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ACF, EdgeHD - I don't get it???

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#26 Starhawk

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Posted 23 April 2013 - 09:35 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 I'm not sure where you've been reading a lot of this since some of it is over a quarter century out of date.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

#27 dscarpa

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Posted 23 April 2013 - 10:24 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

#28 TG

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Posted 23 April 2013 - 11:57 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


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.

#29 coopman

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Posted 23 April 2013 - 06:19 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.

#30 WadeH237

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Posted 23 April 2013 - 10:01 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

#31 Patrick

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Posted 23 April 2013 - 11:13 PM

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

#32 Eddgie

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Posted 24 April 2013 - 08:49 AM

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.

#33 WadeH237

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Posted 24 April 2013 - 09:14 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

#34 jrcrilly

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Posted 24 April 2013 - 09:21 AM

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.

#35 Eddgie

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Posted 24 April 2013 - 09:29 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.

#36 TG

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Posted 24 April 2013 - 03:53 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


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.

Attached Files



#37 WadeH237

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Posted 25 April 2013 - 09:00 AM

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

-Wade






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