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518x from 90mm

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#1 BlueMoon

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Posted 21 June 2021 - 08:21 AM

Yepper. 518x (well, 517.5 to be precise) from my 90mm scope.

 

The Math: 621 f/l / (3mm / 2.5x) = 621 / 1.2 = 517.5 then the "mag per inch rule" yields 517.5 / 3.5 (aperture) = 147.8x per inch.

The Players: the Moon, a Megrez 90FD APO, some DeLite EP's and a 2.5x PowerMate.

The Story: Generally, I'm not a planetary guy but I'd been working Cygnus a lot and Cassiopeia was still too low on the horizon for good cluster hunting. The Moon was up and the air was particularly clear and settled albeit with some light moonglow. So, swapping DeLites around I worked my way down to the 3mm, or 207x which, using the 60x per inch rule, should theoretically be close to the Meg's mag limit of 210x. Just for the heck of it, I put the 2.5x PowerMate in the chain and boy, was I amazed! I had a sharp image at 518x. The clarity blew me away. I literally shook my head a few times thinking "this can't be" as I've never hit that level of magnification with any refractor I've owned before. I had been poking around the terminator and the relief and contrast was incredible. I did the magnification calculation several times manually and used these links as well:  https://astronomynow.com/scope-calc/ and https://skyandtelesc...ope-calculator/ (The only difference I can see between them is one uses focal length and the other focal ratio)

Effective Focal Length:  1555mm   (You are using a 2.5 x Barlow lens)
Effective Focal Ratio: f/17.3
Magnification: 518x
True Field of View: 0.1°   (Less than half of the full Moon's disc would fit into the field of view)
Eyepiece Exit Pupil: 0.2 mm
Theoretical Resolving Power: 1.29 arcseconds
Approx. Limiting Magnitude of Telescope (under dark, moonless skies): +12.6

The 0.2 exit pupil should have been a problem but it wasn't. I had plenty of light and while eye placement was a bit more picky than I'm used to with the DeLites, it wasn't bad at all. There is something about the optics in the DeLite series that I can't explain. They are almost magical in their performance.

 

I have a new appreciation of both William Optics and Tele Vue. If I can get it to work, I intend to try getting a projection image using my cell phone in an upcoming night and splitting some tough binaries.

 

Clear skies.


Edited by BlueMoon, 21 June 2021 - 12:34 PM.

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#2 Asbytec

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Posted 21 June 2021 - 09:31 AM

That's interesting the image of the moon, as bright as it is being a little brighter than 3rd magnitude per square arc second (someone check me if incorrect), offered "plenty of light" to be amazing. I've pushed the moon to 0.2mm exit pupil and found the view a bit dark for my eye. We were both operating well above the point image degradation due to aberration would have been apparent. Any aberration present would have been seen by then. So the image would have either been sharp or not, already. That yours remained sharp is not really surprising because it was sharp at somewhat less (maximum?) magnification.

At 0.2mm exit pupil, the afocal image on the eye is like observing the moon at 0.2mm f/100 +/- (20mm focal length/0.2mm aperture) being 518x closer to it. I had "plenty of light", I guess, to still see the moon with no new detail not already seen at lower magnification. As I recall, some detail may have been more difficult to see as my eye began to lose photopic vision. Since we're both well above magnification needed to resolve small scale contrast and detail, being up that high and image degradation tends to be on the eye due to the very small relative aperture.

That was my experience with ludicrous magnification on the moon. Still, it's not uncommon for observers to report amazing observations with very small exit pupils using hi end refractors. I still don't understand how they (you) do it. We're our views similar but described differently, are we just stunned we can see anything at all up that high, are some folks able to endure very small exit pupils and very slow focal ratios, or does throughput somehow allow a brighter image? As I recall, about the best I can do on the moon is 0.3mm exit pupil. That may not sound significantly different from 0.2mm, but the difference is a couple hundred x magnification.

Edited by Asbytec, 21 June 2021 - 10:11 AM.

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#3 BlueMoon

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Posted 21 June 2021 - 09:59 AM

I appreciate your comments. I was completely gobsmacked when this happened. I hope to take some time and quantify the results more by doing some "critical observing" when I can. Noting clarity, sharpness and contrast are fairly general and subjective so I want to pick out some well-known lunar features and study the results of viewing them for comparisons. Splitting some difficult binaries and hunting down some carbon stars, which is more my usual observing habits, should provide some better comparison data as well.

 

This Megrez 90FD APO is "new to me" scope I acquired within the last month so the optical performance was totally unexpected. As a matter of fact, I was swapping DeLites around in sort of a "test" of the scope's optics to get a feel for the 90's capabilities and the Moon was a convenient target. My previous scope, a fine SW 100ED APO with very good optics was unusable with the 3mm, 5mm being the best it would handle, 180x (900mm/5mm) , reliably.

 

Light levels for me are a boon and a bane. I have blue eyes which are fairly sensitive to light levels. I have to wear "wrap around" very dark sunglasses on a bright day, and my prescription glasses have a dark photo-gray coating. On a dark night, I routinely see fainter stars than some of my observing friends (which drives them a bit crazy).

 

Clear skies.


Edited by BlueMoon, 21 June 2021 - 12:36 PM.

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#4 Asbytec

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Posted 21 June 2021 - 10:16 AM

I'd love to see an image, if possible (small exit pupils are difficult in my experience) and if you manage to do so. I wish I had the skill and done so back then. I also realize there are differences from visual, but I expect your image will be at least sharp as an image can be. Yes, those are subjective things, at least visually. But a successful image may give some idea.
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#5 BlueMoon

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Posted 21 June 2021 - 10:22 AM

 

I'd love to see an image, if possible (small exit pupils are difficult in my experience) and if you manage to do so.

I'm going to give it a try. I have a Tele Vue PhoneMate adapter that I've never used. It's designed to work directly with the DeLite series so that's a plus. My cell is a Moto Z Droid Play which doesn't have the best low-light sensor, but I'll give it a try. waytogo.gif I'm curious too to have a look at what the camera can image. 



#6 Asbytec

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Posted 21 June 2021 - 10:42 AM

I have a Mark I shakey hand adapter with outdated patience v1.0. It's over 60 years old. :)

Edited by Asbytec, 21 June 2021 - 10:44 AM.

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#7 BlueMoon

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Posted 21 June 2021 - 12:41 PM

I have a Mark I shakey hand adapter with outdated patience v1.0. It's over 60 years old. smile.gif

I've got the same model and antiquity myself. It's why I bought the PhoneMate. waytogo.gif


Edited by BlueMoon, 21 June 2021 - 12:49 PM.


#8 CHASLX200

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Posted 21 June 2021 - 01:27 PM

I do crazy powers all the time with all my scopes. Super well made scopes in my super steady seeing eat up power. Sloppy optics fall apart fast at higher power. I have no problem using over 100x per inch in all my Taks. I have done 400x on the moon with my Orion 90mm F/10 Achro, but the Taks soaks up the power better. My bigger Zambuto and OMI mirrors have done 700x to 1150x many times from 11" to 14.5".

 

I also notice Maks take crazy powers as well. Had a freaky sharp older 125 ETX do 600x on the moon and a Sky watcher 150mm do the same.  Some objects don't take super high power well in smaller scopes as they get too dim. But the moon and Venus can keep on going. Mars when big and close can also take more power vs Jupiter.

 

I find 350x the max for Jupiter in a 4" ED or APO,and 450x in a 6" ED.   This is why i love bigger well made Newts to get a crazy image of Jupiter on a dead still nite at over 1000x and see all 4 moons as nice sharp little BB's with shading on two of the moons.


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#9 BlueMoon

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Posted 21 June 2021 - 02:05 PM

 

I have no problem using over 100x per inch in all my Taks.

No surprise there I'd say. However, a WO 90FD APO circa 2006 (with coatings and glass from that time) is a bit of an eye-opener. I was happy with the 207x I was getting ... :)

 

Clear skies. 


Edited by BlueMoon, 21 June 2021 - 02:06 PM.

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#10 CHASLX200

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Posted 21 June 2021 - 02:59 PM

No surprise there I'd say. However, a WO 90FD APO circa 2006 (with coatings and glass from that time) is a bit of an eye-opener. I was happy with the 207x I was getting ... smile.gif

 

Clear skies. 

I had one of them and it was very good.


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#11 barbie

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Posted 21 June 2021 - 04:28 PM

My two fluorite apos can do 100x/ inch of aperture under excellent seeing, a rare occurrence for my location. One night last summer, I had my Tak FC76dcu up to 120x/inch on the moon, planets, and a few select close double stars. This was the only time for such a ridiculous magnification in my 50+ years of observing and it's likely to be the only time!!

#12 CHASLX200

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Posted 21 June 2021 - 06:13 PM

My two fluorite apos can do 100x/ inch of aperture under excellent seeing, a rare occurrence for my location. One night last summer, I had my Tak FC76dcu up to 120x/inch on the moon, planets, and a few select close double stars. This was the only time for such a ridiculous magnification in my 50+ years of observing and it's likely to be the only time!!

Come to FL and crank it up.  Love to show ya a planet at 1100x on one of my still nites with a big Newt.  The most i could go on Jupiter with a FS78 was 275x as the image starts to get dim fast past that power. But Venus and the moon can take 350x or more in the FS78.



#13 russell23

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Posted 21 June 2021 - 06:15 PM

Yes - there is no question that when observing the Moon you can push the magnification and exit pupil well below the 0.5mm threshold.  I’ve done 425x with my 102mm scope when seeing allows and 525x with my 120mm.  I managed 850x with the 120mm the same night as I went to 525x, but the seeing did start to falter at 850x.  

 

You don’t actually see more details at those small exit pupils, but you see the Moon at a stunning image scale that makes it feel like you are orbiting it.  If your optics can handle it then it is fun to do that once in a while.  Most of the time I observe the Moon at 130x-170x, but I enjoy ramping up the magnification when seeing allows. 

 

Good for you!


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#14 CHASLX200

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Posted 21 June 2021 - 07:01 PM

Crazy high powers are fun. But stuff floats around and gets dim and floaters and worms move around and so on at super crazy high powers. But it is like you are a mile above the moon.



#15 barbie

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Posted 21 June 2021 - 08:15 PM

I guess I'm lucky, I don't have any floaters!! Anyway, the object of the game for me is to find the magnification that shows the sharpest, most detailed image, not a big blown up one!! Often, that magnification range is from 140x- 200x under my typical seeing.


Edited by barbie, 21 June 2021 - 08:19 PM.

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#16 russell23

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Posted 22 June 2021 - 07:03 AM

I guess I'm lucky, I don't have any floaters!! Anyway, the object of the game for me is to find the magnification that shows the sharpest, most detailed image, not a big blown up one!! Often, that magnification range is from 140x- 200x under my typical seeing.

Northeast Ohio so you probably deal with the same jet stream issues I deal with in upstate NY.  Seeing conditions around here often don’t allow much more than 250x and rarely allow 500x.  That is the way I look at it. On those rare nights when the seeing allows 500x then why not use that magnification?  It may not be the sharpest, but it is sufficiently sharp with a good objective and it is a jaw dropping image scale on the Moon.


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#17 BlueMoon

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Posted 22 June 2021 - 08:42 AM

I went out last night with the same setup and had a hard look at Albireo in Cygnus. The air wasn't as good as it had been the night before, hazy and unsettled, more moonglow too, but it sufficed. The Moon was in some high cirrus so I skipped a test there.

 

Albireo resolved into distinct, but a slightly fuzzy balls of light so the calculated theoretical resolution of 1.29 arcsec was enough it seems. The colors were fairly well pronounced but again, somewhat muted by the haze. Using the Porta II, I was constantly tweaking the axes controls to keep it in view as was expected. All in all, I'm quite pleased at this point but the effects of atmosphere really do make themselves known at the higher mags.

 

When the sky clears up, time to try a few tough doubles.

 

Clear skies.


Edited by BlueMoon, 22 June 2021 - 11:31 PM.


#18 m9x18

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Posted 22 June 2021 - 11:06 PM

Love hearing good things about this scope. The William Optics Megrez 90FD is a great little grab-n-go. Here's a picture of mine.

 

 

Attached Thumbnails

  • 90fd save.JPG

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#19 daquad

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Posted 23 June 2021 - 07:00 PM

Yepper. 518x (well, 517.5 to be precise) from my 90mm scope.

 

The Math: 621 f/l / (3mm / 2.5x) = 621 / 1.2 = 517.5 then the "mag per inch rule" yields 517.5 / 3.5 (aperture) = 147.8x per inch.

The Players: the Moon, a Megrez 90FD APO, some DeLite EP's and a 2.5x PowerMate.

The Story: Generally, I'm not a planetary guy but I'd been working Cygnus a lot and Cassiopeia was still too low on the horizon for good cluster hunting. The Moon was up and the air was particularly clear and settled albeit with some light moonglow. So, swapping DeLites around I worked my way down to the 3mm, or 207x which, using the 60x per inch rule, should theoretically be close to the Meg's mag limit of 210x. Just for the heck of it, I put the 2.5x PowerMate in the chain and boy, was I amazed! I had a sharp image at 518x. The clarity blew me away. I literally shook my head a few times thinking "this can't be" as I've never hit that level of magnification with any refractor I've owned before. I had been poking around the terminator and the relief and contrast was incredible. I did the magnification calculation several times manually and used these links as well:  https://astronomynow.com/scope-calc/ and https://skyandtelesc...ope-calculator/ (The only difference I can see between them is one uses focal length and the other focal ratio)

Effective Focal Length:  1555mm   (You are using a 2.5 x Barlow lens)
Effective Focal Ratio: f/17.3
Magnification: 518x
True Field of View: 0.1°   (Less than half of the full Moon's disc would fit into the field of view)
Eyepiece Exit Pupil: 0.2 mm
Theoretical Resolving Power: 1.29 arcseconds
Approx. Limiting Magnitude of Telescope (under dark, moonless skies): +12.6

The 0.2 exit pupil should have been a problem but it wasn't. I had plenty of light and while eye placement was a bit more picky than I'm used to with the DeLites, it wasn't bad at all. There is something about the optics in the DeLite series that I can't explain. They are almost magical in their performance.

 

I have a new appreciation of both William Optics and Tele Vue. If I can get it to work, I intend to try getting a projection image using my cell phone in an upcoming night and splitting some tough binaries.

 

Clear skies.

You should also have a new appreciation of what excellent seeing can do for observing.

 

Dom Q.


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#20 BlueMoon

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Posted 23 June 2021 - 08:19 PM

 

You should also have a new appreciation of what excellent seeing can do for observing.

I'd have to say I'm delighted that after 50+ years of looking up, I can still be surprised! :)

 

Clear skies.



#21 Asbytec

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Posted 24 June 2021 - 06:32 AM

You should also have a new appreciation of what excellent seeing can do for observing.

Dom Q.


It's a real eye opener. Literally. You really get to see what a scope can do.
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#22 Sol Robbins

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Posted 24 June 2021 - 06:36 AM

I had the same experience with my 80mm Planet Hunter refractor observing the Moon at 562. Some small scopes can pull it off. 


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