16" F4.5 Dob - Does it need the very best EPs?

[Damo636]

Barry, there is a solution to this and its strikingly obvious! I would be willing to undertake a road trip to Cork, at no cost to yourself, where we could do a straight swap of our two scopes, simples

[umadog]

The ES 82 degree eyepieces are fine at fast focal ratios. I use the 30 mm and the 14 mm at f/4.0 without problems. As Dirk says, it's coma you're seeing. Coma increases with the square of the focal ratio and becomes problematic at f/5 and below, particularly at lower powers. I tried the 30 mm both with and without the Paracorr and it's unusable (for me) at f/4.0 without the corrector. With the corrector, there are some minor issues near the field edges but you'd have to look for it to notice. I've also tried the 30 mm at f/5 with and without a Paracorr and the views were definitely better with the Paracorr. The Paracorr is expensive, but if you want clean low power views at about f/5 and below then it becomes necessity. The good news is that you don't need to buy a new low power eyepiece because the issue you're seeing is almost entirely coma, not astigmatism.

[Naemeth]

The ES 82 degree eyepieces are fine at fast focal ratios. I use the 30 mm and the 14 mm at f/4.0 without problems. As Dirk says, it's coma you're seeing. Coma increases with the square of the focal ratio and becomes problematic at f/5 and below, particularly at lower powers. I tried the 30 mm both with and without the Paracorr and it's unusable (for me) at f/4.0 without the corrector. With the corrector, there are some minor issues near the field edges but you'd have to look for it to notice. I've also tried the 30 mm at f/5 with and without a Paracorr and the views were definitely better with the Paracorr. The Paracorr is expensive, but if you want clean low power views at about f/5 and below then it becomes necessity. The good news is that you don't need to buy a new low power eyepiece because the issue you're seeing is almost entirely coma, not astigmatism.

Can both Coma and Astigmatism be an issue on a low quality eyepiece at long focal lengths at F/5?

[Moonshane]

generally with poor oculars you cannot see the coma for the astigmatism masks it.

[KaStern_Former_Member]

Hello Naemeth,

Can both Coma and Astigmatism be an issue on a low quality eyepiece at long focal lengths at F/5?

eyepiece designrs definitely try to avoid that there is any coma inherent in their eyepiece design.

If you use the 82° ES eyepieces with an f/4,5 Newt there will be coma from the mirror,

and there will be astigmatism from the eyepiece.

If you use a paracorr it will cure coma. Since the paracorr leads to an 1.15 x longer focal length

the eyepiece will virtually work in an f/5.175 scope. Because of this the eyepiece will suffer lesser

from astigmatism than in an f/4.5 scope!

Cheers, Karsten

[umadog]

Can both Coma and Astigmatism be an issue on a low quality eyepiece at long focal lengths at F/5?

You mean do these errors occur at focal lengths longer than f/5? Briefly, yes, they do, but they are less severe. Coma is far less pronounced at f/6. I don't know at what point it effectively vanishes. Eyepiece-induced astigmatism can potentially occur at any focal ratio if the eyepiece is sufficiently badly made.

[Naemeth]

You mean do these errors occur at focal lengths longer than f/5? Briefly, yes, they do, but they are less severe. Coma is far less pronounced at f/6. I don't know at what point it effectively vanishes. Eyepiece-induced astigmatism can potentially occur at any focal ratio if the eyepiece is sufficiently badly made.

What I meant was, do they occur with eyepieces with long focal lengths (25mm, 30mm etc.) enough to be an issue in an F/5 scope that is noticeable either with or beyond Astigmatism (as in, if the EPs had Astigmatism, would a coma corrector improve the view at all? Or if they had no Astigmatism, would the coma still be noticeable at F/5). Sorry I didn't make that clear .

[Bart]

The ES 82 degree eyepieces are fine at fast focal ratios. I use the 30 mm and the 14 mm at f/4.0 without problems. As Dirk says, it's coma you're seeing. Coma increases with the square of the focal ratio and becomes problematic at f/5 and below, particularly at lower powers. I tried the 30 mm both with and without the Paracorr and it's unusable (for me) at f/4.0 without the corrector. With the corrector, there are some minor issues near the field edges but you'd have to look for it to notice. I've also tried the 30 mm at f/5 with and without a Paracorr and the views were definitely better with the Paracorr. The Paracorr is expensive, but if you want clean low power views at about f/5 and below then it becomes necessity. The good news is that you don't need to buy a new low power eyepiece because the issue you're seeing is almost entirely coma, not astigmatism.

Well, Umdog, if you're getting on ok at F/4 with the 82*s, then I'll have to have another look, and see if it was collimation, tiredness etc, etc excitedness at getting out under the stars for the first time in weeks etc Thanks

Edit: Umadog, whats expensive? This any good? http://www.optcorp.c....aspx?pid=10026

Barry, there is a solution to this and its strikingly obvious! I would be willing to undertake a road trip to Cork, at no cost to yourself, where we could do a straight swap of our two scopes, simples

Or,alternatively, you could make the same roadtrip, and I could take good care of your eyepieces here, and you could take my little EPs back to Fermanagh for an extended holiday.

[John]

Coma and astigmatism look subtly different but it can be hard to tell what you are actually seeing. I found this web page helpful:

http://www.umich.edu/~lowbrows/reflections/2007/dscobel.27.html

[Naemeth]

Coma and astigmatism look subtly different but it can be hard to tell what you are actually seeing. I found this web page helpful:

http://www.umich.edu...dscobel.27.html

Thanks for the very useful link .

I've heard the weakest part of the optics will show the errors of the whole system, does that mean that if the mirror coatings are of high quality enough, that errors in Televue eyepieces may be possible, or are they designed so even ultra expensive mirror coatings and mirrors don't show any errors?

[KaStern_Former_Member]

Hello Maemeth,

Or if they had no Astigmatism, would the coma still be noticeable at F/5).

Yes. I can see off-axis coma in my 8"f/6 Newt.

if the EPs had Astigmatism, would a coma corrector improve the view at all?

Yes it would. The combined aberrations would let a star look even worse than one aberration alone would do.

And if you take teh Parakorr it`s mild prolonging of the focal length would let the eyxepiece astigmatism be smaller.

But as John wrote eyepiece astigmatism can be way bigger than the coma from the mirror.

So better try before you buy.

Edit:

I've heard the weakest part of the optics will show the errors of the whole system, does that mean that if the mirror coatings are of high quality enough, that errors in Televue eyepieces may be possible, or are they designed so even ultra expensive mirror coatings and mirrors don't show any errors?

The coatings have nothing to do with aberrations. Please have a look at that superb website:

http://www.telescope-optics.net/

Goodnight at all, Karsten

[Pingster]

I have a 16in F4.5 flextube. With a 31mm Nagler there a lot of coma around the edges. i guestimate it starts about 50% from the centre.... When viewing open clusters such as the double cluster.... the centre FOV is beautiful.... but the view is ruined by the coma, as it starts so early in the FOV.

I noticed that coma reduces with the shorter the eyepiece FL. For example my 13mm Ethos shows what appears like no coma at all. But dont enjoying viewing clusters as the mag is too high.

I have recently bought a Parracor type II, and typically... the weather has been so bad i have not had a chance to try it.

If it fixes my coma in my 31mm Nagler... i will be so dam happy and most likely will have that eyepiece in my scope 90% of the time.

[Moonshane]

I

I have a 16in F4.5 flextube. With a 31mm Nagler there a lot of coma around the edges. i guestimate it starts about 50% from the centre.... When viewing open clusters such as the double cluster.... the centre FOV is beautiful.... but the view is ruined by the coma, as it starts so early in the FOV.

I noticed that coma reduces with the shorter the eyepiece FL. For example my 13mm Ethos shows what appears like no coma at all. But dont enjoying viewing clusters as the mag is too high.

I have recently bought a Parracor type II, and typically... the weather has been so bad i have not had a chance to try it.

If it fixes my coma in my 31mm Nagler... i will be so dam happy and most likely will have that eyepiece in my scope 90% of the time.

I can pretty much guarantee that there will be about the same level of coma as with an f8 scope using your 31mm Nagler - i.e. almost none. with my 26mm the field is perfect in my 16" f4 with Paracorr. the double cluster is astonishing from a dark site (less so from home but still great) with my 26mm Nagler and is one of the first objects I seek out when I have the chance.

[KaStern_Former_Member]

Hello Pingster,

the 31mm Nagler T5 is not free from astigmatism when used with a telescope faster than f/5.

So what you see is both coma and astigmatism.

The Parakorr will fix both, since it makes the f/4.5 to f/5,175 where the 31mm T5 works quite well.

Cheers, Karsten

[alan potts]

We will have to disagree on that point

I'm putting my money on John.

Alan

[umadog]

Well, Umdog, if you're getting on ok at F/4 with the 82*s, then I'll have to have another look, and see if it was collimation, tiredness etc, etc excitedness at getting out under the stars for the first time in weeks etc Thanks

Edit: Umadog, whats expensive? This any good? http://www.optcorp.c....aspx?pid=10026

Yes, I'm getting good views at f/4 but with a coma corrector. It will look nasty without a corrector. I use a Paracorr II. I've not used the corrector you link to. I remember reading that one of those non-TV correctors introduced spherical aberration, but I don't remember which corrector it was and I don't know how severe it is. If you want the best quality, a Paracorr is probably the way to go. If you want to investigate other options then make sure you read a lot of reviews.

[cs1cjc]

Paracorr II is now £460.

If you want something that works well for somewhat less, look at the GSO coma corrector (aka Astro-Tech and fka Altair Astro). I know of no current UK source but Telescope Services (55euro but you need an adapter for visual use) and Agena list it ($120 with adapter, but currently out of stock). The corrector needs correctly spacing (65mm to 85mm from the last lens of the corrector ) to the focal point of the eyepiece (at about the shoulder on most eyepieces except Hyperion, WALER, and some Tele Vue) with Baader fine tuning rings and/or or empty 2" filter rings.

I note that TS also do a coma correcting eyepiece...

[Damo636]

I would keep an eye out on Astromart Barry, there has been a couple of the type 1 Paracorr on there in recent times for a little over $200 each.

[Moonshane]

As I say, I got mine for <£200.

[Bart]

Well, bit embarassed now!

I got out again last night for an hour of observing, scope out for about 2 hours beforehand. Was particular about collimation. Seemed spot on. Was dreading popping in the ESs inot the focusser. I was very surprised to see that what I remember from last Sat night was not really there. The 30mm was very very good almost out to the very edge. The 20mm 68* was pretty good, needed to LOOK for the blurred stars at the edge. The 14mm looked spot on. I was really relieved, but kind of embarassed for making such a mistake. Could it really have been down to cooling/collimation, both of which I thought I had sorted last Sat. Its very puzzling.

What it does mean, though, is that the EP collection I am happy with, is ok, and there appears to be no need to get a coma corrector or TVs, unless I want to!!

Thanks a million for indepth discussion here, I know more now than I did when the post started

PS The Dumbell Neb was great last night, never saw so much detail !!

[alan potts]

Bart,

I have made the cool down, warm up mistake more than once, don't think you get the warm up problem very offen in England. The LX 200 take a long time to get warm or cold 2 hours most times.

Nice to hear that you are being able to use it.

Alan.

[swamp thing]

Just seen this thread, you had me worried for a minute, as there has been some reports of astigmatic secondary mirrors on the 16" LB, mine was fine, but I feared you had bought a bad one for a minute. Phew! Glad it's not that for your sake.

[Pingster]

Coma is a very odd beast! When viewing Nebs and Galaxys... and dimmer star fields... you don't notice the coma... or it does not distract or borther me. But if you spend your evening viewing brighter star clusters/star fields, then coma suddenly becomes really obvious and distracting, for me at least. (6in F4.5 flextube has aweful coma viewed thru a 31nagler).

Thats my experience.

[E621Keith]

I do wonder if you can get scopes with a focal ratio that is faster than 2.7 (fastest I've seen), who knows, F/1 anyone?

You mean like this TEC 300mm F1.44 astrograph

http://www.baader-pl...c/tec_300vt.htm

1 whole stop faster than a hyperstar C11

[Naemeth]

You mean like this TEC 300mm F1.44 astrograph

http://www.baader-pl...c/tec_300vt.htm

1 whole stop faster than a hyperstar C11

Apparently, the Thirty Metre Telescope is going to be F/1.