Primary mirror defect?

[AlexB67]

if patient you'd get a used altair with the spacer for <£80. there would be no need to change the standard SW crayford as it's not partic. heavy.

I thought one of the issues is that the two inch tube that inserts in the focuser of the SW scopes is that has a rim/baffle at the bottom, can't think of the correct term, but it prevents the coma corrector sitting deep enough. I recall reading about it on CN. You need to cut off the rim or something like that, and file it down a bit. There is a way around it to get it working though.  If I recall correctly this was someone trying to get it to work on the 300 or 350 flextube but I believe all the SW scopes have this unique funny focuser design with the separate 1.25 and 2 inch tubes on the Dobs, he got there in the end though.

It is not a must to replace the focuser, but part of the reason me saying so is that for a coma corrector to work well any marginal tilt errors will reduce how effective it will be, with that in mind a higher grade focuser is often recommended before you go down this route on the SW scopes. That being said, it is easy to get carried way by these things, no doubt satisfactory results can be had with the default focuser I would think.

[Moonshane]

if necessary it's easy enough to take out the drawtube and sand/file away the offending ridge. this would not affect the spacing though of course. I have a baader steeltrack and there's quite a bit of flex (if you apply light finger pressure) when I use my paracorr, and 13mm Ethos. whatever you do is likely to be a compromise.

[Rustysplit]

If you get the SW standard CC, you get a new focuser top ring inc, which allows the use of 2" fittings. It is thinner and does not have the step. I used one on my old 200p, and now use the same CC on my dob.

[wolfpaw]

I thought one of the issues is that the two inch tube that inserts in the focuser of the SW scopes is that has a rim/baffle at the bottom, can't think of the correct term, but it prevents the coma corrector sitting deep enough. I recall reading about it on CN. You need to cut off the rim or something like that, and file it down a bit. There is a way around it to get it working though.  If I recall correctly this was someone trying to get it to work on the 300 or 350 flextube but I believe all the SW scopes have this unique funny focuser design with the separate 1.25 and 2 inch tubes on the Dobs, he got there in the end though.

It is not a must to replace the focuser, but part of the reason me saying so is that for a coma corrector to work well any marginal tilt errors will reduce how effective it will be, with that in mind a higher grade focuser is often recommended before you go down this route on the SW scopes. That being said, it is easy to get carried way by these things, no doubt satisfactory results can be had with the default focuser I would think.

Would the Skywatcher CC take that into account?

[wolfpaw]

If you get the SW standard CC, you get a new focuser top ring inc, which allows the use of 2" fittings. It is thinner and does not have the step. I used one on my old 200p, and now use the same CC on my dob.

Could I just use my 1.25 eyepieces in it or does it still require something extra?

[Rustysplit]

It comes with an adapter so you can use 1.25" ep's, but the actual CC barrel is 2" fit, hence the inc adapter ring.

[Rustysplit]

If you adjust your focuser to take out any slack in it, the standard SW focuser will be fine.

[AlexB67]

Not to discourage it, of course you should do what you feel is right for you, but the SW CC and cant tell you how to configure it, other will be able to help better but it is probably a bit easier to get going.

If I am thinking of the right SW CC, it also acts as a reducer, thus making the focal length even shorter before you start correcting asking even more of your eyepieces in other departments to correct the views. 

No doubt there are happy users of the SW CC so I hope I have not offended many happy users of that product  , but my gut feeling after many reviews and experienced users for visual use the Altair astro is hard to beat.

I also believe the SW CC suffers form spherical aberration more compared to some others if I remember rightly, I hear more mixed opinions about the SW or even Baader CC  over the altair astro product, but  your mileage may vary as they say how that annoys or not.

Not that I would know from practice,  I just like the idea of the way the GSO design works on the same principle as that of the paracorr, which is a proven very successful product so many swear by and the GSO can approach that performance at a very reasonable price.  The fact it increases the focal ratio a bit instead of reducing I see as a positive thing for better overall correction too.

[wolfpaw]

Not to discourage it, of course you should do what you feel is right for you, but the SW CC and cant tell you how to configure it, other will be able to help better but it is probably a bit easier to get going.

If I am thinking of the right SW CC, it also acts as a reducer, thus making the focal length even shorter before you start correcting asking even more of your eyepieces in other departments to correct the views. 

No doubt there are happy users of the SW CC so I hope I have not offended many happy users of that product  , but my gut feeling after many reviews and experienced users for visual use the Altair astro is hard to beat.

I also believe the SW CC suffers form spherical aberration more compared to some others if I remember rightly, I hear more mixed opinions about the SW or even Baader CC  over the altair astro product, but  your mileage may vary as they say how that annoys or not.

Not that I would know from practice,  I just like the idea of the way the GSO design works on the same principle as that of the paracorr, which is a proven very successful product so many swear by and the GSO can approach that performance at a very reasonable price.  The fact it increases the focal ratio a bit instead of reducing I see as a positive thing for better overall correction too.

I'd get the Altair Astro one but now things like having to take a file to the focuser have put me off

It seems that few correctors are actually made to work straight out of the box and most seem to require quite a lot of modifying. Why not just sell something that people can put in the focuser and use?!?

[Moonshane]

they do - it's called a paracorr.   that said, I had to get my focuser (Revelation dual speed) drawtube machine to remove the lip to get it to go all the way in. to be fair, this is a bad design on some focusers rather than the coma correctors.

the issue is that if they get the thing to work out of the box, that engineering costs money which increases the price.

[Rustysplit]

I'd get the Altair Astro one but now things like having to take a file to the focuser have put me off

It seems that few correctors are actually made to work straight out of the box and most seem to require quite a lot of modifying. Why not just sell something that people can put in the focuser and use?!?

 Like the SW one?

[wolfpaw]

 Like the SW one?

Perhaps, although talk of added spherical aberration has put me off a little.

I'm surprised more people don't use a coma corrector given that coma affects such a large part of the field of view even in 68 degree EPs (which are hardly the largest wide-angle available).

[mitchelln]

My Baader 31mm aspheric is 72 degrees and it is sharp almost to the edge with my 300P. It is a 2" EP, but not sure what difference that makes. I think your bad experience with your MaxVision may have led to to think most eyepieces are bad in fast scopes or all fast scopes have very serious coma issues. I don't think this is the case. As you've said previously, if it was a general problem you'd see a lot more threads like this on the forum. I've never seen anything remotely near as bad as only 10-20% coma free area with any of my eyepieces or binoviewers. Or I would have started this thread 18 months ago

If the 250PX is new I think you should just send it back. Something just isn't right here. Buying a £460 Paracorr for a £435 scope doesn't strike me as a cost effective way forward, especially if the scope really is faulty.

[John]

I think it's down to how critically you examine the view. During most observing we are enjoying the view, studying the target object and not paying too much attention to some of the more esoteric factors of the optical performance.

Over the past few years since I've been reviewing and comparing eyepieces for this forum, I've tended to look for these small defects and aberrations much more than I used to. In fact it gets to the point when you are glad to get out of "reviewing" mode and getting back to just enjoying the views again !

Issues such as astigmatism, coma, chromatic aberration and others are there to a greater or lesser extent in virtually all the optics I've looked through. Coma and CA can be pretty much predicted as they are inherent in certain optical designs and specifications. 

I've also found that it's when you are able to compare competing designs back to back, switching frequently between them to view the same object through the same scope under the same conditions, that differences start to show themselves more clearly.

When I was comparing the Baader Hyperions to the Vixen LVW's, both very similar designs and specifications, I was quite happy with the views through a Hyperions, until I put the equivalent LVW in the drawtube, and then the astigmatism of the Hyperions was highlighted by the comparative lack of it in the Vixen equivalent. The Vixens do cost nearly twice as much as the Hyperions and, in this case, they were better corrected eyepieces in scopes with a focal ratio of F/7 or faster, to my eyes at least. So thats what I wrote in the review.

It is also much easier to report dispassionately on items that have not cost you any money, to be honest 

[mitchelln]

I agree John But Jupiter's moons that close to the planet (going by the sketch) displaying severe coma is just not right.

I took this with my Baader Zoom before I got my CC:

The shot is cropped to about 75% of the original frame width. If I had the same coma issues the moons would be a shuttlecock shaped blur surely? I am probably over simplifying though

[wolfpaw]

My Baader 31mm aspheric is 72 degrees and it is sharp almost to the edge with my 300P. It is a 2" EP, but not sure what difference that makes. I think your bad experience with your MaxVision may have led to to think most eyepieces are bad in fast scopes or all fast scopes have very serious coma issues. I don't think this is the case. As you've said previously, if it was a general problem you'd see a lot more threads like this on the forum. I've never seen anything remotely near as bad as only 10-20% coma free area with any of my eyepieces or binoviewers. Or I would have started this thread 18 months ago

If the 250PX is new I think you should just send it back. Something just isn't right here. Buying a £460 Paracorr for a £435 scope doesn't strike me as a cost effective way forward, especially if the scope really is faulty.

Tonight and tomorrow are supposed to be clear. I'm planning on doing some detailed sketches looking through the 16mm and 24mm Maxvisions. And my moons don't look anything like the ones in your rather nice photo. I have never seen them off-axis as anything other than tiny shuttlecocks.

I agree that there's something wrong with the telescope's optical train. My problem is how do I ever convince the retailer that there is an issue without them insisting that it's the collimation, or natural coma in a fast scope, etc. etc. etc.. Most people who have contributed to the thread don't think there's a problem with the scope and they have no real vested interest in the issue, unlike the vendor.

Oh well. I'll do the sketches and post them here when I get the chance.

[mitchelln]

I wonder if you can contact SkyWatcher and get them to state an opinion. If they say you shouldn't be seeing the problem, then that gives you leverage with the retailer. A decent retailer would just replace it though...

[wolfpaw]

I wonder if you can contact SkyWatcher and get them to state an opinion. If they say you shouldn't be seeing the problem, then that gives you leverage with the retailer. A decent retailer would just replace it though...

I agree, but me and the retailer are in France... I sometimes think Charles Messier was the last Frenchman ever to look through a telescope.

I've found an Orion XT8 for 297.51 euros which is a pretty good deal. It doesn't come with a 9x50 finderscope though which, despite having a Rigel, I do use. I guess I could use the one from the 250PX but, if I don't get anywhere with the retailer (which I just know is going to be an enormous struggle), I'd be hoping to off-load it via eBay. I know the Synta mirrors are the same in both Orions and Skywatches but, if the issue is indeed with the telescope, I wouldn't touch a SW again with a bargepole.

[wolfpaw]

Mitchelln - I guess you do astrophotography, right? If so, do you know why there are so few images online showing extensive coma in a photograph? I know most astrophotograpers use a CC but I would've expected to find some showing the effect of coma anyway.

[mitchelln]

Hi

Mitchelln - I guess you do astrophotography, right? If so, do you know why there are so few images online showing extensive coma in a photograph? I know most astrophotograpers use a CC but I would've expected to find some showing the effect of coma anyway.

I guess most serious astro photographers use refractors and/or slower corrected scopes. So they will not have coma issues. They have to spend loads to combat CA and SA instead

But you are right. You just don't see your kind of coma issues even on the more snapshot type pictures you see on this site.

I believe SW even sell a 250PX on a HEQ mount (eek!) geared toward AP, so they must be pretty confident in the optics.

I'm not a serious ap, I just posted that image to illustrate that my 300P doesn't show the awful coma with Jupiter's moons that you are seeing. I can't imagine how frustrating this must be for you.

Come on all you 250PX owners out there, what do you see? I would be expecting loads of cried of anguish on Google if this is a fundamental design or quality problem.

Sent from my GT-N7105 using Tapatalk

[Moonshane]

at <f5 coma will appear worse if your collimation is not accurate - are you absolutely sure yours is? you mention early in this thread the black centre of the secondary moving off centre when an object is off axis. this means you are way too far out of focus for a star test. furthermore, the star you are testing must be dead centre for the test and if off axis would make the collimation appear off. when you collimate the primary during a star test, you must then re-centre the star (I don't use star tests to collimate, just every now and again to check it's still looking OK).

sorry if you are competent at collimation just trying to think of cheaper ways to correct the problem for you.

[mitchelln]

Moonshane is right. I think you should go back to first principles. Check focuser is square (mine wasn't). Check secondary is central to the focuser (again, mine wasn't).

Might even be worth backing off all the collimation screws and winding the secondary all the way back to the holder to ensure it's square and then re-centralise to the focuser again by winding each screw by the same number of turns and then aligning. Then barlow laser or Cheshire to align the primary.

At least then you can be confident everything is aligned. Assuming you haven't done this already of course

Sent from my GT-N7105 using Tapatalk

[wolfpaw]

Moonshane is right. I think you should go back to first principles. Check focuser is square (mine wasn't). Check secondary is central to the focuser (again, mine wasn't).

Might even be worth backing off all the collimation screws and winding the secondary all the way back to the holder to ensure it's square and then re-centralise to the focuser again by winding each screw by the same number of turns and then aligning. Then barlow laser or Cheshire to align the primary.

At least then you can be confident everything is aligned. Assuming you haven't done this already of course

Sent from my GT-N7105 using Tapatalk

I'll have another look at the set-up tomorrow during the day.

I've just written an email to Skywatcher asking them what level of apparent aberration they regard as acceptable themselves. Earlier this evening I did two careful tests on the 24mm MV and the 16mm MV using M44 in Cancer. The 24mm started showing 'shuttlecocks' between 45% and 50% from the centre of the field of view (being generous) and the 16mm from about 35%-40% from the centre of the field of view. The aberration in the 16mm is less noticeable when the moons of Jupiter aren't being used as a test object.

I spent a while looking for the Eskimo planetary nebula using the 24mm Maxvision. I gave up eventually. With so much aberration across the field of view (50%+) it was impossible to distinguish between a star and a very small planetary nebula without bringing each part of the field of view into the centre of the field of view. The stars outside of the 45%-50% sharp zone weren't massively affected but exhibited enough aberration to make it difficult to tell if what I was seeing was a naturally 'soft' nebula or just a star with aberration.

[nicks90]

That is absolutely horrendous and my heart goes out to you with the struggles you are having!

Although i'm a 300p user, so its slightly slower - I certainly have no where near as much image quality issues across my field of view. For info i use panaviews at the longer fl and a standard 15mm skywatcher swa. None are what you would consider premium eps, I wouldnt say they are equivalent to the maxvision/meade5000 eps you are using.

But given good collimation the field of view is very good out to the last 10%. Yes, if you concentrate hard and really look for it you can start to see some abherations in the outer 50%, but you have to look really hard and basically ignore what you are actually looking at/for and be super critical.

I found that the main thing with my trusstube is getting the secondary bang on, otherwise the image really does degrade exponentially. I am averse to tightening things too much, so my secondary adjustment screws are only just tight enough to hold it still when moving the scope during observing. But it does mean it regularly twists on its axis whilst being transported on the back seat of the car. I regularly set up and find the secondary pointing at the back of the scope! But a traditional colly cap is absolutely fantastic at recentering the secondary to the focus tube and making sure its properly 'round', much better IMHO than the laser collimator. But then I use the laser for final adjustments.

Once completed I do a quick star test on polaris - I need to point at polaris anyway, as its the tracking version and needs the power turning on pointing north to track - and its always perfectly bang on.

I find that as long as I;m 95% collimated, the view is great and any abherations are kept to a minimum and not really noticeable, but as soon as the collimation is even slightly out of that magic number - it goes horrendous very very quickly!

[Karen]

Hi I guess most serious astro photographers use refractors and/or slower corrected scopes. So they will not have coma issues. They have to spend loads to combat CA and SA instead

But you are right. You just don't see your kind of coma issues even on the more snapshot type pictures you see on this site.

I believe SW even sell a 250PX on a HEQ mount (eek!) geared toward AP, so they must be pretty confident in the optics.

I'm not a serious ap, I just posted that image to illustrate that my 300P doesn't show the awful coma with Jupiter's moons that you are seeing. I can't imagine how frustrating this must be for you.

Come on all you 250PX owners out there, what do you see? I would be expecting loads of cried of anguish on Google if this is a fundamental design or quality problem.

Sent from my GT-N7105 using Tapatalk

Nope, I don't see any coma on Jupiters moons. Not with TMB planetary type ep's anyway. Hyperions are poor towards the edge but still give pleasing widefield views. I did have a problem with astigmatism which turned out to be overtight primary mirror clips. Synta employ gorillas with screwdrivers.