Jump to content


Photo

Focal Reducer for Video SCT SCopes down to F/1.5

  • This topic is locked This topic is locked
118 replies to this topic

#26 Dwight J

Dwight J

    Apollo

  • *****
  • Posts: 1012
  • Joined: 14 May 2009
  • Loc: Lethbridge, Alberta, Canada

Posted 03 April 2013 - 11:59 AM

:roflmao: :roflmao: :roflmao:
 

#27 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10669
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 03 April 2013 - 12:39 PM

When might we look forward to purchasing one, then?
 

#28 budman1961

budman1961

    Apollo

  • *****
  • Posts: 1108
  • Joined: 25 Feb 2011
  • Loc: Springfield, MO

Posted 03 April 2013 - 01:23 PM

Vaporware
 

#29 Devon

Devon

    Sputnik

  • -----
  • Posts: 42
  • Joined: 06 Mar 2010
  • Loc: OK

Posted 03 April 2013 - 02:48 PM

Is BSware a term...?
 

#30 Chris A

Chris A

    Apollo

  • -----
  • Posts: 1139
  • Joined: 03 Feb 2007
  • Loc: Toronto, Canada

Posted 03 April 2013 - 03:04 PM

Glenn, Andy, Dwight and Devon. This is known (M42 Image Matt posted)as Photoshop tricks and I could have easily fixed the stars right out to the very edge and remove the vignetting LOL. This is why I do not like to judge a camera from posted images and prefer see live with my own eyes a broadcast on NSN!!

Chris A
Astrogate
 

#31 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10669
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 03 April 2013 - 05:23 PM

Chris,
If embellishment, adornment or even fakery is intended, then it's a *lot* simpler to substitute an image taken with even a rather different rig but which has the image scale of the intended 'simulated' view...

Matt,
If you wish not to have such speculation continue, then at least let us know if you're indeed developing (or helping to develop) something which eventually will be available commercially. To drop a 'teaser' and then clam up with the "not for the public domain" brush-off is not consistent with a 'Skunk Works' project for which you have not oversight or signing authority. And so it stands to reason that you're the one answerable, and can only do yourself a favor by providing even a smidgen of supportable clarification.

And what about the state of the camera you also 'teased' us with a little while back? Is it to be considered in the same category?
 

#32 budman1961

budman1961

    Apollo

  • *****
  • Posts: 1108
  • Joined: 25 Feb 2011
  • Loc: Springfield, MO

Posted 03 April 2013 - 06:51 PM

Both the FR and camera are made of a rare-earth material.....Unobtanium!

lol

Andy
 

#33 jimthompson

jimthompson

    Vostok 1

  • -----
  • Posts: 102
  • Joined: 06 Oct 2009
  • Loc: Ottawa, Canada

Posted 03 April 2013 - 08:13 PM

:roflmao: You are my hero of the day Glenn.

While we're all gathered here today, I wonder Glenn if you can help me to understand something that I'm unclear on regarding focal reduction or extension. How does the distance from the primary to focus plane change with reduction or extension ratio? I'm guessing that will depend on the relative position of the reducer/telextender in the light cone. Is it correct to assume that the light cone between reducer/telextender and camera will be the same solid angle as a cone subtended by the primary's aperture with height equal to the effective focal length? Or is the cone angle of light exiting the reducer/extender fixed by that element's focal length and only the length/diameter varies? Any insight you can provide would be appreciated.

Best Regards,

Jim Thompson
AbbeyRoadObservatory
 

#34 ccs_hello

ccs_hello

    Fly Me to the Moon

  • *****
  • Posts: 6454
  • Joined: 03 Jul 2004

Posted 03 April 2013 - 09:09 PM

For 0.2x:
Not completely impossible from simple calculation.

It's going to be hard to create a well-corrected fl=70mm and dia = 39mm lens group. Type-1/3" CCD is used in the illustration.

Also note that the calculation is based on the OTA itself can offer a huge image circle to begin with. In real life, it is not. Focal Reducer cannot "fetch" light which is not offered by its feeder (the OTA) to begin with.

P.S. it's also OTA design dependent. In my example, the FR lens assembly has to go VERY far into the inside of the OTA tube (28% from the OTA's original primary focal plane) !!! Not many OTA can support that.

See attached calculation:

Attached Files


 

#35 ccs_hello

ccs_hello

    Fly Me to the Moon

  • *****
  • Posts: 6454
  • Joined: 03 Jul 2004

Posted 03 April 2013 - 09:15 PM

let's try 0.15x:
Not completely impossible from simple calculation.

It's going to be hard to create a well-corrected fl=60mm and dia = 39mm lens group.

P.S. it's also OTA design dependent. In my example, the FR lens assembly has to go VERY far into the inside of the OTA tube (34% from the OTA's original primary focal plane) !!! Not many OTA can support that.
It's parameter "D", in line 36.

See attached calculation:

Attached Files


 

#36 ccs_hello

ccs_hello

    Fly Me to the Moon

  • *****
  • Posts: 6454
  • Joined: 03 Jul 2004

Posted 03 April 2013 - 09:22 PM

Jim,

See links in post #1 and #9 in this CN thread.

Clear Skies!

ccs_hello
 

#37 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10669
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 04 April 2013 - 03:07 AM

Jim,
Good ol' ccs beat me to it, and good thing, too, because I don't have the spreadsheet he's used. I was going to fall back on the basic formulae.

To summarize the particulars for the 0.15X reducer, as calculated by ccs_hello above for a 100mm f/10 scope.

The reducer must itself have a pretty fast f/ratio. The 'recipe' here is for the reducer being a 39mm f/1.54. (Off-the-shelf reducers are typically f/2.5-4.)

Compared to the 1,000mm scope focal length, the reducer must be located fully 340mm inside the f/10 focus position. For a Cat of only 100mm aperture, that would put it essentially in contact with the secondary mirror, which is of course a ridiculous location.

The camera's chip has to be only 51mm behind the reducer, placing it also near the secondary.

Finally, no more than 20% of he chip is fully illuminated, and so vignetting will be notable.

If employing this reducer on other f/10 scopes, the distances the reducer and detector must be moved forward of the 'native' f/10 focus will be essentially identical. Meaning they will certainly be located well up in the primary baffle for even large SCTs.

Now, if one wanted to get really sophisticated, transfer lenses could be installed inside the primary baffle so as to place the 'native' f/10 focus sufficiently far outside the back end so that the reducer could be located at or just behind the exit aperture of the primary baffle. But that's getting quite complicated!


To answer your more general questions.

The light cone behind the reducer has the solid angle appropriate to the final, working f/ratio.

For a given reducer, as the reduction factor is increased:
- The reducer moves closer to the objective.
- The detector moves back farther behind the reducer, but
- The detector does also move toward the objective, just a little more slowly than does the reducer.

In order to not suffer more inward placement than necessary, the reducer must be made as small as possible, commensurate with the detector's size. And the faster the desired working f/ratio at the detector, the faster must be the reducer itself.

To first order, the reducer's clear aperture should not be less than

1 / reduction factor * the detector width.

(At least it holds fairly well for 'simple' reducers; compound reducers (of which the two-piece MFR-5 is a good example) may well introduce some complications.)

For example, a 0.5X reducer used with a 25mm wide CCD should have a clear aperture of

1 / 0.5 * 25 = 50mm

This will keep vignetting to a reasonable level.
 

#38 nytecam

nytecam

    Voyager 1

  • *****
  • Posts: 11462
  • Joined: 20 Aug 2005
  • Loc: London UK

Posted 04 April 2013 - 03:25 PM

Just a thought but anyone considered/ tried John Wall's Hyperchromatic Refractor featured on the ATM Forum last April? Effectively it's a stacked long and short focus refractors giving a fast 'speed' but light collecting power of the large front objective which can be a simple[?] plano-convex lens with apparently minimal CA. May help our light-hungry video cams :rainbow:
 

#39 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10669
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 04 April 2013 - 04:54 PM

Maurice,
Wall's design you point to is a visual scope, primarily. It's speed is determined by the doublet objective (which works exactly like a focal reducer) which in the end gets no faster than about f/3. Moreover, there is significant lateral color present, due to the big singlet lens up front.

I think that in this incarnation it's a dead end as a fast imaging system.
 

#40 nytecam

nytecam

    Voyager 1

  • *****
  • Posts: 11462
  • Joined: 20 Aug 2005
  • Loc: London UK

Posted 05 April 2013 - 03:11 AM

Maurice, Wall's design you point to is a visual scope, primarily. It's speed is determined by the doublet objective (which works exactly like a focal reducer) which in the end gets no faster than about f/3. Moreover, there is significant lateral color present, due to the big singlet lens up front. I think that in this incarnation it's a dead end as a fast imaging system.

Glenn - I read your largely negative contribution to that ATM thread but to be fair you weren't alone in that. However like much in this field it's best-to-test rather than rely on theory alone via raytracing or what is often idle internet comment.

Outside the lab effects of seeing, tracking, pixel scale etc cause some inevitable relaxing of standards below perfection. I delayed my Littrow spectroscope and Lensless Schmidt camera because I saw told "it wouldn't work" but both worked just fine. I'd give the doers credit before any theorist. :grin:

I just thought that astrovideographers hereabouts, who I suspect are not all theoretical perfectionists, would benefit from the Hyper article. :rainbow:
 

#41 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10669
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 05 April 2013 - 04:23 PM

Maurice,
My comments on Wall's Hypo design were based on both the data and images supplied by Jon himself. He clarified that the aberrations limited it to a lowest power visual instrument.

Why make up a long refractor whose light collector is an f/40 singlet, followed by binocular objective acting as a focal reducer, delivering only an f/3 image plagued by wicked lateral color and still-significant longitudinal color?

FAR better optically, readily available, and much more compact; a cheap f/4 Newt with a $30 0.5-0.7X reducer. Or an inexpensive f/5 achromat with the same reducer at 0.5X.
 

#42 mattflastro

mattflastro

    Vendor - Astrovideo Systems

  • -----
  • Posts: 622
  • Joined: 31 Jul 2009
  • Loc: Brevard County , FL

Posted 15 April 2013 - 03:53 AM

Chris,
If embellishment, adornment or even fakery is intended, then it's a *lot* simpler to substitute an image taken with even a rather different rig but which has the image scale of the intended 'simulated' view...

Matt,
If you wish not to have such speculation continue, then at least let us know if you're indeed developing (or helping to develop) something which eventually will be available commercially. To drop a 'teaser' and then clam up with the "not for the public domain" brush-off is not consistent with a 'Skunk Works' project for which you have not oversight or signing authority. And so it stands to reason that you're the one answerable, and can only do yourself a favor by providing even a smidgen of supportable clarification.

And what about the state of the camera you also 'teased' us with a little while back? Is it to be considered in the same category?


Any of the group experts or anyone from this group for that matter, if you happen to be in or travel to south east Florida, I'd be happy to meet and demonstrate one evening the reducer and camera.
Otherwise I'll let you know when production starts and more info will then be available .
 

#43 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10669
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 15 April 2013 - 04:32 PM

If you're willing to demo in person, then why not a *little* detail here, sufficient to allay doubts yet not compromise any proprietary aspects?
 

#44 Lorence

Lorence

    Viking 1

  • -----
  • Posts: 844
  • Joined: 15 Sep 2008

Posted 16 April 2013 - 01:25 PM

Any of the group experts or anyone from this group for that matter, if you happen to be in or travel to south east Florida, I'd be happy to meet and demonstrate one evening the reducer and camera. Otherwise I'll let you know when production starts and more info will then be available .


As I'm not expert enough to speculate about equipment, I'll just buy it. You win some and you loose some but at least you end up knowing. That beats the alternative and I don't have to spend all day on the forums.

I would like to preorder one of your cameras and focal reducers. I used the term "like" simply because you haven't quite achieved Rock's reputation in the astro video camera business yet.

I already have a pile of cheap cameras for Lunar and planetary so I'm sure you can understand I am only in the market for high end products.

If your camera is in the Mallincam Xtreme or Universe class my order stands. Otherwise I wish you the best of luck with this camera, hopefully you will be successful enough with it to move on to bigger and better cameras. If it is a low end class camera just keep the price down. It seems that is the main criteria for camera selection by many.
 

#45 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10669
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 16 April 2013 - 10:59 PM

I would be hard pressed to refrain from countering the nay-saying of 'forum experts' (like myself :grin:) if I had found a way to achieve the Holy Grail of taking an f/10 system down to f/1.5 with a back-end focal reducer.

That a hobbyist can cobble up from existing lenses such an astounding optical wonder which seems to be beyond the capability of professional designers to offer (at anything like a non-astronomical price, if achievable) would be cause to rightfully crow to high heaven and bask in the ensuing avalanche of plaudits and laurels.

My feeling is that it might well be the case that the *image scale* reduction is occurring, but virtually certainly not at full system aperture. For example, a 10" SCT might be working at nearer to 6" aperture. This notwithstanding, the lack of apparent vignetting and any notable off-axis aberration, for such aggressive reduction, raises a big red flag.

I wonder if the device is actually an afocal imaging system, which would employ a fast, somewhat short focal length lens on the camera, shooting into an eyepiece...
 

#46 ccs_hello

ccs_hello

    Fly Me to the Moon

  • *****
  • Posts: 6454
  • Joined: 03 Jul 2004

Posted 16 April 2013 - 11:58 PM

Indeed, placing a lens "before" the primary focal plane is the preferred way to modify an existing optical system's parameters. E.g., (per definition,) placing a negative lens inside focal plane, it is a Barlow; placing a positive lens inside focal plane, it is a Focal Reducer.

Placing a lens "after" the primary focal plane is afocal method. Placing a positive lens (or even a common magnifier), e.g., using an eyepiece, is an eyepiece projection (similar to Barlow effect.)
On the same token, replacing that positive lens with a negative lens will reduce the image scale.

Do an experiment, place a Barlow lens in between a piece of newspaper and your eye, you'll find the text looks tinier.

Clear Skies!

ccs_hello
 

#47 mattflastro

mattflastro

    Vendor - Astrovideo Systems

  • -----
  • Posts: 622
  • Joined: 31 Jul 2009
  • Loc: Brevard County , FL

Posted 17 May 2013 - 06:30 PM

Here are the first 3 prototypes of my focal reducer.
Why 3? Due to lack of enough planning .
I started by making focal reducer #1 to be compatible with my C-mount cam #1.
Then I made focal reducer #2 as a cleaner version ( #1 has a lot of rework and modifications due to experimenting) .
Then I decided to make camera #2 with Peltier cooling and T-mount .
Then I had to redo focal reducer #1 and modify it to work with both the T-mount and the C-mount .
So then I made reducer #3 that works from the beginning with both C and T mounts . I also made it fit any 2" focuser instead of just the MEade 3.25" threads.

Attached Files


 

#48 mattflastro

mattflastro

    Vendor - Astrovideo Systems

  • -----
  • Posts: 622
  • Joined: 31 Jul 2009
  • Loc: Brevard County , FL

Posted 17 May 2013 - 06:33 PM

Another pic below.
I need to mention that these focal reducers that bring an 8 to 12" F/10 SCT down to F/1.85 are NOT a simple doublet or triplet. They are multielement reducers with an ED glass element and were optimised in Oslo before being tested .
The #3 is close to a final version, except for finishing which is still a rough prototype. Final version will get all the normal treatments, blackened lens edges, hard anodizing etc.

Attached Files


 

#49 Atl

Atl

    Viking 1

  • *****
  • Posts: 501
  • Joined: 13 Apr 2012

Posted 17 May 2013 - 11:05 PM

Very cool.
 

#50 nytecam

nytecam

    Voyager 1

  • *****
  • Posts: 11462
  • Joined: 20 Aug 2005
  • Loc: London UK

Posted 18 May 2013 - 02:02 AM

Looks very interesting - thanks for posting ;)
 






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics