Gene Baraff
sage
Reged: 03/22/09
Posts: 245
Loc: Berkeley Heights, N.J.
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Quote:
OK,
Using vee blocks and blue tack I then set it up with the achromat at 1000mm from the objective.
I used a movie camera eyepiece (about 35mm focal length) and focussed up on my sharpness indicator chart at about 45 feet away...
Considering that collimation was rough at best the image was very bright, sharp and clean. CA looked minimal.
Would you be willing to compare the images you got with the long focal length lens in place - with the images you get with everyting else the same but without the front lens? You'd have to adjust the spacing to achieve focus, but do the comparison after you have made that adjustment. Without the front lens, your image scale would be larger of course, but I'm curious about the sharpness and color correction of the image, with and without the front lens.
The effect of that front lens, as far as I can tell, is to up the aperture, but in your case, you have stopped it back down so that there is not much aperture enhancement.
Thanks in advance.
Gene Baraff
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nytecam
Postmaster
Reged: 08/20/05
Posts: 5741
Loc: London UK
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Hi John - if you click edit button on YOUR posts - there's a delete button [at bottom of page] to remove your duplicate posts - but only for a day or so at most then they get locked.
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Nytecam 51N 0.1W
Meade 30cm LX200+ETX-70+e-finder+C8+Ha+CaK PSTs SBIG SGS+homebuilt spectrographs
Starlight SXVF_M9+Lodestar CCDs/Canon 300D DSLR/Fuji E550
My observatory build-ETX-70 imaging-my videos
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Andy Taylor
twistin' by the pool
Reged: 09/24/08
Posts: 431
Loc: t=0 UK
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Quote:
Quote:
OK,
Using vee blocks and blue tack I then set it up with the achromat at 1000mm from the objective.
I used a movie camera eyepiece (about 35mm focal length) and focussed up on my sharpness indicator chart at about 45 feet away...
Considering that collimation was rough at best the image was very bright, sharp and clean. CA looked minimal.
Would you be willing to compare the images you got with the long focal length lens in place - with the images you get with everyting else the same but without the front lens? You'd have to adjust the spacing to achieve focus, but do the comparison after you have made that adjustment. Without the front lens, your image scale would be larger of course, but I'm curious about the sharpness and color correction of the image, with and without the front lens.
The effect of that front lens, as far as I can tell, is to up the aperture, but in your case, you have stopped it back down so that there is not much aperture enhancement.
Thanks in advance.
Gene Baraff
Last post before packing up for the day...
Without the 1st lens everything still looks good, as you say - with a different scale.
Hard to say with a 25mm F7 mini refractor with a 1500 dollar eyepiece on the end.. 
I also tried just the 1st lens and the eyepiece - yuk.
The setup I have here is (I suppose) a half scale mock up.
Other questions/suggestions I'll try to deal with on Monday...
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Equipment list of shame:
A strange heap of assorted junk that when thrown together and dragged out into the dark shows me the wonders of the universe...
And then dews up...
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KI4YUN
sage
Reged: 04/21/08
Posts: 490
Loc: Satsuma, Florida, USA
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I must say that this design seems very promising on view of my wallet. I can't wait to possibly try out this design.
So does this mean that I could just get a 6" lens like this from http://www.surplusshed.com/pages/item/l3919d.html $25 for the 1st lens. Now with this lens being larger than what has been talked about, would it then be best to use an 80mm objective http://www.surplusshed.com/pages/item/l3911.html $32, instead of lets say a 50mm binocular objective? After all we wouldn't want vignetting.
Would be something to think about. I am just thinking and don't know how to use the ray tracing programs *got confused* so am not sure if it would work.
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-Tristan
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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K14YUN. Hi there, sounds good what you are about to do, now you must observe the F:40 rule for the main lens, you can drop it to F:35 but no more, and yes use an 80mm lens, so if you do your sums right, you will find that you can move the reducing lens closer to the primary, this will suppress the lateral chromatic aberration still further and shorten the scope. Now do not have a long focus reduction lens, it should be about the same F:No as for a bino OG not a refractor. the final configuration should not be more than F:6 but F:3 would be ideal, if there is two much color in the system then resort to the duplex version. Re the optical diagram, a 7X50 at half the back focus (BF) distance will give a much more corrected field and low color. A tip for you guys out there, try the scope out on terrestrial objects with plenty of detail, you will see clearly the residual aberrations in the system.
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grendel
sage
Reged: 04/12/09
Posts: 240
Loc: Canterbury, Kent, UK
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would an etx 80 lens work here, its about f5, but I have a spare one that I might be able to use with a surplusshed 6" lens. I love the idea though and its something I might well try - maybe surplushed could be persuaded to make a batch of 150mm f40 lenses if we got together enough requests?
Grendel
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Charl
member
Reged: 02/08/07
Posts: 84
Loc: Hants, UK
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I must admit that with only limited knowledge of optics, I found the idea confusing at first. Your supplied diagram has made the concept clear to me though. In effect you have an achromatic system with slightly converging rays entering the system in place of parallel rays. It would be interesting to see if figuring of the achromatic lens can further improve the quality. Maybe we'll finally find a good use for projector lenses!
Keep up the good work and thanks for sharing it with us. I will be following this thread with great interest.
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Gene Baraff
sage
Reged: 03/22/09
Posts: 245
Loc: Berkeley Heights, N.J.
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Quote:
I must admit that with only limited knowledge of optics, I found the idea confusing at first. Your supplied diagram has made the concept clear to me though. In effect you have an achromatic system with slightly converging rays entering the system in place of parallel rays. It would be interesting to see if figuring of the achromatic lens can further improve the quality. Maybe we'll finally find a good use for projector lenses!
Keep up the good work and thanks for sharing it with us. I will be following this thread with great interest.
Yes!!!!!
I had exactly the same thought. It was for that reason that I had asked Andy Taylor to report back on the difference between views he got using the original concept and views without the big lens in front.
The achromat and the eyepiece together constitute a stand alone scope. It has a certain image scale, aperture, and also has the various aberrations that experts know how to calculate and observe - spherical aberration, longitudinal chromatic abberation, lateral C.A., off axis coma, astigmatism,... whatever.
Place a large but weak lens way out in front of your scope. Clearly you end up with a scope having a slightly smaller image scale and (assuming that the diamter of the achromat is large enough) a much enhanced aperture.
The question then is "what is the effect of that added lens on the errors inherent in the back scope"?
I was about to ask David G. if he would be willing to redo his ray trace for the back scope alone, much as I had asked Andy to observe through the back scope alone. Then I saw your post which captures the concept far more concic=sely than I would have been able to.
David: Are you still willing to run the numbers? If you are, could you interpret them for guys like me that are ignorant of what they mean?
The bottom line, I suspect, is how much gain you get from the extra aperture relative to the (I am guessing here) the slightly greater error relative to that of the back scope alone.
Gene Baraff
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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Charl, Gene:
I feel similarly limited in my knowledge of optics, but I was wondering the same thing myself.
And one more: If the front lens were a doublet, would the system be a petzval? Or do the lenses in a petzval differ enough from a pair of achromats that simply putting two achromats together doesn't make a petzval?
-Tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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As per Gene's request, I used OSLO to look at a Parachromat system made with a 100mm f/40 plano convex lens and Melles Griot 75mmx 150mm achromat. The achromat was place 2000mm from the objective.
Here is the result for that system. If one looks at the Long. Spherical Abberation graph, this tells were the colors come to focus. I had OSLO focus the system for green light. Red focus long by 0.25mm and blue short by 0.37mm.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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Here is the 75mm Achromat by itself. Red and blue focus about equally on each side of green by 0.25mm. Also note that the red,green and blue curves do cross each other so one can refocus the image to achieve a better image. With the parachromat there is no crossing of the curves.
- Dave
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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To get a feel for the performance of this system I compared the spot size it produces vs a 4" f/15 achromat of the Baker design that uses common BK-7 and F2 flint. Note that the black circle is the size of the airy disk.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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GJJim
scholastic sledgehammer
Reged: 09/09/06
Posts: 907
Loc: Western CO
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Seems to me the Parachromat is roughly equivalent to using a binocular objective and looking through a wavy piece of plate glass.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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Hey Guys, compute an objective lens that has a low dispersion glass such as Schotts FK54 Nd 1.437 Vd 90.7 you just might reduce the LCA to a minimum, for the objective lens. I might be right or wrong, are some opthalmic glasses with a Nd 1.9 having a very low dispersion available, this would give very shallow front curve and the low dispersion would have very little prism effect at the edge of the OG reducing the LCA to vanishingly small amount. I found that its LCA in the Hypo is the big problem, I think Corning make it.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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GJJJim. Hi. Build a Hypo (Para has been dropped), and you will find that the scope is far from that, the image is crisp and bright with a little color at the edges of the images, thats all,remember an important point, we are talking a cheap and very simple refractor here, and not a high performance APO!
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Gene Baraff
sage
Reged: 03/22/09
Posts: 245
Loc: Berkeley Heights, N.J.
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Quote:
As per Gene's request, I used OSLO to look at a Parachromat system made with a 100mm f/40 plano convex lens and Melles Griot 75mmx 150mm achromat.
- Dave
Thank you, both for the calculation and for the interpretation. The issue (I still think) is whether the loss of image perfection is a price that those who WANT the extra aperture without bankrupting themselves will be willing to pay.
It does call into question one of the assumptions of the original posting: Although one does get greater light throughput from the larger aperture, it is not clear that one gets the extra resolution as well. I say "not clear" because I am not skilled enough to tell, from Dave G.'s work, what the effect on the overall resolution is. I get the feeling that it is even worse than the achromat alone would have beeen, but that is only an impression - unsuported by any knowledge on my part.
Comments?
Gene
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Dave, Gene - I'm sure my understanding is less than Gene's, probably by a good deal, but I think I'm wondering the same thing. Leaving aside color, isn't resolution partly a function of contrast, and if the light energy is so dispersed the contrast must be quite poor - and wouldn't that pretty much negate the benefit of the larger aperture? Or am I getting it mixed up?
Gary
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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Try this for size folks. Taken through a five inch F:40 OG using a 70mm bino lens at half the focus.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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This about what you see visually with a 15mm Plossl, seeing is believing. House 55m away.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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A night shot of a christmas tree at 55m
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