Primary mirror defect?

[wolfpaw]

I've had an on-going issue with distortion ever since I had the telescope and as time's gone on, and I get more used to using it, it's starting to become more of a concern.

This is one example: I'm using a 16mm Maxvision 68 degree EP to view Jupiter and the four Gallilean moons. Jupiter looks fine through the eyepiece. When the atmosphere calms for a few moments I can see the GRS and clear details within the dark belts of cloud.

However the moons only appear as perfect pinpoints of light in the exact centre of the field of view. Once they deviate away from the centre point by around 10% they start to fray at the edges and can't be brought back to perfect pinpoints. If I then defocus slightly I get a round circle of light with a black spot in the centre (as you do when star-testing). But the black spot is not in the centre of the white circle but displaced far over to one side. It's not diffraction spikes caused by the spider vanes. It's a definite lateral fraying.

This isn't at the edge of the field of view but within the central 20-30% (it continues all the way to the edge, gradually getting worse). Is this coma? I think it is. When I got the telescope I knew that coma at the edges would be apparent but I would never have expected coma to be prevalent with a relatively good EP so close to the centre. At the moment, using the moons of Jupiter as a guide, 80%-90% of the field of view seems to be affected by slight distortion.

I first thought this was a problem with the wide-angle nature of the Maxvision EPs. I also have the 24mm one and it too has a similar issue when looking at star clusters, etc. The cheap Skywatcher plossls that came with the scope resolve the stars into pinpoints of light easier than the Maxvision. The other night I viewed the supernova in M82 and the pinpoint of light looked sharper in my Skywatcher 10mm plossl than in the barlowed 16mm Maxvision.

It's something I can really only work on when the nights are clear so at the moment it's an on-going process of comparing the EPs on different subjects.

What's going on? My big worry is that it's an issue with the primary mirror. I've collimated the scope numerous times and the collimation seems fine. Why would Jupiter appear focused and clear, showing details in the cloud belts, when the Gallilean moons appear as frayed specs of light? The distortion isn't enormous but it's obvious (and getting increasingly annoying).

[ChrisLX200]

Well there will certainly be some coma in a short f-ratio Newt, you can buy coma correctors to reduce this. However, I'm not convinced you have the colimation correct - can you describe how you went about it?  Ideally, you would make the fine adjustments using a star test - with the star in the center of the FOV. Slightly de-focus the star image and the diffraction circles should be perfectly concentric around a black middle (and a faint bright dot in the center). If this is not the case then colimation is out.

ChrisH

[Daniel-K]

Hi Wolfpaw i agree with chris 68deg is not a huge Fov so you should get a little less coma in a 16mm. i have the 28mm maxvision in a F4.5 DOB and i get nealry 85% the FOV in focus at one time. after youve collimated do you then recheck it after the scope ha cooled down as it can shift id check your seconday hasnt got a tilt this can afect focusing aswell. mine does this so i collimate twice.

[moriniboy]

 If I then defocus slightly I get a round circle of light with a black spot in the centre (as you do when star-testing). But the black spot is not in the centre of the white circle but displaced far over to one side. It's not diffraction spikes caused by the spider vanes. It's a definite lateral fraying.

Perhaps I have mis-understood but this sounds like it needs collimating, the black disc should be in the centre?

[wolfpaw]

Well there will certainly be some coma in a short f-ratio Newt, you can buy coma correctors to reduce this. However, I'm not convinced you have the colimation correct - can you describe how you went about it?  Ideally, you would make the fine adjustments using a star test - with the star in the center of the FOV. Slightly de-focus the star image and the diffraction circles should be perfectly concentric around a black middle (and a faint bright dot in the center). If this is not the case then colimation is out.

ChrisH

I agree that a fast scope will have coma. I was fully expecting it around the edges. When something is dead centre in the FoV and I de-focus I get the white circle with the black dot in the centre. No problems. But when I do the same with something slightly off centre then the black dot appears to be pushed over to one side. I did this last night with Jupiter's moons. As a moon moved away from the centre of the FoV to the left it started to 'fray'. When I de-focused it the black dot appeared to be pushed over to the right instead of being in the centre. This is just off centre and nowhere near the edge.

[wolfpaw]

Perhaps I have mis-understood but this sounds like it needs collimating, the black disc should be in the centre?

images.jpg

I do get that when something is directly in the centre of the field of view. When it's dead centre and I de-focus I get concentric circles with the black dot in the middle. It's when something moves slightly away from dead centre that the black dot no longer appears in the centre. Surely the black dot should stay centered within the concentric circles across a larger area of the FoV than the central 10%?

[ChrisLX200]

Sounds almost like under-corrected optics, but again, in the absence of any other method of testing then a Star Test will tell you an awful lot about the mirror's figure - you just have to understand what you are looking at.

To get an idea look at this link: http://www.willbell.com/tm/tm5.htm

ChrisH

[ronin]

Doesn't the appearance of the mirrors etc in a fast newtonian when collimating not match that of a standard newtonian?

Seem to recall that something appeaers "off centre" for best results.

Have a search through Astro-Baby's site as I think she covers it.

Other aspects are possibly a pinched mirror, the collimator needs collimating, how well do Maxvisions work on fast scopes.

What happens with EP's other then the Maxvisions?

[ChrisLX200]

Doesn't the appearance of the mirrors etc in a fast newtonian when collimating not match that of a standard newtonian?

Seem to recall that something appeaers "off centre" for best results.

Have a search through Astro-Baby's site as I think she covers it.

Other aspects are possibly a pinched mirror, the collimator needs collimating, how well do Maxvisions work on fast scopes.

What happens with EP's other then the Maxvisions?

That refers to offseting the secondary, it won't affect the shape of the de-focussed star image

ChrisH

[wolfpaw]

Doesn't the appearance of the mirrors etc in a fast newtonian when collimating not match that of a standard newtonian?

Seem to recall that something appeaers "off centre" for best results.

Have a search through Astro-Baby's site as I think she covers it.

Other aspects are possibly a pinched mirror, the collimator needs collimating, how well do Maxvisions work on fast scopes.

What happens with EP's other then the Maxvisions?

The only other EPs I've got are the Skywatcher Plossls that came with the telescope. I've not used them much as I had the Maxvisions when I first had the scope but recently I've been using them as the Maxvisions were disappointing me. I've found that I can bring the moons of Jupiter into slightly greater sharpness in the central 30%-40% of the FoV with the cheap Skywatchers than I can with the Maxvisions. There is still some distortion but not as much.

I just cannot believe that the Skywatcher Plossls should show less distortion than the Maxvisions. The lack of pinpoint stars in the Maxvisions is leading me to think there's an issue with the optics. I'm thinking of sending the primary mirror away to be tested as I'm starting to think it's substandard. From what others have said the Maxvisions should perform well in a fast scope, at least over 80% of the FoV. I'm only getting pinpoint stars over the central 10%. That's dreadful.

[swamp thing]

If the Skywatcher plossls are showing good star images and the mavisions arent. It's the maxvisions.

If you are getting good point source images with one set of eyepieces and you aren't with the other its kinda obvious it's the eyepieces that are the problem. Astigmatism in wide field eyepieces is common. Specially when used in fast optics.

Don't start worrying about your primary mirror.

BTW you'll get coma in a fast scope like yours no eyepiece can correct for this.

[ChrisLX200]

This is one of the reasons expensive premium eyepieces are just that - it is difficult to design (and make) a wide-field eyepiece that performs well with fast optics.

ChrisH

[Moonshane]

I've had an on-going issue with distortion ever since I had the telescope and as time's gone on, and I get more used to using it, it's starting to become more of a concern.

This is one example: I'm using a 16mm Maxvision 68 degree EP to view Jupiter and the four Gallilean moons. Jupiter looks fine through the eyepiece. When the atmosphere calms for a few moments I can see the GRS and clear details within the dark belts of cloud.

However the moons only appear as perfect pinpoints of light in the exact centre of the field of view. Once they deviate away from the centre point by around 10% they start to fray at the edges and can't be brought back to perfect pinpoints. If I then defocus slightly I get a round circle of light with a black spot in the centre (as you do when star-testing). But the black spot is not in the centre of the white circle but displaced far over to one side. It's not diffraction spikes caused by the spider vanes. It's a definite lateral fraying.

This isn't at the edge of the field of view but within the central 20-30% (it continues all the way to the edge, gradually getting worse). Is this coma? I think it is. When I got the telescope I knew that coma at the edges would be apparent but I would never have expected coma to be prevalent with a relatively good EP so close to the centre. At the moment, using the moons of Jupiter as a guide, 80%-90% of the field of view seems to be affected by slight distortion.

I first thought this was a problem with the wide-angle nature of the Maxvision EPs. I also have the 24mm one and it too has a similar issue when looking at star clusters, etc. The cheap Skywatcher plossls that came with the scope resolve the stars into pinpoints of light easier than the Maxvision. The other night I viewed the supernova in M82 and the pinpoint of light looked sharper in my Skywatcher 10mm plossl than in the barlowed 16mm Maxvision.

It's something I can really only work on when the nights are clear so at the moment it's an on-going process of comparing the EPs on different subjects.

What's going on? My big worry is that it's an issue with the primary mirror. I've collimated the scope numerous times and the collimation seems fine. Why would Jupiter appear focused and clear, showing details in the cloud belts, when the Gallilean moons appear as frayed specs of light? The distortion isn't enormous but it's obvious (and getting increasingly annoying).

everything you say sounds pretty normal to me. if you move a bright object off centre and defocus a lot, the spider will show off centre.

at f4.7 there will be coma quite close to the point of axis even with decent eyepieces. a coma corrector will improve things a lot.

[AlexB67]

While a fast scope will have coma. I have an f4.7 scope, and even at higher mag when the moons spread over larger area they do not noticeably deform, best I can tell anyway. Perhaps you have hyper sensitive eyes to type this defect.

I would say I am one of the harsher critics when it comes to coma compared to some owning this scope, where in many cases they are quite happy to live with 82 degree eyepieces where I think 60 - 70 degree is a sort of limit for me.  It sounds a bit severe to me what you are experiencing.  While everything you describe sounds like coma and combination of astigmatism, though I find the MVs suffer very little from the latter.

Some other things to try.  Perhaps your mirror is too tight on the locking screws, pinched optics ?  With some aberrations you can tell if it is the eyepiece by rotating it in the focuser and bringing it out of focus. If the aberration goes with the eyepiece rotation, that is a hint towards the eyepiece. To test for the various issues this is also quite a useful read..

www.backyardastronomy.com/...files/Appendix%20A-Testing_1.pdf‎

may well be the same to what was already posted earlier.

One point to add, because of the pinpoint sharpness in the MVs (I find anyway for most of the FOV), you may notice aberrations more readily due to brighter images, tighter stars , and therefore star shapes deforming.

Hope you can work out a satisfactory solution that makes you happy, or else you just may have to invest/try a coma corrector and see how you go.

Good luck

[wolfpaw]

If the Skywatcher plossls are showing good star images and the mavisions arent. It's the maxvisions.

If you are getting good point source images with one set of eyepieces and you aren't with the other its kinda obvious it's the eyepieces that are the problem. Astigmatism in wide field eyepieces is common. Specially when used in fast optics.

Don't start worrying about your primary mirror.

BTW you'll get coma in a fast scope like yours no eyepiece can correct for this.

When it's clear again I intend to do an exact comparison between the Maxvisions and the Skywatcher Plossls.

I am surprised though that having 90% of the FoV affected by distortion is regarded as normal in an f4.7 scope using what are essentially Meade 5000 SWA eyepieces. I know others with the same scope and the same EPs have said that they have pinpoint sharpness across most of the FoV.

[John]

Just on a point of clarity, to avoid confusion, coma is generally generated by the scope optics and astigmatism by the eyepiece. While a really well corrected eyepiece will show little or no astigmatism, the coma will still be there (assuming no coma corrector is in use). 

Coma is a distortion that makes stars look like little comets with their tails pointing away from the center of the field of view and the tails increasing in length as you move the star away from the central optical axis towards the edges of the field of view. Astigmatism seems to distort the stars the other way, around the field of view hence the comment sometimes used about "seagulls" - the astigmatism seems to create tiny wings on the stars.

To add to the fun it's quite possible to see a combination of both coma and astigmatism !

It's worth checking to see that the shadow of the secondary mirror remains central within the de-focused star image both sides of focus and that the star image is circular when out of focus on both sides of the focus point.

Some folks seem more sensitive to seeing astigmatism than others I've noticed. From my experience I reckon it's very tough to remove astigmatism in a wide angle eyepiece when the focal ratio gets below around F/6. Those that manage it show the coma in fast newtonians really well ! 

[wolfpaw]

While a fast scope will have coma. I have an f4.7 scope, and even at higher mag when the moons spread over larger area they do not noticeably deform, best I can tell anyway. Perhaps you have hyper sensitive eyes to type this defect.

I would say I am one of the harsher critics when it comes to coma compared to some owning this scope, where in many cases they are quite happy to live with 82 degree eyepieces where I think 60 - 70 degree is a sort of limit for me.  It sounds a bit severe to me what you are experiencing.  While everything you describe sounds like coma and combination of astigmatism, though I find the MVs suffer very little from the latter.

Some other things to try.  Perhaps your mirror is too tight on the locking screws, pinched optics ?  With some aberrations you can tell if it is the eyepiece by rotating it in the focuser and bringing it out of focus. If the aberration goes with the eyepiece rotation, that is a hint towards the eyepiece. To test for the various issues this is also quite a useful read..

www.backyardastronomy.com/...files/Appendix%20A-Testing_1.pdf‎

may well be the same to what was already posted earlier.

One point to add, because of the pinpoint sharpness in the MVs (I find anyway for most of the FOV), you may notice aberrations more readily due to brighter images, tighter stars , and therefore star shapes deforming.

Hope you can work out a satisfactory solution that makes you happy, or else you just may have to invest/try a coma corrector and see how you go.

Good luck

Thanks for the advice I'll try rotating the eyepiece and see what happens. I also need to establish exactly what the nature of the distortion is, whether it's coma, astigmatism or a combination of the two.

I wouldn't consider buying a coma corrector though. There is just no way that 90% of the field of view using a Maxvision EP should be subject to any really noticeable distortions in a normal telescope. If the fault is with the primary mirror then I might as well buy a new primary than lavish a fortune on a coma corrector. If it was merely around the edges then I would live with it.

I'll report back after the next clear night when I get the chance to make detailed notes of exactly how each EP behaves and I can make some sketches of the distortions.

All I can repeat for now is my observations on Jupiter's moon. They are pinpoints in the very centre of the field and when they move away by as little as 10% they start to show distortion and 'fraying'.

[John]

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.

[AlexB67]

I agree that when you really look for It is possible when distortion occurs really quickly. You do not need to go very far off axis at all to see the effect, but the point at which it is obviously detectable and becomes  an annoyance is very much subjective one, and whether the brain just gets conditioned to it. Perhaps sometimes fresh eyes are a better judge than conditioned ones.

Off the top of my head I can't tell you exactly where it occurs in my eyepieces right now, though if I am in the mood and look for it if someone asked, I can tell it is not hard to pick an appropriate target like m42 at low mag and see aberrations easily on bright stars or for that matter most clusters. There is no denying that in a fast newt the spot within which coma stays within the diffraction limit is tiny part of the mirror away from centre, of the order of a few tenths of degrees or less off axis ( or in the  milimeter range for linear ) without coma correction, though you should not be seeing it that soon by eye, unless you got bionic eyes.

It is the results you are getting with the MVs versus the skywatcher eyepieces that puzzles me somewhat, and it would seem unlikely to me that you got two bad ones ( but not impossible).

Also allow at least an hour for the mirror to settle to get the best out of this scope without a fan. I find for planetary views anyway, high mag it takes about that time anyway.

[wolfpaw]

I agree that when you really look for It is possible when distortion occurs really quickly. You do not need to go very far off axis at all to see the effect, but the point at which it is obviously detectable and becomes  an annoyance is very much subjective one, and whether the brain just gets conditioned to it. Perhaps sometimes fresh eyes are a better judge than conditioned ones.

Off the top of my head I can't tell you exactly where it occurs in my eyepieces right now, though if I am in the mood and look for it if someone asked, I can tell it is not hard to pick an appropriate target like m42 at low mag and see aberrations easily on bright stars or for that matter most clusters. There is no denying that in a fast newt the spot within which coma stays within the diffraction limit is tiny part of the mirror away from centre, of the order of a few tenths of degrees or less off axis ( or in the  milimeter range for linear ) without coma correction, though you should not be seeing it that soon by eye, unless you got bionic eyes.

It is the results you are getting with the MVs versus the skywatcher eyepieces that puzzles me somewhat, and it would seem unlikely to me that you got two bad ones ( but not impossible).

Also allow at least an hour for the mirror to settle to get the best out of this scope without a fan. I find for planetary views anyway, high mag it takes about that time anyway.

Would you expect to see non-pinpoint stars over 90% of the field of view using a Maxvision SWA eyepiece? 

[Moonshane]

Would you expect to see non-pinpoint stars over 90% of the field of view using a Maxvision SWA eyepiece? 

I stick by my original comment above and would not expect pinpoint stars for more than about 50% of the field with most eyepieces wider than 50 degrees. you mention spending a lot of money on a coma corrector but I bought a used one recently for £55 and it was pretty good.

obviously rule out the eyepiece but I am pretty sure your 'problem' is coma assuming the fraying is from the centre of field outwards on all stars/moons?

[wolfpaw]

I stick by my original comment above and would not expect pinpoint stars for more than about 50% of the field with most eyepieces wider than 50 degrees. you mention spending a lot of money on a coma corrector but I bought a used one recently for £55 and it was pretty good.

obviously rule out the eyepiece but I am pretty sure your 'problem' is coma assuming the fraying is from the centre of field outwards on all stars/moons?

I'm going to do a thorough test, hopefully tonight if the sky stays clear. I'll record exactly which EP shows what distortion, the nature of the distortion, and what I was looking at, as well as trying out such things as rotating the EP, rotating the way I look through the EP, etc.

I don't have any problem with the telescope showing coma. I knew full well that an f4.7 scope would show coma before I bought it but from everything I read I believed that it was something that usually only affected the edge of the field of view not 90% of the field of view, and not using relatively decent eyepieces. If the EPs cost a tenner from Argos I could appreciate them having an issue with coma. I also read a lot about the Maxvisions before I bought them too and most of the reports from people using them in a 10'' f4.7 scope said that the stars were sharp across most of the field of view. I don't recall reading anything about them that said that 90% of the FoV was affected. That's what makes me think there's something else going on with the set up.

[Moonshane]

This might help explain things better than I can.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4044797/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1

[AlexB67]

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.

[John]

I'm going to do a thorough test, hopefully tonight if the sky stays clear. I'll record exactly which EP shows what distortion, the nature of the distortion, and what I was looking at, as well as trying out such things as rotating the EP, rotating the way I look through the EP, etc.

I don't have any problem with the telescope showing coma. I knew full well that an f4.7 scope would show coma before I bought it but from everything I read I believed that it was something that usually only affected the edge of the field of view not 90% of the field of view, and not using relatively decent eyepieces. If the EPs cost a tenner from Argos I could appreciate them having an issue with coma. I also read a lot about the Maxvisions before I bought them too and most of the reports from people using them in a 10'' f4.7 scope said that the stars were sharp across most of the field of view. I don't recall reading anything about them that said that 90% of the FoV was affected. That's what makes me think there's something else going on with the set up.

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.