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Telescope Focal Ratio

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#1 country sky

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Posted 22 January 2022 - 08:35 PM

Hello All. I'm looking for opinions regarding the focal ratio of telescopes. A little background: My stable includes a 6" and an 8" f/10 sct, a 6" f/8 newtonian and a 4" f/4 newtonian. I'm partial to the slower f ratios as the coma in the faster scopes seem to bother me, but I do like the wider field of views in the faster scopes. My question then is what you think is the 'sweet spot' regarding a focal ratio that doesn't produce noticeable coma in inexpensive Plossl eyepieces and doesn't sacrafice to much of a wide field. I'm not currently in the market for a telescope, but your thoughts might be helpful if I should decide to acquie a new telescope in the future Many thanks for your opinion.



#2 SeattleScott

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Posted 22 January 2022 - 09:02 PM

For a Plossl I would think F5 should work. Now if you want to use an ultrawide you will probably be happier at F6.

Ultimately it is a catch 22 if you go with 2” format and wide field eyepieces. You want wide field but no more than maybe 30mm focal length. So then you are looking at something like 30mm ultrawide or 20mm hyperwide or something in between. And then coma would be an issue. So you can go narrower like Plossl but now you aren’t maximizing the wide view. But given the plan is to just use Plossl, it makes it easy. Just throw a 30NPL or something in there and you are at max FOV for 1.25” format without coma problems or oversized exit pupil.

Scott

Edited by SeattleScott, 22 January 2022 - 09:08 PM.


#3 country sky

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Posted 23 January 2022 - 12:08 AM

Thanks Scott. I cut my teeth on the 6" newtonian and graduated to the 8" sct. I bought the 4" tabletop dob as a grab-n-go scope and thought there was something wrong with the optics when I first looked thru it. I couldn't resolve the stars to pin points as I was accustomed to seeing in the slower scopes. I have since learned what coma is and that it is an issue in fast telescopes. Being just a 4" scope, I didn't want to have to acquire more expensive eyepieces and/or a coma corrector to eliminate the coma. So, if I understand correctly, then an f/5 instrument is about the limit that I could still use my 1.25" plossl's with.



#4 MellonLake

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Posted 24 January 2022 - 11:50 AM

Why not get a coma corrector for those fast Newtonians.   Then the coma won't bother you and you have the focal ratios you want.

 

 

Rob



#5 vtornado

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Posted 24 January 2022 - 05:53 PM

My question then is what you think is the 'sweet spot' regarding a focal ratio that doesn't produce noticeable coma in inexpensive Plossl eyepieces

 

To be accurate, coma is a function of the mirror, and not the eyepiece.  

 As the f ratio decreases, the mirror produces a smaller and smaller coma free zone.

 

Fast optics produce astigmatism in widefield eyepieces.   This has to do with the changing angle of the light cone

as one moves from on axis to the outer field of view.   Usually astigmatism of the eyepice is much worse than the coma in an f/5 scope.  Many times after the astigmatism is fixed with a well corrected eyepiece, the coma becomes more apparent.

 

High power plossl, that look at a very small circle of the image are not bothered by coma of the fast mirror,

but lower powered plossls like a 32mm will start seeing coma, and 2 inch eyepieces (including plossls)

will be even more affected.


Edited by vtornado, 24 January 2022 - 09:35 PM.


#6 SeattleScott

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Posted 24 January 2022 - 07:53 PM

Why not get a coma corrector for those fast Newtonians. Then the coma won't bother you and you have the focal ratios you want.


Rob

Very hard to find 1.25” coma corrector. And for a bigger Dob one would want a 2” one. So in theory it’s not a bad idea but doesn’t work in practice.
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#7 SeattleScott

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Posted 24 January 2022 - 07:57 PM

Thanks Scott. I cut my teeth on the 6" newtonian and graduated to the 8" sct. I bought the 4" tabletop dob as a grab-n-go scope and thought there was something wrong with the optics when I first looked thru it. I couldn't resolve the stars to pin points as I was accustomed to seeing in the slower scopes. I have since learned what coma is and that it is an issue in fast telescopes. Being just a 4" scope, I didn't want to have to acquire more expensive eyepieces and/or a coma corrector to eliminate the coma. So, if I understand correctly, then an f/5 instrument is about the limit that I could still use my 1.25" plossl's with.

Yes Plossl is ok at F5 but probably wouldn’t go lower. Granted options are limited with a tabletop Dob. A 4” tabletop Dob is pretty much an exercise in compromise. Very portable but very real limitations. There really isn’t a great solution there. But the coma should just be an edge distortion issue. If you are seeing it in the middle of the view, that isn’t coma. It’s probably a collimation issue.

Scott

#8 Dwight J

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Posted 24 January 2022 - 08:31 PM

Newer eyepiece designs cope with coma better.  I used a TeleVue Pantopic and Naglers at f4.5 and they provided a nice field.  There might have been coma around the edges but I did not notice it.  I used an Erfle and wow, what seagulls almost to the centre.  



#9 Tony Flanders

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Posted 24 January 2022 - 09:28 PM

Newer eyepiece designs cope with coma better.  I used a TeleVue Pantopic and Naglers at f4.5 and they provided a nice field.  There might have been coma around the edges but I did not notice it.  I used an Erfle and wow, what seagulls almost to the centre.  

Technically, you're talking about eyepiece astigmatism, not coma. Coma is inherent in the image; it's obvious in a photograph. No eyepiece can make it go away; to do that you need to intercept the light cone a good deal closer to the source. The fewer aberrations the eyepiece has, the more obvious the underlying coma will be.

 

Eyepiece astigmatism is what you get when you use a simple eyepiece or a cheap wide-field eyepiece in a scope with a fast focal ratio. You will see it even in scopes that are very well corrected for coma. In general, it's much more prominent than coma.


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#10 SeattleScott

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Posted 24 January 2022 - 09:28 PM

Newer eyepiece designs cope with coma better. I used a TeleVue Pantopic and Naglers at f4.5 and they provided a nice field. There might have been coma around the edges but I did not notice it. I used an Erfle and wow, what seagulls almost to the centre.

Yes, but seagulls are not coma.

Modern designs do better in fast scopes but they can’t correct coma. However there can be worse things than coma. Like you saw in the Erfle.

Certainly there would have been coma present with the Nagler or Panoptic at F4.5. But yeah it might not have been distracting to you. A lot of people would be distracted by coma with superwide or ultrawide at F4.5. So some subjectivity and personal preference here.

Scott

#11 Tony Flanders

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Posted 24 January 2022 - 09:33 PM

But the coma should just be an edge distortion issue. If you are seeing it in the middle of the view, that isn’t coma. It’s probably a collimation issue.


More precisely, it is coma, but it's not supposed to be in the center of the field. The whole point of collimation is to align the optics so that the center of the field of view coincides with the spot where coma is smallest. In theory a Newtonian has zero coma along the optical axis.

 

For what it's worth, I used to use Plossls frequently when I owned a 4.5-inch f/4 StarBlast, and although the eyepiece astigmatism was certainly visible, it never really bothered me much.


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#12 SeattleScott

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Posted 25 January 2022 - 12:18 AM

I thought seagulls were astigmatism and coma was just bloated ovals? Maybe I’m remembering wrong.

Yes Plossl likely about the best compromise for a Starblast. Used those in one some myself. Go wider and you just get coma so really not an ideal situation any way you slice it. The Starblast just is optimized for portability, not performance, so there isn’t much of any way to get great performance out of it. Maybe Orthos? For the price they are good I suppose. It’s just a different type of system than my 4” Vixen Apo, which is optimized for performance first and foremost, and then they made it as portable as they could within those performance constraints. As opposed to a 4” Starblast that is designed for portability first, and then as good of performance as possible given the portability (and price) constraints.

Scott

#13 Asbytec

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Posted 25 January 2022 - 05:04 AM

I thought seagulls were astigmatism and coma was just bloated ovals? Maybe I’m remembering wrong.
 

My understanding is seagulls are a combination of field coma and eyepiece astigmatism. See here. Otherwise you see bloated ovals, which I believe gives the seagull its tail, or astigmatism on either side of best focus. I believe sagittal astigmatism spreads the gull's wings. When both are present, you get a flock of seagulls near the edge of the field of view. 


Edited by Asbytec, 25 January 2022 - 05:23 AM.


#14 Tony Flanders

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Posted 25 January 2022 - 06:38 AM



My understanding is seagulls are a combination of field coma and eyepiece astigmatism. See here. Otherwise you see bloated ovals, which I believe gives the seagull its tail, or astigmatism on either side of best focus. I believe sagittal astigmatism spreads the gull's wings. When both are present, you get a flock of seagulls near the edge of the field of view. 


Symmetric "wings" on either side of the bright center -- or sometimes on four sides -- are astigmatism. Coma stretches away from the bright part in one direction, like a comma or a comet. All three words are derived from the same root.

 

Wikimedia has a nice image of ideal coma here.


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#15 Jon Isaacs

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Posted 25 January 2022 - 06:39 AM

My understanding is seagulls are a combination of field coma and eyepiece astigmatism. See here. Otherwise you see bloated ovals, which I believe gives the seagull its tail, or astigmatism on either side of best focus. I believe sagittal astigmatism spreads the gull's wings. When both are present, you get a flock of seagulls near the edge of the field of view. 

Edit:

 

While I was writing my post, Tony posted his..  I am not sure whether I should just delete this post as Tony's is very simple and easy to understand.  Read this if you want to be further confused. 

 

 

The three off-axis aberrations are coma, astigmatism and field curvature. Coma is present in Newtonians and is a function of focal ratio and distance off-axis. Astigmatism basically means that there are two focal lengths, one tangential and one radial. Field curvature means that the focal plane is curved, that the focus at the edge of the field is not the same as the focus in the center. This is an issue with shorter focal length refractors and SCTs and similar scopes. 

 

When one of these aberrations is present and one is using a scope that is corrected for coma and field curvature, identifying the eyepiece aberrations is pretty easy.  But when all three are present, it can be a real mess.  

 

Norme's image is pretty good. However, astigmatism means there are two focal lengths and off-axis in an eyepiece, it is not normally at best focus for both directions as shown in the image. In my experience, normally the star is close to focus in the radial direction and out of focus tangentially,   With one direction in focus and one out of focus, it focuses to a line. This can be thought of as a combination of field curvature and astigmatism but my interpretation is that one direction is in focus and one is out of focus.

 

 

comaflare.gif

 

So what I see most often in an eyepiece that is not well corrected for off-axis astigmatism in a fast scope that is corrected for all the aberrations is a blur that is longer tangentially than it is radially.  

 

Now combine this the pure coma and you get the seagulls as Norme suggested. 

 

Jon


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#16 SeattleScott

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Posted 25 January 2022 - 07:11 PM

Gotcha thanks guys

#17 country sky

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Posted 25 January 2022 - 09:01 PM

Yes Plossl is ok at F5 but probably wouldn’t go lower. Granted options are limited with a tabletop Dob. A 4” tabletop Dob is pretty much an exercise in compromise. Very portable but very real limitations. There really isn’t a great solution there. But the coma should just be an edge distortion issue. If you are seeing it in the middle of the view, that isn’t coma. It’s probably a collimation issue.

Scott

Scott, Could you clarify for me what you are considering as limitations. Are you speaking of limitations in a 4" newtonian or of newtonians in general? Unless I'm missing something, I would think the limitation of a small newt would be the same as in a large newt. -Thanks.



#18 Jon Isaacs

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Posted 25 January 2022 - 10:39 PM

Scott, Could you clarify for me what you are considering as limitations. Are you speaking of limitations in a 4" newtonian or of newtonians in general? Unless I'm missing something, I would think the limitation of a small newt would be the same as in a large newt. -Thanks.

 

Newtonians scale up nicely.  With a small Newtonian, because of the focuser height and general tube sizes, the secondary obstructions have to be relatively large.  As they become larger and larger, the proportional size of the secondary obstruction becomes smaller and smaller.  With a 130mm F/5 it might be 30% by diameter.  With a very large scope, It can be 15%.

 

When you get to scopes big enough for 2 inch focusers, then coma correctors become practical.. 

 

Jon



#19 SeattleScott

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Posted 26 January 2022 - 03:32 AM

Jon has a valid point. In addition, the Starblast is a F4 scope (clearly coma corrector territory) but has 1.25” focuser which is incompatible with 95% of coma correctors. Not that many would bother spending money on a coma corrector for a $200 scope. So pretty much any eyepiece will have significant edge distortion, except for maybe an Ortho. With the Starblast, coma is basically a design feature. There is no avoiding it. Only trying to ignore it.

On top of that the very short focal length can make high power viewing more difficult, requiring barlows or very short focal length eyepieces.

And of course at F4 it is very sensitive to collimation. Granted some VERY nice, VERY expensive scopes are also F4 or even faster. Just ask Jon. But most of those people have collimation tools that cost as much as a Starblast. Many Starblast users just use a collimation cap or something to ballpark collimation. Fancy collimation tools don’t seem to make a lot of sense for such a cheap scope.

Also there are some limitations regarding the price point and build quality. The focuser isn’t the greatest, there can be some slop to make it hard to get precise focus, or collimation for that matter. Also it is hard to make a high precision F4 mirror at this price point. Shoot Jon might pay $200 for a nice secondary mirror for one of his scopes. Between the crude focuser, the questionable optical quality, the typical rudimentary collimation tool used at this price point, it can be challenging to get the rated 200x or so magnification out of these scopes.

On the other hand my much more expensive Vixen Apo isn’t as portable, but it can exceed 300x and never needs collimation. Why did Vixen chose F7.7? Likely because that is about the point where field curvature isn’t an issue with refractors and it is about the point where you can control for CA with a high end doublet. The F ratio was chosen with performance in mind. They chose the fastest F ratio that would not compromise performance. As opposed to the Starblast where they chose a blazing fast F ratio for portability even though it compromises performance.

That being said the Starblast is one of the best $200 scopes available. 4.5” aperture at that price and in such a small package is an impressive achievement.

Scott

#20 country sky

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Posted 26 January 2022 - 05:46 PM

Thanks Scott / Jon. You both make some points that I've missed. I was obviously expecting the same performance out of this small scope as I was getting out of the 6" & 8" scopes. I bought this little scope as a grab-n-go scope and I think I can still use it as such.....If I can learn to ignore the coma at the edges. I'm always learning something new about telescopes!


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