33 replies to this topic

[Pluto 134340]

When I try observing Mars through a 90mm (f7) refractor and some of my other telescopes, it is messy and difficult to focus. But, I get a good image in my Meade Infinity 70mm (f10) all the way up to 140x (I paid $12 for the Meade at a thrift store).

 

Is it a combination of a very bright object with atmospheric turbulence? Maybe the larger aperture scopes are a bit too fast?

[AstroApe]

Depending on where you're located and the local conditions, smaller aperture, especially well corrected premium instruments, can often out preform larger aperture instruments. This is often caused by user-error (not thermally acclimated or out of collimation) but another major issue is seeing conditions, especially if located under the jet stream or if observing before a front moves in.

[Pluto 134340]

Depending on where you're located and the local conditions, smaller aperture, especially well corrected premium instruments, can often out preform larger aperture instruments. This is often caused by user-error (not thermally acclimated or out of collimation) but another major issue is seeing conditions, especially if located under the jet stream or if observing before a front moves in.

I did allow for cool down. I remember seeing a YouTube video from a channel named "Astrobiscuit" (or something like that) where he mentioned a similar experience when comparing various telescopes. A cheap 70mm had the best view of the target planet.

[triplemon]

You wont notice these effects wirh such small aperture differences. A scope 2 or 3 times larger than what it takes to resolve for a given seeing may look in direct comparisons a tad worse.
But this here is likely a basic optical quality issue.
What are the tyopes and focal ratios of these scopes?

Edited by triplemon, 02 February 2025 - 01:55 PM.

[Pluto 134340]

You wont notice these effects wirh such small aperture differences. A scope 2 or 3 times larger than what it takes to resolve for a given seeing may look in direct comparisons a tad worse.
But this here is likely a basic optical quality issue.
What are the tyopes and focal ratios of these scopes?

I tried most of what I have listed in my signature (below). It is all small "budget" stuff. I would expect the fast telescopes to not do so well on planets, but I thought the 90mm (f7) refractor to be the best.

[havasman]

Reverse confirmation bias?

The slightly larger but still small scope has something wrong with it?

I expect a good OneSky would be better than either.

[Inkie]

I tried most of what I have listed in my signature (below). It is all small "budget" stuff. I would expect the fast telescopes to not do so well on planets, but I thought the 90mm (f7) refractor to be the best.

This is often reported by people relatively new to the hobby.....and.....wait for it!!....by people with scads of experience using many large and fine telescopes.  

 

A well-figured  F10 and higher refractor is almost always going to outperform other scopes of a kind, just with the limits imposed by the 'seeing' and by the quality of the optics, both objective lens and the eyepiece.  But, also by the aperture.  It turns out that larger apertures, even on premium refractors, also gather more misinformation caused by the atmosphere, and they put it all down the tube toward the eyepiece and thence your eye.  Your smaller scope sees less of the roiling and stratification of the air above it, so you see less of it, and you still see Jupiter or the moon and you see it that much better.  

 

In the conditions where you can see how much better your smaller refractor is, if you would have been able to look through a 20" Dob, you'd find that it was a bust of a night, at least at that time.  Maybe 0200 next morning would be vastly improved, and for many of us who tough it out until the 'wee hours', we are rewarded with very nice views.

[Pluto 134340]

This is often reported by people relatively new to the hobby.....and.....wait for it!!....by people with scads of experience using many large and fine telescopes.  

 

A well-figured  F10 and higher refractor is almost always going to outperform other scopes of a kind, just with the limits imposed by the 'seeing' and by the quality of the optics, both objective lens and the eyepiece.  But, also by the aperture.  It turns out that larger apertures, even on premium refractors, also gather more misinformation caused by the atmosphere, and they put it all down the tube toward the eyepiece and thence your eye.  Your smaller scope sees less of the roiling and stratification of the air above it, so you see less of it, and you still see Jupiter or the moon and you see it that much better.  

 

In the conditions where you can see how much better your smaller refractor is, if you would have been able to look through a 20" Dob, you'd find that it was a bust of a night, at least at that time.  Maybe 0200 next morning would be vastly improved, and for many of us who tough it out until the 'wee hours', we are rewarded with very nice views.

You made a good argument for us grab'ngo, cheapskates to keep old-fashioned, long-tube 60-70mm refractors around for moon and planets.

[sevenofnine]

+1 on Inkie's reply. On a good night of Seeing, my 8" f/5.9 Dob vastly outperforms my smaller scopes 

[Keith Rivich]

When I try observing Mars through a 90mm (f7) refractor and some of my other telescopes, it is messy and difficult to focus. But, I get a good image in my Meade Infinity 70mm (f10) all the way up to 140x (I paid $12 for the Meade at a thrift store).

 

Is it a combination of a very bright object with atmospheric turbulence? Maybe the larger aperture scopes are a bit too fast?

Were these observations made on the same night? Side by side?

 

I looked at Mars last week with my 18" and it blows away any view a small scope can offer. 

Edited by Keith Rivich, 02 February 2025 - 11:37 PM.

[Pluto 134340]

That settles it. I need one of these. 130ft primary and 14ft secondary. 😂

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[areyoukiddingme]

Lots of possibilities. The "easy" ones are cooling and collimation.

 

But another one that is often overlooked is the larger scope is often running higher magnification. So comparing like-for-like is important.

 

Another I have noticed using my 8" F7 reflector from my balcony, I get better views of planets when I mount it on a tripod, getting the scope up off the ground in comparison to when I have ran it using a simple Dob structure. This may be peculiar to my balcony, with air flowing about with different temperatures lower than higher.

 

Optical quality is also a consideration. Some bigger scopes just don't have good figuring and that's going to show up in views of the planets when the power is high.

 

Another subtle difference is body heat. Again with the 8", I notice if I get a tiny bit of breeze blowing my heat away from the scope, I generally get slightly tighter views of planets and doubles. I suspect refractors are more immune to this than my reflectors with the optics up high and shielded from my body heat.

 

And if you are comparing across eyepieces, sometimes there's dust and gunk on an eyepiece that needs cleaning. I noticed this the other night when I decided to see how my 12.5 Morpheus would compare to the 12.5 Noblex in my 2.5 Powermate. The Morpheus view when de-focused showed splatters of detritus all over the place, so I abandoned the comparison to a later time.

[Daveatvt01]

The 90mm f7 (larger and faster) will also have more CA than a 70mm f10. Here’s a chart:

https://www.cloudyni...s-chart-source/

[Shorty Barlow]

Conditions have a lot to do with it. I had a superb view of Mars recently with my 72mm Evostar at 140x. I recognised the Syrtis Major plateau. The 72mm Evostar has a Dawes Limit resolution of 1.61 arcsec. You will get better resolution with bigger apertures of course.

 

[Tony Flanders]

When I try observing Mars through a 90mm (f7) refractor and some of my other telescopes, it is messy and difficult to focus. But, I get a good image in my Meade Infinity 70mm (f10) all the way up to 140x (I paid $12 for the Meade at a thrift store).
 
Is it a combination of a very bright object with atmospheric turbulence? Maybe the larger aperture scopes are a bit too fast?

I'm baffled. Optical quality being equal, a 90-mm f/7 achromat should show significantly more planetary detail than a 70-mm f/10. But maybe the optical quality isn't equal? How does the 90-mm do on other targets? And does it have mechanical issues, such as a crude focuser? An f/10 scope is very forgiving as far as focus is concerned; you have to move the eyepiece quite a long way to alter the focus significantly. A jerky or sloppy focuser is much more of a problem at f/7 than at f/10.

 

When you say you got a good image with the 70-mm scope at 140X, what does that mean? Can you describe the details that you saw?

 

Mars tends to be annoyingly bright at low magnification, which tends to bring out the defects of your own eyes. And of course at any given magnification, it's going to appear brighter in a 90-mm scope than in a 70-mm. But I wouldn't expect that to be a problem at 100X or higher.

[Pluto 134340]

I'm baffled. Optical quality being equal, a 90-mm f/7 achromat should show significantly more planetary detail than a 70-mm f/10. But maybe the optical quality isn't equal? How does the 90-mm do on other targets? And does it have mechanical issues, such as a crude focuser? An f/10 scope is very forgiving as far as focus is concerned; you have to move the eyepiece quite a long way to alter the focus significantly. A jerky or sloppy focuser is much more of a problem at f/7 than at f/10.

 

When you say you got a good image with the 70-mm scope at 140X, what does that mean? Can you describe the details that you saw?

 

Mars tends to be annoyingly bright at low magnification, which tends to bring out the defects of your own eyes. And of course at any given magnification, it's going to appear brighter in a 90-mm scope than in a 70-mm. But I wouldn't expect that to be a problem at 100X or higher.

The 90mm should be better quality. It has a metal focuser, etc. The Meade Infinity 70mm is a plastic focuser cheapie (but a surprisingly decent one). It doesn't even shake that much at 140x. It looks just like all those other yoke mount 60-70mm that are unusable with mounts that shake like those paint can shakers at the hardware store. ‍♂

 

If it is clear tonight, I will do another experiment and leave them to cool down longer (1 hour+). I will put them all side by side.

 

When I said I had a good image, I meant I had a nice round sharp planet with just very faint markings, not really detail. And, that is with a 5mm Kellner! These are small scopes at 150x or less. 

 

In my other scopes, it is a bit of an effort to focus without bloating or double image. I expect that in the ST80s at over 100x, but not the 90mm.

 

The planet was somewhere around 45 degrees.

 

It could be my eyes. I am in late 60s. Eyes not the best. I see floaters and other annoying eye issues at higher mag.

Edited by Pluto 134340, 02 February 2025 - 09:27 PM.

[Pluto 134340]

I found this funny video of this guy comparing super-cheap, beginner scopes. The 60mm (f10) was the winner on Jupiter beating out 100mm fast reflector (I would expect that). He likes his ST80 for planets. My ST80s are "OK" if you don't try to push the magnification too high.

 

https://youtu.be/S9A...6i2UGe2RlUX3_8p

Edited by Pluto 134340, 02 February 2025 - 07:55 PM.

[Pluto 134340]

This other guy (studied astrophysics and likes math) analyzed the video above.

 

https://youtu.be/BtF...KWH6tHcqUzzV2Y1

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Edited by Pluto 134340, 02 February 2025 - 07:31 PM.

[gnowellsct]

I tried most of what I have listed in my signature (below). It is all small "budget" stuff. I would expect the fast telescopes to not do so well on planets, but I thought the 90mm (f7) refractor to be the best.

Well I will disagree with some of the opinions offered here.  A 90mm f/7 achromat has a larger objective and a faster focal ratio than the f/10 70mm.  On a small bright object like Mars the achromatic color cloud will significantly obscure details in comparison to the f/10 70mm which will have pretty good color correction, a function of the longer ratio and smaller diameter objective. 

 

Shorter focal ratio, bigger aperture = significant disadvantage in color correction.

 

The 90mm f/7 achromat should have the edge on deep sky wide field views.

 

Incidentally I observe with smaller apertures on top of larger ones (92mm on c8, 92mm on c14, 92mm on 130mm) all the time, by which I mean almost every time out, and my money is on aperture every time.  Even in bad sky conditions I see more in the larger apertures.  A small aperture is not a ticket to making a good viewing night out of a bad one.

 

I assure you there are 90mm f/7 refractors out there that will blow away your 70mm f/10 achromat. Every time, regardless of seeing conditions.  But there is a $ entry fee.  The good news is that you don't have to get a triplet apochromat to have the experience (though it doesn't hurt).  An ED doublet will do.

 

Greg N

Edited by gnowellsct, 03 February 2025 - 06:14 PM.

[gnowellsct]

You can look at the Conrady chart here.  It is frequently posted on CN: 

 

https://www.cloudyni...erration-chart/

 

As you will see, your 70mm f/10 is well "into the green."  That's where you want your refractor to be.  This is why your little refractor is f/10!  If your 70mm were f/7 the color correction would be more similar to your 90mm, and the overall performance greatly reduced.

 

Your 90mm is not in the red colored junk zone, but it certainly is well into the "this really isn't very good" zone.    

 

The focuser quality has nothing to do with it.  The intrinsic (lack of) quality of the optics guarantees that the 90mm will be at a disadvantage.  

 

On the good side you are learning why people who like refractors pay mega bucks for apos (and get fantastic focusers into the bargain).  

 

Remember that the poor color correction is "information that has been lost."  It is not brought into focus with the other light and so, you don't see as much.  In the very old days they went to extraordinary lengths to extend focal ratios.  to wit: https://en.wikipedia...ial_telescope  

 

Fortunately they figured out how not to do that in the 18th c.

 

Greg N

Edited by gnowellsct, 03 February 2025 - 06:36 PM.

[gnowellsct]

I have an f/10 Newtonian 100mm that would easily outperform any 60mm out there.   Well actually it is 4.25 inches or 108 mm.  

 

The problem with four inch Newtonians is that they are considered budget scopes and not a lot of effort goes into making them top tier, not now, not in the distant past.  That was actually the problem with my scope.  The original mirror was horribly defective because the company had very little quality control (Optical Craftsman, back in the 1960s).  But I nosed around in the early 00s, as an adult, and managed to score an f/10 mirror, vintage 1978 or so, from the same company.  And all of a sudden I was seeing things in that Newt that I had never seen before: Jupiter's bands, Great Red Spot, festoons, etc.    One mirror was outstanding, the other was a hound dawg.

 

 But in terms of ease of use you just can't beat a good a 92mm or 100mnm refractor with a 100mm class Newt.  

 

Greg N

[Pluto 134340]

You can look at the Conrady chart here.  It is frequently posted on CN: 

 

https://www.cloudyni...erration-chart/

 

As you will see, your 70mm f/10 is well "into the green."  That's where you want your refractor to be.  This is why your little refractor is f/10!  If your 70mm were f/7 the color correction would be more similar to your 90mm, and the overall performance greatly reduced.

 

Your 90mm is not in the red colored junk zone, but it certainly is well into the "this really isn't very good" zone.    

 

The focuser quality has nothing to do with it.  The intrinsic (lack of) quality of the optics guarantees that the 90mm will be at a disadvantage.  

 

On the good side you are learning why people who like refractors pay mega bucks for apos (and get fantastic focusers into the bargain).  

 

Greg N

Last night I lined-up the Meade Infinity 70mm (f10) refractor, one of my ST80s, and the Celestron 90mm (f7) refractor. I let them sit for over an hour. I used the same eyepieces (sets of Kellners, Plossls, Goldlines) on all three (I had to use a barlow in the ST80 for the highest magnification).

 

I used Mars again for the test. I was just looking for a sharp image (not surface detail in these small scopes). 

 

The Meade had the sharpest image by far and the Celestron 90mm the worst. The image in the ST80 was pretty decent. For highest magnification I used 140x in the Meade, 133x in the ST80, and 132x in the Celestron. 

 

Even though the Meade had the sharpest image, I was surprised how well the ST80 performed at 133x. The image was somewhat soft, but it was still useable/serviceable. If all I had was the ST80, the image at 133x was still worthwhile and better that naked eye and binoculars.

 

I own two Orion ST80s (an early one and a late model) and a Gskyer 80/400. I had heard that the Gskyers are not as good as the Orion ST80s. 

 

Last night, I compared those also. I could not see a difference between the Gskyer and Orions. If there is a difference, it is slight (to my old eyes).

 

I paid about $75 for the whole Gskyer kit used (like new). The tripod mount with the Gskyer performed better than my ES Twighlight Nano Mount. The Nano was a little shaky by comparison. But, they all work well enough.

Edited by Pluto 134340, 03 February 2025 - 07:05 PM.

[gnowellsct]

Everything you are recounting is written large in the Conrady chart. It is almost as if someone put a lot of thought into sorting out the differences in performance as a function of aperture and focal ratio, summarized his findings in a chart, and made it available for the world to see.

It's great that you have found entry into the hobby at a price point that satisfies you. Your experiences are verifying what is known about the relationship between achromats, focal ratio, and aperture. There's no mystery here, in other words.

Greg N

Edited by gnowellsct, 03 February 2025 - 08:47 PM.

[gnowellsct]

It should be noted that as you increase magnification The inability of the fast focal ratio optics to bring red green and blue to the same focus point is exaggerated. Your eyepiece is like a little magnifying glass or even a microscope. You are examining the precise point at which the colors are supposed to all come together but they don't because of the glass choices (which keep costs way down). At higher magnifications the color aberration is even more exaggerated.

As you push magnification on the planets and the moon the color problems will grow more exaggerated not less. It pretty much doesn't matter what eye pieces are involved the determining factor is the focal length of the eyepieces which is what determines the magnification.

You can develop the habit of two scope observing where you keep one out for the planets, that would be your 70 mm, and use the others for wide field views. That would work pretty well although many of us draw the line at planet observing in 70 mm apertures. Not everyone. There are small aperture cults.

It's been a long time since I've had an ST80 but I seem to recall that for me the color aberration became bothersome at magnifications below one X per millimeter, call it 0.6 or 0.7 X per millimeter. Say 48x to 56x.

Anyhow the color aberration is not just an aesthetic thing. Your telescope gathers light and all the wavelengths of light that it gathers deliver information to your eye which you perceive as details on the object. When one of those colors is unavailable It will show as a blur or haze and that is lost detail. That's what's going on with your 90 mm and your 80 mm too for that matter. That's what the conrady chart is trying to say.

Edited by gnowellsct, 03 February 2025 - 07:52 PM.

[Pluto 134340]

It should be noted that as you increase magnification The inability of the fast focal ratio optics to bring red green and blue to the same focus point is exaggerated. Your eyepiece is like a little magnifying glass or even a microscope. You are examining the precise point at which the colors are supposed to all come together but they don't because of the glass choices (which keep costs way down). At higher magnifications the color aberration is even more exaggerated.

As you push magnification on the planets and the moon the color problems will grow more exaggerated not less. It pretty much doesn't matter what eye pieces are involved the determining factor is the focal length of the eyepieces which is what determines the magnification.

You can develop the habit of two scope observing where you keep one out for the planets, that would be your 70 mm, and use the others for wide field views. That would work pretty well although many of us draw the line at planet observing in 70 mm apertures. Not everyone. There are small aperture cults.

It's been a long time since I've had an ST80 but I seem to recall that for me the color aberration became bothersome at magnifications below one X per millimeter, call it 0.6 or 0.7 X per millimeter. Say 48x to 56x.

Anyhow the color aberration is not just an aesthetic thing. Your telescope gathers light and all the wavelengths of light that it gathers deliver information to your eye which you perceive as details on the object. When one of those colors is unavailable It will show as a blur or haze and that is lost detail. That's what's going on with your 90 mm and your 80 mm too for that matter. That's what the conrady chart is trying to say.

According to that chart, shouldn't the ST80 have the worst image of the three? The CA wasn't really all that bad in the ST80 (I actually didn't see much rainbow color) and none in the Meade 70. I have observed Venus at over 100x with the ST80 and have found that stopping the aperture helps (removing that cap in the middle of the lens cap). 

Edited by Pluto 134340, 03 February 2025 - 11:47 PM.