Coma correctors

[neptune1]

here's my OOUK 12" f4 on home made base with paracorr and 26mm Nagler

Many thanks Shane. It doesn't look unbalanced which was my worry. So that's good. Do you use your paracorr with all your EPs or is it mostly needed for the wide field ones?

[neptune1]

p.s. there was a dead chicken on your grass!

[mitchelln]

This might be true of large apparent objects but not with smaller ones. For example, the last time I used my f/4.7 telescope was to look for the Eskimo planetary nebula. I was expecting it to be very small, almost like an out-of-focus star and so I put in the 24mm 68 degree eyepiece and tried finding it. Unfortunately, with 50% of the field of view obscured by coma, it was impossible to work out what was a faint planetary and what was just a coma-affected star. The only option was to being every part of the field of view into the central 50% of the field of view. Scanning the entire field of view simply wasn't possible because of the aberration. I gave up after a while and haven't used the telescope since.

Seriously, as generally agreed on the other thread, there is something wrong with your scope. If all 250PX owners saw that level of coma it would be all over Google. It isn't.

Sorry to be blunt, but telling everyone they won't be able to distinguish nebula from coma even after all the input from other owners of fast newts is misleading at best.

The EU has the cooling off legislation for Internet purchases. Perhaps you could try a MPCC and send it back if it doesn't work. Or keep an eye on the classifieds here. A MPCC went on here this week for £88. Surely better than not using the scope and being clearly upset about it.

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[Dreadz]

........... It doesn't look unbalanced which was my worry. So that's good. Do you use your paracorr with all your EPs or is it mostly needed for the wide field ones?

You can always add some weight to the primary end to counterbalance heavy EPA's and the various other accessories you may end up using

[Moonshane]

other than the rare occasion when I am using both my big scopes (never as far as I recall) or trying to squeeze and extra last few arc seconds out of the view I have the paracorr in for all eyepieces. I never think about it once in so have not really tested it against higher magnification eyepieces to be fair. good spot re the chicken!

a good thing about the f4 version is that you have a lot more capacity to send the scope down further in the rings which allows you to balance more easily without adding weights. mine is balanced for what you see i.e. telrad, 9x50 finder, paracorr and 26mm nagler which is max weight and it stays stable if I take out the nagler and add a small ortho eyepiece. the only time I need a counterweight (a small box filled with copper coins inserted just inside the end ring) is for the heavy set up towards the horizon.

I find the 4" telrad riser very useful too.

[wolfpaw]

Seriously, as generally agreed on the other thread, there is something wrong with your scope. If all 250PX owners saw that level of coma it would be all over Google. It isn't.

Sorry to be blunt, but telling everyone they won't be able to distinguish nebula from coma even after all the input from other owners of fast newts is misleading at best.

The EU has the cooling off legislation for Internet purchases. Perhaps you could try a MPCC and send it back if it doesn't work. Or keep an eye on the classifieds here. A MPCC went on here this week for £88. Surely better than not using the scope and being clearly upset about it.

Sent from my GT-N7105 using Tapatalk

Well, with respect, the general consensus wasn't that there was something wrong with the telescope. The general consensus was that it was coma. When I asked if 50% coma was expected in an f/4.7 telescope I received the reply: "pretty much, unless you use a coma corrector". You can read the exchange here:

http://stargazerslounge.com/topic/207286-primary-mirror-defect/?p=2245869

I was told on another forum that 80% was expected as well. Either way, it seems strange to read that there's "something wrong" with the telescope in one paragraph and then be advised to fork out for a coma corrector in the following paragraph. Either there's something wrong with the telescope or there isn't. I've been told that there is and that there isn't (sometimes by the same person). As you said, if coma was as bad generally as it is in my scope then the forums would be full of people complaining, and yet they're not. But apparently 50%+ coma is perfectly normal.

As for the EU legislation, it's a bit of a joke IMO. I've contacted the company from whom I ordered the telescope here in France. After a rather protracted exchange of emails they now believe this 'fault' with the telescope can only be a product of a collimation error. They want me to make an eight-hour round trip to Toulouse so they can check it out. There is zero chance of that happening.

Caveat emptor indeed, especially when buying a Skywatcher Skyliner 250PX.

[John]

Caveat emptor indeed, especially when buying a Skywatcher Skyliner 250PX.

Shouldn't that be ",,,,,, when buying an F/4.7 newtonian" ?. It's a feature of the optical design, not that particular brand. 

[wolfpaw]

Shouldn't that be ",,,,,, when buying an F/4.7 newtonian" ?. It's a feature of the optical design, not that particular brand. 

Coma might be a feature of the optical design but I believe the jury is still out on the extent to which it is visible.

[RikM]

 I've contacted the company from whom I ordered the telescope here in France. After a rather protracted exchange of emails they now believe this 'fault' with the telescope can only be a product of a collimation error. 

I also own an f/4.7 Newtonian on a big white MDF base and I would agree with your supplier. Miscollimation brings the 'edge of field' more into play so coma is much more visible. I only <cringe> use superwide 70° rather than ultrawide 82° + eyepieces but they are well corrected for astigmatism. Coma is visible in my lower magnification eyepieces but nowhere near 50% of the field and I certainly don't find it a problem when finding objects.

If it helps, the Ekimo nebula isn't very 'starlike' in appearance. This is how it looked in my 6" Newtonian. This is f/5 but I used an 84° aFOV eyepiece and there was no visible coma at all at that focal length.

[AndyWB]

Well, FWIW, I just looked up my notes on finding the Eskimo nebula in my 250px. It read "Easily identifiable as a fuzzy blue tinged star in 28mm [Maxvision, 68 deg]. A bit hard to find from Wasat. 8mm [bST] showed round blue/green nebula with central star"

[wolfpaw]

I also own an f/4.7 Newtonian on a big white MDF base and I would agree with your supplier. Miscollimation brings the 'edge of field' more into play so coma is much more visible. I only <cringe> use superwide 70° rather than ultrawide 82° + eyepieces but they are well corrected for astigmatism. Coma is visible in my lower magnification eyepieces but nowhere near 50% of the field and I certainly don't find it a problem when finding objects.

If it helps, the Ekimo nebula isn't very 'starlike' in appearance. This is how it looked in my 6" Newtonian. This is f/5 but I used an 84° aFOV eyepiece and there was no visible coma at all at that focal length.

I don't personally agree with the miscollimation theory. What would, specifically, have to be wrong to make "coma much more visible" if the centre of the field of view is already sharp?

[Moonshane]

coma starts from (just off) the central point of axis and increases the further out you go. if you look at the image in this article you'll see what I mean http://www.umich.edu/~lowbrows/reflections/2007/dscobel.27.html

the slower the scope/the narrower the eyepiece field/the higher the magnification the less you get/see.  this article gives more detailed information on coma http://web.telia.com/~u41105032/kolli/kolli.html

a coma corrector does what it says on the tin. they are not overly expensive (I bought one recently for £55 used and delivered) and improve views dramatically. Most people would not consider a BST an expensive purchase at £50?

[wolfpaw]

coma starts from (just off) the central point of axis and increases the further out you go. if you look at the image in this article you'll see what I mean http://www.umich.edu/~lowbrows/reflections/2007/dscobel.27.html

the slower the scope/the narrower the eyepiece field/the higher the magnification the less you get/see.  this article gives more detailed information on coma http://web.telia.com/~u41105032/kolli/kolli.html

a coma corrector does what it says on the tin. they are not overly expensive (I bought one recently for £55 used and delivered) and improve views dramatically. Most people would not consider a BST an expensive purchase at £50?

So, again, how would miscollimation increase the amount of coma seen if the centre of the field of view is clear and sharp?

Also, as has been mentioned by others, if 50%+ apparent coma was normal in an f/4.7 telescope then why is there so little discussion of it on the internet and why is coma only referred to as affecting the "edge of the field" rather than "half or more of the field"?

[scarp15]

Coma should not be an issue with your scope anywhere near the extent that you are implying and prevent you from locating and enjoying views of any DSO or planetary object, in particular an easy to locate and observe object such as NGC 2392. I would assume that your scope has cooled is collimated, that your eyepieces are of reasonable quality and you are observing under adequate dark sky circumstances. Quiet frankly, whilst attractive to have, a coma corrector with this scope is but an optional extra.

[andyman]

Coma should not be an issue with your scope anywhere near the extent that you are implying and prevent you from locating and enjoying views of any DSO or planetary object, in particular an easy to locate and observe object such as NGC 2392. I would assume that your scope has cooled is collimated, that your eyepieces are of reasonable quality and you are observing under adequate dark sky circumstances. Quiet frankly, whilst attractive to have, a coma corrector with this scope is but an optional extra.

+1

[jetstream]

coma starts from (just off) the central point of axis and increases the further out you go. if you look at the image in this article you'll see what I mean http://www.umich.edu/~lowbrows/reflections/2007/dscobel.27.html

the slower the scope/the narrower the eyepiece field/the higher the magnification the less you get/see.  this article gives more detailed information on coma http://web.telia.com/~u41105032/kolli/kolli.html

a coma corrector does what it says on the tin. they are not overly expensive (I bought one recently for £55 used and delivered) and improve views dramatically. Most people would not consider a BST an expensive purchase at £50?

Great link Shane with an excellent explanation of the "sweet spot" and the effects of being near its edges.It also points out the importance of accurate primary mirror center spotting,particularly in faster scopes.Going to read this article a few times.Thanks

[jetstream]

So, again, how would miscollimation increase the amount of coma seen if the centre of the field of view is clear and sharp?

Also, as has been mentioned by others, if 50%+ apparent coma was normal in an f/4.7 telescope then why is there so little discussion of it on the internet and why is coma only referred to as affecting the "edge of the field" rather than "half or more of the field"?

Wolfpaw,when you checked the position of your primary mirror center spot was it in the proper location?Within how many mm of exact center?

[wolfpaw]

Great link Shane with an excellent explanation of the "sweet spot" and the effects of being near its edges.It also points out the importance of accurate primary mirror center spotting,particularly in faster scopes.Going to read this article a few times.Thanks

If the apparent sweet spot was really so small then everyone with a fast Newtonian for visual use would use a coma corrector. AFAIK, they are primarily used by people doing astrophotography. This is my big issue with the claim that the coma I'm seeing in my telescope is remotely normal.

Wolfpaw,when you checked the position of your primary mirror center spot was it in the proper location?Within how many mm of exact center?

I didn't measure it exactly when I took the mirror out. I tried but couldn't find a way of stretching the measure across the face of the mirror without the tape touching the mirror's surface. It seemed reasonably centralised but I couldn't say for sure.

[mitchelln]

Well, with respect, the general consensus wasn't that there was something wrong with the telescope. The general consensus was that it was coma. When I asked if 50% coma was expected in an f/4.7 telescope I received the reply: "pretty much, unless you use a coma corrector". You can read the exchange here:

http://stargazerslounge.com/topic/207286-primary-mirror-defect/?p=2245869

I was told on another forum that 80% was expected as well. Either way, it seems strange to read that there's "something wrong" with the telescope in one paragraph and then be advised to fork out for a coma corrector in the following paragraph. Either there's something wrong with the telescope or there isn't.

I suggested trying a CC to eliminate that possibility. If you still see the problem then it is the scope and you can return the CC. If the CC cures the problem then you are sorted.

EU law is very clear on Internet purchases. You have a cooling off period where you can return the item for a full refund. You do not even have to give a reason.

Sent from my GT-N7105 using Tapatalk

[Moonshane]

So, again, how would miscollimation increase the amount of coma seen if the centre of the field of view is clear and sharp?

Also, as has been mentioned by others, if 50%+ apparent coma was normal in an f/4.7 telescope then why is there so little discussion of it on the internet and why is coma only referred to as affecting the "edge of the field" rather than "half or more of the field"?

miscollimation means that the coma free field is not accurately aligned with the central axis and on an f5 scope the coma free field is about 2.2mm, therefore the area affected by coma increases the more inaccurate your (usually primary) collimation is (and of course vice versa to the maximum). see e.g. http://www.catseyecollimation.com/pensack.pdf there's info all over the internet if you just google it.

as to why you think there has been so little discussion about coma in f4.7 scopes I cannot say but again if you search it there's a lot. personally, I just feel you'd be happier with another scope design, however all of them have their own idiosyncracies and as I have said a few times, it's always going to be a compromise. there is not a single scope that gives affordability, large aperture, manageable size and weight and pin sharp views across wide field (without a flattener/coma corrector) with little or no aberration of any kind.

[AlexB67]

I still think that for a budget solution keeping the existing scope, buying a GSO or Baader CC would be a good way to go for Wolfpaw to try out, as others and I also tried to suggest before as when he raised the coma topic.  It may not be paracorr, but the much cheaper GSO, Baaader alternatives go a large way there to tidying up the view to a very good level I feel from everything I've read.

I suspect the maxvisions he has would still show a small bit of astigmatism perhaps but I suspect ( without having tried it )  it will do a very decent job to sort out the view and go a long way for  a fraction of the cost, instead of  going the entire paracorr TV route if that is too expensive. If he is not bothered about aperture loss perhaps think of something else but it will always be expensive anyway to buy SCTs Maksutov in that size new with 10 inch or so aperture.

I guess a careful eye on the second hand market and who knows what that may bring.

[wolfpaw]

I still think that for a budget solution keeping the existing scope, buying a GSO or Baader CC would be a good way to go for Wolfpaw to try out, as others and I also tried to suggest before as when he raised the coma topic.  It may not be paracorr, but the much cheaper GSO, Baaader alternatives go a large way there to tidying up the view to a very good level I feel from everything I've read.

I suspect the maxvisions he has would still show a small bit of astigmatism perhaps but I suspect ( without having tried it )  it will do a very decent job to sort out the view and go a long way for  a fraction of the cost, instead of  going the entire paracorr TV route if that is too expensive. If he is not bothered about aperture loss perhaps think of something else but it will always be expensive anyway to buy SCTs Maksutov in that size new with 10 inch or so aperture.

I guess a careful eye on the second hand market and who knows what that may bring.

I appreciate that you think a coma corrector would improve the telescope but I'm not prepared to try and patch up a potentially faulty telescope with a further outlay on money. No-one would. If the telescope's faulty (as has been suggested by a number of people with a similar set-up) then why would a coma corrector work as it would in a non-faulty telescope? I'd be more prepared to try a CC if I hadn't received so many contradictory opinions on here.

miscollimation means that the coma free field is not accurately aligned with the central axis and on an f5 scope the coma free field is about 2.2mm, therefore the area affected by coma increases the more inaccurate your (usually primary) collimation is (and of course vice versa to the maximum). see e.g. http://www.catseyecollimation.com/pensack.pdf there's info all over the internet if you just google it.

as to why you think there has been so little discussion about coma in f4.7 scopes I cannot say but again if you search it there's a lot. personally, I just feel you'd be happier with another scope design, however all of them have their own idiosyncracies and as I have said a few times, it's always going to be a compromise. there is not a single scope that gives affordability, large aperture, manageable size and weight and pin sharp views across wide field (without a flattener/coma corrector) with little or no aberration of any kind.

I must disagree with this. There is very little discussion on the internet of people using an f/4.7 and seeing 50%+ coma (let alone the 80%+ coma I was getting with the MV16mm on Jupiter's moons). Nearly every single discussion about coma in an f/4.7 telescope refers to it as appearing at the edge of the field of view, not over halfway towards the centre. And most of it also relates to astrophotography. I have no spent weeks and weeks reading about people's experiences of coma online and almost none of them matches my own.

As for miscollimation being the cause, would that still hold true if the centre of the field of view was perfectly sharp? I thought miscollimation resulted in poor images across the field of view, not good in the centre and then quickly getting worse.

Again, here's the view through the focuser:

http://oi61.tinypic.com/szd5sl.jpg

When I posted this pic up in another thread the general response was that the collimation looked fine.

[Scooot]

My scopes f4.8, I've only just started reading this thread so I apologise if I've missed something. I only noticed coma to an extent that it was a problem when I used my 26mm nagler which gives a 1.67° true field of view. It isn't noticeable with the17.3mm Delos (about 1° TFOV) unless I look for it. So f you're seeing significant coma or what appears to be coma with eyepieces with a 1° TFOV or less I'd suspect something else was wrong.

[Moonshane]

I appreciate that you think a coma corrector would improve the telescope but I'm not prepared to try and patch up a potentially faulty telescope with a further outlay on money. No-one would. If the telescope's faulty (as has been suggested by a number of people with a similar set-up) then why would a coma corrector work as it would in a non-faulty telescope? I'd be more prepared to try a CC if I hadn't received so many contradictory opinions on here.

I must disagree with this. There is very little discussion on the internet of people using an f/4.7 and seeing 50%+ coma (let alone the 80%+ coma I was getting with the MV16mm on Jupiter's moons). Nearly every single discussion about coma in an f/4.7 telescope refers to it as appearing at the edge of the field of view, not over halfway towards the centre. And most of it also relates to astrophotography. I have no spent weeks and weeks reading about people's experiences of coma online and almost none of them matches my own.

As for miscollimation being the cause, would that still hold true if the centre of the field of view was perfectly sharp? I thought miscollimation resulted in poor images across the field of view, not good in the centre and then quickly getting worse.

Again, here's the view through the focuser:

http://oi61.tinypic.com/szd5sl.jpg

When I posted this pic up in another thread the general response was that the collimation looked fine.

all I was doing was explaining how collimation can induce more coma as you asked how. if your 'sweet spot' is not aligned with the optical axis (i.e. primary accurately collimated mainly) then you could have a field which is completely lacking a coma free zone. it looks like your secondary needs moving up the tube a bit (this won't make much difference to the views) and I cannot see the primary collimation in the pics. I presume the central dot of the cheshire is accurately centred in the primary ring? but I agree if the centre of field is sharp then your collimation may not be the cause.

I don't think we will ever agree. whilst not specifically aimed or referring to f4.7 scopes you presumably read the links I posted which explained where and why coma appears. here's a good additional one. http://stargazerslounge.com/topic/154798-coma-correctors/?p=1570288

unfortunately many people refer to e.g. 'buying televue eyepieces as they will be sharp to the edge' in a fast newtonian when discussing coma. This is not right and I suspect many people posting on forums are regurgitating from things they have read rather than experiences they have enjoyed. even the best eyepieces will show some coma across the field but as already explained, increasing in apparency the further off axis you go. only a coma corrector will correct this.

this situation would possibly be resolved if you could get someone to have a look at your scope. I genuinely hope you eventually get to a situation where you have the scope you want that works how you want it to work but as I have already said, maybe a newtonian is not one that meets your needs.

[wolfpaw]

Well the way I see it there are three options:

1) The telescope is faulty.

2) The telescope hasn't been collimated properly despite showing a good image in the centre of the FoV.

3) The telescope's 50%+ coma is entirely normal in an f/4.7 telescope and I'm just one of handful of people who can see it.

I've spent hours and hours collimating my telescope, trying to get it a little better each time, so I find it hard to believe that the collimation is now so far out that it's producing such extensive aberration. And I don't believe 50%+ apparent coma is remotely normal (or the 80% coma I was getting with the MV16mm). So that leaves me with the 'faulty telescope' option which will be almost impossible to prove to the retailer without taking it on an eight-hour round trip, especially as they've already stated that the only 'fault' with it can be miscollimation.

The bottom line is that I'm lumbered with a 500 euro telescope that I don't use and, until I can get it back to the UK to flog on eBay, I'm stuck with it.