Planetary Eyepiece advice

[Don Pensack]

Originally Posted by SkyWatch

I suspect all this is getting quite confusing for the OP.

For the record, I have a 12" f5 scope, and the eyepieces I mentioned (even the cheap LP ones) are well corrected for coma at that f-ratio, and give sharp images across virtually the whole field, so you can watch a planet pretty much from one edge of the field to the other without having to move it into the centre of the field for the whole time.

For visual use they do a great job, and are a lot cheaper than a Paracorr or an equatorial platform...

- Dean

Sure, your *eyepieces* are corrected for coma, as all good eyepieces are, but your *scope* is not. The coma free zone in your scope is only 2-3mm wide in the center of the field, and adds coma linearly as you move toward the edge of the field. Even if you don't notice appreciable coma at the edge of the field (and, in 50 degree eyepieces you probably won't), the sharpness of the planetary image will slowly diminish in a linear way as it nears the edge of the field. A 10mm Plossl has about a 7.5-8mm field stop, so coma certainly won't be severe at the edge in that eyepiece. But there is coma in the image as soon as you are 1.1-1.3mm away from the center of the field. That's why tracking is so important.

Or a coma corrector if tracking isn't present.

Having said all that, though, the variation in seeing that occurs will totally swamp the best correction possible:
Perfect optics + Perfect collimation + Perfect equilibrium temperatures in the optics + perfect eyepiece + tracking + bad seeing = bad image.
Alas.

[astro744]

Originally Posted by Don Pensack

Perfect optics + Perfect collimation + Perfect equilibrium temperatures in the optics + perfect eyepiece + tracking + bad seeing = bad image.
Alas.

Exactly what I was getting too earlier in the discussion, I just didn't add all the parameters as I had done so in a previous similar thread a while back.

You can also add bad eyesight to the mix. If any one is bad the result is bad.

[bratislav (Bratislav)]

This was very educational thread; I have certainly learned a couple of things, and for that I am grateful.

[209herschel (Herschel)]

Originally Posted by Don Pensack

I agree with Bratislav, except with these caveats:
If your scope is undriven and you are pushing it to follow a planet that drifts from side to side across the field, then definitely a coma corrector like the TeleVue Paracorr is called for. Otherwise, the image will be unsharp outside of a very small area in the center.
IF, on the other hand, you add a tracking platform and you now can hold the planets dead center, then, for planets, you don't need a coma corrector and just about any decent eyepiece will show you all the scope can see in the seeing conditions under which you're observing. Seeing is the greatest factor in determining the quality of the image, not the eyepiece. There is no magic eyepiece that suddenly makes seeing better.

For wider, extended, objects, then merely having tracking is not enough, and the coma corrector becomes more important again. We have all read innumerable accounts how such and such an eyepiece is "terrible in my dob", but "the images clean right up when a coma corrector is added". Well, duh. Of course they do--you just eliminated the coma from the mirror and, in the case of a Paracorr, also flattened the field slightly.

The primary difference you'd see between the top tier eyepieces like the Delos, and a lower-tiered one like the Hyperion, is at the edge of the field. Being free from induced astigmatism, the star images in the outer 50% of the field will be a lot better looking with the Delos. Will that matter for planets held in the center? Not really. Will it matter when you are letting the planet drift across the field? Yes. But we still get back to the coma corrector once again.

Where would I put my money if I were a hardcore planetary and lunar viewer?
1) tracking platform.
2) coma corrector
3) THEN maybe a better eyepiece, though it is hard to beat the lowly Plossl for planetary images.
so, maybe:
4) Gasoline. To drive me and my scope to sites where the seeing is better.
We have a few sites around here that are that much better than average. Everyone, with every eyepiece in every scope remarks that the images are better at those sites. Just points out how important seeing is.

Thanks very much for all of that information. I bought my 10" dob from Andrews so I know there are mirrors that are a lot better but I have to say I'm really enjoying my scope. I've never had an eyepiece with greater magnification that 9mm so I thought that something like 5mm or 6mm would make a difference with something like Saturn? At the same time, I know I might sacrifice some clarity. High on my list is to try to get to a watching event where I can talk to some more experienced people. Thanks for the advice.

[SkyWatch (Dean)]

Just to weigh in again: I haven't had the scope out for a while, but managed a really nice view of Saturn tonight. Very good seeing for once!

Some of the previous comments about coma in an f5 scope and the "unlikelihood" of a sharp fov with a planetary eyepiece like the Long Perng 5mm made me doubt my memory, so it was nice to have a chance to check. I understand the fov must deteriorate past the centre couple of mm due to uncorrected coma, but high power eyepieces only effectively use the centre of the field anyway. Coma is certainly present at the edge of the fov at low power, and that is where a coma corrector shines, but I must say I am not sure that one is absolutely necessary at higher power, even with an undriven scope like mine. Especially when one like the Paracorr can cost as much as the entire scope, and more than 10x as much as the cheaper high power eyepieces...

Tonight's viewing showed me the following:

The Long Perng 5mm showed a beautiful view at 300x (12" f5 scope), and I could see a sharp Cassini division and nicely contrasting cloud bands from edge to edge of the 60 degree apparent field, letting it drift in and out. The contrast and sharpness was perhaps slightly better at the centre, but it was very hard to notice any difference apart from a tiny bit of lateral colour near the edge of the fov: but almost all eyepieces show some lateral colour near the edge.

My Nagler T6 13mm (115x) was beautifully sharp and contrasty right across the 82 degree apparent fov.

I compared with a 6mm Vixen LV (45 degree fov, 250x), a 7.5mm Takahashi (50 degree, 200x), and a 3-6mm Nagler zoom.
All showed a beautiful, sharp image from edge to edge.

The Long Perng 5mm was certainly not put to shame, at roughly 1/2 to 1/4 the price of the others, and it has a wide enough fov to enjoy the view for a while before it drifts out of the field and requires moving the scope again. Much more so than (say) the Vixen. Its 20mm eye relief is a great asset as well if you have to use glasses- way better than a standard plossl. Well worth a look.

It would be nice to use a driven scope, especially at 300x(!), but because of the sharpness of the eyepieces it didn't really matter much at all if the image drifted off centre.

All the best,

Dean