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

[Bart]

Hello All,

I got to bring out my new 16" last night for about an hour between clouds. I enjoyed it and objects such as M13 were fantastic. I could resolve stars to the very core, amazing. M57 was good, I saw dust lanes in the Andromeda galaxy.

However, I noticed something disturbing. My Explore Scientific 82* EPs looked pretty soft about 60-70% out from centre. This doesn't happen in my 10" F5.

Didn't like that. I thought they were supposed to be not as good as, say the Naglers, in fast scopes, but not TOO far off. From what I could see, the F4.5 gave them a bit of a beating.

Does this mean the inexorable, fund depleting transition to TV

God help me, i thought my big outlays were over? Would the ES 100* fare better at F4.5, they'd have a bigger FOV and still cheaper?

Any thoughts?

Thanks

[DirkSteele]

That softness you report is likely down to coma. Coma in an optical system refers to aberration inherent to certain optical designs which results in off-axis point sources (i.e. stars) appearing distorted, appearing to have a tail like a comet, hence why it is known as coma. Coma is an inherent property of telescopes using parabolic mirrors, which Newtonian telescopes do. In a system using a parabolic mirror, only incoming light rays that strike the mirror exactly parallel to the axis of the parabola will be precisely focused. When looking at a point that is not perfectly aligned with the optical axis, some of the incoming light from that point will strike the mirror at an angle. This results in an image that is not in the centre of the field looking wedge-shaped. The further off-axis (i.e. the further from the centre of the field), the worse this effect of Coma is.

The effect is worse the faster the mirror is, so F5 and faster, the performance of the scope could benefit from the use of Coma corrector. The effect is inversely proportion to the cube of F-number, so becomes more pronounced in telescopes wit fast focal ratios. So the Coma in an F/4 is approximately double that in an F/5.

In addition, the faster a scope, the more pressure it places on an eyepiece design, however, better quality eyepieces will not solve this problem, though other optical problems in cheaper EPs are also more likely to manifest in faster scopes. As such it might be worth looking into a coma corrector. I am not too familiar with many that are available. The Televue Paracorr is a well known example of a Coma corrector. However it is approx £460 so rather expensive. So before you buy one, you should decide whether the problem with Coma in your telescope system is one that you find bothersome and worth the expense to correct. Though I do believe there are much cheaper versions available from other manufacturers though I do not know how well they perform.

[johninderby]

That's why the wide angle Televue or Pentax eyepieces cost more, they'll work in any scope. It's surprising what a difference going from f/5 to f/4.5 makes. I think the WO UWANs and SkyWatcher Nirvanas are a bit better than the ES at f/4.5, but still not as good as the Televue or Pentax. I would expect the 100 degree ES to be worse for edge softness than the 82 degree.

BTW coma is more evident in slower eyepieces.

Have you considered the Televue Delios? Not ultra wide but will work well in an f/4.5.

John

[Bart]

It's surprising what a difference going from f/5 to f/4.5 makes.

Have you considered the Televue Delios? Not ultra wide but will work well in an f/4.5.

John

Thanks gents.

John, you're not joking. I didn't expect this, in my naivety. Hadn't up to now considered TVs because the much better priced ESs worked great at F5. Might have to start saving.

[Moonshane]

you have to bear in mind that it's only when you use good quality eyepieces that you see coma. otherwise it's likely to be astigmatism in the eyepiece. assuming the Explore come under the good quality eyepieces bracket (I have never used one) then you are likely to be seeing coma. I use a 16" f4 and with e.g. my 26mm Nagler there is quite a bit of coma at the edges of field without a paracorr. with the paracorr (I got mine for less than £200 used) the view is like an f8 scope. the whole field is pretty much sharp edge to edge even with my 13mm Ethos.

given your comments I'd strongly recommend a paracorr (or maybe just a CC attached 'permanently' to your widest eyepiece). mine resides in my dob and is used for all eyepieces.

[Merlin66]

See if you can find a good s/h Klee Pretoria eyepiece.....

[Andrew*]

I notice you have four ES eyepieces from 11mm to 30mm.There is a very easy way to work out the source of the problem with a fairly high confidence level. Coma is more visible the larger true field you have, so simply compare the level of the aberration in each eyepiece. If it's coma, it will be almost invisible in the 11mm, and steadily get worse as you go up the focal lengths. If it's the eyepieces' quality, it will be pretty much the same in every eyepiece. Note however that there can be performance differences between eyepieces in the same series.

You'd be lucky if it was coma, but my gut feeling is that it's the eyepieces. Unfortunately both fixes are costly. A coma corrector costs anything from £100 to £300. Eyepieces of a similar field and higher quality cost at least that each...

I spent many months swapping eyepieces to find ones that work well in my f/4.5 16" dob, desperately trying to keep costs down and field of view up - clearly a contradiction in terms! At one point I had a 20mm Type 2 Nagler, which was the ONLY one I tried that I could not fault! The 28mm William Optics UWAN was astonishing for the price. Both the latter were under £200, but hefty 1kg lumps that will mess with your balance. The BIG unsung hero is the 8.5-12mm Antares Speers-WALER zoom. It's long, ugly, and hard to find, but a superb ultra-wide performer in fast scopes. Ones that didn't work well were the 16mm UWAN, 21mm Baader Hyperion/Orion Stratus and Meade Series 5000 SWA. The in-betweeners include the Skywatcher Aeros, Meade Series 5000 UWAs, and the TS TMB Planetary's, which are not perfect, but very good for their cost.

HTH

Andrew

[Bart]

I notice you have four ES eyepieces from 11mm to 30mm.There is a very easy way to work out the source of the problem with a fairly high confidence level. Coma is more visible the larger true field you have, so simply compare the level of the aberration in each eyepiece. If it's coma, it will be almost invisible in the 11mm, and steadily get worse as you go up the focal lengths. If it's the eyepieces' quality, it will be pretty much the same in every eyepiece. Note however that there can be performance differences between eyepieces in the same series.

You'd be lucky if it was coma, but my gut feeling is that it's the eyepieces. Unfortunately both fixes are costly. A coma corrector costs anything from £100 to £300. Eyepieces of a similar field and higher quality cost at least that each...

I spent many months swapping eyepieces to find ones that work well in my f/4.5 16" dob, desperately trying to keep costs down and field of view up - clearly a contradiction in terms! At one point I had a 20mm Type 2 Nagler, which was the ONLY one I tried that I could not fault! The 28mm William Optics UWAN was astonishing for the price. Both the latter were under £200, but hefty 1kg lumps that will mess with your balance. The BIG unsung hero is the 8.5-12mm Antares Speers-WALER zoom. It's long, ugly, and hard to find, but a superb ultra-wide performer in fast scopes. Ones that didn't work well were the 16mm UWAN, 21mm Baader Hyperion/Orion Stratus and Meade Series 5000 SWA. The in-betweeners include the Skywatcher Aeros, Meade Series 5000 UWAs, and the TS TMB Planetary's, which are not perfect, but very good for their cost.

HTH

Andrew

HTH? Fantastic, very informative. Depressing, but informaive. Thanks Andrew.

[Andrew*]

I'm glad. Don't be disheartened, Bart! Try the new Celestron X-Cel LXs. I have a feeling they will be quite good in fast scopes. Yes, they are only 60°, but only £70 each.

Andrew

[rwilkey]

HTH = hope that helps.

[John]

I agree with the feedback given above. Below F/5 scopes really stretch eyepiece performance. Tele Vue eyepieces are tested to work well down to F/4 - and thats each and every eyepiece before they go out for sale. I would expect to see some outer field astigmatism in Nirvana's / UWAN's / Meade UWA's / ES 82's and 100's in an F/4.5 scope, great eyepieces though they are. I guess the question is how bothersome it is to the observer, which will vary person to person.

If your ES's are OK over 70% of the FoV then you are seeing an undistorted field of nearly 60 degrees so switching to the Celestron X-Cel LX's may not make much sense. The ES's deliver great sharpness and contrast in that central 60 degrees I'm sure.

[alan potts]

Fast scopes need Tele-Vue eyepieces, I don't know how many times now in the short time I have been a member I have seen John write something similar to what he has writen above.

The fastest scope I have is F 5.3 and all my TV's are fine at this, as for faster I don't know. However I can see cracks in the armour of SWA and UWA at F 7, not big ones but they are there.

Alan.

[Banana head]

Or for fast scopes try the BSTs. Often overlooked because they are not expensive, but they work exceptionally well.

Sometimes you don't need to spend a fortune to get good results.

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[Mike73]

I've used BST and Xcel LX'S in my f/6 and they were fine but as soon as I stepped up to f/5 they showed exactly the same problem that Bart mentions in his original post.

My TV's and XW cured that problem but at a price, just depends on if you are willing to fork out to cure that edge softness IMO.

[Banana head]

I've found them very good in my f5 Newt. No hint of problems.

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[John]

I've found them very good in my f5 Newt. No hint of problems.

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As has been said, F/4.5 is a somewhat different proposition from F/5 if you want pinpoint stars right to the edge of a wide field of view, as the OP has found out through comparing the performance of his ES eyepieces.

[KaStern_Former_Member]

Hi Bart,

what you see is most likely the combination of two seperate aberrations:

1) Coma originated from the mirror. As Dirk Steele pointed out coma increases rapidly toward faster mirrors.

2) Astigmatism caused by the eyepiece that is not siuted for very fast scopes.

To cure coma you can either use a coma corrector or a coma correcting University Optics Klee Barlow

or a Klee designed comacorrecting Pretoria eyepiece.

To cure eyepiece astigmatism you can use a barlow or eyepieces better suited for fast telescopes.

Cheers, Karsten

[Banana head]

As has been said, F/4.5 is a somewhat different proposition from F/5 if you want pinpoint stars right to the edge of a wide field of view, as the OP has found out through comparing the performance of his ES eyepieces.

Actually, I was replying to the post immediately before mine; which mentioned f5 scopes, but thank you for pointing that out.

In fact, when it comes to Eps and fast scopes, there is not a lot of difference between f5 & f4.5.

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[John]

...In fact, when it comes to Eps and fast scopes, there is not a lot of difference between f5 & f4.5.....

We will have to disagree on that point

[Naemeth]

Actually, I was replying to the post immediately before mine; which mentioned f5 scopes, but thank you for pointing that out.

In fact, when it comes to Eps and fast scopes, there is not a lot of difference between f5 & f4.5.

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Not a lot of difference? The OP highlights at least 30-40% of edge sharpness lost when upgrading to an F/4.5 from an F/5.

[Banana head]

Yes, in the EPS he was using, which he admits aren't good in fast scopes.

Had he used those more suited to fast scopes then he would notice little difference between an f5 & f4.5.

I hope I've explained it clearly?

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[Bart]

I suppose it comes down to what you call 'fast' scopes. Explore Scientific 82* series would generally be classified as good in fast scopes, I found this to be true in my fast scope of F5. However, in my even faster F4.5 they seem to struggle, something I haven't come accross in many of the reviews I'd previously read. I suppose you can't beat real life experience. However, before I go off and sell my kidneys to fund TVs (my kidneys are a bit like the ESs, not as good as I thought), I'll wait to have a session or two more..

Thanks for all the input, interesting.

Barry

[Naemeth]

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?

[Bart]

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?

Jasus, good luck getting an eyepiece for that.

[Naemeth]

Jasus, good luck getting an eyepiece for that.

I saw the F/2.7 and thought... "hang on, TeleVue tests as fast as F/4, how could it work to that standard at F/2.7?