Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Video and Electronically Assisted Astronomy

Pages: 1 | 2 | 3 | 4 | 5 | 6 | (show all)
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: GlennLeDrew]
      #5772157 - 04/02/13 04:39 AM

In reviewing the M42 image, I'm struck by the lack of field curvature and coma. The 0.33X reducers *designed specially* for SCTs so as to correct these two aberrations are probably at just about the limit of what's possible for a 3-lens unit.

That a cobbled up, home made rig can provide 0.15-0.2X reductions with such evenness of illumination and lack of off-axis aberrations is astonishing. To the point that I'm really beginning to doubt the whole thing.

Any further details to quell my misgivings?


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: GlennLeDrew]
      #5774312 - 04/02/13 10:19 PM

To further build upon the thinking inspiring my doubts...

Small-chip cameras have been around since the first CCD cameras. And Meade designed a suitable reducer for those small formats, it being the f/3.3 unit for f/10 SCTs. An even faster device would have been something of a Holy Grail, but seems to not have been easily achievable. And so the jump to 'extreme' speed was realized by the front-end corrector, whose current and most popular incarnation is Starizona's Hyperstar.

Now comes along a hobbyist claiming to achieve a focal reducer of simply astonishing capability not heretofore seen. A reduction factor of 0.15X which delivers good correction and little vignetting. This is a *very great* amount of wavefront shaping; no mean feat to achieve. And we're given to understand (by reading between the lines) that such had been obtained at no great expense and with existing lenses.

It might pose a difficult problem for an optical designer to come up with such a device, it requiring computation and numerous elements to deliver such performance. The chance of matching up a couple or few off-the-shelf lenses or existing reducers is not expected by chance to come close.

And if the 0.15X worked well for the 10" SCT, why bother to make up a different (0.2X, or so was thought) version for the 8" SCT when the already proven 0.15X should work as well on the 8"?

In the end, the numbers just seem to be *too* good, based on what has been achieved prior, and having some understanding of what it takes to perform such optical tasks.

A number of questions are wanting answering.


Post Extras: Print Post   Remind Me!   Notify Moderator  
budman1961
Pooh-Bah
*****

Reged: 02/25/11

Loc: Springfield, MO
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: GlennLeDrew]
      #5774465 - 04/02/13 11:47 PM

This was just another threadstarter with no finish thread.........

Too bad!

andy


Post Extras: Print Post   Remind Me!   Notify Moderator  
mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: GlennLeDrew]
      #5774951 - 04/03/13 09:10 AM

Quote:

To further build upon the thinking inspiring my doubts...

Small-chip cameras have been around since the first CCD cameras. And Meade designed a suitable reducer for those small formats, it being the f/3.3 unit for f/10 SCTs. An even faster device would have been something of a Holy Grail, but seems to not have been easily achievable. And so the jump to 'extreme' speed was realized by the front-end corrector, whose current and most popular incarnation is Starizona's Hyperstar.

Now comes along a hobbyist claiming to achieve a focal reducer of simply astonishing capability not heretofore seen. A reduction factor of 0.15X which delivers good correction and little vignetting. This is a *very great* amount of wavefront shaping; no mean feat to achieve. And we're given to understand (by reading between the lines) that such had been obtained at no great expense and with existing lenses.

It might pose a difficult problem for an optical designer to come up with such a device, it requiring computation and numerous elements to deliver such performance. The chance of matching up a couple or few off-the-shelf lenses or existing reducers is not expected by chance to come close.

And if the 0.15X worked well for the 10" SCT, why bother to make up a different (0.2X, or so was thought) version for the 8" SCT when the already proven 0.15X should work as well on the 8"?

In the end, the numbers just seem to be *too* good, based on what has been achieved prior, and having some understanding of what it takes to perform such optical tasks.

A number of questions are wanting answering.



Glenn, thanks for pointing out the value of such a reducer. At this point it is not consistent wiht my goals to place such a reducer desgn in the public domain.


Post Extras: Print Post   Remind Me!   Notify Moderator  
budman1961
Pooh-Bah
*****

Reged: 02/25/11

Loc: Springfield, MO
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: mattflastro]
      #5775293 - 04/03/13 11:14 AM

He's tell us, but he's have to kill us.......lol

Post Extras: Print Post   Remind Me!   Notify Moderator  
Dwight J
scholastic sledgehammer
*****

Reged: 05/14/09

Loc: Lethbridge, Alberta, Canada
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: budman1961]
      #5775516 - 04/03/13 12:59 PM



Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: Dwight J]
      #5775601 - 04/03/13 01:39 PM

When might we look forward to purchasing one, then?

Post Extras: Print Post   Remind Me!   Notify Moderator  
budman1961
Pooh-Bah
*****

Reged: 02/25/11

Loc: Springfield, MO
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: GlennLeDrew]
      #5775697 - 04/03/13 02:23 PM

Vaporware

Post Extras: Print Post   Remind Me!   Notify Moderator  
Devon
member


Reged: 03/06/10

Loc: OK
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: budman1961]
      #5775855 - 04/03/13 03:48 PM

Is BSware a term...?

Post Extras: Print Post   Remind Me!   Notify Moderator  
Chris A
scholastic sledgehammer


Reged: 02/03/07

Loc: Toronto, Canada
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: GlennLeDrew]
      #5775883 - 04/03/13 04:04 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: Chris A]
      #5776132 - 04/03/13 06:23 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?


Post Extras: Print Post   Remind Me!   Notify Moderator  
budman1961
Pooh-Bah
*****

Reged: 02/25/11

Loc: Springfield, MO
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: GlennLeDrew]
      #5776303 - 04/03/13 07:51 PM

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

lol

Andy


Post Extras: Print Post   Remind Me!   Notify Moderator  
jimthompson
member


Reged: 10/06/09

Loc: Ottawa, Canada
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: GlennLeDrew]
      #5776502 - 04/03/13 09:13 PM

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
ccs_hello
Postmaster
*****

Reged: 07/03/04

Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: budman1961]
      #5776612 - 04/03/13 10:09 PM Attachment (18 downloads)

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:


Post Extras: Print Post   Remind Me!   Notify Moderator  
ccs_hello
Postmaster
*****

Reged: 07/03/04

Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: ccs_hello]
      #5776625 - 04/03/13 10:15 PM Attachment (16 downloads)

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:


Post Extras: Print Post   Remind Me!   Notify Moderator  
ccs_hello
Postmaster
*****

Reged: 07/03/04

Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: jimthompson]
      #5776641 - 04/03/13 10:22 PM

Jim,

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

Clear Skies!

ccs_hello


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: ccs_hello]
      #5776994 - 04/04/13 04:07 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
nytecam
Postmaster


Reged: 08/20/05

Loc: London UK
Re: Focal Reducer for Video SCT SCopes down to F/1.5 new [Re: GlennLeDrew]
      #5778066 - 04/04/13 04:25 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

Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Focal Reducer for Video SCT SCopes down to F/1.5 [Re: nytecam]
      #5778219 - 04/04/13 05:54 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
nytecam
Postmaster


Reged: 08/20/05

Loc: London UK
Re: Focal Reducer for Video SCT SCopes down to F/1.5 [Re: GlennLeDrew]
      #5778973 - 04/05/13 04:11 AM

Quote:

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.

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | 5 | 6 | (show all)


Extra information
3 registered and 6 anonymous users are browsing this forum.

Moderator:  csa/montana, JayinUT 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 6505

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics