Eyepieces

[BLiTZWiNG (Trent)]

Ok I've had my scope for 6 years and its 4 plossl eye pieces, 6mm, 9mm, 12.5mm and 20mm.

Can I do better than these and get a better image? I think the 6 and 12.5 are GSO plossl and the other 2 were unbranded.

[mental4astro (Alexander)]

Stepping into a mine field, my friend -eyepieces!!!

There are essentially two camps: 1, the big bucks crew, and 2, the cheaper crew.

I'm in the second group.

This doesn't mean cheap and nasty. I see it as practical and logical.

Eyepiece technology has come a very long way since the plossl design ruled king. Today, even 'cheap' eyepieces use glass materials, lens coatings and lens designs that make plossls look just aweful.

My main obs. eyepieces are GSO's. They come from the Superview range. I have the 2" 30mm, and the 20mm & 15mm in the 1.25".

For higher magnification, I've got the TMB Planetary Type II 9mm & 6mm. These are sensational, and as there is a nasty bum fight between the estate of the designer & the Chinese manufacturer, they are dirt cheap. The eyepieces classifieds here in IIS sees Sylvain giving us IIS members a great deal on the 6mm & 9mm. I've had the 6mm for nearly a year. When Sylvain posted the TMB deal, I jumped at getting the 9mm.

If you like the idea of cheap and practical, I can suggest for your 8" dob the Superview 30mm & 15mm & the TMB 6mm (read the classified ad on the topic of these and contact Sylvain). These will serve you very well. Not bad for less than $150.

Barlow? I've got one, I don't really use it though. For your 8", with these eyepieces, taking into account the effects of the atmosphere on maximum magnification (200X at the best of times), you really don't need a barlow as the 6mm will give you 200X.

Prices then go up, but not practicality necessarily.

[BLiTZWiNG (Trent)]

Thanks again Alex

I don't mind cheap, I am quite happy with my 12.5mm, but not knowing enough about optics and reading how the ones that usually come with the scope aren't that great I decided its time to increase my viewing pleasure, seeing I can't see colour nebulas (yeah I'm one of those crew), but the best Jupiter I've seen I was only able to make out 4 bands of colour. The 6mm is garbage even to my eye. I have to wonder whether a 9mm would be better than a 6mm, but then maybe my current 6mm is way worse than even I think it is.

So now I will probably take your advice and try these eye pieces on for size. A barlow probably wont do me much good with all my light pollution!

[BLiTZWiNG (Trent)]

Oh, what's this 1.25" and 2" stuff? Is that the size of the eyepiece tube?

[erick (Eric)]

Yep, essentially the outside diameter of the bottom of the eyepiece that inserts into the focusser draw tube - it's called the barrel.

There are some old focussers smaller than 1" and there are some new focussers designed to fit imaging cameras that are 3" (or even more!!).

But most focussers these days will have a draw tube designed for 2" barrel eyepieces. Some focussers are designed for 1.25" barrel eyepieces, but these are not very common in other than the cheap-end scopes.

So most eyepieces that you can buy come with a 1.25" barrel or a 2" barrel. Some (Televue is an example) come with a dual barrel that will fit into either size focusser. Usually, however, if you have a 1.25" barrel eyepiece, you use the 2"-->1.25" adapter that would have been supplied with the 2" focusser.

So do you want to buy 2" eyepieces or 1.25" eyepieces? Sometimes, for convenience, you might stick with one barrel size (2" so that you don't have to fiddle with the adapter), but most people don't mind doing that. Sometimes you cannot avoid going to a 1.25" barrel to get the magnification and field of view that you want for an object (eg. planets).

Intuitively, you would think that a longer focal length eyepiece (eg fl of 30 or so mm and longer) with a wide apparent field of view (say AFOV of 68 deg or greater) would only be possible in a 2" barrel, and, guess what, you would be mostly right. But narrower AFOV eyepieces (eg,. plossl at 49-50 deg) can come in a 1.25" barrel for a long focal length (eg. Televue's 40mm fl plossl has a 1.25" barrel.)

So if your focusser takes 2" eyepieces and you are happy to use the 2"-->1.25" adapter, don't worry about the barrel size.

[BLiTZWiNG (Trent)]

Thanks Eric!

I actually have this funny feeling that the barrels of my eyepieces are all 1.25", but I'm not at home to check that. Just going from memory they are roughly 1" and I'm pretty sure they aren't 2".

Looking at the 8" on Andrews comes with 1.25" and 2" -> 1.25" adapters. Mine is an 8" GSO that I got from Andrews 6 years ago, so it's probably safe for me to say it's 1.25", though I guess its not much of a hassle to get an adapter!

[erick (Eric)]

Originally Posted by BLiTZWiNG

Thanks Eric!

I actually have this funny feeling that the barrels of my eyepieces are all 1.25", but I'm not at home to check that. Just going from memory they are roughly 1" and I'm pretty sure they aren't 2".

Looking at the 8" on Andrews comes with 1.25" and 2" -> 1.25" adapters. Mine is an 8" GSO that I got from Andrews 6 years ago, so it's probably safe for me to say it's 1.25", though I guess its not much of a hassle to get an adapter!

Trent, if you have a 1.25" focusser, you will only use 1.25" eyepieces. You can use smaller barrel diameter eyepieces in a larger focusser (using the adapter), but not vice versa.

But it is a fairly easy job to swap your 1.25" focusser for a 2" focusser. A secondhand 1:1 2" GSO Crayford focusser can be had for a handful of dollars from IceTrade classifieds.

[BLiTZWiNG (Trent)]

OIC... the 8" that has those adapters also comes with the 2" GSO Crayford focuser. That makes sense now.

[koputai (Jason)]

Trent, it will depend on which version of the 8" GSO you have. They come in f/4, f/5, and f/6. If yours is f/6 you might get away with Superview's, but any faster and they are a bit iffy.

I've owned a couple of Superview 42's and found them rather nasty, even in an f/6 scope. Good quality Plossl's perform better.

The shorter ones, like the 15mm and 20mm Alex mentions may be ok, I've tried the 15mm in a refractor and it's not bad.

Cheers,
Jason.

[BLiTZWiNG (Trent)]

Hi Jason, I've actually been wondering about the speed of my scope. I think the new versions are faster than mine but I can't be sure. I'm sure mine wasn't a 1200 focal length either, I'm pretty sure it's 1000, but I don't really have a way to check that that I know of, other than measuring the length of the OTA.

I checked, my GSO Plossls are 25mm and 9mm, and the Super 500 are 12.5 and 6.

I think the 12.5mm and the 9mm work the best in my scope, 6 was too blurred and 25 was probably not magnified enough. Edit: That being said I probably don't "get" the purpose of a low mag EP.

[koputai (Jason)]

Measure the total length of you OTA and let me know. I'll compare it with mine and we can work it out.

Cheers,
Jason.

[jenchris (Jennifer)]

You show me yours and I'll show you mine???
C'mon guys - comparing lengths...MEASURING?

[BLiTZWiNG (Trent)]

It's the only way to know for sure Jen. You women just don't understand!

[mental4astro (Alexander)]

Originally Posted by BLiTZWiNG

I think the 12.5mm and the 9mm work the best in my scope, 6 was too blurred and 25 was probably not magnified enough. Edit: That being said I probably don't "get" the purpose of a low mag EP.

Hi Trent,

A scope, terrestrial or astronomical, is not all about magnification.

You may have noticed that the higher the magnification, the narrower the patch of sky you are looking at. If you are looking for a particular target, or a target that is very large, or even a very faint target, AND if conditions aren't great, using low power is your most useful.

As an example, the Orion nebula. In just about ANY scope, this bugger is so big, it will fill the entire field of view with its extensions. The only way to make the most of this massive size is to use your lowest power.

Some targets are also large, but so faint as their light is spread over a large area. The Helix nebula is one example. Here, the lowest power available to you is the only way to see this gassie ball. Actually, binocular probably offer the best view of it. It comes to the relationship ratio between the eyepiece's focal length, the scope's f/ratio, the scope's focal length all working together.

Galaxies. Low power is your best friend here. It is the best way to first catch a glimpse of most of them, but if conditions are spectacular, the only way to see them the whole night.

My 30mm is the most used EP. Then my 15mm. Then my 9mm. My least used is my 6.

Jason, yes the Superviews do show their short comings in fast scopes. Yet, for their price, I am happy enough with their performance in my f/4.5 & f/5 scopes. ONEDAY I'll look at getting my hands on a better quality EP. But that is the point of my discussion: if you are prepared to both recognise shortcomings and to work with them, the eyepiece options increase.

As an added note, I can't use the superviews at all on my very fast 8" f/4 scope. Instead, the best eyepiece for it (without using any type of optical corrective aid) is a 30 year old RKE 28mm.

Oh, AND one more thing, their is also a minimum useable power in reflectors. This is because of the influence of the secondary mirror's obstruction. Make it too low, and you can't eliminate the shadow of the secondary from the view through the EP.

The lowest power eyepiece in mm is given by this:

longest EP in mm = f/ratio of scope X 6

So, for my 17.5" f/4.5 scope, the longest EP is 28mm (30mm is still ok, 42 too long), giving me 67X.


There is also another thing about cheap eyepieces, that Trent has alluded to- difficulty of using. The shorter the focal length, the smaller the eyelens becomes and the closer your eye needs to be to it to see anything at all.

That is why I suggest the TMB Planetary TYPE II's. Their design and materials combine to make a superb high power eyepiece that is easy to use & with a relatively wide field of view.

Also, the shorter the focal length, the more obvious the prevailing atmospheric conditions become, REGARDLESS of the quality of the EP or scope as it is purely an atmospheric limit. Most times, the atmosphere only allows for between 100 & 120X to be used. 200X is very occasionally. 300X you should be buying lottery tickets.

That is the reason I jumped at getting the 9mm TMB for my 17.5". The 6mm gives me 333X, way too much for any practical use (I've managed only a handful of good times that it was useable in the last two years). The 9mm gives me 222X which is much more useful, and does get used much more than the 6.

Last note (really!), maximum power in a scope : 50 X per inch of apeture.

BUT, the ultimate limiting factor to maximum magnification is the atmosphere. I "should" be able to take my 17.5" to 875X, not a hope in hell! Not unless I'm 5000m up a mountain.

And you thought this telescope stuff was straight forward!

[BLiTZWiNG (Trent)]

Once again awesome info Alex!

No, really, if I thought this stuff was easy I wouldn't be here asking questions.

I had a good viewing last night in ordinary seeing conditions. I had a go with all of my EPs, and I have to say the 6mm problem I had was all user error. Jupiter was a little wobbly (hence my noting that seeing wasn't that great) but I could still see the dark brown band. I could barely see it with the 9mm. I think I was using the 6mm for the Tarantula nebula as well, though I spotted it with the 9mm. Wierd given it took me 15 minutes to find Jupiter even though I could clearly see it with my eyes. Come on RDF where are you!

So, I also measured my scope this morning. The barrels of my EPs are 32mm (1.25") wide and the length of the OTA is 1113mm. At a guess I'd say that the focal length is probably 1000.

[ZeroID (Brent)]

Hi Trent, I'll backup the GSO SuperViews as good for the money. I've got a 20 and 15 I use in the 10" f5 and both are excellent (IMO) and both barlow well at 2x.
I have 12, 9 and 6 plossls which only get used when I want some specific details.

[erick (Eric)]

Originally Posted by BLiTZWiNG

..... the length of the OTA is 1113mm. At a guess I'd say that the focal length is probably 1000.

OK, the distance you need to measure is from the primary mirror surface to about the upper third of the draw tube in the focusser, doing a right angle turn at the surface of the secondary mirror. I think you'll find it is close enough to 1200mm - an f6. (The other options are f5 - 1000mm and f4 - 800mm so you are just trying to get to the nearest 200mm. Eg, if you measure 1172mm - it's probably 1200mm focal length.) There isn't a straightforward way to find that spot in empty air where the primary mirror focuses, hence my suggestion of using the upper part of the draw tube - that should get you close enough to 1200, or 1000, or rarely 800.

(don't scratch the primary mirror with the end of the tape measure!! Measure from the centre spot position to be safe - a scratch or two there does not matter to your viewing. With the secondary mirror, try to not touch any of the surface.)

[BLiTZWiNG (Trent)]

Yeah I'm not gonna stick a tape measure in there. I'm not quite sure where you mean either, but wouldn't measuring to the center of the focuser be about right?

Oh, I vaguely recall this scope being an F6.

[koputai (Jason)]

OK, it sounds like you've got an f/6. My f/6 OTA is 1130mm total length (113cm).

Funnily enough, using Erick's method, from the centre spot, turning a right angle through the secodary, and out to the focuser, I get 1100mm, and this is definitely an f/6 (1200mm) scope.

Cheers,
Jason.

[BLiTZWiNG (Trent)]

Does the right angle turn at the secondary though include the distance out to the eye?