Jump to content


Photo

C14HD full illuminated field

  • Please log in to reply
27 replies to this topic

#1 pbsastro

pbsastro

    Viking 1

  • *****
  • Posts: 551
  • Joined: 21 Mar 2007

Posted 11 February 2013 - 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

#2 bilgebay

bilgebay

    Skylab

  • *****
  • Moderators
  • Posts: 4200
  • Joined: 06 Nov 2008
  • Loc: Turkiye - Istanbul and Marmaris

Posted 11 February 2013 - 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

#3 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12696
  • Joined: 01 Feb 2006

Posted 11 February 2013 - 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.

#4 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10672
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 11 February 2013 - 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.

#5 Lee Jay

Lee Jay

    Apollo

  • *****
  • Posts: 1193
  • Joined: 27 Feb 2008
  • Loc: Westminster, CO

Posted 12 February 2013 - 08:53 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.


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?

#6 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10672
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 13 February 2013 - 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.

#7 Steve Cobb

Steve Cobb

    Sputnik

  • -----
  • Posts: 33
  • Joined: 26 Apr 2010

Posted 13 February 2013 - 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!

#8 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12696
  • Joined: 01 Feb 2006

Posted 13 February 2013 - 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.

#9 JMW

JMW

    Apollo

  • -----
  • Posts: 1413
  • Joined: 11 Feb 2007
  • Loc: Nevada

Posted 23 February 2013 - 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.

#10 ewave

ewave

    Apollo

  • *****
  • Posts: 1190
  • Joined: 16 May 2009
  • Loc: northwest NJ

Posted 23 February 2013 - 04:31 PM

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.

#11 Lee Jay

Lee Jay

    Apollo

  • *****
  • Posts: 1193
  • Joined: 27 Feb 2008
  • Loc: Westminster, CO

Posted 23 February 2013 - 05:34 PM

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.

Attached Files



#12 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10672
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 23 February 2013 - 06:44 PM

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

#13 Lee Jay

Lee Jay

    Apollo

  • *****
  • Posts: 1193
  • Joined: 27 Feb 2008
  • Loc: Westminster, CO

Posted 23 February 2013 - 06:46 PM

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.

#14 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10672
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 23 February 2013 - 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.

#15 wolfman_4_ever

wolfman_4_ever

    Apollo

  • -----
  • Posts: 1245
  • Joined: 15 Jul 2011
  • Loc: El Segundo, Ca, So. Cal

Posted 24 February 2013 - 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.

#16 Lee Jay

Lee Jay

    Apollo

  • *****
  • Posts: 1193
  • Joined: 27 Feb 2008
  • Loc: Westminster, CO

Posted 24 February 2013 - 08:08 AM

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

#17 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12696
  • Joined: 01 Feb 2006

Posted 24 February 2013 - 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.

#18 Lee Jay

Lee Jay

    Apollo

  • *****
  • Posts: 1193
  • Joined: 27 Feb 2008
  • Loc: Westminster, CO

Posted 24 February 2013 - 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.

#19 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10672
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 24 February 2013 - 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.

#20 Lee Jay

Lee Jay

    Apollo

  • *****
  • Posts: 1193
  • Joined: 27 Feb 2008
  • Loc: Westminster, CO

Posted 24 February 2013 - 04:10 PM

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."

#21 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12696
  • Joined: 01 Feb 2006

Posted 24 February 2013 - 04:41 PM

Unless specifically stated as being the diameter of the image circle, this figure (21mm) is almost always given as a linear distace from the center of the focal plane.

If I say "21mm Off Axis" it means radially from the center of the focal plane.

In most cases, if they just specify an amount in millimeters, they are talking "from the center of the focal plane" (implied).

Otherwise, it is common practice to always say "Image Circle diameter" if that is the measurement being provided.

The math all adds up though, and this sounds about right. It is in fact better than the standard C14 which when used with the 3.25" to SCT thread back (which is part of the baffle system). With this system and the 38mm focal reducer with a camera at the recommended back focus, the illumination was only 15% at the edge of a 30mm circle, or 15% at 15mm off axis.

So the new EdgeHD Focal Reducers are light years better than the old system which was full vignetted at the outside of a 38mm image circle.

#22 Lee Jay

Lee Jay

    Apollo

  • *****
  • Posts: 1193
  • Joined: 27 Feb 2008
  • Loc: Westminster, CO

Posted 24 February 2013 - 04:47 PM

Unless specifically stated as being the diameter of the image circle, this figure (21mm) is almost always given as a linear distace from the center of the focal plane.

If I say "21mm Off Axis" it means radially from the center of the focal plane.


That's how I interpreted it.

I'm wondering if my system will do a bit better yet, given that I'm not using the T-adapter approach but am using the 2" barrel instead.

#23 wolfman_4_ever

wolfman_4_ever

    Apollo

  • -----
  • Posts: 1245
  • Joined: 15 Jul 2011
  • Loc: El Segundo, Ca, So. Cal

Posted 24 February 2013 - 08:19 PM

Celestron has a write up about the light fall off..
Is there vignetting with the EdgeHD optics and reducers on a full-frame image...


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

#24 Lee Jay

Lee Jay

    Apollo

  • *****
  • Posts: 1193
  • Joined: 27 Feb 2008
  • Loc: Westminster, CO

Posted 24 February 2013 - 08:23 PM

Celestron has a write up about the light fall off..
Is there vignetting with the EdgeHD optics and reducers on a full-frame image...

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


I know - I was the one that asked them the question.

Thing is, that's with their T-adapter. I still wonder if my 2" barrel solution will do better.


#25 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12696
  • Joined: 01 Feb 2006

Posted 25 February 2013 - 10:19 AM

Unlikely. The vignetting is a function of the baffles and the apertuer of the reducer.

Nothing you do downstream of them will change this in any meaningful way.

This is why Astrographs have 40% to 45% obstructions (or 4" focusers in refractors). It is what you have to do to get even 80% illumination over a 42mm image circle.

So back to your original post...

Clearly, the choice of apeture size in an SCT is important because it estabilishes the image scale and usable true field size.

For someone wanting to image bigger objects using an SCT, then obviously, they should consider a smaller SCT.

For those wanting to go after smaller targets, they should consider a bigger SCT.

Both produce more or less the exact same image brightness (C11 a bit dimmer because f/11 and f/7 configurations, but litte difference in exposure time), so the difference then is image scale and the size of the true field that is at least 80% illuminated.

For DSOs then, just like with cameras and mounts, the buyer really needs to match the equipment to the target.

This should not be a surprise to anyone, in the same way that the poor off axis illumination of the standard SCT using a focal reducer is. It is well known that SCTs and many general purpose reflectors have very poor off axis illumination and reducing the field doesn't fix this in either design.

So, one does their homework and buys the one best suited to the kinds of targets they want to image.

But if a high percentage of field illumination (80%) is required over a very large image circle, a better choice is an astrograph or a refractor with a large focauer and not an SCT.

That is why astrogrpahs exist (and why people like big refractors for imageing).






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics