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What is the difference here with the views...

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#1 fellers

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Posted 26 June 2006 - 10:14 AM

I have been noticing a different type of view in the fov of my binocs and my refractor. When I look through my stellarvue b-85 binocs the fov almost looks like I am looking at a flat piece of paper. Especially when I am glassing around the star fields around cygnus. It seems like the more stars in the fov the more pronounced it looks to me. But when I look through my refractor the fov does not look this flat. Though it is not a high quality refractor nor binocular for that matter. I am wondering what makes the fov's look different from each other?

Also I was hoping someone could explain to me the difference between distortion and field curvature? At the very edge of fov in my binocs I am noticing the stars stretched out a hair compared to very tight dots in the center to about 90% out.
I have only been into astronomy for about a year and my binocualr experience is nill. Mostly military, and I was an assaulter not an optical man. So I am trying to figure out how to judge if what I am looking at is a decent view or low quality view. :question:
Is my clearsky clock sweet today or what :crazy:

#2 Glassthrower

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Posted 26 June 2006 - 11:22 AM

Jason -

The difference in views could be in the eyepieces. What kind of ep are you using in the scope?

It sounds like your Stellarvue binoc has a better view than my Celestron 25x100. I only get a nice flat area of pinpoints in the center "sweet spot" of my FOV - the rest exhibits increasing degradation as one approaches the edge. In my case, it's a softening of the focus and increased flaring near the edges, although this could actually be coma.

As for "distortion" and "field curvature", I'll leave that to the experts. I have my opinion of what they are, but I'll keep it to myself because I'm sure my explanation is flawed or overly simplistic.

Clear dark flat pin-pointy skies...

MikeG

#3 btschumy

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Posted 26 June 2006 - 11:22 AM

I have been noticing a different type of view in the fov of my binocs and my refractor. When I look through my stellarvue b-85 binocs the fov almost looks like I am looking at a flat piece of paper. Especially when I am glassing around the star fields around cygnus. It seems like the more stars in the fov the more pronounced it looks to me. But when I look through my refractor the fov does not look this flat. Though it is not a high quality refractor nor binocular for that matter. I am wondering what makes the fov's look different from each other?

Since you don't say how the refractor image appears (other than not as flat), it is a bit hard to say. I would hazard a guess that it has to do with the distortion (se below) being different in the two cases, probably because of the eyepieces involved.

Also I was hoping someone could explain to me the difference between distortion and field curvature? At the very edge of fov in my binocs I am noticing the stars stretched out a hair compared to very tight dots in the center to about 90% out.

Distortion is of two types: barrel distortion and pincushion distortion. It isn't related to sharpness of the image, but rather image scale across the FOV. If you look at a square centered in the FOV, with pincushion distortion, the sides of the square will be bowed inwards. With barrel distortion, the sides will bow out. Generally, when looking at a star field, the distortion is not much of an issue. Sometimes, when panning, barrel distortion can give a "rolling" effect that makes some people dizzy.

Field curvature is when the outside of the field of view has a different focal point than the center. You can't get both in focus at the same time but you can get one or the other in focus.

What you describe as the stars being stretched out near the field sound more like coma to me, yet another aberration.

#4 Glassthrower

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Posted 26 June 2006 - 11:24 AM

Field curvature is when the outside of the field of view has a different focal point than the center. You can't get both in focus at the same time but you can get one or the other in focus.


That sounds exactly like my Celestron 25x100.

You must have been typing your reply at the same time I was.

MikeG

#5 EdZ

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Posted 26 June 2006 - 11:59 AM

Also I was hoping someone could explain to me the difference between distortion and field curvature? At the very edge of fov in my binocs I am noticing the stars stretched out a hair compared to very tight dots in the center to about 90% out.



Bill T dedcribed distortion and curvature pretty well. As Bill said, "It (Distortion) isn't related to sharpness of the image". Distortion does not affect the image of the star, it simply moves it in the field of view. Generally it cannot even be seen in astronomy, but can usually be seen when the binocular is used for looking at terrestrial objects where we are accustomed to seeing nice straight line edges of things like buildings and poles.

As for how you notice stars in the outer fov;

If the stars seem slightly bloated, but it can be refocused down to a finer point, then it is curvature as Bill T described.

If the stars seem flared as wedges with the point towards the middle and the flared wedge toward the outer edge, them it is coma. It cannot be focused out.

If the stars seem slightly bloated and are still circular, but it cannot be focused out it is spherical aberration. This can be present even in the center of the field.

If the stars seem slightly bloated and slightly elongated or eliptical then it is astigmatism. it can be focused to a minimum, but not completely out. Althoughit is more of a problem at larger exit pupils than smaller, it generally is not a problem at very low magnifications, say 7x or 8x, but it is a problem at 25x+. This also can be present even in the center.

edz

#6 Joad

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Posted 26 June 2006 - 12:00 PM

Thanks, BTS, for the clarification.

Really serious astrophotographers purchase Ritchey Chretien scopes precisely because of their flat fields and lack of coma. Costs a pretty penny though.

#7 Glassthrower

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Posted 26 June 2006 - 02:40 PM

As for how you notice stars in the outer fov;

If the stars seem slightly bloated, but it can be refocused down to a finer point, then it is curvature as Bill T described.

If the stars seem flared as wedges with the point towards the middle and the flared wedge toward the outer edge, them it is coma. It cannot be focused out.

If the stars seem slightly bloated and are still circular, but it cannot be focused out it is spherical aberration. This can be present even in the center of the field.

If the stars seem slightly bloated and slightly elongated or eliptical then it is astigmatism. it can be focused to a minimum, but not completely out. Althoughit is more of a problem at larger exit pupils than smaller, it generally is not a problem at very low magnifications, say 7x or 8x, but it is a problem at 25x+. This also can be present even in the center.


Thanks for the clarification. What I see in my 25x100 is curvature. The stars can be refocused to sharp, but then the center of the field becomes unfocused.

Clear dark skies...

MikeG

#8 fellers

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Posted 26 June 2006 - 03:06 PM

Ok, that helps to clarify things for me. I never realized how many things are involved in making and judging the quality of optical equipment. Never gave it any thought till lately. Can be quite confusing at first as to what I am really seeing in the optics.
I didn't really explain the view in my refractor because I don't know how to really explain it with the different ep's. I guess the one closest would be the 2" 35mm stratus prototype. That ep still does not appear quite as flat but it does not appear curved either. There are more stars and they look great through it, lol. It is a nice image out to about 80 to 85% of the image. Then they get a little soft. Kind of bloated but not strtched out like the binoculars.

#9 BluewaterObserva

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Posted 26 June 2006 - 04:40 PM

Field flatness is virtually an entire optics field unto itself. Not sure how much you want to get into this.


Better eye pieces may help some, may not though too because it depends on the exact nature of the field not being flat.

#10 fellers

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Posted 26 June 2006 - 05:16 PM

As deep as you want to explain it all to me. I am asking to understand. So I am all ears. ;)


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