Primary mirror defect?

[AlexB67]

I don't entirely agree you should be seeing coma as soon as you hit the coma free field. The coma free field often mentioned is the one that predicts whether or not the coma lies within the diffraction limit, assuming perfect collimation and often people apply this formula

CFF =  0.0004333 * fratio ^ 3  in inches

and then use that to work out the percentage of that the total field, but this will yield a figure that is smaller to the visually perceptible coma, which depends on how much blurring you need to get, depends on other factors too, but anyway.  As a very crude rule regardless magnification this comes to about 3 times that before the eye can begin detection for good eyes.  

I must admit the way you have drawn the sketches makes the moon look rather astigmatic. The other night, having  read your post,  I had a quick look through my maxvision 20mm again at Jupiter (I don't use it for that normally)  but it was not nearly as bad as your sketch presents to my eyes.   

I'll refer you to some of the articles and links I put up here

http://stargazerslounge.com/topic/207739-coma-astigmatism-and-newts-some-useful-notes-perhaps/?hl=%2Bcoma+%2Bastigmatism

Suffice it to say, it is a  complex and tricky subject.

[hippie]

Dob's don't have to have perfect collimation,

 http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Better quality EP's would help with your fast scope, I found the cheaper EP's wouldn't focus ease with my F4.7,  You can pick up S/H EP's on astrobuysell, Or from other members on SGL for sale,

 http://www.astrobuysell.com/uk/propview.php?minprice=0&maxprice=1000000000000000&typechoice%5B%5D=Eyepiece-Barlow-Diagonal-Reducer&cur_page=1&sort=id+DESC

Hope you get it sorted soon   

O'h welcome to the Dob mob   .

[AlexB67]

I meant to say, following on from what I said, just a gut feeling that something is wrong in the optical train somewhere. I would not rule out  a badly figured mirror here, or the eyepieces.  I'd really tempted to compare to some other scopes at a meeting if you can arrange it before selling or before taking further action, and get other independent opinions on the equipment.

Good luck

[wolfpaw]

I don't entirely agree you should be seeing coma as soon as you hit the coma free field. The coma free field often mentioned is the one that predicts whether or not the coma lies within the diffraction limit, assuming perfect collimation and often people apply this formula

CFF =  0.0004333 * fratio ^ 3  in inches

and then use that to work out the percentage of that the total field, but this will yield a figure that is smaller to the visually perceptible coma, which depends on how much blurring you need to get, depends on other factors too, but anyway.  As a very crude rule regardless magnification this comes to about 3 times that before the eye can begin detection for good eyes.  

I must admit the way you have drawn the sketches makes the moon look rather astigmatic. The other night, having  read your post,  I had a quick look through my maxvision 20mm again at Jupiter (I don't use it for that normally)  but it was not nearly as bad as your sketch presents to my eyes.   

I'll refer you to some of the articles and links I put up here

http://stargazerslounge.com/topic/207739-coma-astigmatism-and-newts-some-useful-notes-perhaps/?hl=%2Bcoma+%2Bastigmatism

Suffice it to say, it is a  complex and tricky subject.

Thanks for the links, although some of them are far over my head! If the sketch makes the moons look astigmatic then I think it's probably a reflection on my sketching ability more than anything. I looked again last night (briefly admittedly as I'm sick of the whole subject) and the 16mm Maxvision is definitely making the moons comatic. It increases in size the closer the moons move to the edge of the field.

I compared the view again with the Skywatcher 10mm Plossl that came with the telescope. The SW Plossl shows a lot of 'seagulls' around the outer 20%-30%, getting worse as it moves towards the edge, and it's pretty clear that it's not a great eyepiece. Maybe defects in the SW Plossl hide the coma shown in the allegedly better-quality Maxvision but, to my eyes, the moons show less aberration in the SW Plossl than they do in the Maxvision 16mm.

I just hate the MV16mm now! I find it exceptionally uncomfortable to use and it's almost impossible to get my eye near to the lens without constant vignetting and blacking out, no matter how far in or out I rotate the rubber eye shield. The slightest movement when looking through it causes spikey 'hairs' to flare out from the stars at random angles. For me, the SW 10mm Plossl is significantly more comfortable/easy to look through.

I used the Maxvision 24mm a lot last night too as I was trying (and failing!) to find the Rosette and Crab nebulae. Although there is apparent aberration towards the edges of the field, it's a lovely eyepiece to use in comparison with the 16mm. It doesn't magnify the moons of Jupiter enough to tell if it is repeating the same aberration that I see using the 16mm but the fact I've not even noticed any aberration affecting more than 30% of the field of view suggests that there's not a significant issue.

I wish I'd never bought the Maxvision 16mm as its performance has made me doubt the whole optical set-up. If I'd just got the Maxvision 24mm and stuck with the two EPs that came with the telescope (SW 25mm and 10mm Plossls) then an issue with the telescope itself wouldn't have occurred to me. I'd have put any aberration seen in the Plossls down to the fact that they're not great EPs (although perhaps better than people often say) and any aberration in the Maxvision 24mm down to the fact that it's a relatively fast focal ratio.

Two things are a bit of a shame though. One is my lack of experience. I've only ever owned and used a 60mm refractor, with 0.925 eyepieces(!), so a 10'' Newtonian is a big step up, both visually and technically, and I've no idea what I should or should not be seeing at the focuser. The other misfortune is that it is simply not feasible for me to share my scope/EPs with anyone with greater experience. I live in the middle of nowhere, a very rural backwater in France. It's a two-hour round trip down a motorway just to get to the nearest medium-size town or city. It's just me, my eyes and my telescope trying to work through what are complicated, optical issues.

[bomberbaz]

I know this is going a little around the topic but I have the 24mm 82^o and following from reading this decided to run some tests and I have found that the eyepiece is not as good as I one thought. Scope is perfectly collimated and was cooled during tests but I have a lot of seagulls all around. I compared it to a Axiom 32mm, Revelation 42mm and Delos 17.3mm (those are all I had ) and I suddenly felt decidely unsure about the eyepiece.

^{I am going to try again when conditions are better and compare it to a SW stock 25mm one I have and see what that brings us! I shall make a full report in another area of forum.}

[wolfpaw]

I know this is going a little around the topic but I have the 24mm 82^{o and following from reading this decided to run some tests and I have found that the eyepiece is not as good as I one thought. Scope is perfectly collimated and was cooled during tests but I have a lot of seagulls all around. I compared it to a Axiom 32mm, Revelation 42mm and Delos 17.3mm (those are all I had ) and I suddenly felt decidely unsure about the eyepiece.}

^{I am going to try again when conditions are better and compare it to a SW stock 25mm one I have and see what that brings us! I shall make a full report in another area of forum.}

Feel free to post your findings on this thread if you wish. I guess the wider field of view in the 82 degrees will produce a little more aberration compared with the 68 degree but it's interesting that you've changed your mind a little on the quality of the view through the 24mm 82 degree.

Maybe something similar but in reverse operates when I use my cheapo SW Plossls that came with the telescope. I'm expecting them to be really bad (especially the 10mm) as I'd read that they were really bad. Perhaps I let them off the hook when I find that the view isn't as bad as I was anticipating leading me to ignore obvious aberrations.

It does seem that despite 'optics' being such a scientific and precise subject quite a lot actually comes down to the subjectivity of the person using them. For me the coma shown in the 16mm is unacceptable and stands out like a sore thumb. For others it might hardly be noticeable even when looking through the same telescope with the same eyepiece.

I've pretty much decided to put the Maxvision 16mm back in its box for now. I want to go out and observe different things, not spend too much time agonising over whether there's a problem with the mirrors or the eyepiece or my own eyes. The constant deliberation has dominated, and spoilt, my observing more than the coma ever did. I'm going to keep a look out for a coma corrector though and if I can pick one up secondhand for a reasonable price then I will do.

[Jules Tohpipi]

I've just come across this thread and wolfpaw's experiences have mirrored my own recently.

I'm currently transitioning between scopes. I started with a Skyliner 200p during which time I bought the MaxVision 16mm and 24mm. The 24mm was exceptional; clear to the edge, comfortable, etc. The 16mm was a pig due to very tight ER.

Recently bought a 250px FlexTube. Last night I had both scopes out on the patio alongside each other...

The 16mm MV looks so much better in the 200p than the 250px. Stars are more pinpoint for longer in the 200p. Even when dead-centre in both scopes, details in Jupiter were clearer in the 200p. Of course, the image was brighter in the 250px, but it was noticeably easier to discern the details in the 200p.

As the 250px is new to me, but used, then it similarly got me fearing for the scope's optics (I've carefully collimated it).

Swapping to the 24mm MV, the difference is less pronounced. Certainly, stars are pinpoint practically to the edge through the 200p -whereas they are not through the 250px, but I was expecting that. Other than that the views were fairly similar except for brightness.

Unfortunately, the seeing was too fizzy here last night to draw any meaningful conclusions when comparing the two scopes using my 6mm.

So, I'm somewhat glad to hear that another forum member is struggling with the 16mm MV in the 250px. Because I certainly am. But I need some more clear skies to test the other EPs more thoroughly across a wider range of objects than just Jupiter. But the 200p is going shortly, so I might not get opportunity if it keeps raining.

[wolfpaw]

I've just come across this thread and wolfpaw's experiences have mirrored my own recently.

I'm currently transitioning between scopes. I started with a Skyliner 200p during which time I bought the MaxVision 16mm and 24mm. The 24mm was exceptional; clear to the edge, comfortable, etc. The 16mm was a pig due to very tight ER.

Recently bought a 250px FlexTube. Last night I had both scopes out on the patio alongside each other...

The 16mm MV looks so much better in the 200p than the 250px. Stars are more pinpoint for longer in the 200p. Even when dead-centre in both scopes, details in Jupiter were clearer in the 200p. Of course, the image was brighter in the 250px, but it was noticeably easier to discern the details in the 200p.

As the 250px is new to me, but used, then it similarly got me fearing for the scope's optics (I've carefully collimated it).

Swapping to the 24mm MV, the difference is less pronounced. Certainly, stars are pinpoint practically to the edge through the 200p -whereas they are not through the 250px, but I was expecting that. Other than that the views were fairly similar except for brightness.

Unfortunately, the seeing was too fizzy here last night to draw any meaningful conclusions when comparing the two scopes using my 6mm.

So, I'm somewhat glad to hear that another forum member is struggling with the 16mm MV in the 250px. Because I certainly am. But I need some more clear skies to test the other EPs more thoroughly across a wider range of objects than just Jupiter. But the 200p is going shortly, so I might not get opportunity if it keeps raining.

Thank you!!

If you get the chance to compare the 16mm in the 200P and 250PX again then please report back.

[Jules Tohpipi]

Yes, will do. There's a very small window of opportunity before the 200p is delivered to its new home. So I'm praying for clear skies.

Glad you found the post useful - as I did yours.

[John]

At F/6 the 200P dob will be more forgiving of eyepieces than the F/4.7 250PX. As newtonians get faster they stretch the correction of eyepieces further, show more coma and are less tolerant of collimation errors. These changes happen exponentially rather than in a linear fashion so the differences between an F/6 and an F/4.7 are really quite marked.

Although the scope would be larger (taller) I do often think that a 10" dob at F/5.3 would be more "easy going" than the F/4.7's that the chinese have decided to serve us up.

[Ags]

The GSO 10" ones are F5 - don't know if that makes much of a difference...?

[wolfpaw]

At F/6 the 200P dob will be more forgiving of eyepieces than the F/4.7 250PX. As newtonians get faster they stretch the correction of eyepieces further, show more coma and are less tolerant of collimation errors. These changes happen exponentially rather than in a linear fashion so the differences between an F/6 and an F/4.7 are really quite marked.

Although the scope would be larger (taller) I do often think that a 10" dob at F/5.3 would be more "easy going" than the F/4.7's that the chinese have decided to serve us up.

I agree. The 10'' seems like a poor combination of a really nice mirror size and a really unsuitable tube length. I know the GSOs are a little longer than the Skywatcher/Orions but it's not really enough to make a big difference to the focal ratio. Most people have long enough arms to manage a longer tube and long enough legs to look through its focuser.

[wolfpaw]

The GSO 10" ones are F5 - don't know if that makes much of a difference...?

It'll be a little kinder on eyepieces but it's much closer to the f4.7 of the 250s compared with the f6 of the 200s. An f.5 speed seems to be the cut off point for when people really start considering getting a coma corrector.

[John]

It'll be a little kinder on eyepieces but it's much closer to the f4.7 of the 250s compared with the f6 of the 200s. An f.5 speed seems to be the cut off point for when people really start considering getting a coma corrector.

I think thats true. My 12" is F/5.3 and I can only start to see traces of coma right out by the field stop of my widest field eyepieces. Nowhere near enough to tempt me into buying a coma corrector.

[nicks90]

its just a shame skywatcher didnt take the brave plunge and utilise their flextubes to go with an F6 in both the 250 and 300 versions. They would still pack down small enough for transporting in cars, and the overall tube length wouldnt be that much of an issue unless at the zenith. Even then, nothing a small £5 plastic step from ikea wouldnt sort.

plus the views through a 12" f6 would be superb with even a moderately good eye piece. My 38 and 32mm panaviews would be epic, instead of just 'good'...

[mitchelln]

Very interesting thread this. I have to say that if I saw Jupiter's moons going out of whack that close to the center I'd be extremely hacked off too!

However, I've never seem anything approaching that bad with any of my eyepieces with my 300P, which is f4.9. My Baader Hyperion Zoom and 31mm Aspheric give great views across most of the field, especially the latter. Ditto my Baader orthos. Even pretty standard Revelation Astro EP's in my binoviewers are coma free across around 75% of the field I'd say.

I am a perfectionist however So I bought a Baader coma corrector via the classifieds. It really does work and doesn't affect the focal length or field size like most other correctors. My zoom is pretty much coma free with it and the 31mm Aspheric is just awesome. Noticeable improvement with the binoviewers too.

[wolfpaw]

Very interesting thread this. I have to say that if I saw Jupiter's moons going out of whack that close to the center I'd be extremely hacked off too!

However, I've never seem anything approaching that bad with any of my eyepieces with my 300P, which is f4.9. My Baader Hyperion Zoom and 31mm Aspheric give great views across most of the field, especially the latter. Ditto my Baader orthos. Even pretty standard Revelation Astro EP's in my binoviewers are coma free across around 75% of the field I'd say.

I am a perfectionist however So I bought a Baader coma corrector via the classifieds. It really does work and doesn't affect the focal length or field size like most other correctors. My zoom is pretty much coma free with it and the 31mm Aspheric is just awesome. Noticeable improvement with the binoviewers too.

I am hacked off about it! I spent so long researching which telescope to get, weighing up the pros and cons of the 200P and the 250PX, and now when I use my 250PX or look at it I'm aware of the fact that I made the wrong decision. If I had the choice again I would get either an Orion XT8 or a SW 200P as the 250PX just isn't worth the hassle it's giving me.

Based on my own experience I would never recommend that anyone get an f4.7 telescope unless i) they're not fussy about aberration or ii) have the money to spend on high-end eyepieces and a Paracorr. Amazing how quickly a one-off purchase of a telescope can turn into a money pit just in an attempt to push aberration to the outer edge of the field of view. Neither option is particularly appealing.

I'm in a bit of a dilemma at the moment. The aberration is an annoyance (in the 24mm Maxvision as well as the 16mm) and it's spoiling my enjoyment of the scope so I either have to get a coma corrector (which may or may not be effective depending on what the issue is) or call it quits and sell it, taking a financial hit, and get a f6 scope instead.

If me and the telescope were in the UK I'd probably put it on eBay but I live in a very sparsely populated area. I hate to say I'm stuck with 250PX, but that's pretty much the situation.

One question. Surely my 250PX should exhibit the almost same amount of coma as every other 250PX? If it's showing more coma than other 250PXs, what would be the cause?

[AlexB67]

You could get the altair astro GSO coma corrector, it is not very expensive and works very well by all accounts, It works similarly to a paracorr by increasing the focal length by 1.1 or so, thus changing the scope from f4.7 to effectively f5.2 before you even start, and on top coma correcting the image, this will come at the expense of a slight reduction in FOV, and increased magnification for each eyepiece by the same multiplier.

https://www.altairastro.com/product.php?productid=16363

In fact there was one here recently in the classifieds for 60 pounds.  Be aware though that you will need spacers ( extension rings ) and need to configure each eyepiece to work with it.  Form what I read since I considered getting one is that it will not sink deep enough into the 2 inch extension tube either that comes with the 250. The long shot is you need to do some modding.  The extra weight may also begin to become problematic with the focuser with heavier 2 inch eyepieces I suspect.  So all in all it will take a bit of working out, but once configured it works well by all accounts. I would think for the MVs this will work quite well I suspect, though of course any astigmatism that is left behind by the eyepiece will still be visible. 

[wolfpaw]

You could get the altair astro GSO coma corrector, it is not very expensive and works very well by all accounts, It works similarly to a paracorr by increasing the focal length by 1.1 or so, thus changing the scope from f4.7 to effectively f5.2 before you even start, and on top coma correcting the image, this will come at the expense of a slight reduction in FOV, and increased magnification for each eyepiece by the same multiplier.

https://www.altairastro.com/product.php?productid=16363

In fact there was one here recently in the classifieds for 60 pounds.  Be aware though that you will need spacers ( extension rings ) and need to configure each eyepiece to work with it.  Form what I read since I considered getting one is that it will not sink deep enough into the 2 inch extension tube either that comes with the 250. The long shot is you need to do some modding.  The extra weight may also begin to become problematic with the focuser with heavier 2 inch eyepieces I suspect.  So all in all it will take a bit of working out, but once configured it works well by all accounts. I would say for the MVs this may work quite well, though of course any astigmatism that is left behind by the eyepiece will still be visible. 

I've read about the need to use spacers on different eyepieces. Funnily enough last night I was reading a thread/tutorial on SGL about how to use spacers in a coma corrector and I didn't have a clue what the fellow was talking about. There were a load of technical terms that I didn't even understand. It sounded extremely complicated and at the end of it I still wasn't sure how it was supposed to work. Do the spacers stay in the CC all the time? Is there a spacer for each eyepiece? Do the spacers stay on the eyepieces? Do they need putting in every time you use different EP? I appreciate that the Paracorr has a turntable that needs adjusting according to each eyepiece so how does that work in a CC without a turntable?

Another concern is the fact I'd be putting a minimum of four lenses into the focuser even before I add an eyepiece. Doesn't that impact on the quality of the view? I was looking at the Skywatcher CC too and, according to FLO, it comes "supplied complete with necessary adaptors for visual use". Are these adaptors the same as the spacers? Or would I have to buy something else as well as the SW corrector itself?

There's a review of the SW CC here:

http://stargazerslounge.com/topic/58587-the-new-skywatcher-f5-coma-corrector-review/

I was a bit bemused though when the reviewer posted a pic of almost invisible coma in an image he'd taken. If my coma showed like that this thread would never even have been started.

I'm really surprised that so few people seem to use a CC with an f4.7 telescope or even regard coma as a big issue.

[AlexB67]

Other will be able to explain better that actually used them, but the gist is this, the tunable top is a much neater solution, the way the GSO works with spacers is what effectively the tuneable top gives you in a much more convenient  manner with continuous adjustment, but at a horrendous cost !. 

There are variable length spacer rings too you can use instead.  There are different ways you can do it, you can fit the spacers between the eyepiece barrel if I recall correctly and do it that way for each eyepiece so it is always ready for the CC once inserted.   It is a bit if a hassle to get it all set optimally, but there are plenty threads on it on CN and here.

Without having tried them, I did much reading on budget CCs and I feel the altair astro in that price range for visual use is amongst the best solution you can get. The other issue is that coma correctors can induce other type of aberrations, though on the whole it is considered to be worth while since coma is the dominant one that bothers people in fast Newts, the GSO does a good job on the whole as I understand it, hassles not withstanding to get it l configured.

As for the number of elements introduced in the optical train, yes there is that, how much of that really matters is hard to say without having tried them myself, but no doubt it will reduce transmission, introduce extra scatter perhaps, in the end it is all about trade-offs but I suspect don't think it is a big deal a lot of the time.

[John]

Maybe it's simpler just to wean yourself off wide angle eyepieces ?

Not so much fun though if you like that perspective on the Universe 

[AlexB67]

There is a bit of a rundown here

http://stargazerslounge.com/topic/158529-astro-techaltair-astrogso-coma-corrector-and-user-guide/

I also believe Moonshane one of the moderators owned one briefly to try it and said it did a decent job, and he owns a paracorr so would have a comparison to go by, he may be able to throw in a couple of helpers how to get it setup better than I can. 

[Moonshane]

I did have the Altair one as I wanted to compare it with the paracorr. the paracorr was definitely better (in my scopes which are f4) but the altair did a really great job for the price, being excellent value for money.

as for the differences, I found it hard to be specific as to why but the paracorr just seemed 'cleaner' and 'crisper' (I realise these are pretty valueless terms) after taking out the altair. I found a 28mm baader fine tuning ring (about £14) between the coma corrector housing and the eyepiece housing created the right sort of gap and needed no further changes for any of my eyepieces.

I didn't do an intensive review but would recommend an altair unit while you save for and watch for / pounce on a used Paracorr. I got my PC for £165 used and it's the best 'eyepiece' I have bought. you may even be completely delighted with the altair unit. if I had not already had the paracorr, I'd have been quite content I think even at f4.

a 10" scope is a great tool and personally, I'd spend another £50-60 on the altair.

[AlexB67]

Wolfpaw,  given how you feel I'd be tempted to buy one good eyepiece like a TV plossl second hand or something that is very well corrected for a fast newt, say something in the 15- 25 mm to start with, sees how you find that and you could sell the MV to budget it, and then reconsider after that. 

The thing is once you go that CC route, it may well be cheap to buy, but by the time you add spacers,  possibly having to consider upgrading the focuser, you are already beginning to spend spend bit by bit. 

We'll make a happy costumer out of you yet I hope 

[Moonshane]

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.