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Focal Reducer for Video SCT SCopes down to F/1.5

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#1 mattflastro

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Posted 23 March 2013 - 09:09 PM

Hyperstar is an option that gets down to F/2 .
Meade .33x gets down to F/3.3 .
I am not aware of any other options that could be used , which should not involve disassembling the corrector and buying $1000 worth of mechanical+optical parts .
The image below is shot with my home made reducer and my Meade 10 inch F/10 SCT .
The result is F/1.55 apparently (I might be off by a little, actual range based on transit time is 1.5 to 1.6 ) .
Is there anything like this available commercially ?
Please note that my reducer can NOT work with cameras that have a CCD any larger than 1/3" ( 6mm diagonal).

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#2 mclewis1

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Posted 23 March 2013 - 09:52 PM

I don't know of any add on commercial product that's specifically intended to be used below .3x. It's tough to maintain image quality at those levels of reduction and there's also very severe vignetting outside of a small image circle.

The Mallincam MFR-5 if "abused" with spacers both between the lens elements and behind it (between it and the sensor) can get you down below .3x but the image quality isn't great outside an area about the size of a 1/3" sensor.

I can also get down to around .25x with my SCTs by stacking the SCT f6.3 reducer and a .5x reducer, but again poor image quality at the edges of a 1/2" sensor.

Stacking focal reducers is a viable solution particularly if you relax the spacing a bit (don't push them to their limits) and aren't looking for high quality wide fields that cover larger sensors.

Getting down under .2x is quite impressive, can you show us how you did it? A shot of a star field will tell you more about the image quality (it's really tough to keep stars small and tight right across a fov). Also how about a shot at a longer exposure on a 1/2" sensor so we can see the vignetting?
 

#3 highfnum

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Posted 24 March 2013 - 03:47 AM

What you did is tuff
I do have sc5 and sc8 @ 1.6 1.5 those are special cameras
 

#4 Moromete

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Posted 24 March 2013 - 02:46 PM

Excelent stuff. Bravo!
 

#5 ippiu

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Posted 24 March 2013 - 04:35 PM

Is there anything like this available commercially ?
Please note that my reducer can NOT work with cameras that have a CCD any larger than 1/3" ( 6mm diagonal).


Yes! There is...
A normal focal reducer like this: http://www.teleskop-...-Focal-reduc...

with 74 mm extension tubes spacing. I have a watec 120 n+, that is 1/2" sensor size.

Total factor reduction: f1.75, more or less. With huge coma at the borders, obviously :)
 

#6 GlennLeDrew

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Posted 24 March 2013 - 05:29 PM

It's necessary to check that aperture reduction is not occurring. To obtain such aggressive reductions, the reducer optics must themselves be 'fast', else the on-axis light cone (to say nothing of off-axis light cones) will not be fully accommodated. One can get significant reductions in image scale with large reducer-to-focus spacings, but if the reducer's clear aperture is not taking in the full light cone the working f/ratio will be longer than assumed. In other words, image brightness is less.

There are easy ways to check if aperture reduction is happening...

I'd love to see what you've cobbled up!
 

#7 mpgxsvcd

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Posted 25 March 2013 - 10:34 AM

Here is the fastest telephoto lens I could find. It was made for medium format so it will probably work even with larger sensors.

Imagine this thing on a large crop factor camera like the Nikon 1 series.

240mm f/1.2 with an image circle that covers 6x7 medium format

http://photography-o...ad.php?t=872418
 

#8 nytecam

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Posted 26 March 2013 - 04:58 AM

Hyperstar is an option that gets down to F/2 .
Meade .33x gets down to F/3.3 .....The image below is shot with my home made reducer and my Meade 10 inch F/10 SCT .
The result is F/1.55 apparently ....

What Glenn says is spot-on :bow: I've my FR daylight tests somewhere on distant chimney pots - why waste precious night skies! Whilst it's possible to stack FRs like my Meade 3.3, 6.3 and my DIY 7.0 FRs, in decending order of power, 'speeds' faster than the fastest f/3.3 optic is improbable.

Whilst scaling the image, compared to the native f/10, it seems possible to reach f/1.5 but this is done by severely vignetting the scope's full aperture - the actual 'speed' remains ~f/3.3. Good try but there ain't no gain without pain :lol: I've optomised my 12" SCT to run a f/3.6 via the Meade f/3.3 FR and a near perfect balance for my Lodestar camera in speed and coverage on DSOs :grin:
 

#9 GlennLeDrew

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Posted 26 March 2013 - 04:43 PM

My own tests indicate that for f/10 systems using various 'off-the-shelf' reducers and combinations yields a limit near f/2.8 before aperture reduction has reared its head, but the poor image quality (on-axis spherical aberration, to say nothing of the worse off-axis coma and field curvature) are becoming intolerable.

A multi-element reducer of rather fast aperture ratio of about f/2, if not faster), and perhaps/probably aspherized, would be required in order to get to the f/1.5 level at the focus. And of course the inherent field illumination must support the significant reduction of nearly 7X (from f/10); a 40mm image circle is shrunk to 6mm.

Hmmm... I wonder what a fast camera lens could do. Something like my 50mm f/1.7 Minolta. I can see some experimenting in the near future...

Again, Matt, I'd love to see what you've made!
 

#10 nytecam

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Posted 27 March 2013 - 03:23 AM

Hmmm... I wonder what a fast camera lens could do. Something like my 50mm f/1.7 Minolta. I can see some experimenting in the near future...

I've used a variety of fast short fl lenses on my e-finder that also doubles as a widefield astrograph with longer exposures. My favourite lens is a 85mm f/2 Jupiter lens + red filter. The filter seems to supress my LP and act as a poorman's H-alpha filter. Checkout my e-finder page [down page] and sample pics N America Neb - M31 and below :grin:

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#11 GlennLeDrew

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Posted 27 March 2013 - 03:48 AM

Maurice,
I meant a fast camera lens used as a focal reducer in a long f.l. system, not by itself (which we know works quite well.)

Matt's absence after his initial teaser is keenly felt. Hint, hint, Matt!
 

#12 nytecam

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Posted 27 March 2013 - 04:42 AM

Maurice, I meant a fast camera lens used as a focal reducer in a long f.l. system, not by itself (which we know works quite well.)

Glenn - you are normally pedantically precise but fell on that one :o I don't think FRs will improve on ~f2.8 or less bearing in mind the horrid aberrations they incur. f/2 Hyperstar seems the proven if expensive answer but then too much 'speed' can be a disadvantage as we're all aware under commonplace LP :grin:
 

#13 GlennLeDrew

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Posted 27 March 2013 - 05:58 AM

Indeed, Maurice, my pedantic precision fall by the wayside. :grin:

For the benefit of all, it must be stressed that the roughly f/2.8 limit via focal reduction applies to longer systems in the range of f/10.

I use a dual lens reducer--the Mallincam MFR-5--on an f/5 refractor to achieve f/2.5, delivering full aperture performance.

I provide this example merely to illustrate that the final, effective f/ratio depends also on the objective's f/ratio.

Incidentally, the Hyperstar system is not at all a focally-reduced configuration, for the objective f/ratio is already at or near to f/2 (depending on the specific scope.) The Hyperstar optics merely correct the off-axis aberrations and field curvature, retaining the system speed provided by the fast objective.
 

#14 mattflastro

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Posted 29 March 2013 - 02:38 AM

Maurice,
I meant a fast camera lens used as a focal reducer in a long f.l. system, not by itself (which we know works quite well.)

Matt's absence after his initial teaser is keenly felt. Hint, hint, Matt!

Sorry for the delay. The Moon pic I posted at the beginning of this thread was taken with my home made reducer and a Meade 10" F/10 SCT, working at around F1.5 with no vignetting .
Since posting that image I was busy making another reducer to work with my Meade 8" F10 SCT , which has a smaller baffle tube and is a much more portable scope .
Of course as soon as I finished assembling the parts and setting up , thin clouds covered most of the sky.
Here's a pic taken with the Meade 8" at F2 . Please ignore the skyglow due to being shot thru the (thin) clouds . Also didn't have time for focusing properly so it's just eyeballed . Both images are unprocessed frames from my live video feed , taken with a cheap digital DVR .

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#15 mattflastro

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Posted 29 March 2013 - 02:49 AM

Just to clarify, both the F1.5 reducer for the 10" F10 SCT and the F2 reducer for the 8" F10 SCT are working without vignetting with 1/3" ccd's .
The reducers attach to the back of the scope , not in front Fastar/Hyperstar style.
Even with this reduction, the cameras have a field of view that's still narrower than the scopes , so there's still some margin , although not much .
 

#16 nytecam

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Posted 29 March 2013 - 06:28 AM

Sorry to doubt your DIY unvignetted FRs - practise trumps theory - BTW checked your M42 starfield for 5.3x3.8mm chip =6mm diagonal = f/1.75 eg faster than f/2 :lol:

My own experiments I found the secondary caused vignetting at fast FRs by dimming the image :ooo:
 

#17 GlennLeDrew

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Posted 29 March 2013 - 06:07 PM

Matt,
Have you verified that you're indeed working at or at least near to full aperture? Enquiring minds want to know! :grin:

An image by itself will not tell you this.
 

#18 GlennLeDrew

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Posted 30 March 2013 - 12:25 AM

I tried a 50mm f/1.7 camera lens on an 80mm f/5 achromat. The spherical aberration was notable, enough to quash that notion. I merely held the camera behind the lens, and so could not measure the reduction factor from the shaky image during the quick test. From the image scale, I'd guess a reduction factor of about 0.3X. The lens was certainly at its limit for the acceptance of the light cone, if not very slightly clipping the aperture.
 

#19 mattflastro

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Posted 30 March 2013 - 02:26 PM

Matt,
Have you verified that you're indeed working at or at least near to full aperture? Enquiring minds want to know! :grin:

An image by itself will not tell you this.

A reduction from a 40mm image circle to a 6mm image circle is theoretically possible, I hope you agree. That would be a reduction factor of 6.66 . A reduction factor of only 6 down to a 6mm image circle would need to start from a 36mm image circle, same as a full frame sensor or film . F10 SCT's are capable of larger than 35mm image circle.
Simply because people haven't built such reducers doesn't mean it is impossible, it's not even pushing the limit .
 

#20 GlennLeDrew

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Posted 30 March 2013 - 03:44 PM

Indeed. Now, obtaining the reduced image scale is one thing. But doing this while preserving full aperture performance is another. You can check for this with everything assembled and focused at infinity. Aim the scope toward the daytime sky, or while indoors shine a light down the front end, so that you can see the detector. Move your gaze toward the outer edge of the corrector, noting the point at which the *center* of the CCD becomes clipped by some obstructor. If you can see the chip's center until it it reaches the edge of the primary mirror, you're working at full aperture.

It's virtually certain that the chip's corners will be clipped well before your sight line reaches the edge of the objective. This means you'll have to estimate the location of the chip's center, which is facilitated by quickly moving your eye back nearer to the center (looking just past the edge of the secondary.)
 

#21 GlennLeDrew

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Posted 02 April 2013 - 03:39 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?
 

#22 GlennLeDrew

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Posted 02 April 2013 - 09:19 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.
 

#23 budman1961

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Posted 02 April 2013 - 10:47 PM

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

Too bad!

andy
 

#24 mattflastro

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Posted 03 April 2013 - 08:10 AM

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.
 

#25 budman1961

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Posted 03 April 2013 - 10:14 AM

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






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