Primary mirror defect?

[Jules Tohpipi]

I had a long session with the 200p and 250px alongside each other on Tuesday night. The results will require a long post - to clearly explain the findings with relevant background information - which unfortunately I don't have time to post tonight. Hopefully tomorrow.

The top-line conclusion is that I am deeply disappointed with the coma this scope is producing. It is such a significant backwards step compared with my 200p. Frankly speaking, regardless of price, I'd take the 200p over the 250px because the negative points of this faster scope heavily outweigh the positive points.

When I write up the full details I'll be choosing and using my words far more carefully because it's still early days with the 250px. I don't underestimate the need to perhaps fettle and familiarise myself better with the bigger scope. Plus I need to give some deeper context to what I was seeing - such that others might be able to better interpret and explain what I was seeing.

Right now though, despite my very best collimations efforts in situ, anything more than half way out from centre ain't worth looking at, alas.

[nicks90]

Karen makes an extremely good point about the mirror clips. Dare I say it, but is it worth you taking a breath and go at this with a more 'mechanical' approach?

You know the EPs work, its great in the 200. You are confident in collimation, if you couldn't collimate then the view in the 200 would be pants too. There you are back with something being wrong with the 250.

So go back to basics. Take the primary cell out and loosen every nut, bolt and screw. Then retighten them to a maximum of finger tight ( and I mean finger tight! Use one finger and thumb on your screw driver to tighten until you just feel resistance) then reinstall it.

Take the flex tube struts off and check they are straight. Check the top tube is round and not bent oval. Take the focussed off and check the focus tube sits straight in the housing of the body. Refit it all and double check the focusser sits square on to the secondary.

Sorry if you've already done this, or teaching you to suck eggs etc but it could be worth giving the lot a quick going over before going back to looking through the eye piece again and getting more frustrated.

[Jules Tohpipi]

Yes, they are all very good points and why I'll not be giving up just yet. I'll give them a go when time allows (tricky) and taking a vvvvery deep breath....

[John]

F/4.7 is a fast newtonian focal ratio, challenging on eyepieces, collimation tolerances are tight, coma is apparent and the mirror figure is harder to produce accurately. F/6 will be notably less demanding in all these areas. These issues apply to all brands of course.

[Karen]

Karen makes an extremely good point about the mirror clips. Dare I say it, but is it worth you taking a breath and go at this with a more 'mechanical' approach?

You know the EPs work, its great in the 200. You are confident in collimation, if you couldn't collimate then the view in the 200 would be pants too. There you are back with something being wrong with the 250.

So go back to basics. Take the primary cell out and loosen every nut, bolt and screw. Then retighten them to a maximum of finger tight ( and I mean finger tight! Use one finger and thumb on your screw driver to tighten until you just feel resistance) then reinstall it.

Take the flex tube struts off and check they are straight. Check the top tube is round and not bent oval. Take the focussed off and check the focus tube sits straight in the housing of the body. Refit it all and double check the focusser sits square on to the secondary.

Sorry if you've already done this, or teaching you to suck eggs etc but it could be worth giving the lot a quick going over before going back to looking through the eye piece again and getting more frustrated.[/quote

If Synta produce all the 250 PX like mine it will have tight mirror clips, focuser off square and secondary mirror not centred.

To square the focuser you have to remove the secondary mirror and mark a spot on the inside of the tube directly opposite the centre of the focuser. Then using a Cheshire site tube to align on the spot adjust the focuser alignment with the three adjuster Allen screws and locking screws. It is similar to adjusting the collimation screws on a primary mirror.

[mitchelln]

There is a quick way to work out if your focuser is off centre before you take everything to bits.

Put a piece of white card onto the OTA wall behind the mirror and opposite the focuser. This is just to give you some contrast. You should also mask the primary. Easy on a Flextube - you just leave the primary cover on, otherwise carefully insert some card behind the secondary to block off the primary. Wind the focuser half way out and insert your collimation cap or sighting tube. Once you have got the secondary central and a nice circle (following the numerous guides out there!), you then slowly wind your focuser all the way in and then out whilst observing the secondary.

If your focuser is square, the white gap around the secondary will get bigger and smaller, but the proportion visible will remain even all the way round it. If the focuser is not square the white area will change out of proportion. On mine more white appeared top left and less bottom left as I wound the focuser in and out. I actually managed to square it using this method alone. Probably not as accurate as removing the secondary and measuring, but still pretty good I think.

I did the same method when I installed my MoonLite focuser. I found a sight tube inserted about half its length worked best to give a nice clear view of the white area around the mirror.

Best way for all focuser and secondary alignment stuff is to use a webcam and nosepiece, but with the webcam lens still in place to get an image of the mirror. You can then check the secondary circle and white area proportions very accurately using sharpcap (or similar) with circles overlaid using Al's collimation aid http://www.iceinspace.com.au/forum/showthread.php?t=62696

[wolfpaw]

I had a long session with the 200p and 250px alongside each other on Tuesday night. The results will require a long post - to clearly explain the findings with relevant background information - which unfortunately I don't have time to post tonight. Hopefully tomorrow.

The top-line conclusion is that I am deeply disappointed with the coma this scope is producing. It is such a significant backwards step compared with my 200p. Frankly speaking, regardless of price, I'd take the 200p over the 250px because the negative points of this faster scope heavily outweigh the positive points.

When I write up the full details I'll be choosing and using my words far more carefully because it's still early days with the 250px. I don't underestimate the need to perhaps fettle and familiarise myself better with the bigger scope. Plus I need to give some deeper context to what I was seeing - such that others might be able to better interpret and explain what I was seeing.

Right now though, despite my very best collimations efforts in situ, anything more than half way out from centre ain't worth looking at, alas.

Did you get the telescope new or used? If new, is there the possibility of returning it? I'm not sure why you should have to contemplate taking the entire thing to pieces if it's still under warranty.

[wolfpaw]

F/4.7 is a fast newtonian focal ratio, challenging on eyepieces, collimation tolerances are tight, coma is apparent and the mirror figure is harder to produce accurately. F/6 will be notably less demanding in all these areas. These issues apply to all brands of course.

So aberration across 50% of the field of view in an f4.7 telescope is both normal and to be expected.

[Moonshane]

So aberration across 50% of the field of view in an f4.7 telescope is both normal and to be expected.

pretty much

http://stargazerslounge.com/topic/207286-primary-mirror-defect/?p=2202728 unless you use a CC

[wolfpaw]

pretty much

http://stargazerslounge.com/topic/207286-primary-mirror-defect/?p=2202728 unless you use a CC

Maybe members of SGL should bear that in mind then when merrily handing out advice as to which telescope people should buy. From now on it would perhaps be advisable if anyone considering getting an f4.7 telescope is told that it will produce aberration across 50% of the FoV and they'll either have to get a coma corrector to make the views workable.

[Moonshane]

personally, I would generally recommend a CC (only about as expensive used as an average quality eyepiece like a BST would be new) for a newt faster than f5. that said, a lot of users of f4.5 scopes seem quite happy with no cc in place - it depends on your 'tolerance'I suppose. even on my f4 scopes, you could get away without using one but the views are definitely enhanced with one.

in general terms I tend to see people on SGL when talking about 10" skywatchers as giving advice similar to that of John above, or at least stating it will be much more demanding on eyepieces than a f6 scope. I am also of the view that the extra aperture is worth it, and of course for the same aperture, a faster focal ratio = more field of view - you have to compromise somewhere.

did you eliminate the other possible sources of distortion like tight mirror clips and collimation?

my advice would be to continue to use the scope and see if you can get on with it. if the worst effects are seen on Jupiter and moons then maybe try another target at that magnification. I think you were using the 16mm on Jupiter which gives 75x? this is quite low power and you may find the effects of coma reduced at higher powers which is what I'd normally use (perhaps 120-180x is my most common range on this target). I'd also recommend if you get a chance, to borrow a premium eyepiece and see the difference as there usually is one. as referred to above, other 'defects' can hide the coma but when you get rid of the defects the coma then becomes more apparent.  

the best views will always rely on adequate cooling, accurate collimation and good quality eyepieces and optics. even then the humidity, seeing and transparency can affect things too, as well as your own eyes - e.g. I wear contact lenses and sometimes I get optical effects from using them such as flares on bright objects. it' a wonder we see anything at all

[faulksy]

personally, I would generally recommend a CC (only about as expensive used as an average quality eyepiece like a BST would be new) for a newt faster than f5. that said, a lot of users of f4.5 scopes seem quite happy with no cc in place - it depends on your 'tolerance'I suppose. even on my f4 scopes, you could get away without using one but the views are definitely enhanced with one.

in general terms I tend to see people on SGL when talking about 10" skywatchers as giving advice similar to that of John above, or at least stating it will be much more demanding on eyepieces than a f6 scope. I am also of the view that the extra aperture is worth it, and of course for the same aperture, a faster focal ratio = more field of view - you have to compromise somewhere.

did you eliminate the other possible sources of distortion like tight mirror clips and collimation?

my advice would be to continue to use the scope and see if you can get on with it. if the worst effects are seen on Jupiter and moons then maybe try another target at that magnification. I think you were using the 16mm on Jupiter which gives 75x? this is quite low power and you may find the effects of coma reduced at higher powers which is what I'd normally use (perhaps 120-180x is my most common range on this target). I'd also recommend if you get a chance, to borrow a premium eyepiece and see the difference as there usually is one. as referred to above, other 'defects' can hide the coma but when you get rid of the defects the coma then becomes more apparent.  

the best views will always rely on adequate cooling, accurate collimation and good quality eyepieces and optics. even then the humidity, seeing and transparency can affect things too, as well as your own eyes - e.g. I wear contact lenses and sometimes I get optical effects from using them such as flares on bright objects. it' a wonder we see anything at all

very good point shane

[mitchelln]

I am really struggling getting my head around this thread now.

I took my 300P to a Wessex Astronomical Society public event on Saturday. Got Jupiter in the binoviewers with a pair of decent (but nothing particularly special) 10mm EPs. No CC fitted. Viewing was good. Had Jupiter and 3 moons in view over pretty much the full FOV. Ganymede was nearing the edge (about 75% I'd say) and still looked like a moon to me. About 100 people took a look and I counted 34 "Wows". Everyone was commenting on the moons and not one said "why are they all fuzzy?" which I would have expected at least one person to say if they were.

Other Wessex Astro members also took a look to check out the the binoviewers and were similarly impressed. No "pity about the coma" type comments, just very positive feedback and admiration of good optics.

Okay, this is at high mag, but at a previous event seriously experienced members said my view of M13 @31mm was about the best they had ever seen. It was performing up there with the Society's 14" Meade LX200R that night. I can't remember if I had the CC at that point or not, but it does go to show you can get superb results with a fast Dobo.

[John]

I am really struggling getting my head around this thread now.

I took my 300P to a Wessex Astronomical Society public event on Saturday. Got Jupiter in the binoviewers with a pair of decent (but nothing particularly special) 10mm EPs. No CC fitted. Viewing was good. Had Jupiter and 3 moons in view over pretty much the full FOV. Ganymede was nearing the edge (about 75% I'd say) and still looked like a moon to me. About 100 people took a look and I counted 34 "Wows". Everyone was commenting on the moons and not one said "why are they all fuzzy?" which I would have expected at least one person to say if they were.

Other Wessex Astro members also took a look to check out the the binoviewers and were similarly impressed. No "pity about the coma" type comments, just very positive feedback and admiration of good optics.

Okay, this is at high mag, but at a previous event seriously experienced members said my view of M13 @31mm was about the best they had ever seen. It was performing up there with the Society's 14" Meade LX200R that night. I can't remember if I had the CC at that point or not, but it does go to show you can get superb results with a fast Dobo.

What AFoV did the binoviewer / eyepieces give and was a barlow end piece used to get the image to focus ?

I'm asking this because the amount of distortion seen will increase as the field of view widens and includes more that is off the central axis of the scope. 

Also, though there does not seem much difference, F/4.9 and F/4.7 are a step apart with regard to aberration as their progression is exponential not linear.

[mitchelln]

The binoviewers were at prime focus (with the truss locked back to it's 2nd set of holes). Not sure what the FOV is to be honest, but I guess it was pretty tight. Around 50 degrees or so, maybe less. In fact, I've just realised that by dropping the truss back I'm shortening the focal length making the scope nearer f4.6.

My view of M31 was 72 degrees, but that was at f4.9.

However, Wolfpaw says it is bad from 20% of center. Even an exponential difference does not explain that. Moons that close to Jupiter (going by the sketch) should not be showing bad coma surely? Google would be awash with complaints otherwise.

I wish Wolfpaw was near to me. I'm itching to see this problem first hand and try a few things out!

[John]

I wish Wolfpaw was near to me. I'm itching to see this problem first hand and try a few things out!

Yes, it would be helpful to try another 10" F/4.7 alongside Wolfpaw's scope and see how they compare. 

By the way, moving the secondary closer to the primary (as you did) does not alter the focal ratio of the optical system. It pushes the focal plane outwards a bit so the binoviewer can reach focus but the focal ratio of the primary and it's light cone stays the same and may actually vignette a bit if it's larger in diameter than the secondary diameter where the two intercept.

[mitchelln]

By the way, moving the secondary closer to the primary (as you did) does not alter the focal ratio of the optical system. It pushes the focal plane outwards a bit so the binoviewer can reach focus but the focal ratio of the primary and it's light cone stays the same and may actually vignette a bit if it's larger in diameter than the secondary diameter where the two intercept.

Fair enough, I did wonder actually. Thanks for the info I think I'll hit the binoviewer's prism limit for vignetting before the mirror.

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[wolfpaw]

personally, I would generally recommend a CC (only about as expensive used as an average quality eyepiece like a BST would be new) for a newt faster than f5. that said, a lot of users of f4.5 scopes seem quite happy with no cc in place - it depends on your 'tolerance'I suppose. even on my f4 scopes, you could get away without using one but the views are definitely enhanced with one.

in general terms I tend to see people on SGL when talking about 10" skywatchers as giving advice similar to that of John above, or at least stating it will be much more demanding on eyepieces than a f6 scope. I am also of the view that the extra aperture is worth it, and of course for the same aperture, a faster focal ratio = more field of view - you have to compromise somewhere.

did you eliminate the other possible sources of distortion like tight mirror clips and collimation?

my advice would be to continue to use the scope and see if you can get on with it. if the worst effects are seen on Jupiter and moons then maybe try another target at that magnification. I think you were using the 16mm on Jupiter which gives 75x? this is quite low power and you may find the effects of coma reduced at higher powers which is what I'd normally use (perhaps 120-180x is my most common range on this target). I'd also recommend if you get a chance, to borrow a premium eyepiece and see the difference as there usually is one. as referred to above, other 'defects' can hide the coma but when you get rid of the defects the coma then becomes more apparent.  

the best views will always rely on adequate cooling, accurate collimation and good quality eyepieces and optics. even then the humidity, seeing and transparency can affect things too, as well as your own eyes - e.g. I wear contact lenses and sometimes I get optical effects from using them such as flares on bright objects. it' a wonder we see anything at all

No. I see very little point in taking apart a five-month-old, 500 euro telescope in the hope of 'fixing' something which, according to the sages, isn't actually a problem in the first place. Apparently aberration across 50% of the FoV is to be expected, which is what I'm getting with both MV eyepieces (the strange anomaly of Jupiter's moons excepted). So why screw around with checking on mirror clips, secondary mirrors, etc. etc. It seems my telescope is operating exactly as it's supposed to. Maybe the ones who don't see coma in their f4.7s are ones who should be getting their eyes tested.

[mitchelln]

Sounds like a CC is the cheapest way to go then rather than lose money selling it. Did you see the SW one in the classifieds?

You should check the secondary though. It's easy enough and will affect the overall performance of the scope.

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[wolfpaw]

Sounds like a CC is the cheapest way to go then rather than lose money selling it. Did you see the SW one in the classifieds?

You should check the secondary though. It's easy enough and will affect the performance of the scope.

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Apparently Skywatcher CCs are Rubbish and introduce spherical aberration.

[mitchelln]

Keep your eye out for a Baader then

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[nicks90]

Ooof, erm......... OK

You are right, fast scopes do have abheration of various degrees...but what you are describing seems much more than that. A half hour loosening and correctly tightening a few screws might make a vast improvement to the point you don't 'notice' it anymore.

I can appreciate the frustration and upset, but if its that bad surely its worth checking.

[Moonshane]

No. I see very little point in taking apart a five-month-old, 500 euro telescope in the hope of 'fixing' something which, according to the sages, isn't actually a problem in the first place. Apparently aberration across 50% of the FoV is to be expected, which is what I'm getting with both MV eyepieces (the strange anomaly of Jupiter's moons excepted). So why screw around with checking on mirror clips, secondary mirrors, etc. etc. It seems my telescope is operating exactly as it's supposed to. Maybe the ones who don't see coma in their f4.7s are ones who should be getting their eyes tested.

the point of doing so would be to possibly correct or at least eliminate a problem.  it's entirely up to you what you do in the end. all I can do is advise what I'd do which is check the mirror clips and redo the collimation from scratch, it would take less than 10 minutes to do. taking a newtonian to pieces is not an uncommon thing when undergoing general maintenance and it teaches you a lot about how the scope works. the difficulty here is we are doing our best to help but it's always based on a description/artist's impression of the problem from a relatively inexperienced user (this is not a criticism, just an observation/assumption based on what you have said - sorry if this is inaccurate).

I genuinely hope you get sorted and stress all of my comments have been well meaning and intended as helpful.

[mitchelln]

I agree. Newton famously shoved a bodkin behind his eye to observe the way it distorted his vision. If your mirror clips are squeezing your mirror it will have the same effect. It's worth checking you have no "bodkin issues"!

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[Karen]

No. I see very little point in taking apart a five-month-old, 500 euro telescope in the hope of 'fixing' something which, according to the sages, isn't actually a problem in the first place. Apparently aberration across 50% of the FoV is to be expected, which is what I'm getting with both MV eyepieces (the strange anomaly of Jupiter's moons excepted). So why screw around with checking on mirror clips, secondary mirrors, etc. etc. It seems my telescope is operating exactly as it's supposed to. Maybe the ones who don't see coma in their f4.7s are ones who should be getting their eyes tested.

Checking and adjusting the primary mirror clips takes just a few minutes and then you will be sure the mirror is optimised. Coma will happen in a f4.7 'scope with medium and low power EPs. I mostly go 170x upwards for Jupiter and just about everything else and don't notice any coma. A CC must be the way to go but coma doesn't bother me that much.