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Equipment Discussions >> Cats & Casses

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Eddgie
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Reged: 02/01/06

Re: C14HD full illuminated field new [Re: Lee Jay]
      #5698564 - 02/24/13 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.


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Lee Jay
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Reged: 02/27/08

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Re: C14HD full illuminated field new [Re: Eddgie]
      #5698572 - 02/24/13 04:47 PM

Quote:

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.


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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: wolfman_4_ever]
      #5698868 - 02/24/13 08:19 PM

Quote:

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...


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Lee Jay
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Reged: 02/27/08

Loc: Westminster, CO
Re: C14HD full illuminated field new [Re: wolfman_4_ever]
      #5698872 - 02/24/13 08:23 PM

Quote:

Quote:

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.


Edited by Lee Jay (02/24/13 08:25 PM)


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Eddgie
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Re: C14HD full illuminated field new [Re: Lee Jay]
      #5699579 - 02/25/13 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).


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Lee Jay
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Reged: 02/27/08

Loc: Westminster, CO
Re: C14HD full illuminated field new [Re: Eddgie]
      #5699599 - 02/25/13 10:32 AM

Quote:

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.




That's most certainly untrue. You can restrict it, to any arbitrary degree you like.

The inside diameter of a T-adapter is in the 1.5" or 38mm range. My sensor is 43.3mm corner to corner. It's entirely possible that a 38mm restriction can add additional shading that wouldn't be present with the roughly 45mm ID restriction of the 2" barrel.


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Eddgie
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Re: C14HD full illuminated field [Re: pbsastro]
      #5699610 - 02/25/13 10:38 AM

Quote:

1” scopes, because they are all the same…




Far from correct. While a 1" f/6.3 scope would give the same brightness as an 8" f/6.3 scope, there would be a huge difference in how they would image.

The true field would be far larger in the 1" scope, but everything in the view would be far smaller.

The role of a telescope is to magnify the image. This is true both visually and photographicly.

While the brightness at the focal plane for these two scopes (1" and 8", both f/6.3) for extented objects is identical, the larger aperture in this case would give 8 times the image scale. Objects that were simply smudges in the 1" scope would be large enough to see structure and detail in the 8" scope.

If they were all the same, Hubble would be an 80mm triplet APO (if you asked on the refractor forum )

You pick the scope for the target. Small scopes are good for large targets and big scopes are good for small targets.

And scopes in between are good for a little of each.

This is why the large APO is so well respected as an astrograph. It provides surperb off axis illumition over a very large image circle, has high transmission, high contrast, and can be used with focal reducers and focal extenders to get a large variety of image scales.

And by the way.. I have a great paper written by Ken Hutchinsen that has ray traces for the C8, C9, C11, and C14. These ray traces show the illumination level for all these designs over different size image circles using both the redcuer and native configurations. It is an excellent paper, and if you would like a copy, PM me your email address and I will send it to you.

It does not cover the EdgeHD designs, but native they should be somewhat similar. The new Reducers though are far better than the old ones in terms of vignetting in the larger scopes.

One of the main compromises of the standard SCT design is th off axis illumination tradeoff.

Other scopes make even more extreme tradoffs. The Mak Newts have a field that is only fully illuminated over a tiny image cirlce and have illumiontion characteristics that make the SCT look great by comparison.

So, one has to pick one's poison, but if a very wide illuminated field with a large image scale is what you want, get a big astrograph. That is why people sell them.

Don't expect to see much detail in the Ring Nebula if you use a 1" scope.


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pbsastro
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Re: C14HD full illuminated field new [Re: Eddgie]
      #5724669 - 03/10/13 08:10 PM

Sorry for creating this thread and then disappearing. I was off with health problems, but am back now.
Meanwhile I checked Celestron EdgeHD white paper pointed out often by Eddgie. It is a wonderful and unique document in that it is very rare that mass-market manufacturers get so honest and open about a product:
http://www.celestron.com/portal/technologies/edgehd-optics/

It is stated that 11HD and 14HD both have 100% illumination on the central 16mm circle, and 83% illumination at 42mm circle edge , the corners of full-frame chips.
Using the supplied cutout diagrams I made some calculations and got the following:
The diagram focus plane is exactly at 146mm from flange (the recommended distance) for both 11HD and 14HD.
The segment we see at focus plane is exactly 42mm (full-frame diagonal).
If I draw a light cone (using Paint tool) touching the baffle entrance full opening I get exactly 16mm circle for both 11HD and 1HD4.
So all matches perfectly right with the stated values!
Therefore I am quite confident that the stated values are to be trusted.
I any case I would like to check some full-frame flats to confirm the corner 83% value for both 11HD and 14HD.

Pedro


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