Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

25mm, for reverse aperture fever.

  • Please log in to reply
49 replies to this topic

#1 Mr Onions

Mr Onions

    Two Time International Photographical Competition Winner

  • *****
  • topic starter
  • Posts: 6,711
  • Joined: 14 Apr 2007
  • Loc: Newcastle upon Tyne.

Posted 07 October 2018 - 05:42 AM

Thomas, recently mentioned the Borg 25mm telescope.

Last night I decided to try the 25mm aperture for myself on Mizar in Ursa Major and it's companion of about 14" separation.

The telescope used was my giant 66mm WOZS66

The lowest I usually use is 60mm and more recently 40mm.

It was just a case of using cardboard and cutting out a 25mm diameter hole { easier said than done }.

Three pieces of blue tak hold the cardboard mask against the edge of the dew shield.

 

The split was easy but what was surprising was that I could resolve the two components into two beautiful airy discs.

I used a TV3-6mm zoom at the 6mm setting for about 64X.

This was all the more impressive as I was looking through double glazing for inside the house.

When I removed the 25mm cardboard mask, the two stars were all spiked and mushy.

 

So if you can't get out due to bad weather, freezing temperatures or very high winds, it's well worth trying this little experiment.

I love messing around like this.

Once again, thank you Thomas for idea of trying this.

Attached Thumbnails

  • WOZS66.jpg

Edited by Mr Onions, 07 October 2018 - 05:44 AM.

  • Daniel Mounsey, zjc26138, Vesper818 and 13 others like this

#2 gnowellsct

gnowellsct

    Hubble

  • *****
  • Posts: 18,278
  • Joined: 24 Jun 2009

Posted 07 October 2018 - 06:53 AM

25? Why not 10?
  • Daniel Mounsey, Sarkikos and dd61999 like this

#3 Mr Onions

Mr Onions

    Two Time International Photographical Competition Winner

  • *****
  • topic starter
  • Posts: 6,711
  • Joined: 14 Apr 2007
  • Loc: Newcastle upon Tyne.

Posted 07 October 2018 - 08:55 AM

Give me a chance.

It took me a while to cut the 25mm hole.

I will give it a go though.


  • Colin exraaf likes this

#4 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 13,914
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 07 October 2018 - 09:31 AM

As I also mentioned in a PM to Mr. Onions, I made a 25mm cardboard aperture stop to fit my 72mm f/6 ED after reading about the 25mm Borg "Pencil scope". I was curious about how much one could actually see with such a small aperture and the simplest way to do so was simply to stop down an existing scope, preferably of relatively short focal length. 

 

Turns out you can actually see surprisingly much. The Moon shows quite a lot of details, Jupiter shows a belt or two, as well as his moons. I haven't looked at Saturn, but I suspect the ring is visible. Bright double stars, such as Mizar, Gamma Andromedae and Gamma Leonis are fabulously beautiful. Under a dark sky, a surprisingly large number of deep-sky objects are visible. Many more than in a binocular of comparable aperture, thanks to the possibilty of much increased magnification. I suspect all the Messiers should be visible, as well as a good number of the bright NGC. 

 

What I didn't mention was that I've also experimented with a 10mm aperture! I originally intended to try just 7mm, the diameter of the dark adapted eye, but making a 7mm aperture mask with an exacto knife turned out to be surprisingly difficult and I ended up with a 10mm hole. 

 

Obviously, things are faint, even at low magnifications (duh!), but with added magnification of 10x or so (the lowest I could get on the 72mm ED at the time), you can already see MUCH more than with the naked eye alone. It's been a long time, so I don't remember anything else than a surprisingly detailed view of the Moon, showing FAR more details than visible to the naked eye, and that I tried to split Albireo, but failed.

 

I guess it's time to try again. 

 

 

Clear skies!
Thomas, Denmark


  • Mr Onions, Castor, Uwe Pilz and 2 others like this

#5 Mr Onions

Mr Onions

    Two Time International Photographical Competition Winner

  • *****
  • topic starter
  • Posts: 6,711
  • Joined: 14 Apr 2007
  • Loc: Newcastle upon Tyne.

Posted 07 October 2018 - 10:48 AM

I had the opposite experience in making a 10mm hole and coming up with 8mm.

I used a compass to make the 10mm circle and then used the point of the compass to make holes around the circumference.

It came out at 8mm.

I just tried this this afternoon.

Yes it's very dim but I will try it out on moon when next in view.

 

After using 8mm a 25mm aperture is like the 60" on Mount Wilson!


  • Castor, Astrojensen, Rick-T137 and 1 other like this

#6 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 13,914
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 07 October 2018 - 12:45 PM

After using 8mm a 25mm aperture is like the 60" on Mount Wilson!

grin.gif That was my experience as well. 

 

 

Clear skies!

Thomas, Denmark


  • Mr Onions and andycknight like this

#7 Alan French

Alan French

    Night Owl

  • *****
  • Posts: 6,124
  • Joined: 28 Jan 2005
  • Loc: Upstate NY

Posted 07 October 2018 - 01:37 PM

When you get to such small apertures, you're limiting useful magnification considerably. An 8mm aperture gives a 1mm exit pupil at 8 power. At 16 power you're down to a 0.5mm exit pupil. 

 

Clear skies, Alan



#8 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 13,914
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 07 October 2018 - 01:43 PM

When you get to such small apertures, you're limiting useful magnification considerably. An 8mm aperture gives a 1mm exit pupil at 8 power. At 16 power you're down to a 0.5mm exit pupil. 

 

Clear skies, Alan

That goes without saying. The reason I personally wanted to experiment with it was to find out what would be possible to see, if the human eye was diffraction limited and also operated at that level. I can tell you that if it did, lunar craters would have been known since the dawn of time. No telescope would have been needed to see them.  

 

 

Clear skies!
Thomas, Denmark


  • Castor, daquad, Axunator and 1 other like this

#9 Mr Onions

Mr Onions

    Two Time International Photographical Competition Winner

  • *****
  • topic starter
  • Posts: 6,711
  • Joined: 14 Apr 2007
  • Loc: Newcastle upon Tyne.

Posted 07 October 2018 - 02:35 PM

Hopefully I'll get a clear night later this coming week to see what I can see on the first quarter moon.

I'm going for 3 plato craterlets.


  • Astrojensen likes this

#10 Rick-T137

Rick-T137

    Apollo

  • *****
  • Posts: 1,376
  • Joined: 28 Feb 2015
  • Loc: Canada

Posted 07 October 2018 - 02:47 PM

Hmmmm... for those of you old enough to remember the TV Show "The Six Million Dollar Man", he had a 20.2:1 zoom in his bionic left eye. It would be interesting to put a 7mm aperture mask on and put the power at 20X and see what is visible. Maybe Steve Austin (the Colonel, not the wrestler) was an amateur astronomer! :)


  • Mr Onions likes this

#11 25585

25585

    Voyager 1

  • *****
  • Posts: 12,312
  • Joined: 29 Aug 2017
  • Loc: In a valley, in the UK. Dark end of the street.

Posted 07 October 2018 - 04:15 PM

With a camera lens that has a large enough inner tube diameter, and which has an aperture ring that can be stopped down manually, you have a versatile small telescope.


Edited by 25585, 07 October 2018 - 04:16 PM.

  • Astrojensen likes this

#12 Sketcher

Sketcher

    Surveyor 1

  • *****
  • Posts: 1,594
  • Joined: 29 Jun 2017
  • Loc: Under Earth's Sky

Posted 07 October 2018 - 04:40 PM

I often "play around with" small apertures.  I have 1-inch (25.4mm) masks for three of my refractors.  For others, I have masks from 10mm through full aperture, at 10mm increments.

 

For one 'scope, I often use a 1/2-inch mask.  That telescope is a 42mm 'singlet' refractor.  I've seen galaxies, nebulae, star clusters, and double stars with the 1/2-inch aperture.  My notes are too disorganized to regularly go back to, but I did start a posting a while back on CN sharing some of the 1/2-inch observations . . .

 

More recently I posted a (daytime) Venus observation using a 1-inch aperture.  The thin crescent was very nicely defined at "high" power.

 

Some of the reasons for using/experimenting with small apertures:  One can see far more than most would guess (or imagine).  Some key observers from the past made outstanding use of small apertures.  Most modern instruments, when stopped down to apertures in the 25mm range, perform very much like an optically 'perfect' telescope -- zero CA, perfect airy disks, etc.  Seeing conditions are often found to be 'perfect' for such apertures.  F-ratios tend to be 'long', so even the cheapest of modern eyepiece will perform wonderfully.

 

Sadly though, aperture-fever sometimes calls for me.  During my last observing session I used a Galileoscope (47mm clear aperture) at full aperture!


  • Vesper818, Mr Onions, Castor and 4 others like this

#13 daquad

daquad

    Surveyor 1

  • *****
  • Posts: 1,918
  • Joined: 14 May 2008

Posted 08 October 2018 - 10:54 AM

That goes without saying. The reason I personally wanted to experiment with it was to find out what would be possible to see, if the human eye was diffraction limited and also operated at that level. I can tell you that if it did, lunar craters would have been known since the dawn of time. No telescope would have been needed to see them.  

 

 

Clear skies!
Thomas, Denmark

By "...at that level..." I assume you are talking about the level of magnification.  Early people without telescopes did not have the advantage of magnification.  So, since the human eye is diffraction limited at 1 - 2 mm exit pupils, we should look at the moon without a telescope, but through a 1 - 2 mm aperture, essentially stopping down the iris opening.  (Make a hole in a thin piece of cardboard with a sharpened pencil to the desired diameter.) Would craters then be "readily" visible?  

 

Let's assume the eye peering through such an aperture has a resolving power of 2 minutes of arc, about I/15th the diameter of the full moon.  Seems to me the larger craters should easily be visible.  I'm going to try it as soon as I get some clear skies.  It's been a month now without them.  

 

Should be a fun experiment.  If someone has already tried it, I would like to hear about the results.

 

Dom Q.


Edited by daquad, 08 October 2018 - 10:57 AM.


#14 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 13,914
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 08 October 2018 - 01:01 PM

By "...at that level..." I assume you are talking about the level of magnification.  Early people without telescopes did not have the advantage of magnification.  So, since the human eye is diffraction limited at 1 - 2 mm exit pupils, we should look at the moon without a telescope, but through a 1 - 2 mm aperture, essentially stopping down the iris opening.  (Make a hole in a thin piece of cardboard with a sharpened pencil to the desired diameter.) Would craters then be "readily" visible?  

 

Let's assume the eye peering through such an aperture has a resolving power of 2 minutes of arc, about I/15th the diameter of the full moon.  Seems to me the larger craters should easily be visible.  I'm going to try it as soon as I get some clear skies.  It's been a month now without them.  

 

Should be a fun experiment.  If someone has already tried it, I would like to hear about the results.

 

Dom Q.

No, craters at 1x magnification and 60" resolution (2mm aperture) aren't readily visible. I know, because I've looked for them. Some of them, notably Clavius, when its floor is still in shadow, can be seen as a tiny "bite" in the terminator, but it isn't readily identifiable as a crater. The three bright patches of ejecta rays from Copernicus, Aristarchus and Kepler are easily visible, but they're not craters per se. 

 

The naked eye at 7mm aperture has a theoretical resolution of 17" and that is actually good enough to recognize some craters as such, but the receptors in our eye aren't small enough to resolve details that small and the optical quality of the human eye at full aperture usually leaves much to be desired.  

 

 

Clear skies!

Thomas, Denmark


  • Castor, daquad and Axunator like this

#15 Mike Spooner

Mike Spooner

    Vendor (mirrors)

  • *****
  • Vendors
  • Posts: 796
  • Joined: 06 Aug 2010

Posted 08 October 2018 - 01:03 PM

I've seen a couple of different sizes of paper punches available for ring binders that would provide small apertures. Might be fun to see the effects of different patterns in front of apertures on both day and night time viewing (vertical, horizontal, spiral or random for contrast and resolution effects, etc.). For the recently retired and really bored it would be a definite CN posting project to present a photo thread showing a scientific result. Just instigating as I have plenty of projects (and a personal project manager). wink.gif

Best,
Mike Spooner
  • Astrojensen likes this

#16 Axunator

Axunator

    Viking 1

  • *****
  • Posts: 708
  • Joined: 23 May 2015
  • Loc: Helsinki, Finland

Posted 08 October 2018 - 01:21 PM

 

The naked eye at 7mm aperture has a theoretical resolution of 17" and that is actually good enough to recognize some craters as such, but the receptors in our eye aren't small enough to resolve details that small and the optical quality of the human eye at full aperture usually leaves much to be desired.  

 

I like the idea and the theoretical thinking behind these experiments, but is it physiologically even possible to look at the moon with a naked eye with a pupil dilated to 7 mm (without some pharmacological aid, mainly anticholinergic eyedrops)? Even at night, wouldn't the brightness of the moon make the pupil shrink? Haven't tested this myself, though...



#17 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 13,914
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 08 October 2018 - 01:44 PM

I like the idea and the theoretical thinking behind these experiments, but is it physiologically even possible to look at the moon with a naked eye with a pupil dilated to 7 mm (without some pharmacological aid, mainly anticholinergic eyedrops)? Even at night, wouldn't the brightness of the moon make the pupil shrink? Haven't tested this myself, though...

I think the Moon would be bright enough to make the pupil shrink to ~2 - 3mm, depending on the phase. It's more of a "what if?" kind of experiment. What would we be able to see IF our eyes were diffraction limited at 7mm aperture and IF the receptors in our eyes were small enough to take advantage of it?

 

Could we see craters on the Moon? Would all four Jovian satellites be visible? Could we split Albireo with the naked eye? 

 

There's lots of fun things to try out!

 

 

Clear skies!
Thomas, Denmark



#18 Paul Hyndman

Paul Hyndman

    Apollo

  • -----
  • Posts: 1,312
  • Joined: 13 Jul 2004
  • Loc: Connecticut Shoreline USA

Posted 08 October 2018 - 02:01 PM

… What I didn't mention was that I've also experimented with a 10mm aperture! I originally intended to try just 7mm, the diameter of the dark adapted eye, but making a 7mm aperture mask with an exacto knife turned out to be surprisingly difficult and I ended up with a 10mm hole. ..

 A wood backer can be used to drill cleanly through the mask material using an appropriate sized bit (woodworking trick to prevent tear out).



#19 Sasa

Sasa

    Mercury-Atlas

  • *****
  • Posts: 2,580
  • Joined: 03 Nov 2010
  • Loc: Ricany, Czech Republic

Posted 08 October 2018 - 04:55 PM

Both diameter (7mm and 25mm) bring back the memories of my first observations. These were done with plastic ~7mm lens from Optic Cabinet (the red one on the top):

 

https://thumbs.img-s...z-348918137.jpg

 

Here is a sketch I made through it, apparently I could go down to 6.5 magnitude with it...

 

19881008_Pleiady.jpg

 

Later on, in secondary school in the hostel, my roomate had Russian monocular 20x30

 

https://www.picclick...20-X-30-_57.jpg

 

With this one I measured Jupiter's density.


  • Castor, Astrojensen and Rick-T137 like this

#20 Sky Muse

Sky Muse

    Cosmos

  • *****
  • Posts: 8,167
  • Joined: 26 Oct 2012
  • Loc: Mid-South, U.S.

Posted 08 October 2018 - 11:14 PM

25? Why not 10?

See?  There's considerable merit to be had with your 60mm f/11 achromat; and there it was all along, under-foot.


  • Mr Onions, Castor and Astrojensen like this

#21 Mr Onions

Mr Onions

    Two Time International Photographical Competition Winner

  • *****
  • topic starter
  • Posts: 6,711
  • Joined: 14 Apr 2007
  • Loc: Newcastle upon Tyne.

Posted 09 October 2018 - 02:28 AM

The main point I was trying to get over in the first place was that you can gain beautiful images of double stars *through dirty double glazing* at 25mm aperture.

I might put a 25mm mask over the 60/1000 for f/40.


  • Castor likes this

#22 BillP

BillP

    ISS

  • *****
  • Posts: 21,489
  • Joined: 26 Nov 2006
  • Loc: Spotsylvania, VA

Posted 09 October 2018 - 07:33 AM

Doubles can be quite nice at small apertures - https://www.cloudyni...diet/?p=8833872


  • Mr Onions and Castor like this

#23 Mr Onions

Mr Onions

    Two Time International Photographical Competition Winner

  • *****
  • topic starter
  • Posts: 6,711
  • Joined: 14 Apr 2007
  • Loc: Newcastle upon Tyne.

Posted 09 October 2018 - 08:08 AM

Good write up that.

I've found the 25mm mask makes a 3 on pickering scale into a 9.

I was actually binoviewing this morning at 60X with the 25mm mask on with very pleasing results.

Can't wait to try that on the moon.


  • Castor likes this

#24 highfnum

highfnum

    Fly Me to the Moon

  • *****
  • Posts: 5,170
  • Joined: 06 Sep 2006
  • Loc: NE USA

Posted 09 October 2018 - 10:06 AM

these small aperture have been around for a while

1790 P Dollond  25.4mm triplet

about F/7 which was fast for that time

and moon with it and s8 camera

 

doll1frnt.jpg

pdollond1moonc.jpg

 

 


  • Paul Hyndman, Mr Onions, Castor and 2 others like this

#25 Mr Onions

Mr Onions

    Two Time International Photographical Competition Winner

  • *****
  • topic starter
  • Posts: 6,711
  • Joined: 14 Apr 2007
  • Loc: Newcastle upon Tyne.

Posted 09 October 2018 - 11:28 AM

What a find that is.

Thanks for posting.


  • highfnum likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics