Primary mirror defect?

[Moonshane]

The views through most telescopes at 4.5 or slower will be very acceptable with average or better eyepieces and if you have known nothing else then you may be happy for all of your life. A good coma corrector will definitely improve the sharpness of images across the newtonian field and in my opinion are well worth the investment. I got a used paracorr for £165 and it's the best investment I have ever made as it makes my scopes and my eyepieces perform better than without.

if you get a chance to try one at a star party sometime then I suggest you take it.

[wolfpaw]

Designing a wide field eyepiece that is free from astigmatism (not coma - thats from the scope optics) over even 90% of the field of view in an F/4.7 scope is difficult and takes expensive glass types, sophisticated optical design and high levels of quality testing. 

The Maxivision eyepieces are I believe the Meade 5000 SWA's under different branding. The Meade SWA's were decent but not in the Tele Vue league for edge sharpness in fast scopes.

I expect the Maxivisions are very good eyepieces for their cost though but just be realistic in your expectations in the demanding context of an F/4.7 newtonian optical system.

My issue isn't based around a desire for 90% of the field of view to be free of coma but that 90% of the field of view appears to be affected by coma. I'd be happy with 70% free of coma, or even 60%. At the moment, using the Maxvisions, I have 10% free of coma at the most.

[wolfpaw]

Hi Wolfpaw,

as I was out last night for a bit observing I did a quick test at the end, only remembered to check later on, I was too engrossed inviewing, a very brief and quick check.  The scope had well settled about an hour in.

I had a 28mm Maxvision in the scope, and pointed at a fairly bright star such somthing you may typically see in a cluster, but not overly bright like the ones you may see in Orion or M42.  

When I went 10 degrees or so of axis I really could not tell with any confidence. about a third off axis it was obvious something was not symmetrical anymore, at 2/3 or so it was more obvious that a little trail  was beginning to show.  Still. for every day use I'd say it was vey acceptable an useable for about 75 - 80 percent where I would not get annoyed by it, note that it really depends on star brightness, mag  etc how these things show. There was virtually no field curvature worth speaking about in this eyepiece since that can make things worse, with out of focus stars near the edge becoming astigmatic looking if there astigmatism in the eyepiece.

Now into something like my highest quality eyepiece, an 10.5mm pentax XL ( it has a 65 degree FOV).  Really nothing to show for about 80 percent of FOV going wrong to my eyes on this same star, of course it will be dimmer and a bit more noise due to atmosphere and seeing, after that I notice a more obvious dropoff.  Only in the last 5-10 percent or so in this eyepiece does it go a bit wonky due to coma.  Out of all the eyepieces I own the best I can tell, this eyepiece has next to no astigmatism and gives me the best measure of pure coma in the scope I feel.  Of course on a brighter star I may get different results and notice it differently.

Don't take these numbers as a very hard test but more of an indicator

Hope that helps a bit

On a side note:

I agree that coma correctors are good things, it has frequently been on my mind too for the following reason ( without having used  one ) You will see a lot of discussion about diffraction limited optics, the grandest mirrors with 1/10 PV wave optics giving great details and so on and the highest quality eyepieces. The fact is that such numbers only apply to a very small central portion of the mirror where you can extract that performance, coma not only affects stars but also things such as planetary details on the belts of Jupiter and so on.  

You do not need go to go very far off axis for that nice 1/10 mirror to be just a so so mirror in terms of optical performance without coma correction. There is another article somewhere that Shane linked to by John Isaacs, he swears by coma correction from what I have read. John Isaacs is a seasoned observers that will know 10 times more than I ever will in the short time I have been observing. but the moral of it, when you have all those high quality eyepieces and a great mirror and then short-change yourself to not coma correct is a bit of a strange thing in his view.

I'd say theoretically at least, without ever having been privi to a coma  corrector , it makes a lot of sense to me what he preaches. certainly theory support his argument too. that said, theory is theory and what matters is what makes us happy in the views. 

All I do is not use to big FOV eyepieces in such a scope that you and I own, it keeps it acceptable in my view.  In an ideal world if money no object I think I would have a parracor sitting in my scope a lot of the time however.

Thanks for trying out the eyepieces, Alex It's really appreciated. It's raining and blowing a gale here at the moment so my own observations will have to wait. It's forecast to be clear tomorrow so hopefully I'll be able to do it then.

Unfortunately nothing I've read on the subject has convinced me that there isn't something wrong with my set up. I just don't believe that the image in the Maxvisions (the ones I've used the most and so the ones I'm most familiar with) should start to degrade beyond the central 10% of the field of view.

[Tiny Small]

 I just don't believe that the image in the Maxvisions (the ones I've used the most and so the ones I'm most familiar with) should start to degrade beyond the central 10% of the field of view.

That as may be, it's still cheaper and easier to test the eyepiece first. You are correct though, you shouldn't be experiencing what you are with either that scope or those eyepieces. Something is wrong.

[Ags]

Hi Wolfpaw,

as I was out last night for a bit observing I did a quick test at the end, only remembered to check later on, I was too engrossed inviewing, a very brief and quick check.  The scope had well settled about an hour in.

I had a 28mm Maxvision in the scope, and pointed at a fairly bright star such somthing you may typically see in a cluster, but not overly bright like the ones you may see in Orion or M42.  

When I went 10 degrees or so of axis I really could not tell with any confidence. about a third off axis it was obvious something was not symmetrical anymore, at 2/3 or so it was more obvious that a little trail  was beginning to show.  Still. for every day use I'd say it was vey acceptable an useable for about 75 - 80 percent where I would not get annoyed by it, note that it really depends on star brightness, mag  etc how these things show. There was virtually no field curvature worth speaking about in this eyepiece since that can make things worse, with out of focus stars near the edge becoming astigmatic looking if there astigmatism in the eyepiece.

Now into something like my highest quality eyepiece, an 10.5mm pentax XL ( it has a 65 degree FOV).  Really nothing to show for about 80 percent of FOV going wrong to my eyes on this same star, of course it will be dimmer and a bit more noise due to atmosphere and seeing, after that I notice a more obvious dropoff.  Only in the last 5-10 percent or so in this eyepiece does it go a bit wonky due to coma.  Out of all the eyepieces I own the best I can tell, this eyepiece has next to no astigmatism and gives me the best measure of pure coma in the scope I feel.  Of course on a brighter star I may get different results and notice it differently.

Don't take these numbers as a very hard test but more of an indicator

Hope that helps a bit

On a side note:

I agree that coma correctors are good things, it has frequently been on my mind too for the following reason ( without having used  one ) You will see a lot of discussion about diffraction limited optics, the grandest mirrors with 1/10 PV wave optics giving great details and so on and the highest quality eyepieces. The fact is that such numbers only apply to a very small central portion of the mirror where you can extract that performance, coma not only affects stars but also things such as planetary details on the belts of Jupiter and so on.  

You do not need go to go very far off axis for that nice 1/10 mirror to be just a so so mirror in terms of optical performance without coma correction. There is another article somewhere that Shane linked to by John Isaacs, he swears by coma correction from what I have read. John Isaacs is a seasoned observers that will know 10 times more than I ever will in the short time I have been observing. but the moral of it, when you have all those high quality eyepieces and a great mirror and then short-change yourself to not coma correct is a bit of a strange thing in his view.

I'd say theoretically at least, without ever having been privi to a coma  corrector , it makes a lot of sense to me what he preaches. certainly theory support his argument too. that said, theory is theory and what matters is what makes us happy in the views. 

All I do is not use to big FOV eyepieces in such a scope that you and I own, it keeps it acceptable in my view.  In an ideal world if money no object I think I would have a parracor sitting in my scope a lot of the time however.

You can't really compare a 10mm and 28mm EP as regards coma. The 10mm field is basically all on axis because of the magnification, regardless of EP quality. Also, why would the star be dimmer at higher magnification - star brightness is not affected by magnification (except at very low magnification, such as 28mm in a F4.7 scope, where low mags will make the star dimmer due to exit pupil).

[wolfpaw]

That as may be, it's still cheaper and easier to test the eyepiece first. You are correct though, you shouldn't be experiencing what you are with either that scope or those eyepieces. Something is wrong.

Thank you. I've posted quite a lot on SGL about collimation as I've spent a long time trying to get it right. The reason, I think, that collimation became an issue for me was because I knew instinctively that what I was seeing wasn't normal and so I obviously thought it was a collimation problem. I wasn't happy with the collimation because I wasn't happy with what I was seeing but over recent weeks I've proved to myself that the collimation of the scope is absolutely fine and yet despite getting it as accurate as possible the images haven't improved.

[wolfpaw]

P.S - and yet the supernova in M82 was pinpoint sharp, most noticeably in the 10mm Skywatcher Plossl that came with the scope...

[bomberbaz]

I think you would benefit from a fresh pair of experienced eyes giving things the once over. Sometimes we get ourselves so bogged down we may be missing the downright obvious.

steve

[wolfpaw]

Unfortunately, despite the forecast claiming 'clear skies' in my area, it's totally cloudy and starting to rain. The short-term forecast doesn't look good so god only knows when I'll get back outside with the damn thing. I'll resurrect this thread when I have something new to report.

Many thanks to everyone who has given feedback

[Moonshane]

where (approximately) are you in the UK? maybe in the meantime someone could take a look and rule out collimation / confirm you are spot on with it as you say. even if you are then it's another thing that's not creating the issue.

[mdstuart]

The best plan is to get your scope alongside another similar one and compare.

I get sharp stars in my max vision 24mm across my view in a f5

Mark

[Rustysplit]

I would have to wholeheartedly agree with Shane about coma correctors. You do not need to go out and buy a paracorr if the price is out of your budget, it sure is mine. The Altair/TS/Revelation one at about £65-70 new works well. I use the standard SW .9corrector/reducer and find that it works really well. I know a lot of people find that they are quite happy with fast scope and no CC, but I have yet to find someone who does not immediately see the improved view when using one. Even Pentax/Televues will still show the coma if it is there.

I have had nights where I have been very disappointed with the views I was getting. I thought the collimation was good when tested with the cheshire so just started observing. After a while I decided to check and low and behold, it was not as close as I had thought. I now always start a session with a quick check of the secondary alignment and a star test for primary adjustment. Fast scopes only have a relatively small sweet spot for precise collimation, and it does not take much deviation to degrade the sharpness quite a bit, especially if you are really looking for it.

[AlexB67]

You can't really compare a 10mm and 28mm EP as regards coma. The 10mm field is basically all on axis because of the magnification, regardless of EP quality. Also, why would the star be dimmer at higher magnification - star brightness is not affected by magnification (except at very low magnification, such as 28mm in a F4.7 scope, where low mags will make the star dimmer due to exit pupil).

Appreciated. I can see why you say that hope it wasn't to misleading what I wrote.  I am just going by what my eye detects in that post in two different eyepieces.  To me at lower mag it is always easier to see coma where stars appear as brighter points at low mag, but spread out a bit more due to noise and atmosphere at high mag, and subtly become more like extended sources, so coma is more easily seen at low mag. as for astigmatism, that is how I find it in practice.

I Agree that for a perfect point source brightness is brightness and that's it.

I appreciate that doing a comparison wit a 10.5m and a 28mm is not an absolute measure of it to say one is better or worse anyway.  In fact my understanding is that coma is coma, the eyepiece has nothing to with it anyway, it only depends on the mirror, but what I see is what I see. That aside, I am pretty sure the 10.5mm I own is a better optically corrected eyepiece overall I own out of those two, but that's another story. 

[Ags]

Ah, I see now.... that makes more sense.

[wolfpaw]

Well it was clear last night, at last! No moon and moderate(ish) seeing.

I had about four hours at the eyepiece using the 24mm Maxvision, the 16mm Maxvision, the 10mm Skywatcher 'Super' Plossl and my Baader 2.25x barlow.

First off, I'm now fairly sure the optics of the telescope itself are fairly good. I managed to get a good Airy disc by using Sirius and the SW 10mm plossl. I'm no expert at interpreting the disc but I did see about five, well-defined concentric rings, all evenly spaced out with a round, white disc in the centre. It appeared to be the same both sides of the focusing. There was no hint of a pinched mirror or astigmatism.

I looked at M46 using the 16mm Maxvision and the 24mm Maxvision and could easily see the planetary nebula NGC 2438, especially noticeable with averted vision. The cluster looked relatively pinpoint in both EPs over around 60% of the FoV although I believe the 24mm was sharper. They then started to smear slightly from 60% to 80% and were showing definite signs of coma around the edges.

The 24mm Maxvision just about showed six stars in the Trapezium but they weren't visible in the 16mm Maxvision. I suspect it was because of the seeing conditions.

The lovely little multiple star system of Sigma Orionis showed well in both eyepieces. I wanted to try and split the Rigel double but Orion had moved behind a tree by the time I got around to it!

Back to Jupiter, and the moons were still showing quite pronounced coma in the 16mm Maxvision starting within 20% or 30% of the centre of the FoV. Placing the moons in the centre of the FoV and they were pinpoint but as they drifted across the coma would begin to show. It is definitely coma. The pinpoints looked like tiny shuttlecocks with the 'feathers' pointing towards the edges of the field of view. Moons to the right of the centre of the FoV had their 'feathers' pointing right and those to the left pointed to the left. Only the central 15% or so is pinpoint sharp. The Galilean moons are the only subject that I've found it very noticeable on, perhaps because they are so extremely bright. They just won't focus into pinpoints beyond the centre of the FoV.

I also think the problem isn't helped by the eye placement. If I moved my eye even fractionally while looking through the 16mm Maxvision then the distortion would either move slightly or I'd get some flaring in the opposite direction. I don't find it a comfortable EP to use at all.

I'm tempted to contact Explore Scientific and send the EP back as I don't think it's quite right. The alternative is to get a coma corrector but it seems I should be getting better results with the 16mm Maxvision without one.

[AlexB67]

FWIW. I did some back of the envelope scribbles ( with some arguably hand waving assumptions I might add ) , based on that eyepiece and the field stop of the 16mm, which I am assuming is the same as the ES model, it is only a guide anyway I expect to see coma at about 30% off axis becoming visible to the eye, even if in theory it lies well within that, that is assuming good eyesight and assuming no astigmatism overriding to see it in its pure form, but I think that astigmatism is also going to be a factor in that eyepiece, at least to some degree however small also adding to the distortion. 

So, it does not seem entirely unreasonable for a 68 degree eyepiece what you say. Reading my own post again when I tested it without any bias or knowing this figure  I see I said about a third off axis in my 20mm I began to see what I would call definite asymmetry in the eyepiece star shape, my eye sight is less than perfect I'd say not getting any younger    

If at all possible and you could get an expert to look through it and compare with other eyepieces, it would be interesting to see it compared to a TV plossl for example at 15mm or some other premium eyepiece with similar FOV and focal length with very good correction and see what you can observe there before spending on extras like coma correctors.   Of course a TV plossl will only give 50 degrees so will perform better as a percentage of the whole field anyway, but it should give you a good reference indicator of pure coma, since it is better corrected for a fast scope such as yours.  After all the MV is recommended for scopes of f/5 and above, so I would expect it to give some issues besides coma anyway.  

Hope we can make you a satisfied customer and work it out    

[wolfpaw]

FWIW. I did some back of the envelope scribbles ( with some arguably hand waving assumptions I might add ) , based on that eyepiece and the field stop of the 16mm, which I am assuming is the same as the ES model, it is only a guide anyway I expect to see coma at about 30% off axis becoming visible to the eye, even if in theory it lies well within that, that is assuming good eyesight and assuming no astigmatism overriding to see it in its pure form, but I think that astigmatism is also going to be a factor in that eyepiece, at least to some degree however small also adding to the distortion. 

So, it does not seem entirely unreasonable for a 68 degree eyepiece what you say. Reading my own post again when I tested it without any bias or knowing this figure  I see I said about a third off axis in my 20mm I began to see what I would call definite asymmetry in the eyepiece star shape, my eye sight is less than perfect I'd say not getting any younger    

If at all possible and you could get an expert to look through it and compare with other eyepieces, it would be interesting to see it compared to a TV plossl for example at 15mm or some other premium eyepiece with similar FOV and focal length with very good correction and see what you can observe there before spending on extras like coma correctors.   Of course a TV plossl will only give 50 degrees so will perform better as a percentage of the whole field anyway, but it should give you a good reference indicator of pure coma, since it is better corrected for a fast scope such as yours.  After all the MV is recommended for scopes of f/5 and above, so I would expect it to give some issues besides coma anyway.  

Hope we can make you a satisfied customer and work it out    

I do appreciate that. I'm just surprised that there have been quite so many glowing reviews of the same EP from people with the same telescope (or similar). Such reviews are often characterised by phrases like 'clear to the edge', or 'pinpoint sharp for most of the field of view'. So I don't know what's going on that I should have such obvious coma so close to the centre of the field of view when I use it. It is simply impossible to resolve the moons of Jupiter into pinpoint dots of light without coma except within the central 15%-20% (maximum). They just cannot be focused to sharpness without distortion occurring.

Interestingly enough, another member of SGL has contacted me to say that he has discovered a similar problem using exactly the same EP in the same telescope.

To have bought the EP new as a Meade 5000 SWA, which is allegedly what the Maxvisions are, would've cost me about £140. I don't see the point in forking out for a relatively premium eyepiece when it performs no better in a fast scope than something a quarter of the price. This is what makes me believe the coma so close to the centre of the field of view isn't remotely normal for the EP and that it should be performing much better. The blurb accompanying the Meade version states:

The new Series 5000 Super Wide Angle eyepieces boast a 68° apparent field-of-view and image resolution that is razor sharp from edge-to-edge.

Because of its innovative six and seven element design the objects you observe will be rich in detail and contrast with virtually no chromatic aberration.

These eyepieces are made of the finest materials available, and are a perfect optical match for Schmidt-Cassegrains, fast Newtonians and Schmidt-Newtonians.

First light with the new Series 5000 Super Wide Angle eyepieces will be an event you always remember.

I'll give it one more try anyway. I never tested the Maxvision 16mm on Sirius or Betelgeuse and I meant to. It seems to be the brightness of Juptier's moons that is making the effect so noticeable. It's much less noticeable when viewing star fields and clusters.

[John]

I do appreciate that. I'm just surprised that there have been quite so many glowing reviews of the same EP from people with the same telescope (or similar). Such reviews are often characterised by phrases like 'clear to the edge', or 'pinpoint sharp for most of the field of view'. So I don't know what's going on that I should have such obvious coma so close to the centre of the field of view when I use it. It is simply impossible to resolve the moons of Jupiter into pinpoint dots of light without coma except within the central 15%-20% (maximum). They just cannot be focused to sharpness without distortion occurring....

Over the years I've been browsing forums I've noticed that there can be an element of "over-optimism" in initial reporting. I put it down to enthusiasm and I've done it myself from time to time and then found with longer acquaintance with a piece of equipment, or perhaps the opportunity to compare it with it's peers, you get a more rounded experience of it's abilities and downsides.

[itmo]

I only have the 24mm and 34mm MV's but I am really happy with both of them. Cant remember the point where the coma starts(Month of clouds..), but the coma free field of view is large and certainly not just in the middle. The reason why I dumped by hyperions and got MV's is that the hyperions had  a huge issue with coma. They were just like you described, only 10% coma free in the middle. 

[itmo]

As a note: by scope is only 4.9 and something. not 4.7. So this might be one of the reasons why you are seeing coma where others arent. 

[AlexB67]

The reasons Hyperions are not recommended is because of astigmatism in such eyepieces in fast scopes, the main reason why you would find the MVs better more than likely. I qualify that by saying I've never used hyperions, but I expect it is that and probably a bit of field curvature as well not helping when out of focus stars will look astigmatic. Coma will be the same in both eyepieces, whether it is a hyperion or a maxvision. 

[wolfpaw]

I finally got around to making a sketch of the moons as seen through the 16mm Maxvision. In reality Jupiter was much much brighter than it appears in the sketch. As you can see, pinpoint in the centre with rapid deterioration away from the centre.

[wolfpaw]

If it still happens when the scope is cooled and reasonably well collimated then I'd suspect the eyepiece over the scope. Eyepiece optical systems are complex (many elements, many curves, precise spacing) and it's much more likely that the fault lies there, particularly if other eyepieces don't show the issue.

The issue was coma after all. I've posted elsewhere about this and have been given the following information regarding my telescope and the Maxvision 16mm:

At f/4.7, your coma free field is only 1.85mm wide in the center of the field. Your eyepiece has an 18.2mm field stop, almost 10X as wide as the coma-free zone, so only the central 1/10 of the width of field is coma-free.

"Only the central 1/10 of the width of field is coma-free". This pretty much matches my initial description of the coma affecting around 80% of the field of view and the sketch I made of Jupiter and its moons. "1/10" isn't 'periphery', or 'round the edges', where coma is so frequently cited as appearing. It seems that a mere 10% coma-free field of view is actually built into the mechanics of the Maxvision 16mm itself when used in a f4.7 telescope and no amount of endless collimation will change it.

[John]

The issue was coma after all. I've posted elsewhere about this and have been given the following information regarding my telescope and the Maxvision 16mm:

At f/4.7, your coma free field is only 1.85mm wide in the center of the field. Your eyepiece has an 18.2mm field stop, almost 10X as wide as the coma-free zone, so only the central 1/10 of the width of field is coma-free.

"Only the central 1/10 of the width of field is coma-free". This pretty much matches my initial description of the coma affecting around 80% of the field of view and the sketch I made of Jupiter and its moons. "1/10" isn't 'periphery', or 'round the edges', where coma is so frequently cited as appearing. It seems that a mere 10% coma-free field of view is actually built into the mechanics of the Maxvision 16mm itself when used in a f4.7 telescope and no amount of endless collimation will change it.

I saw your thread on this on the other forum.

Is your conclusion that this is a faulty eyepiece then or that it is inevitable when you use wide angle eyepieces with a fast newtonian and no coma corrector ?.

[wolfpaw]

I saw your thread on this on the other forum.

Is your conclusion that this is a faulty eyepiece then or that it is inevitable when you use wide angle eyepieces with a fast newtonian and no coma corrector ?.

Hi John - I guess I'm moving away from it being an issue with the primary or the eyepiece. If the Maxvision 16mm only has a 1/10 coma-free field in my f4.7 telescope then clearly the issue is the parabolic design of the mirror and the fast speed combined with that particular eyepiece. I am only seeing a 10%-20% coma-free field of view which is almost exactly what is predicted based on the various factors.

That said, and I cannot begin to reconcile the numerous testimonies from other people with the same eyepiece in the same speed scope with what I'm seeing myself. I don't see pinpoint sharp moons and yet other people, using the same set-up, apparently do.

I'm not sure I ever read anything before buying my telescope that suggested for a moment that I would be getting 80%+ coma in the FoV, no matter what eyepiece I used. The discussions of coma in an f4.7 telescope that I read were almost entirely restricted to issues around the "edge of the field" or the "periphery". It's probably the biggest reason why I'm so narked with the performance of the Maxvision 16mm on Jupiter's moons.  "Edge of field" aberration I was expecting. I wasn't expecting "80% or 90% aberration" which is on a totally different level, but it seems that is exactly what is predicted based on the speed of the scope and the field stop of the eyepiece. Why do I see it and others don't? I have absolutely no idea.