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Achieving a Pin-sharp focus on a 12" Dob


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Hi All,

Spending a number of hours last night with my 12" Dob & after leaving the scope outside an hour prior I still noticed the focus wasn't quite perfect, although it could pass as a pretty sharp image, adjusting the focuser both sides of the 'sharp' image into the blurry areas & back again to find the optimum focus I did find it impossible to achieve that perfect pin-sharp image. Now I maybe really nit-picking here because the image across the range of stars looked very sharp, but if you look really closely & single out a star for example you can see a very small blur or smudge to one edge (very small).

My questions is - is this where the expensive EP's come in to play?

I was mostly using a 2" 32mm Panoview (Skywatcher) & also the standard 10mm & 28mm Plossl's that came with the scope.

I'm just curious to know whether the image sharpness could ever be improved (again it is pretty impressive as is) with either expensive top-grade EP's or focuser upgrade or could it even be down to the type of scope I'm using?

BW

PJW

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I think what you are seeing is astigmatism, if I use cheaper eyepieces in my dob, I see lots of it. When I use the UWAN's it's only in the last 5-10%.

Two pictures below.

First one shows astigmatism (Seagulls) the second shows coma (comet tails) which is in your primary mirror.

post-13619-133877471814_thumb.gif

post-13619-133877471818_thumb.gif

Edited by Doc
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Thanks Doc,

It does resemble the 2nd picture more but nowhere near as bad as the pic - maybe I'm just expecting too much with my setup & I guess it's part of the learning curve.

I'll look through some more EP's next time theres an observing session with my local group.

BW

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These pictures are over exaggerated. If it's the second then it shows coma and as far as I know, thats inherrant in your primary mirror.

A coma corrector will remove most of it, or there is a eyepiece called a Pretoria but I have never seen one of these.

I personally just live with mine, it's not that bad and dissappear when searching for DSO's.

All dobs have coma to some extent, just the cheaper mirrors have alot of it.

Edited by Doc
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You are not going to get perfection.

You have a production mirror and the accuracy of it, while good, there will be simple machining tolerances and owing to the laws of reflection they get amplified through the system.

If the errors are in the eyepiece then a better eyepiece will obviously help.

One thought is that we take these outside and cool them down, often to fairly low temperatures, do any state the operating temperature at which they were designed to be optimum?

Just thinking that if the curvature is designed to be correct at 20 deg, then at 5 deg it will not be correct.:)

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These pictures are over exaggerated. If it's the second then it shows coma and as far as I know, thats inherrant in your primary mirror.

A coma corrector will remove most of it, or there is a eyepiece called a Pretoria but I have never seen one of these.

I personally just live with mine, it's not that bad and dissappear when searching for DSO's.

All dobs have coma to some extent, just the cheaper mirrors have alot of it.

Thanks for the info again Doc, I'm sure I can live with it too - just thought I would ask to see if it could be possible. There is another 12" SW Dob owned by one of the other members in my group, I'll see what his looks like & compare!

Are the stars in the centre affected as well? Sorry if it's stating the obvious but you have nailed the collimation haven't you?

Yes, they are also affected in the middle but only a tiny amount, I doubt it would trouble most people. No I haven't & that was going to be my next question!

Are there any recommendations on one for a Dob?

BTW - Is that an Enterprise in your avatar?

You are not going to get perfection.

You have a production mirror and the accuracy of it, while good, there will be simple machining tolerances and owing to the laws of reflection they get amplified through the system.

If the errors are in the eyepiece then a better eyepiece will obviously help.

One thought is that we take these outside and cool them down, often to fairly low temperatures, do any state the operating temperature at which they were designed to be optimum?

Just thinking that if the curvature is designed to be correct at 20 deg, then at 5 deg it will not be correct.:)

That's what I was thinking Capricorn, but all 3 x EPs were outside with me the whole time, & were the same when I finished the obs.

I can live with it but will sort the collimation in the meantime.

BW

Edited by PJW
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Did your 10mm eyepiece (shorter focal length) show less off it?

The Panaview is OK in a slower scope but at f/5 or so it will show some distortion.

John

Thanks John, I must admit I couldn't really tell - the 10mm Plossl I used last night seemed to be much darker & poorer quality compared to the panaview (no suprises there) & therefore, very difficult to tell.

I guess being a noob, I really am probably clutching at straws & maybe expecting too much as mentioned earlier from a production mirror.

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Thing is I am not 100% sure that a parabola is the optimum curve. It is better (lots) then spherical but I have an odd suspicion that one of the other conic sections is actually better. Just a lot more costly and difficult to produce. No idea from where I have this thought, but seem to recall it from years ago.

And I do not even want to think about working out the equation that would produce the ideal surface.

If the bit of mirror that the light hits is say 1 sec out then after reflection the reflected ray has 2 sec error. Up this travels to the secondary and another set of reflections occurs and this doubles the errors again. So you have at the eyepiece 4 sec of error on something that started with 1 sec error at the mirror surface.

That is why a very accurate mirror is difficult to make and costly. If you think of machine tolerances then low f number mirrors are more difficult to produce, more errors and so after bouncing round what is seen can easily be less then ideal.

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The 12" SkyWatcher dob has very good optics so it's unlikely that the mirror is at fault. How did you collimate the scope? That's the first thing to get sorted.

The 10mm eyepiece as supplied isn't that good and should be replaced, but the 25mm is quite reasonable. For about £30.00 to £50.00 you could pick up a decent eyepiece of about 10mm that should work well in the dob.

I had a quick look around and there's a William Optics 12.5mm SPL eyepiece on eBay for £50.00. Great little eyepiece with plenty of eye relief that will work well in a fast dob. The SPLs are as sharp as a UWAN but a smaller field of view.

http://cgi.ebay.co.uk/William-Optics-SPL-12-5mm-Planetary-eyepiece-boxed-/300456470002?pt=UK_Telescopes

John

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If you are going to start looking for a better eyepiece then make a solid decision on price per eyepiece first.

TV plossl are good and about £74 a piece.

I have decided to slowly get almost the lot one by one.

WO Swans are £60 each, just 3 in the set.

Vixen do a range of plossl's just cannot locate a cost.

FLO seem to do the higher spec ones not the plossl's.

Orion do 2 plossl ranges (SCS Astro) £52 or £44.

Astronomica do ED eyepieces at £50 (think Orions).

Seems that after these the costs go up and while good I am not sure that an eyepiece for £200 will be 4x better then the ED ones from Astronomica. So I'll stick to the types mentioned.

If eye relief is a factor the ED's and Orions state 20mm eye relief.

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Thanks for the info Capricorn, certainly a lot more to it than meets the eye (no pun intended).

John,

thanks for that, you are absolutely right about the 26mm Plossl, it really isn't bad in terms of clarity, I just wanted a little more eye relief & a little more FOV hence the 32mm pano.

I'll look at the WO 12.5mm that was going to be my natural size replacement for the 10mm, in the meantime I will get the collimation sorted.

BW

pjw

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If you are going to start looking for a better eyepiece then make a solid decision on price per eyepiece first.

TV plossl are good and about £74 a piece.

I have decided to slowly get almost the lot one by one.

WO Swans are £60 each, just 3 in the set.

Vixen do a range of plossl's just cannot locate a cost.

FLO seem to do the higher spec ones not the plossl's.

Orion do 2 plossl ranges (SCS Astro) £52 or £44.

Astronomica do ED eyepieces at £50 (think Orions).

Seems that after these the costs go up and while good I am not sure that an eyepiece for £200 will be 4x better then the ED ones from Astronomica. So I'll stick to the types mentioned.

If eye relief is a factor the ED's and Orions state 20mm eye relief.

Struth, now beginning to see why EPs are the things that make a good scope!

I will bear that lot in mind when looking at around 13mm replacement.

Can you recommend a collimater?

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A cheshire eyepiece works well and costs about £30 or you can go for a Hotech Laser collimator for £120 both when used correctly do a similar job the laser being easier to get the primary sorted with.

Edited by Chris H
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