Eyepiece distortion

[Connor brad]

Hi, I have a 200p skywatcher explorer I was seeing astigmatism when looking through the eyepiece I got tested and they found astigmatism in my eyes anyways they provided me glasses but I'm still seeing the same distortion sometimes the stars look like a deformed diamond or a elongated kite a bit like the umbro symbol at the egdes they look a bit like seagulls anyone know what it could be could be because I have cheap eyepieces I use the eyepieces that came with the scope or could it be a collimation problem? 

[Louis D]

When star testing, astigmatism of the eyepiece will appear as a tangential line on one side of best focus and a radial line on the other side.  At best focus, it will look like an enlarged star with the possibility of small spikes in both directions.

You may also be dealing with field curvature.  Focus on a star in the center and then move it to the edge.  Does the tightness of the star image improve by refocusing?  If so, you've got field curvature in either or both the telescope and eyepiece.

Without a coma corrector, you could also be seeing coma from the mirror near the edge, though it isn't commonly seen in the narrow field of view eyepieces commonly packaged with telescopes.  Generally, the eyepieces have enough aberrations of their own to overwhelm any mirror coma.

What eyepieces came packaged with the scope?  Are you wearing your new eyeglasses at the scope?  To eliminate it being eye astigmatism, rotate your whole head around the eyepiece to see if the aberrations rotate with your eye.  If they don't, they're in the eyepiece.  If they do rotate, they're in your eye.

[John]

Low cost eyepieces used with scopes of F7, F/6, F/5 etc often do show astigmatism in the outer parts of the view.

It looks sort of like this:

[Don Pensack]

4 hours ago, Connor brad said:

Hi, I have a 200p skywatcher explorer I was seeing astigmatism when looking through the eyepiece I got tested and they found astigmatism in my eyes anyways they provided me glasses but I'm still seeing the same distortion sometimes the stars look like a deformed diamond or a elongated kite a bit like the umbro symbol at the egdes they look a bit like seagulls anyone know what it could be could be because I have cheap eyepieces I use the eyepieces that came with the scope or could it be a collimation problem? 

There are aberrations, and there is distortion.

 

Aberrations you might see include:

--astigmatism at the edge of the field.  The star goes from --- on one side of focus to | on the other, with best focus looking like a +  Not all eyepieces work well below f/7-f/8.  If you have a short f/ratio, the number of eyepieces that won't show astigmatism is a highly reduced number.

--coma.  This is from the scope, not the eyepiece.  It makes stars at the edge appear a little like fans, with the point closer to the center of the field.  If this is a problem, a coma corrector becomes a needed accessory.

--chromatic smear.  the star appears to spread with red/orange on one side of the star image and violet/blue on the other.  Some eyepieces have more of it than others.

--light scatter.  the star images are surrounded by a small fuzz of light, which grows brighter as the star grows brighter.  Some eyepieces have superior light control and some don't.

--field curvature.  Usually from the scope, but also possible in the eyepiece.  The stars at field edge are out of focus when the center is focused and vice-versa.  The key is to focus half-way from center to edge to reduce this to the amount your eye can accommodate.

--edge of field brightening.  The edge of the field appears to have a brighter sky background than the center.  Not all that common, but endlessly reported if it's there.

 

Distortion you might see:

--rectilinear distortion (RD).  Makes a straight line running across the center of the field change as it does.  Pincushion (positive RD) looks like ) | ( across the field.  Barrel distortion (negative RD) looks like ( | ) across the field.

--Angular magnification distortion (AMD).  Makes things appear smaller in the center or larger in the center than the edge of the field, like A A A   or A A A.  It's most commonly the latter and looks like the field is rolling over a ball or globe.

Orthoscopic means without distortion and since distortion increases with the width of the apparent field, keeping the eyepiece to 40° or so is just about the only way to make an eyepiece orthoscopic.  All wide eyepieces have distortion and the preferable type for astronomy is RD because it is the least injurious to the image we see.

[Connor brad]

Hi, anyone who is experienced and knows about eyepieces when I look at stars through me telescope they look like elongated diamond shapes or seagull shapes I use the eyepieces that came with me scope I have a 200p skywatcher explorer anyway even in the centre they look elongated and I also see color fringing around them I got tested for astigmatism I got given glasses but I still see these elongated stars and color fringing me mam also see these Abberations could anyone tell me what could be causing this? 

Thanks 

[John]

The stock eyepieces will probably show a distortion called astigmatism in the F/5 200P Explorer. This distortion causes stars to look elongated as they move away from the centre of the field of view.

The colour fringing could be caused by the atmosphere if you are observing objects that are close to the horizon but eyepieces can also introduce false colour as well.

Astigmatism looks sort of like this:

The false colour issue is called lateral colour and looks a bit like this:

The solution may well be to consider moving to better corrected eyepieces.

[Connor brad]

Hi could this be caused by the quality of the eyepieces aswell and I may need to buy a better eyepieces? 

[John]

2 minutes ago, Connor brad said:

Hi could this be caused by the quality of the eyepieces aswell and I may need to buy a better eyepieces? 

Quite possibly.

I've posted in your other thread on this (just started):

Perhaps I should merge these two threads to avoid confusion ?

 

[Connor brad]

On 25/08/2020 at 18:55, John said:

Low cost eyepieces used with scopes of F7, F/6, F/5 etc often do show astigmatism in the outer parts of the view.

It looks sort of like this:

Yes it looks similar to that the eyepieces I use is super 25 and super 10 the eyepieces that came with the scope 

[Connor brad]

On 25/08/2020 at 18:45, Louis D said:

When star testing, astigmatism of the eyepiece will appear as a tangential line on one side of best focus and a radial line on the other side.  At best focus, it will look like an enlarged star with the possibility of small spikes in both directions.

You may also be dealing with field curvature.  Focus on a star in the center and then move it to the edge.  Does the tightness of the star image improve by refocusing?  If so, you've got field curvature in either or both the telescope and eyepiece.

Without a coma corrector, you could also be seeing coma from the mirror near the edge, though it isn't commonly seen in the narrow field of view eyepieces commonly packaged with telescopes.  Generally, the eyepieces have enough aberrations of their own to overwhelm any mirror coma.

What eyepieces came packaged with the scope?  Are you wearing your new eyeglasses at the scope?  To eliminate it being eye astigmatism, rotate your whole head around the eyepiece to see if the aberrations rotate with your eye.  If they don't, they're in the eyepiece.  If they do rotate, they're in your eye.

The super 25 and super 10 eyepieces and I do wear glasses when viewing but I still see these Abberations 

[Connor brad]

On 25/08/2020 at 18:55, John said:

Low cost eyepieces used with scopes of F7, F/6, F/5 etc often do show astigmatism in the outer parts of the view.

It looks sort of like this:

 

These are the shapes I see 

[Connor brad]

Just now, Connor brad said:

 

These are the shapes I see 

I sometimes see like a red fringe around them aswell 

[Louis D]

As long as they get progressively worse from center to edge as you move a bright star from center to edge, it's the eyepiece causing these issues.  The eyepieces are probably simple designs from the past intended for f/10 to f/15 or slower scopes.

[Connor brad]

On 25/08/2020 at 18:55, John said:

Low cost eyepieces used with scopes of F7, F/6, F/5 etc often do show astigmatism in the outer parts of the view.

It looks sort of like this:

Stars look more like pins of light through the Barlow 

[Don Pensack]

The faster the scope (the lower the f/ratio), the more obliguely the rays at the edge of the light cone enter the eyepiece at the edge of the field.

An eyepiece designed to handle light rays at more parallel entry angles sometimes have problems with the lateral light rays.

This results in astigmatism in the star images in the outer field.

 

Now, some eyepieces just have astigmatism, period, in all focal ratios of scopes.

Others are good down to some Critical F/ratio, which is almost never specified by the maker, since they don't want to lose a lot of sales by being honest their eyepieces don't work in dobs.

But, if you want to correct the astigmatism at the edge of the field in short f/ratio scopes, it can be done.  It entails adding more lenses in the eyepiece to control it.

It entails using more expensive glass types.  Or it entails making the field much narrower.

Because wide fields are more popular these days, the more expensive glass types and more elements routes are the methods followed.

 

The problem is that very few manufacturers actually make astigmatism-free designs (or have astigmatism so low you don't see it).

The absence of lateral field astigmatism is one of the reasons TeleVue is such a popular brand.  I have also seen some recent eyepieces from APM, Baader, Pentax, and Explore Scientific that addressed

the problem.  Note that it isn't every eyepiece from any manufacturer that is good down to f/4.

 

The issue grows worse the longer the focal length of the eyepiece and the shorter the f/ratio of the field.

If you want an astigmatism-free 30mm eyepiece for an f/4.7dob, you have a choice of maybe 3 eyepieces in the entire market.

If, on the other hand, you want a 3.5mm eyepiece for that same dob that is also free of lateral field astigmatism, you have many choices.

Eyepieces that come free with scopes won't likely be as well-corrected as eyepieces you buy in the after-market.

 

If you see astigmatism in the center 50% of the field, it is likely in your eye and you may need to wear glasses at the scope.  There is an easy way to tell if wearing glasses will improve the image quality:

Take a star in the center of the field and defocus it.   If it becomes oval when out of focus, and becomes oval at an angle 90° from that on the other side of focus, you need to wear glasses to eliminate the astigmatism caused by your eye.

Note that this will not correct the astigmatism in the eyepiece, just that caused by your eye.  But I have several eyepieces for which all astigmatism over the entire field disappears with glasses on that are horrible without them.

That is certainly not the fault of the eyepiece.

[Don Pensack]

Read the replies to your query on the other thread on the same subject:

 

[John]

I've merged the two threads that @Connor brad has started into this one. There may be some duplication though.