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Fast focal reduction testing

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#1 GlennLeDrew

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Posted 27 May 2013 - 03:30 PM

In an effort to achieve faster than f/3.3 on my f/10 SCT, I tried the combination of an Alan Gee Mk II reducer and a two-lens reducer I cobbled up. First, the Alan Gee.

It's a 2-element reducer designed for Baader by Roland Christen. It's optimized for 'standard' f/10 SCTs, and delivers a flat, well corrected field. The clear aperture is 36mm, maximized in a quite thin-walled barrel which ever so snugly slips up into the scope's primary baffle. The focal length of the reducer (by itself, not in conjunction with any other optics) is ~280mm, which is pretty long. It nominally works at a reduction of 0.59X, but is reportedly able to accommodate a range of spacings to get down to 0.35X territory. I've not tested this, mainly because I already have the Meade 0.33X reducer.

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  • 5887423-Alan Gee II.jpg


#2 GlennLeDrew

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Posted 27 May 2013 - 03:36 PM

Now for the 'home brew' reducer. It's very much like Mallincam's 2-lens MFR-5. I combined a common 0.5X, 1.25" format reducer (Antares brand) with Mallin's MFR-3 (I believe that's its designation). The result is somewher between 0.45-0.5X (yet to quantify). The overall physical length is identical to the MFR-5, but the front lens is farther down and the rear lens is farther up.

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  • 5887430-GCL_MFR-5.jpg


#3 GlennLeDrew

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Posted 27 May 2013 - 03:42 PM

With the Alan Gee II inserted into the primary baffle, I use a pretty common 2" visual back and 2" to 1.25" adapter to attach my VSS+ (12.5mm sensor depth) and 'home brew' reducer. The length extending beyond the scope's rear is 7" (~180mm). Here we see also an exploded view, with components laid out at their correct separations. The 'light cone rays' overlaid show the Alan Gee's intrinsic focal length of ~280mm.

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  • 5887435-reducers_assembled_800W.jpg


#4 GlennLeDrew

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Posted 27 May 2013 - 03:49 PM

Now for comparative results. This three panel construct is a series of single-frame captures of the M84/86 group of galaxies. The labeling tells the tale. These were taken from my 9th floor balcony, in a red zone and with a nearly 1st quarter Moon up.

The relevant frame is the middle one, where the measured image scale, based on a 6.45mm wide active area on the CCD, yields a geometric f/2.73. The vignetting and aberrations are not bad.

For the bottom frame, I've added a 5mm spacer between camera and ol' 'home brew.' The speed gain is considerable, but the result is not so aesthetically pleasing!

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  • 5887442-FR_compare_20sec.jpg


#5 GlennLeDrew

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Posted 27 May 2013 - 03:53 PM

In similar vein, a triple comparison as before, but with gain at maximum and integration at only 1 second. This shows how useful these high-speed systems--in conjunction with a high gain camera--can be for very near real time viewing and quick finding/framing. At a focal length of about 500mm and less, tracking isn't even required! But of course noise is very considerable, and color is completely washed out.

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  • 5887446-FR_compare_1sec.jpg


#6 GlennLeDrew

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Posted 27 May 2013 - 04:09 PM

Earlier I alluded to the "geometric" f/2.73 obtained. In actuality, such aggressive reduction often results in aperture reduction. This arises from one or more causes.

For SCTs, the primary mirror must be moved closer to the secondary in order to throw the focus the required farther distance outside the back end of the scope. And so the light cone may not be fully fielded by the primary baffle.

The reducer front lens, being pushed farther in, may not be large enough to fully field the light cone.

Here is a series of images of the Alan Gee II and 'home brew' 2-lens reducer setup, with video camera in place. I've aimed the camera into the front end of the scope, from a distance of 2-3 feet (this distance is not at all critical). Because the scope setup is at infinity focus, I've set my camera lens to infinity focus (the scope presents a collimated image of the at-focus CCD).

To clearly see the CCD, it was necessary to shine an LED flashlight into the front end. In this case, I held the light against the top edge of the camera, adjusting its angle as required while altering my viewpoint during the sequence.

From top to bottom, I'm moving my viewpoint farther off axis, toward the left. In all images, the (out of focus) primary mirror edge is aligned vertically. It's most easy to see in the top image, it being the transition from black to bright about 1/3 the distance from L to R.

In the third image, we see that the CCD's center is just about to be clipped. This indicates the zone on the primary where the effective aperture edge lies. I didn't measure this, but it seems the real working aperture is perhaps 7" (on an 8" scope).

By the final image, it's quite clear that when our sight line puts the visible edge of the CCD in line with the edge of the primary, only some portion of the opposite side of the CCD is illuminated by the primary's edge zone. In other words, the center of the chip is not being illuminated by the primary edge zone, which is absolutely indicative of aperture reduction.

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  • 5887476-CCD_baffle_cutoff_800W.jpg


#7 A. Viegas

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Posted 27 May 2013 - 05:46 PM

Glenn,

That is really interesting. I think I tried using my celestron F6.3 reducer with my MFR5 once but I had a difficult time getting focused... I think using a diagonal in the image train was the culprit. As many of us have those F6.3 reducers around, maybe it's worth experimenting ??

Al

#8 GlennLeDrew

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Posted 27 May 2013 - 06:03 PM

Al,
Indeed, the presence of a diagonal can be a killer when striving for aggressive reduction. In my setup, I'm not that far from running out of mirror travel; a diagonal would be utterly not feasible, methinks.

#9 mega256

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Posted 27 May 2013 - 07:22 PM

Glen interesting...best I can do is F3.3 with the meade,what did you finaly come up with...allaround best ?

#10 GlennLeDrew

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Posted 27 May 2013 - 07:59 PM

The Meade is certainly the cleanest, as it's just the one reducer optimized for the job. Inside and outside focus, the de-focused 'donuts' for stars are quite symmetrical (indicative of good spherical correction on the wavefront), and are uniform across the field.

This f/2.73 resulting from the reducer combination naturally does suffer poorer spherical correction and vignetting, as well as a somewhat less flat field.

I'm still on the fence as to which option I would more often use, myself. I'll probably undertake even more careful comparisons. My primary goal here was to show the differences, and to provide an example of one way to look for aperture reduction which requires nothing more complicated than merely looking down the front end of the scope.

A little more about that last... I forgot to mention (but that's now cirrected, in edit) that in order to see the CCD at all clearly, it was necessary to shine a LED flashlight into the front of the scope. In this case, I held the light against the top edge of the camera, adjusting the angle as required while changing my viewpoint for the series of pictures.

#11 jchaller

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Posted 28 May 2013 - 08:29 PM

Glen,

So the point at which aperture reduction takes place negates any advantage to the speed gained?

In laymen's terms (me) - I could achieve the same result using less aperture and less aggressive focal reduction?

#12 GlennLeDrew

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Posted 28 May 2013 - 09:13 PM

Jim,
If the rate of aperture reduction as focal reduction is increased is still small enough to provide a brighter view, then there is a real gain.

If it's found that making the reduction more aggressive is not really providing a brighter image, then the exercise is largely futile, unless the increase in field is deemed useful.

And of course, the extent of aberrations and vignetting will impose a limit in any event.

As to a smaller aperture and less aggressive reduction achieving the same level of image brightness... That depends. If the scope is merely a scaled down version, and the same reducer optics are used, results will be fundamentally similar. It would take a different configuration, for one or both the scope and reducer system, to realize a different result.

One thing I will try is a combination of Meade f/3.3 reducer and something like a 0.7X single-lens reducer (which would give about f/2.3.) This might well push the Meade beyond its corrected field size, but it's fun to experiment!

#13 jchaller

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Posted 28 May 2013 - 09:42 PM

Jim,
If the rate of aperture reduction as focal reduction is increased is still small enough to provide a brighter view, then there is a real gain.


Thanks, that's what it looked like. Looking forward to more testing results.

#14 mattflastro

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Posted 28 May 2013 - 09:53 PM

Jim,
If the rate of aperture reduction as focal reduction is increased is still small enough to provide a brighter view, then there is a real gain.

If it's found that making the reduction more aggressive is not really providing a brighter image, then the exercise is largely futile, unless the increase in field is deemed useful.

And of course, the extent of aberrations and vignetting will impose a limit in any event.

As to a smaller aperture and less aggressive reduction achieving the same level of image brightness... That depends. If the scope is merely a scaled down version, and the same reducer optics are used, results will be fundamentally similar. It would take a different configuration, for one or both the scope and reducer system, to realize a different result.

One thing I will try is a combination of Meade f/3.3 reducer and something like a 0.7X single-lens reducer (which would give about f/2.3.) This might well push the Meade beyond its corrected field size, but it's fun to experiment!

Guys, I'm glad I started this move to use aggresive focal reducing with videocams and SCT's down into the F1.5 to F2.2 range .
At first Glenn was ready to say that I photoshopped it, but now he's getting close if not the same results as I did. Considering that I reduced for a 1/3" 6mm diagonal CCD and Glenn's experiments are for the 1/2" 8mm diagonal, I think that's about the maximum that can be obtained from an F10 SCT . 40mm image circle down to 8mm diagonal, reduction ratio is 5 , meaning down to F2 is an absolute value hard if not impossible to get for 1/2" CCD's . But F2 is nothing to scoff at, considering that it's 2.7 times faster than F3.3 and 25 times faster than F10 .

#15 mclewis1

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Posted 28 May 2013 - 11:05 PM

Guys, I'm glad I started this move to use aggresive focal reducing with videocams and SCT's down into the F1.5 to F2.2 range

Matt,

You're very likely the first to commercialize a product in this area but you didn't start this move. Many of us have for a number of years been working with (and documented) stacked focal reducer setups to achieve sub .3x reduction factors with video cameras.

#16 Moromete

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Posted 28 May 2013 - 11:14 PM

What was first, the egg or the hen? :foreheadslap:

#17 GlennLeDrew

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Posted 29 May 2013 - 12:03 AM

Matt,
Could you provide a series of images looking into the front end of the scope, along the lines of my last 4-frame series?

#18 Moromete

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Posted 29 May 2013 - 01:42 AM

Glenn, congrats for posting images the reducer combo.

What is your distance in millimeters between the back lens of Alan Gee and the lens of Antares reducer you used to achieve F/2.73? Is it 70mm?

What's the distance between Antares reducer back lens and camera sensor? Is it 37mm?

Actually I think your images look quite nice compared to what I've seen when testing Alan Gee + x0.5 GSO reducer and reached F/2.4 with a C11. My stars looked like exploded bombs. I had supernovas allover the image. :grin:
But I had a different spacing and no Antares reducer but a GSO one.
I had much smaller distance (under 20mm I think) between Alan Gee back lens and the front lens of GSO reducer and a bigger distance between GSO back lens and camera ccd (around 50mm I think). I've noticed that the reduction becomes noticebly stronger only when increasing the distance between GSo back lens the ccd and between the Alan Gee back lens and GSO front lens.
Have you experinced the same or I'm doing something wrong?

#19 nytecam

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Posted 29 May 2013 - 03:00 AM

One thing I will try is a combination of Meade f/3.3 reducer and something like a 0.7X single-lens reducer (which would give about f/2.3.) This might well push the Meade beyond its corrected field size, but it's fun to experiment!

Thanks Glenn for your interesting experiments - I'll retry my Meade f/3.3 + my DIY x0.6 FR and see what happens - edge vignetting seems the worst offender ;)

I've run the question of flats removing vignetting effects with a top CN AP imager but not got an answer, satifactory to me. I feel such flats, in evening out field illumination, act as a variable ND filter suppressing the central field to match the outer field which effectively means a 'slower' optical system. Still puzzled :confused:

#20 ccs_hello

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Posted 29 May 2013 - 07:30 AM

Maurice,

In minor vignetting, flat can help which is to boost the edge values a bit to match central region.

In severe vignetting (or close to total darkness near the edge), there is nothing to boost from. If the adjustment algorithm's dynamic range is not good enough (e.g., 16 bit), then it ran into the risk of reducing the central region fidelity (white clipping or inappropriate gamma curve companding it too much.)

Clear Skies!

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#21 Moromete

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Posted 29 May 2013 - 07:39 AM

Glen, the back focus for Alan Gee is 121mm and focal length is 259mm (quite close to your measurement of 280mm).

Here is a Google translation from German: "In accordance with the recommendations of Alan Gee by Roland Christen (Astrophysics) specifically for Schmidt
Cassegrain telescopes newly designed optics, a 2-Linser air-spaced and with a flattened field has a
Focal length of 259mm and should according to the theory - in order to achieve the best degree of correction - maximum in
half of its focal length is positioned in front of the focal point of the eyepiece, or the image plane of the camera
are. The ideal image distance from the camera side lens surface to the focus (field aperture /
Film plane, etc.) are Christians by 121 mm. The individual tubes are therefore depending on the used
Accessories otherwise combined (or in some cases omitted)."

I hope it will help you the detailed manual for Alan Gee telecompressor which can be found here: http://www.baader-pl...alangee/alan...

#22 GlennLeDrew

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Posted 29 May 2013 - 05:11 PM

I wasn't overly concerned with monitoring spacings, letting the bits and pieces come together as they may.

In such combinations, where each reducer has a rather different focal length, image scale is more rapidly changed when altering the spacing for the shorter focal length unit. The focal length of my cobbled up 2-lens unit is 45-50mm, as compared to the 259mm of the Alan Gee. Adding a mere 5mm spacer between the little guy and camera has a profound impact. It might require moving camera/after reducer rearward some 90mm to achieve a similar effect on image scale.


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