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Equipment Discussions >> Cats & Casses

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maroubra_boy
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Reged: 09/08/09

Loc: Sydney, Australia
ACF, EdgeHD - I don't get it???
      #5811706 - 04/20/13 08:28 PM

Hi all,

All my reading on commercial SCT's tells me that they all experience spherical aberration, despite the best efforts of the corrector plate. Coma has never been an issue (& at f/10, really????). When anyone has mentioned coma with these, it has always been a misinterpretation of spherical aberration.

So why all this gee-up with 'new' coma correction when coma has never been the problem?

I love my orange tube C8. It doesn't have the special coatings option of the time, but as my main lunar & planetary scope it does alright for me.

I've also had a good look through a new 8" ACF Meade, and as far as transmission is concerned, it leaves my old girl for dead on DSOs. Edge performance wise I can't see any difference. And I've tried!

Am I missing something, or are we being sold the Emperor's New Clothes?


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Bill Barlow
Carpal Tunnel
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Reged: 12/03/07

Loc: Overland Park KS
Re: ACF, EdgeHD - I don't get it??? new [Re: maroubra_boy]
      #5811833 - 04/20/13 09:20 PM

I guess it varies individual to individual on how much coma interferes with the view through the eyepiece. I also own Meade 8" and 12" ACF SCT's that have excellent views, but when I owned Celestron traditional/non-HD SCT's, they were also very good. I guess a little bit of come around the edge of the field doesn't bother me that much since most of my viewing is in the center of the FOV. But others can't tolerate it as much and find the Celestron Edge scopes eliminate it totally.

Bill


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Eddgie
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Re: ACF, EdgeHD - I don't get it??? new [Re: maroubra_boy]
      #5811879 - 04/20/13 09:37 PM

First, they don't all have spherical aberration.

Second, the Edge and ACF designs improve performance not at the center of the field but off axis. At the center there is no coma. You see the coma as you look further from the center of the field.


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maroubra_boy
professor emeritus


Reged: 09/08/09

Loc: Sydney, Australia
Re: ACF, EdgeHD - I don't get it??? new [Re: Eddgie]
      #5811955 - 04/20/13 10:17 PM

I realise coma is only along the edge. My original post noted this.

But I've never heard of coma being visible at f/10 in a reflector, at least not in an f/10 Newtonian. My f/4.5 & f/4 Newts show some, but then again, echoing Bill's post, it doesn't bother me, so I don't use or need a coma corrector. I also have a couple of little Newts with spherical primaries, so I know what they are capable of.

Spherical mirrors (which is what SCT's use as a primary) don't show coma. So how can a scope be described as "coma free" if it isn't there in the first place.

I'm glad to read not all SCTs show SA. This implies that there are deficiencies in those designs that exhibit SA. SA would be seen only at high magnification with the whole spectrum of colours not being pulled in to one point. You don't see this at low power which is where you see coma.

Hmmm, would a spherical mirror show coma in addition to SA? I can't say I've noticed it when using good quality EPs.

The corrector plate deals with SA. The hyperbolic secondary with what edge aberrations there could be with such a fast primary (I think I could be answering some questions for myself).

Also, where coma could be seen in an f/10 SCT, it would exceed the field of view afforded by the set of baffles and the f/ratio of the scope. The baffles act as field limiters, so one could never achieve the same true field as a fast Newt. So I'm at the same problem - I don't see where coma comes into an SCT...


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SeattleScott
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Reged: 10/14/11

Re: ACF, EdgeHD - I don't get it??? new [Re: Eddgie]
      #5811956 - 04/20/13 10:17 PM

A standard SCT absolutely will suffer from some coma. An F10 refractor would not suffer from coma. An F10 reflector would not suffer from coma. But an F10 cassegrain will. For example, my 6" Mak has an F3 mirror with a 4x corrector lens if I understand right. SCT's are similar. So the primary mirrors are very fast. It is only the corrector that gives a high F ratio. Consequently SCT's are quite sensitive to collimation, and subject to coma despite their high F ratio. Granted, the corrector takes care of about 50% of the coma, or 70% if you get a Mak. But some can still get through. So the additional corrector lens in the Celestron Edge series, etc., cleans up the remaining coma and edge distortions.

You might be interested in knowing that my economy widefield eyepiece performs the worst in my F5 reflector, and the best in my F9 refractor. No surprise, right? Keep in mind I mentioned having a Mak, which is F12 but really operating at F14 with the 2" diagonal. Even at F14, even with the thicker corrector lens, edge distortions are still fairly obvious with the Mak. Not so with the F9 refractor. But again, thanks to the Mak corrector lens, the edge distortions are quite a bit better than in my F5 reflector.


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Eddgie
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Re: ACF, EdgeHD - I don't get it??? new [Re: SeattleScott]
      #5811973 - 04/20/13 10:29 PM

Assuming good collimation and fabrication, there is no coma or spherical aberration at the center of the field of the standard SCT.

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maroubra_boy
professor emeritus


Reged: 09/08/09

Loc: Sydney, Australia
Re: ACF, EdgeHD - I don't get it??? new [Re: Eddgie]
      #5812145 - 04/21/13 12:23 AM

Thanks Scott. So, there's an extra set of elements in these. Bit of a shame for me - more glass & surfaces. It's getting to the stage that I may as well as use a brick for a telescope the trend is now to just stick in more & more glass. I'm glad at least Celestron is still offering "standard" SCT optics. Like Bill said, not everyone is bother by coma. Nor am I. But if you use an SCT for photography, then I see the advantage. For visual, I'd rather more photons.

-Edit- typo

Edited by maroubra_boy (04/21/13 12:51 AM)


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jrcrillyAdministrator
Refractor wienie no more
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Reged: 04/30/03

Loc: NE Ohio
Re: ACF, EdgeHD - I don't get it??? new [Re: maroubra_boy]
      #5812154 - 04/21/13 12:31 AM

Quote:

the teen is now to just stick in more & more glass. I'm glad at least Celestron is still offering "standard" SCT optics.




Celestron offers both options, extra glass or no extra glass. Meade offers only the "no extra glass" option. Thus, if the extra glass is the issue then either can meet the requirement.


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rcdk
super member


Reged: 11/13/10

Re: ACF, EdgeHD - I don't get it??? new [Re: maroubra_boy]
      #5812191 - 04/21/13 01:06 AM

Quote:

For visual, I'd rather more photons.

-Edit- typo




Eliminating the coma will have a bigger positive effect on detection than the negligible loss through more glass. Coma spreads out light.

And I don't think the Meade has more glass, but I wouldn't let it deter me from an Edge either.


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coopman
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Loc: South Louisiana
Re: ACF, EdgeHD - I don't get it??? new [Re: rcdk]
      #5812842 - 04/21/13 11:33 AM

Some people are more sensitive to the coma than others, and some SCTs have more coma than others. The 8SE that I used to own was pretty bad with the coma, IMO. All of the comments about the EdgeHD performance that I've seen online are very positive.

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Arizona-Ken
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Reged: 08/31/08

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Re: ACF, EdgeHD - I don't get it??? new [Re: coopman]
      #5812898 - 04/21/13 11:53 AM

I bought my C11 before the release of the Edge version and am pretty happy with it. A friend has a C11 Edge version and we regularly set up next to each other. He bought it primarily for AP and uses Hyper-star.

I have looked through his scope and compared it to mine and do see a difference, but see no reason in my case to upgrade. My views are fine and I see what I want to see. As noted by others, it is a bigger deal to them.

If I had the choice originally to get a regular C11 or an Edge C11 I'm not sure I would spend the extra $$. Then again I'm only a visual observer.

Arizona Ken


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korborh
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Reged: 01/29/11

Re: ACF, EdgeHD - I don't get it??? new [Re: maroubra_boy]
      #5813015 - 04/21/13 12:55 PM

Quote:

Thanks Scott. So, there's an extra set of elements in these. Bit of a shame for me - more glass & surfaces. It's getting to the stage that I may as well as use a brick for a telescope the trend is now to just stick in more & more glass.
-Edit- typo




Do you worry about the number of glass elements in your EP's?


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mistyridge
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Reged: 10/28/05

Loc: Loomis, CA
Re: ACF, EdgeHD - I don't get it??? new [Re: korborh]
      #5813111 - 04/21/13 02:06 PM

I like to use my 82* and 100* EPs for wide field scanning with my Edge C8HD. When I use the Same EPs in my standard SCT (now sold) the coma was a distraction from the wide field experience.

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maroubra_boy
professor emeritus


Reged: 09/08/09

Loc: Sydney, Australia
Re: ACF, EdgeHD - I don't get it??? new [Re: korborh]
      #5813180 - 04/21/13 02:30 PM

Actually I do think about the number of elements. I only do visual, so my concerns are different from photography.

In my search for my own "keep forever" set of EPs, I've gone through a lot of EPs. In the end, I've come down to keeping the number of elements down as much as possible & an EP design that gives the most reasonable image.

Things such as astigmatism, pincushion, field curvature can all be eliminated with correct EP selection. Coma is an atrifact of reflectors, and some EPs deal with it better than others. I'm happy to deal with coma as it only lies along the edge of the FOV at low power, and serious observing is really only done at the centre of the FOV - you move the scope if the object is close to the edge, don't you!? I don't use a coma corrector - for me these only kill photons.

I also limit my AFOV to no more than 70*. These tend to have fewer elements - not always as the Delos range can attest to.

I did a side-by-side comparison between an 8mm LVW & my 9mm TMB Type II Planetary. Yes, the Vixen gave a sharper (marginally) image, but my lowly TMB showed more stars. I've kept my TMB & never more entertained the idea of getting the LVW.

Don't get me wrong, I've seen some ultra wide EPs deliver amazing images. But as I also sketch at the EP, usually perched up a ladder, using these ultra wide EPs (even if they were able to transmit as much light as a smaller EP), the risk of loosing balance is just too great as you need to move your head to use your averted vision for detailed observation. Not a big deal if just looking, but I can go to-and-fro from EP to page hundreds of times during a sketch, loosing balance is a big deal.

No EP selection is a simple one.

But, please keep in mind, all my DSO observation is done with fast Newtonians. For the Moon & planets, I only use my C8 at high power, so coma with it would never be a concern!


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SeattleScott
Pooh-Bah


Reged: 10/14/11

Re: ACF, EdgeHD - I don't get it??? new [Re: maroubra_boy]
      #5813432 - 04/21/13 03:56 PM

One other consideration - the Celestron Edge series has a vent with a microfilter to enable faster cool down without letting dust in the tube. Nothing to do with coma, but it could be a factor in determining which to buy, especially in the larger apertures where cool down is a bigger issue. I understand there is an accessory you can get to cool down an SCT fairly quickly now too.

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wolfman_4_ever
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Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: ACF, EdgeHD - I don't get it??? new [Re: SeattleScott]
      #5813795 - 04/21/13 06:41 PM

that so called microfilter let's just as much dust in as if it was open... The vents are an evil to some users in high humidity areas... I would rather deal with cool down than internal dew issues any day..

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frolinmod
Carpal Tunnel


Reged: 08/06/10

Loc: Southern California
Re: ACF, EdgeHD - I don't get it??? new [Re: wolfman_4_ever]
      #5814317 - 04/21/13 10:30 PM

Quote:

that so called microfilter let's just as much dust in as if it was open...



Hey, at least it keeps out the cat hair, something only us cat owners could appreciate. Damn cat hair gets everywhere and I do mean everywhere. Hack, cough, wheeze.


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coopman
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Reged: 04/23/06

Loc: South Louisiana
Re: ACF, EdgeHD - I don't get it??? new [Re: frolinmod]
      #5814823 - 04/22/13 08:46 AM

Shave the cat!

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Eddgie
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Re: ACF, EdgeHD - I don't get it??? new [Re: maroubra_boy]
      #5814826 - 04/22/13 08:47 AM

Quote:

and serious observing is really only done at the centre of the FOV




If this is your feeling, then obvousulty you have defined the criteria for you own observing routine and no one of us can possibly say that you are incorrect.

How couuld we?


Over 15 years, I have heard people say that they only look at what is at the center of the field and don't really care about off axis performance.

Some people accept very bad coma or field curvature, some accept poor off axis illuminattion, and some even except meaningful aperture reduction. These can be attributes of many other different telescope designs by the way, so this is not limited to only SCTs.

And if you asked any of them, my guess is that they would also agree that they are serious about observing.

But clearly other obesrvers must believe that there is value in having a telecope that provides diffraction limited performance across the entire field of view.

For them, perhaps they beleve that seeing the area around the very center of the field with the most possible fidelity enhances the astethics of the view.

I know I do, and I know that the EdgeHD telescopes are among the best designs on the market today for ensureing that the entire field is presented with as much fidelity as possible.

A good analogy to me is the low rider car with giant bass speakers. The driver clearly only cares that the bass is rendered with as much power and possible and ignores the balance provided by a full range system.

My neighbor has a speaker system (just the speakers) that costs $40,000. Not only does he get better base than any car stereo you could ever listen to... The entire range of music is produced with a rich, fully detailed, natural sound.

If I can have both an excellent image at the center of the field, and an equallly excellent performance at the edge, and I can afford it, and I enjoy it, than that makes the extra money spend on the telescope worth it to me.

For you personally, apparently it would be wasted.

Not for many of us though.


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Qwickdraw
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Re: ACF, EdgeHD - I don't get it??? new [Re: Eddgie]
      #5815093 - 04/22/13 11:58 AM

Quote:


If I can have both an excellent image at the center of the field, and an equallly excellent performance at the edge, and I can afford it, and I enjoy it, than that makes the extra money spend on the telescope worth it to me.

For you personally, apparently it would be wasted.

Not for many of us though.




Ed,

You just summed up the law of diminishing returns using several paragraphs...


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WesC
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Reged: 02/06/13

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Re: ACF, EdgeHD - I don't get it??? new [Re: Qwickdraw]
      #5815452 - 04/22/13 02:32 PM

I really don't understand why anyone would tolerate any distortion or degradation of an image when there's an alternative.
When you buy a 14" scope you are paying a premium for that big mirror. Why would you not want to get the most out of every inch of it? Especially if you're using expensive, wide field eyepieces?


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Seiko4169
sage


Reged: 07/04/09

Re: ACF, EdgeHD - I don't get it??? new [Re: WesC]
      #5815481 - 04/22/13 02:53 PM

Quote:

I really don't understand why anyone would tolerate any distortion or degradation of an image when there's an alternative.
When you buy a 14" scope you are paying a premium for that big mirror. Why would you not want to get the most out of every inch of it? Especially if you're using expensive, wide field eyepieces?




It's simply down to cost and perceived value. Why buy a slower pc when a faster one is available. Why a Ford and not a Ferrari? We all would want the best but life is never that easy.

Edited by Seiko4169 (04/22/13 03:49 PM)


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Eddgie
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Re: ACF, EdgeHD - I don't get it??? new [Re: Seiko4169]
      #5815562 - 04/22/13 03:47 PM

Well, I am in fact on the OPs side on this.

Many people simply don't really care about the off axis performance, and if this is of no concern, then I totally agree that there is absolutly no beneift at all to the EdgeHD over the standard SCT.

It is not that I am trying to talk him into something that he clearly won't value, but rather I was trying to help him understand why others are willing to pay more for a scope with these qualities.

I mean after all, the OP clearly didn't get it. He could not understand why people would care.

And I do see that sentiment repeatedly. I have been told over the last 20 years by many observers that they simply could care less about off axis performance because they only look at what is at the center of the field.

Once someone starts using scopes with superb off axis performance though, it becomes harder to accept the limitations of the standard SCT.

This is what I learned form big refractors... The "Refractor-like" view is (in my opinion) justly deserved because refractors are most often coma free. While they have some field curvature, it is much more tolarable if there is no coma present.

And when I used these big 6" refractors, the field was beautifully sharp right to the edge.

This is why we hear so much bragging from the little toy telescope owners ... They really do have pinpoint stars across the field.

But so does the EdgeHD. Mine is every bit as satisfying to use as my 6" APO. Not bad for $1300.

It only matters if it matters though, so if the OP really doesn't care about off axis performance, there is no amount of justification we can provide to change his mind.

I am OK with that..I will sleep well either way.


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TG
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Re: ACF, EdgeHD - I don't get it??? new [Re: maroubra_boy]
      #5815671 - 04/22/13 04:41 PM

Quote:

I realise coma is only along the edge. My original post noted this.

But I've never heard of coma being visible at f/10 in a reflector, at least not in an f/10 Newtonian. My f/4.5 & f/4 Newts show some, but then again, echoing Bill's post, it doesn't bother me, so I don't use or need a coma corrector. I also have a couple of little Newts with spherical primaries, so I know what they are capable of.

Spherical mirrors (which is what SCT's use as a primary) don't show coma. So how can a scope be described as "coma free" if it isn't there in the first place.

I'm glad to read not all SCTs show SA. This implies that there are deficiencies in those designs that exhibit SA. SA would be seen only at high magnification with the whole spectrum of colours not being pulled in to one point. You don't see this at low power which is where you see coma.

Hmmm, would a spherical mirror show coma in addition to SA? I can't say I've noticed it when using good quality EPs.

The corrector plate deals with SA. The hyperbolic secondary with what edge aberrations there could be with such a fast primary (I think I could be answering some questions for myself).

Also, where coma could be seen in an f/10 SCT, it would exceed the field of view afforded by the set of baffles and the f/ratio of the scope. The baffles act as field limiters, so one could never achieve the same true field as a fast Newt. So I'm at the same problem - I don't see where coma comes into an SCT...




From what I read here, you would benefit from reading this book:

https://www.willbell.com/tm/tm6.htm

E.g., it will teach you, among other things, that:

* Coma is not a property of whether the optic is a reflector or a refractor but its design.

* A spherical mirror can provide coma-free views if the aperture stop is placed at 2x the f.l but it will have SA.

* SA is the inability of the optic to focus rays striking the edge of the optic to the same point as those striking the center.

* Chromatic aberration is the inability of the optic to focus different colors to the same point.

* Due to its design, an traditional spherical primary-secondary f/10 SCT has about the same coma as an f/5 Newtonian.

But go read the book already, it can teach you these things far better than any post here.

Tanveer.


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maroubra_boy
professor emeritus


Reged: 09/08/09

Loc: Sydney, Australia
Re: ACF, EdgeHD - I don't get it??? new [Re: Eddgie]
      #5816046 - 04/22/13 06:29 PM

Thanks for answering my question folks

Tanveer, I was across most of your points, but totally unfamiliar with some. You've explained a lot. Ta, mate,


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Starhawk
Space Ranger
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Reged: 09/16/08

Loc: Tucson, Arizona
Re: ACF, EdgeHD - I don't get it??? new [Re: maroubra_boy]
      #5817154 - 04/23/13 10:35 AM

It seems you are applying a diverse set of rules of thumb which really aren't valid in this case. I'm not sure where you've been reading a lot of this since some of it is over a quarter century out of date.

The entire flat field question for SCT optics is because they have a strongly curved Petzval surface (i.e.the image plane is an image sphere). This is what causes the coma in them, and it is possible to correct it for photography with some minor improvement for visual use. Coma has nothing to do with reflectors- any type of optic can cause it.

There are many scenes where the entire field has to be resolved to get a good image. Examples include star clusters like the Pleiades, M13, M8, M20, double cluster, and so on. This business about serious viewing is only at the center sounds like you've bumped into a pushy salesman.

Minimizing optical elements is a viewpoint predating modern multicoated optics. The technology is in a very different place, now. Don't fear good glass.

Head over to an observing event and try some current SCTs out. From what you have written, it should be a very pleasant surprise.

-Rich


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dscarpa
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Re: ACF, EdgeHD - I don't get it??? new [Re: maroubra_boy]
      #5817243 - 04/23/13 11:24 AM

My regular C9.25 has some coma but it doesn't prevent me from enjoying 100 FOV eyepieces in it and usually I hardly notice it. That said after looking through Jon's 12.5" Discovery with Paracorr next time I used my SCT the coma seemed much more noticeable. I'm going with SIPS in my new 11" F/5 newt and if I get another SCT someday it will be coma free. David

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TG
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Re: ACF, EdgeHD - I don't get it??? new [Re: dscarpa]
      #5817408 - 04/23/13 12:57 PM

Quote:

My regular C9.25 has some coma but it doesn't prevent me from enjoying 100 FOV eyepieces in it and usually I hardly notice it. That said after looking through Jon's 12.5" Discovery with Paracorr next time I used my SCT the coma seemed much more noticeable. I'm going with SIPS in my new 11" F/5 newt and if I get another SCT someday it will be coma free. David




To my eyes, the coma and field curvature in a C9.25 is significantly less than, say, in a (vanilla) C11. The first thing I noticed when I got my first C11 was how curved the field was compared to the C9.25. With the EdgeHD C11, it's immediately obvious how a planet stays sharp all across the field or how you can place the two pairs of the Double-Double near field edge at high power and have them present clean splits and disks. This was never possible with even the C9.25.

Tanveer.


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coopman
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Loc: South Louisiana
Re: ACF, EdgeHD - I don't get it??? new [Re: TG]
      #5818470 - 04/23/13 07:19 PM

When I used my 8SE to view the moon, for example, the field curvature was very obvious. I could either focus the outer portion of the FOV or the center of the FOV, take your pick. It really bothered me.

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WadeH237
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Reged: 02/24/07

Loc: Snohomish, WA
Re: ACF, EdgeHD - I don't get it??? new [Re: coopman]
      #5818855 - 04/23/13 11:01 PM

I am certainly no expert, but I don't think that coma is necessarily linked to field curvature. I have a Meade ACF scope that has easily measurable field curvature, but no coma. I also have a regular Celestron SCT that has coma.

The thing with the Celestron scope is that there is no coma on axis, but as you move off axis, it rapidly manifests (the self guiding chip on my ST-10 guides on little comets). Also, as you move off axis, the onset of coma does not seem proportional to field curvature. Coma increases much more aggressively than the field curvature as you move from the center of the field.

I would welcome any correction on the topic.

Thanks,
-Wade


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Patrick
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Re: ACF, EdgeHD - I don't get it??? new [Re: maroubra_boy]
      #5818985 - 04/24/13 12:13 AM

Coma or reduced coma is not necessarily the big thing. The much flatter field of the Edge scopes is what's important.

A flat field may not be important to you but it will cause star bloating at the outer edges of the field of view. Also, depending on your age, you might find it more difficult to focus at the center of the field of view and still keep the outer edges in focus. If you're younger, you might be able to accommodate the focus range. You said that you don't care about imaging, but with photography it becomes pretty obvious that field curvature bloats stars.

One thing I've noticed with my Edge scope (and I've had a bunch of scopes) is that I no longer have to keep recentering an object to enjoy it. For instance I can observe the trapezium in the Orion nebula off center with the gas clouds filling the fov. I don't have to put it on center for the best views. I also don't have to keep fidgeting with re-centering objects.

The other thing is that with 82 deg and greater AFOV eyepieces, the edge characteristics of the scope become pretty important. I've always enjoyed looking through my standard C11 with my Panoptic 27mm eyepiece (68 deg AFOV), but with a Nagler 31 (82 deg AFOV), the edge of the FOV really takes a beating.

Patrick


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Eddgie
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Re: ACF, EdgeHD - I don't get it??? new [Re: WadeH237]
      #5819397 - 04/24/13 09:49 AM

Quote:

I am certainly no expert, but I don't think that coma is necessarily linked to field curvature. I have a Meade ACF scope that has easily measurable field curvature, but no coma. I also have a regular Celestron SCT that has coma.





In optics, it seems that everything is linked together in some way.

In the standard SCT design, there is indeed a relationship between coma and field curvature.

It would be possible to design a scope with reduced coma using aspheric optics, but the penalty is increased field curvature.

This appears to be the path that Meade has taken. They have allowed a bit more field curvature, but reduced the coma.

Back when the standard SCT design was first introduced to the public, the standard design's coma and curvature was not really an issue because the designer controlled it just enough so that it would not be conspicious when used with the eyepecies of the day.

But that was then and this is now, and 2" wide field eyepeices have become the norm.

Anyone that wants to see the difference shoud put a 55mm Plossl in their SCT. The field will appear very flat and very pinpointy. This is because increasing the focal lenght of the eyepeice flattens the field (so does a focal redcuer simply by lowering the magnification). The decreased magnificaiton also makes the comatic blur small enough that it becomes difficult to resolve.

All the abberations are still there, but they are literally to small to be easily seen as being abberated.

It is universal use of wide field eyepeices that have brought these design deficiences of the 40 year old SCT design to the forefront. If we were all still using 55mm Plossls, we would not be having this converstaion.


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WadeH237
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Re: ACF, EdgeHD - I don't get it??? new [Re: Eddgie]
      #5819427 - 04/24/13 10:14 AM

Thanks for the explanation, but perhaps "linked" was the wrong word. I certainly get that design factors affect many aberrations each.

A statement was made a few posts above that implies that field curvature itself is the cause of coma. It's that relationship that I don't get.

-Wade


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Re: ACF, EdgeHD - I don't get it??? new [Re: WadeH237]
      #5819440 - 04/24/13 10:21 AM

Quote:

A statement was made a few posts above that implies that field curvature itself is the cause of coma. It's that relationship that I don't get.

-Wade




The statement probably meant that defocus due to field curvature can make coma more noticeable (or induce it in aplanatic designs, where coma is not present in focus, but appears when out of focus). It's a secondary effect.


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Re: ACF, EdgeHD - I don't get it??? new [Re: WadeH237]
      #5819449 - 04/24/13 10:29 AM

The field curvature does not cause the coma though it does aggrevate it by having it defocused, but it does not cause the coma.


Maybe (just maybe, because I don't know for sure) that is what the other poster was talking about????

This is why the coma in the SCT can appear worse than the coma from an f/5 Newtonian.

For example, an 8" f/5 Newtonian would have about the same coma as an 8" SCT, but the field is much flatter in the f/5 Newtonain.

A C8 has a radius of curvature of about -271mm. The 8" f/5 Newtoninan though has a field that is much flatter, with an RC = ~ -1015mm.

When you view the field in each scope, the comatic blur for a star the same distance from the center of the field will be much further out of focus in the C8. Both have about the same coma, but because of the defocus, the comatic blur appears much larger in the SCT.

That may be what the other poster meant??

I don't know if that is the case, but the situation is as I described it. The coma can be the same, but if the field is more steeply curved, the defocused blur simply gets large enough to be easily resolved.

The dark adapted eye will struggle to resolve a blur that is smaller than aout 2 arc minutes of apparent field. It can be horribly abbreated, but keep it below this size and you don't see it.

But what happes to a star when you defocus it? Simple answer in the context of this thread is that it gets bigger in diameter.

And when it gets big enough, we see that it is both defocused and comatic.

So while there is no relationship bewtween defocus and coma, defocus may indeed make the presence of coma easier to see.


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TG
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Re: ACF, EdgeHD - I don't get it??? new [Re: WadeH237]
      #5820222 - 04/24/13 04:53 PM Attachment (5 downloads)

Quote:

I am certainly no expert, but I don't think that coma is necessarily linked to field curvature. I have a Meade ACF scope that has easily measurable field curvature, but no coma. I also have a regular Celestron SCT that has coma.

The thing with the Celestron scope is that there is no coma on axis, but as you move off axis, it rapidly manifests (the self guiding chip on my ST-10 guides on little comets). Also, as you move off axis, the onset of coma does not seem proportional to field curvature. Coma increases much more aggressively than the field curvature as you move from the center of the field.

I would welcome any correction on the topic.

Thanks,
-Wade




You are correct, coma is not linked, per se, to field curvature. However, coma is an off-axis aberration. Any optical system showing it on-axis is simply miscollimated. I've attached an image showing what a star image in a scope with a large amount of coma would look like and also when you add defocus (due to field curvature). This assumes no accomodation in the eye and a perfect eyepiece with no coma itself, both of which are faulty assumptions.

Optical systems such as the ACF correct coma so you would never see the comatic pattern on the left. Your flat CCD chip would see the bloated pattern on the right off-axis but it would be symmetrical, just like an on-axis defocused one.

Tanveer.


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WadeH237
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Re: ACF, EdgeHD - I don't get it??? new [Re: TG]
      #5821694 - 04/25/13 10:00 AM

Thanks for the last few posts. That makes sense to me.

-Wade


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