days?  The result here is certainly spectacular. But what happened to taking CCD video clips and selecting and stacking the best images using something like Registax.  Has that become obsolete these days or do people still go through all that trouble.  I think I used to get pretty good results with that method some years ago.  Rick

[Tom Glenn]

I've been away from lunar imaging for a long time.  Does everyone take photos at the eyepiece with smart phones these days?  The result here is certainly spectacular. But what happened to taking CCD video clips and selecting and stacking the best images using something like Registax.  Has that become obsolete these days or do people still go through all that trouble.  I think I used to get pretty good results with that method some years ago.  Rick

Visit the solar system imaging forum for many examples using CMOS cameras and stacking.  This method is still the best way to get the most detailed images from Earth using amateur equipment.   

[Tyson M]

The finest lunar views for me tend to be from refractors. 

What size really depends largely on the seeing conditions. 

The TSA102S and NP127 strike me as being the most memorable.  

[revans]

The finest lunar views for me tend to be from refractors. 

What size really depends largely on the seeing conditions. 

The TSA102S and NP127 strike me as being the most memorable.  

If you most enjoy the grand vistas of the moon and don't get caught up in trying to see finer and finer detail then a good quality 4-6 inch refractor is as good as it gets.  That was the case for me with my Meade 5 inch F9 ED/apo doublet.  On exceptional nights I could even push it up to 300x.  About 1995 I became determined to see the central rille on the floor of  the Alpine Valley.  I tried to see it at the eyepiece for many years using many scopes of different types up to 16 inch aperture.  I never succeeded but was certain that a minimum of 300x, good seeing or better, and the right solar angle were very important.  Some people claim to have seen it with smaller aperture telescopes.  In my case the best I could do was capture it on a stack of best video frames processed in the old Registax version 4 and that was on April 14 and 15 of 2008.  The telescope I was a Mewlon 250 mm F10 instrument used with a focal reducer to F7.3.  I haven't duplicated that accomplishment since and anyway I've never seen the rille just at the eyepiece.  So there is a challenge for anyone that wants to try.  My guess is you might need an 8-10 inch scope, 300-400x, fine seeing, and a nice contrasty solar angle.  

[Keith Rivich]

My favorite scope for lunar visual is my 8" f/9. When seeing is good the view is tack sharp. The best view ever was with my 25" on a extremely steady night. I was hitting 1000x and still had a sharp image. 

 

Like Jon, no filter. 

[Magnetic Field]

Some of the sharpest images I’ve ever viewed of the Moon were through a 60mm f/16 Tasco. 

 

Did the 4 craterlets in Plato pop out?

[barbie]

My 3" and 4" apos have shown me the best views of the moon out of all the scopes I've owned.  When seeing conditions permit, they show me craterlets, rilles, and many other features too numerous to mention here. When I can push them to 100X per inch of aperture or even slightly above that as was the case with my 3" F8 Takahashi this past Summer, the craters and craterlets take on an entirely different appearance and look 3 dimensional.