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Equipment Discussions >> Cats & Casses

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pbsastro
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Reged: 03/21/07

C14HD full illuminated field new
      #5673585 - 02/11/13 07:39 AM

Anyone knows the C14HD full (100%) illuminated field (in mm)?
It depends on the focus position, but let’s say at focus position required for a Pan41, XW40 or Ethos 21 with the 2” diagonal included with C14HD.

It would also be interesting to compare it with smaller HDs.

This thread is intended for people that care about it. I know a lot of people say it does not matter because we cannot see it, etc. But there people like me who can see it for who it does matter a lot. We also may not be able to see a difference from a 13” or 14” scope, nor from a 13” and a 12”, nor from a 12” and 11”, etc. So we could all be using 1” scopes, because they are all the same…

Pedro

Edited by pbsastro (02/11/13 08:08 AM)


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bilgebay
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Loc: Turkiye - Istanbul and Marmari...
Re: C14HD full illuminated field new [Re: pbsastro]
      #5673864 - 02/11/13 11:21 AM

It is 42 mm for all the Edge scopes. However, you can ensure this only when your back focus is 146 mm from the shoulder of the 3.25" SCT thread. The only exception is C8. The back focus is 133.35mm for this scope.

Clear skies


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Eddgie
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Re: C14HD full illuminated field new [Re: pbsastro]
      #5673957 - 02/11/13 12:20 PM

In the past, the C14 has only been "Fully" illuminated (100%) over a 10mm image circle when the back focus was 100mm, and I would doubt that the EdgeHD 14" is any different.

I am not aware of a Celestron SCT that has ever been fully illumninated over more than a 30mm circle (C11).

For imaging though, you don't really need 100% illumination. Usually 80% for the outside of the image is considered more than acceptable. Only when it falls off more than this does it get to be an issue.

With 100mm of back focus, the standard C14 is illuminated to 80% out to a 40mm image circle.

I have all of the figures for the standard SCTs, but since the EdgeHDs use almost identical mirror spacing and baffle dimensions, I don't think it is going to be able to be better than this. The off axis illumination is a function of the baffle shape and diameter and that is essenitally the same in the EdgeHDs.

The C11 has the widest 100% illuninated image circle, which is right at 33mm. with 100mm of back focus.

I have a vignetting analysis if you want it. Just PM me your email address and I will send it along.

Once again, I feel compelled to point out that 80% illumiation at the edge of the field is pretty typical of most reflectors and CATs and is more than good for most imageing.

But with ever bigger chips, many people are moving to astrographs with big secondary mirrors and baffles. If for some reason you really do desire a very large 100% illumiated field, that is the way to go.

But 80% is fine. The C11 and C14 both provide 80% illumination out to a 40mm circle and that should be more than good.

Again, I don't know how the EdgeHDs could be that much different because the seem to have the same baffle measurements, but I don't know of any ray traces that have been done for the EdgeHDs. They might be a bit better but I don't know how they could be a lot better..

With a focal reducer though they should be far far better than in the past. The 80% illuminated circle with the old focal reducers was pitiful in the C14, but I think the new focal reducers offer a huge well illuminated circle.

People have complained about the cost, but there is no comparison between the old f/6.3 reducer and the new reducer in terms of off axis illumination.

Edited by Eddgie (02/11/13 12:30 PM)


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GlennLeDrew
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Re: C14HD full illuminated field new [Re: Eddgie]
      #5674003 - 02/11/13 12:45 PM

Who needs raytracing If someone has an example in hand, set the focus appropriately, place a ruler or piece of paper across the image surface so that its edge bisects the field, and note at what points along one's sight lines opposite edges of the primary's aperture just become clipped. The distance between these two points define the circle of full illumination.

It's so darned simple to do this, it astonishes me that virtually no one seems to be aware of it, and instead wring their hands waiting for a ray trace.

Ray tracing is simple geometry, which follow the paths of certain defined rays through a system. You can locate the intercepts of some key rays with nothing more than your eyes, for you are seeing directly the action of the optics and obstructors.


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Lee Jay
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Reged: 02/27/08

Loc: Westminster, CO
Re: C14HD full illuminated field new [Re: GlennLeDrew]
      #5676587 - 02/12/13 08:53 PM

Quote:

Who needs raytracing If someone has an example in hand, set the focus appropriately, place a ruler or piece of paper across the image surface so that its edge bisects the field, and note at what points along one's sight lines opposite edges of the primary's aperture just become clipped. The distance between these two points define the circle of full illumination.

It's so darned simple to do this, it astonishes me that virtually no one seems to be aware of it, and instead wring their hands waiting for a ray trace.

Ray tracing is simple geometry, which follow the paths of certain defined rays through a system. You can locate the intercepts of some key rays with nothing more than your eyes, for you are seeing directly the action of the optics and obstructors.




My plan is to do it the way I did it here. Is there anything wrong with that approach, where I just shoot blue-sky shots with a full-frame camera?


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GlennLeDrew
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Re: C14HD full illuminated field [Re: Lee Jay]
      #5676949 - 02/13/13 02:01 AM

This approach is completely valid, as long as the focus is set at least near to infinity. With software such as MaxIm DL, you can easily graph the illumination with field angle.

Note that the more gradual component of vignetting is mainly due to the inner end of the primary baffle. The sharper component of vignetting near the field edge/corners is due to a restricted aperture rather nearer to the focal surface, such as the the rear opening of the baffle, T-ring or reducer clear aperture.


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Steve Cobb
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Reged: 04/26/10

Re: C14HD full illuminated field new [Re: Eddgie]
      #5677896 - 02/13/13 03:43 PM

When did the "new" focal reducers you mentioned start coming out so I can figure out if mine is new or old? Thanks!

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Eddgie
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Re: C14HD full illuminated field new [Re: Steve Cobb]
      #5677983 - 02/13/13 04:43 PM

As far as I know, the C14 HD reducer is already on the market and has been for some time.

It is a much more expensive reducer than the old one, but it is a far better system for imaging than the old one was.


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JMW
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Reged: 02/11/07

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Re: C14HD full illuminated field new [Re: Eddgie]
      #5696796 - 02/23/13 03:12 PM

Thanks for the information. I just bought a Canon 6D and haven't had a chance to use it yet on my C11 EdgeHD with .7 focal reducer. I am glad to know that the image circle will be large enough for a full frame DLSR. I plan on using the C11 EdgeHD and my AT65EDQ with the 6D.

Hopefully I will have a chance to purchase the Hyperstar for the C11 EdgeHD and use my DSLR with it. I figure using wifi to focus and download I can avoid any cables in front of the corrector plate.


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ewave
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Re: C14HD full illuminated field new [Re: JMW]
      #5696934 - 02/23/13 04:31 PM

Quote:

I figure using wifi to focus and download I can avoid any cables in front of the corrector plate.



Boy is this technology long overdue. Something I'm sure many of us are interested in seeing.


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Lee Jay
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Loc: Westminster, CO
Re: C14HD full illuminated field new [Re: GlennLeDrew]
      #5697019 - 02/23/13 05:34 PM Attachment (22 downloads)

Quote:

This approach is completely valid, as long as the focus is set at least near to infinity. With software such as MaxIm DL, you can easily graph the illumination with field angle.

Note that the more gradual component of vignetting is mainly due to the inner end of the primary baffle. The sharper component of vignetting near the field edge/corners is due to a restricted aperture rather nearer to the focal surface, such as the the rear opening of the baffle, T-ring or reducer clear aperture.




So, I have an EdgeHD 11 and an EOS camera adapter that uses the 2" barrel instead of a T-adapter. Attached is a blue-sky shot taken with a Canon 5D (full-frame 35mm digital) camera. This looks like an amazingly small amount of vignetting to me.

My reducer is on order so I don't have a shot like this with that in place yet.


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GlennLeDrew
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Re: C14HD full illuminated field new [Re: Lee Jay]
      #5697100 - 02/23/13 06:44 PM

That is quite impressive! Can you measure image brightness, sampling a circle of, say 50 pixels diameter, at various radial distances?

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Lee Jay
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Re: C14HD full illuminated field new [Re: GlennLeDrew]
      #5697104 - 02/23/13 06:46 PM

Quote:

That is quite impressive! Can you measure image brightness, sampling a circle of, say 50 pixels diameter, at various radial distances?




I suppose, but this is an out-of-camera JPEG that's been downsampled to post here. You can measure it if you like, or I could measure the original, but since it's not a raw image, I'm not sure of the validity of any such measurement.


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GlennLeDrew
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Re: C14HD full illuminated field new [Re: Lee Jay]
      #5697546 - 02/23/13 11:14 PM

If the image is reasonably noise-free, one might assume a near zero value to be black. A sampling of several ~50-pixel circles in the zone of interest will indicate the variance due to noise/compression artifacts, which due to the large sample spot (which nicely averages pixel-to-pixel variations) is expected to be low. If this is the case, sample the image center, a corner, and a couple places between these extremes. Normalize the other readings to the central brightness (that is, make the central measurement 100, and scale the others by the same ratio), and let us know. This will be an interesting, first real-world report, for me at least.

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wolfman_4_ever
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Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: C14HD full illuminated field new [Re: GlennLeDrew]
      #5697619 - 02/24/13 12:11 AM

Celestron has a write up about the light fall off.. At least the field is flat!

Is there vignetting with the EdgeHD optics and reducers on a full-frame image...

A full-frame image sensor is based on the size of 35mm film and therefore has a 42mm diagonal. This corresponds to a 21mm radius circle at the image plane.

Yes, there is vignetting. Just like any real optical system, an EdgeHD OTA doesn’t have 100% illumination across the image plane. In other words, the EdgeHD optics don’t fully illuminate the 21mm radius image circle.

With the reducer, the vignetting falls to a value of about 40-50% at 21mm. The exact value depends on the specific EdgeHD OTA (8, 9.25, 11 or 14 inches).

The problem of vignetting in astroimages is usually handled by the technique of flat fielding during post processing of astroimages.


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Lee Jay
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Re: C14HD full illuminated field new [Re: wolfman_4_ever]
      #5697866 - 02/24/13 08:08 AM

A full frame sensor has a diagonal dimension of 43.3mm.

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Eddgie
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Re: C14HD full illuminated field new [Re: wolfman_4_ever]
      #5698035 - 02/24/13 10:43 AM

Well, the idea of a focal reducer is not to make the field bigger, it is to make the system faster.

When you use a focal reducer, you simply "Compress" what is there.

If you start with a field that is 80% illuminated at 40mm image circle (20mm radius) which is common in the SCT design and you reduce the field, this means that you compress the image circle where 80% illumination occurs.

So, if at f/10, the field were illuminated to 80% over a 40mm image circle (20mm radius), then when using the focal reducer, the field will be 80% illuminated over an image circle/radius that is .63% of what it would be at f/10.

So, the 80% illumination would occur at .63 of the 40mm image circle (20mm radius).

In other words, using the reducer you would only have 80% illumination at the edge of a 25mm image circle (12.5mm radius).

This is the way focal reducers work. They speed up the system, but basically you also compress the area of the focal plane that is illuminated to "X" level by the compression factor of the lens.

I get the impression that most people somehow think that a focal reducer is intended to make a scope a wide field instrument.

It is not. It is intened to reduce exposure times.

The tradeoff is that the image scale is smaller, but if you looked at the illimination of the true field, it would be the same (for distance in arc minutes of true field of the image) in both photos.

Focal resucers can only make syetem faster at the expense of image scale. You can get a wider true field, but the illuination falls off badly, and in most cases if you use the same exposure time, you would see vignetting in the reduced image.

Not saying at all that you can't get a bit wider field, Everyone makes their choice.

Only attempting to explain how focal reducers work. Every detail of the image at the focal plane is shrunk down to a circle .63x of the native circle size so illumination figures for the full circle at f/10 represent illumination at a circle .63x that size with the focal reducer in place.


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Lee Jay
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Re: C14HD full illuminated field new [Re: Eddgie]
      #5698063 - 02/24/13 10:59 AM

According to Celestron the illuminated field is 42mm with the reducer too, indicating a much wider field of view than that which I posted will be available, albeit with more vignetting outside the area I posted. But that's the only way I can get at that area as I am already using the largest sensor I can.

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GlennLeDrew
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Re: C14HD full illuminated field new [Re: Lee Jay]
      #5698506 - 02/24/13 04:06 PM

How is Celestron defining the illumination? I doubt they claim 100% over that 42mm circle. If there exists some amount of fall-off inside this 42mm circle, a focal reducer can only compress this vignetting pattern in step with the image scale. And the fall-off just outside the 42mm circle will be brought into the outer field, resulting in greater fall-off. Furthermore, for the reducer to not introduce its own outer-field component to vignetting, its lenses must be suitably (over)sized.

As I outlined earlier, anyone having this kit can conduct their own measurements. This is best done with a camera whose body does not interfere with the edge-of-field illumination. DSLRs, at least at faster f/ratios, can cause shadowing near the field edge due to the somewhat restricted, rectangular opening.


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Lee Jay
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Re: C14HD full illuminated field new [Re: GlennLeDrew]
      #5698512 - 02/24/13 04:10 PM

Quote:

How is Celestron defining the illumination?




I asked, and here's the answer:

"With the reducer, the illunination (depending on the OTA size and reducer combo) drops off by about 40-50% at 21mm."


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