Eyepiece selection for my new dobsonian

[khodd]

Although this is my first post I've been reading the forums for a few weeks now. This has been immensely useful and I've learnt heaps.

Like many others with a new scope I'm wondering what I can do to enhance its capabilities. Yet even though there have been volumes written on eyepieces I'm still confused about a few things.

I have what seems to be a fairly standard setup - an 8" Bintel dobsonian. It's 1200mm long which makes it about an F6, I think. I'm not sure whether this makes it "fast" or "slow", (or what these terms even mean for what appears to be a stationary object) but from many of the posts I've read this seems to make a big difference to EP selection. Is this just because the light gathering capacity limits magnification?

The eyepieces they gave me with the dob are a 2" GSO SP 26mm "Wide Field", which seems OK, and two 1.25" GSO Super Plossls at 9mm and 15mm. Since going out and looking at Saturn (sharp, but no cassini) and Jupiter (bands visible, but no red dot) I am stongly tempted to get a better EP for planets. My question is, do I go for a 5mm EP of some kind, or go for a longer focal length and use barlows for higher magnification? Most people seem to recommend putting a barlow or two in the kit so what's the point in having an EP at the limit of the scope's magnification?

Some of the posts seem to suggest that there are viewing advantages in using a barlow: are there any disadvantages? If you were to use an expensive wide field EP in a barlow would you still get a wide view? Can you use a 1.25" EP in a 2" barlow?

Cheers

Khodd

[Starkler (Geoff)]

Originally Posted by khodd

Some of the posts seem to suggest that there are viewing advantages in using a barlow: are there any disadvantages? If you were to use an expensive wide field EP in a barlow would you still get a wide view? Can you use a 1.25" EP in a 2" barlow?

The disadvantages to using a barlow, namely light scatter and optical degradation can be minimised by choosing a good quality barlow over a cheap one. A cheap or mediocre one will noticeably degrade any eyepiece you care to stick in it. Buy a good one and you are likely to keep it for life

The Orion Shorty-PLUS at around $140 seems to be the universal choice for those wanting a good 1.25" barlow at a not-so-astronomical price. This barlow along with your 15mm plossl and another 12mm one will see you right for planetary viewing

[AstroJunk (Jonathan)]

The conditions in Brisbane have been really poor for a while - you may just be suffering from that.

Cassini is obvious in the smallest of scopes, so I suspect other things are at play here. Maybe allignment, maybe the poor recent conditions. How long have you had the scope?

[janoskiss (Steve H)]

Wecome Khodd! I'll second Geoff's advice. A good barlow and a 12mm plossl in addition to what you have will give you a lot more flexibility. But you don't need these to see Cassini. It should be easily visible with your 15mm and 9mm. Just go and look a again a few times. Here is a preview courtesy of Asimov :
http://www.iceinspace.com.au/forum/s...ead.php?t=9040

[RapidEye]

Originally Posted by AstroJunk

Cassini is obvious in the smallest of scopes, so I suspect other things are at play here.

Agreed! On a good night, I can easily spot the CD at 90X in my 4.5" Dob.
On a bad night - no combo of scope and/or EP will pull it in for more than a fleeting second or two.

Doublecheck your collimation then watch what the jetstream is doing!

G'Luck!

[wavelandscott (Scott)]

You mentioned in your post

"Like many others with a new scope I'm wondering what I can do to enhance its capabilities."

I'll suggest what I regularly do to new found enthusiasts...and this is not meant to be insulting by any means as I do not know your experience level...

Get to know the gear you have for a while...the good folks at Bintel (and some other retailers) have put together a pretty good package for you that was up until a few years ago, better than what most astronomers had (amateur or professional).

Use what you have and gain some veiwing experience...figure out what you like and don't like but do it over a period of time...it does take some time to pull out detail and figure out what you like and don't like...

As you gain some eyepeice time, take the opportunity to look through other people's gear and then (and only then IMHO) start window shopping for your next eyepeice...

And when that time comes, buy the "best" you can...although I am not really sure myself if the difference in price between the good functional gear and the top end is always justified or needed...but that is another post...

So in a nut shell, in my opinion the best and most important thing that you can do to improve/enhance your scopes capabilities is to look through it...that is, practice viewing yourself (and get good at collimation if you have a reflector)...as you gain experience and time at the glass, you will see more and then (and only then) it will be time to consider buying more stuff..

Cheers!

[rmcpb (Rob)]

First WELCOME to IIS

Next I'll give my standard answer to all newbies with their first scope. The set of eyepieces you have been given by Bintel is a great start and for the short term there is no need to expand it or to get better lenses.

The things you need to do now to get better views are:

• Use it often and get used to handling it.
• Allow it to cool for at least an hour in the place you are going to view from to allow the mirror to get near ambient temperatures. This will reduce the effects of thermal currents in your tube which degrade views.
• Ensure it is collimated. I would recommend your first purchase to be an Orion Collimating Eyepiece from Bintel. If the scope is not collimated it will never give you great views.
• Allow for bad seeing, if the stars are twinkling then the seeing is bad and you will never see the Cassini Division. It will be a bit like trying to see the bottom of a swimming pool when someone has dived in. This is caused by many factors but lately the jet streams have been over us and messing up the air. On nights like this go for the low power stuff like clusters, etc and avoid trying to get detail on planets and splitting doubles, etc.
• A good barlow should be your other first purchase. I have used the Orion Shorty Plus but now I have a Meade #140 and I prefer it. Pick your poison but make it a good one otherwise forget it.
• Did I mention, use your scope??
• Practice!!
• Join the local astro society and go to their observing sessions. Lots of great experience there for you to plug into directly.

Apart from that enjoy the views.

As for the fast and slow scopes, shorter focal length ones f5 and below are fast and give great wide angle views f8 and above are slow and are more suited to planetary observing where high magnification is important. Our f6 scopes are great middle of the road scopes.

Above all else, keep looking up.

[AstroJunk (Jonathan)]

Originally Posted by rmcpb

Join the local astro society and go to their observing sessions. Lots of great experience there for you to plug into directly.

And no finer in brisbane than the AAQ: http://www.aaq.org.au/

We've got a Deep sky night on Mt Glorious on the 29th and a Lunar and Planetary night the week after. There is usually a dozen or so members and guests with scopes of all types and sizes.

And don't forget the BAS and SAS if our meeting times don't suit!

[ving (David)]

hi and welcome aboard
I have the same scope as you. I have had it for a while now. your 9mm eyepiece should show the cassini division (CD) quite easily in good conditions. the fact that you couldnt see it sugests that either the "seeing" was bad or maybe your scope isnt collimated. is your scope collimated properly?

if you are determined to go for a more powerful eyepiece tho a 7.5mm would be as high as i'd go (unbarlowed). you'd get to use to often as opposed to a 5mm which you would only get to use when the seeing is exeptional....

i am guessing the next question is "what is seeing?"

[Hammerman]

I have the Orion version of the same scope as you have. Been using it for years. Like Ving said.... I have a 7mm Nagler that I use in this scope, but I don't get to use it often. Seeing has to be good.

For the recoed. I own an Orion Shorty Plus, which is a nice barlow to use with this scope. However I have to agree with the folks here who suggested you wait before you buy anything else until you get a lot more experience under your belt. You will have a much better feel for what you really want or need after a while.

Patience is not only a virtue.... it can also save you a lot of cash!

Enjoy the new scope.

[khodd]

Thanks for all the advice.

Astrojunk - I've had the scope since xmas. (Ordered it Dec 23 PM, Bintel had it delivered in 24 hours ready to go under the tree.) Since then I've started to realise just how many clouds there are of an evening. It's certainly been an exercise in patience. I'm certainly intending to join a society, the one at Chermside would be closest but I'll have to see whose meeting times fit in with familly demands.

Rapideye - I had assumed that the collimation was OK since stars were sharp and blurred in a symetrical way either side of focus. Still, I just ordered the Orion collimating EP from Bintel, so I'll soon see. I've noted some discussion of the jetstream: I assume there's a link somewhere to a weather site so I'll go and chase up those threads.

Wavelandscott - Yes, I must do more viewing, though the rare allignment of weather and opportunity is frustrating that aim. I'm somewhat comforted by the assessment that you and others have made of my gear, that it's OK and probably not the main obstacle for now. Some of the comments in other posts had left me thinking that EPs supplied with scopes generally weren't up to much.

rmcpb - now that I look more carefully the stars do twinke - more obvious on the fainter ones. But I thought they always did that, hence the nursery rhyme. So much to learn. I'm afraid to ask just how often you do get good seeing in Brissy. I've been counting myself lucky just to get a patch of cloud free sky. At least out my way past the Bunya State Forrest the sky is darker than the suburbs. I generally see the Milky Way fairly easilly on moonless nights.

Ving - I've certainly come across the concept of "seeing" but haven't made a study of it yet. I guess I just figured that since atmospheric conditions were out of my control I'd just try to optimise the things I could influence. But maybe I should just be more patient. And practice more.

Hammerman - a Nagler is certainly on my wishlist but I 'll probably only ever afford to buy one so I'll agonise over focal length for at least a year before ahnding over my cash. I certainly have enjoyed using my scope. Just tonight I tried using my nephew's $70 refractor he got for xmas. He hadn't been able to find anything other than the moon, but since the spotter was out of focus and pointing in a whole other direction that wasn't surprising. Tried to allign it using a distant streetlight for starters, but even this was arduous owing to the wobbliness of the mount. Got it roughly set up but by then the clouds had rolled over. The whole experience has certainly made me appreciate what I've got. It's a pity, but his parents got a cheap scope mostly because they weren't sure if he'd persevere with astronomy. Because he's found it so frustrating he probably wont.

Once again, thanks a heap for all the help.

Khodd

[chunkylad (David)]

Originally Posted by khodd

Ving - I've certainly come across the concept of "seeing" but haven't made a study of it yet. I guess I just figured that since atmospheric conditions were out of my control I'd just try to optimise the things I could influence. But maybe I should just be more patient. And practice more.

Hammerman - a Nagler is certainly on my wishlist but I 'll probably only ever afford to buy one so I'll agonise over focal length for at least a year before ahnding over my cash. I certainly have enjoyed using my scope. Just tonight I tried using my nephew's $70 refractor he got for xmas. He hadn't been able to find anything other than the moon, but since the spotter was out of focus and pointing in a whole other direction that wasn't surprising. Tried to allign it using a distant streetlight for starters, but even this was arduous owing to the wobbliness of the mount. Got it roughly set up but by then the clouds had rolled over. The whole experience has certainly made me appreciate what I've got. It's a pity, but his parents got a cheap scope mostly because they weren't sure if he'd persevere with astronomy. Because he's found it so frustrating he probably wont.

Once again, thanks a heap for all the help.

Khodd

Hi Khodd, and welcome.

I do believe you've hit the nail on the head with a couple of your obsevations about this wonderful hobby.

Wrt 'the seeing', the more time you spend at the eyepiece, the better your appreciation for the prevailing conditions, and the better understanding you'll have for your eyepiece requirements: ie, which focal length Nagler to fork out those hard-earned $$$ on lol . A point to note is that when viewing objects near the horizon, poor atmospheric conditions are exascerbated, as there is more air for you to observe through. Try to concentrate on objects closer to zenith.

The issue of the cheapo refractor and wobbletronic mount is a classic case of how 'not to go about getting and keeping someone (especially a junior) interested in astronomy'. Very high on the frustration scale.

These two issues, imho, have taken up more beginners' forum space than any others, and are the main thrust of many of the posts in this thread.

I believe that's why we have so many dobsonians spread through the forum members - especially for those who joined this forum looking for advice on how to set up from the beginning. Great aperture, stable and easy to use mount, easy(er) on the hip pocket. Low frustration value. I know that's why I bought and greatly enjoy mine.

Good luck, enjoy!

[janoskiss (Steve H)]

Originally Posted by khodd

I'm somewhat comforted by the assessment that you and others have made of my gear, that it's OK and probably not the main obstacle for now. Some of the comments in other posts had left me thinking that EPs supplied with scopes generally weren't up to much.

IMO those sorts of comments can only come from those who have been spoilt with many years of using the best premium eyepieces. So when they go back to inexpensive plossls, their assessment tends to be rather harsh. The GS plossls are very capable eyepieces. Sure, they are not the pinnacle of optical performance, but most newbies would be hard pressed to appreciate much difference between a GS and say a Televue Plossl. The difference in how much you can see through them is marginal.

If you want better planetary performance then the 12mm HD ortho in a good 2x barlow would be an excellent choice for your scope (or the 18mm HD ortho in a ~3x barlow, say the 2.8x klee). If you can spend a bit more, the 12mm Pentax XF should give you similar performance across a larger field of view and with more eye relief. This focal length will be ideal for deep sky as well. An absolute bargain of a premium quality eyepiece.

a Nagler is certainly on my wishlist but I 'll probably only ever afford to buy one so I'll agonise over focal length for at least a year before ahnding over my cash.

For your scope, the 13 or 11mm T6 or the 12mm T4. I used to own the 13mm, and it was a very useful focal length for the 8" f/6 Dob. But Nagler is not worth the cost IMO: it has too many shortcomings. For the money I expected something much closer to perfection. They are worth about 2/3 what they cost IMO. They are worth buying second hand for that sort of price, and you can always resell with no loss.

[khodd]

Once again thanks all. Now if the clouds would only go away. A couple of friends are coming over on Friday night to join me in some star gazing so I hope it clears up by then. (Part of the reason for the timing is to also look at the pi-Puppids. I'm assuming I should look at Pisces since I can't find another constellation beginning in "pi".) I know what you'll all be thinking but these guys know even less than I do so I should be able to bluff my way along without too much embarrassment.

The deplorable conditions do at least show how bright Jupiter is at the moment - you can still see it despite moderate cloud cover.

Anyway, the one question I forgot to ask is can anyone suggest the ideal book for a guide to southern skies? I'd like something that doesn't assume too much. If it had a range of star-hopping guided tours that would be good. But also if it would describe the objects I'm looking at. How far away, whether they are associated with the Milky Way or are not, that sort of thing. Also do people have any sort of preference for "brand" of planisphere? OK, that was two questions.

Cheers

Khodd

[chunkylad (David)]

HI Khodd

I believe the pi-Puppids originate near the star(s) pi Puppis - around halfway between the two brightest stars in our sky - Sirius and Canopus.

Plenty of info on Southern Hemisphere books on the forum.

So FWIW, my two bob's worth: Pretty much a must-have is "Astronomy 2006" by Quasar publishing. Runs out at around $23.oo. This will give you a month-by-month guide to your night sky, as well as heaps of other useful stuff. Also, Hartung's "Astronomical Objects for Southern Telescopes" is very useful and very informative for the beginner (I got mine from Bintel Sydney). My wife bought a book called "Encyclopedia of Discovery - Skywatching" by The City Press. Although it's published in the USA, it still has some very good Southern Hem star hop guides. I use it at my observing table all the time.

As far as planispheres go, my Philips Planishere is still good as new after over a year's worth of observing. I use it as a rough guide, but prefer to star hop with more detailed maps or charts to find those elusive DSOs etc.
I also use Astrovisuals' star charts by Will Tirion, and their Lunar maps. Very useful , Aussie made, and won't break the bank.
http://www.astrovisuals.com.au/skymaps.html

I hope this helps.

Dave

[janoskiss (Steve H)]

Originally Posted by khodd

Anyway, the one question I forgot to ask is can anyone suggest the ideal book for a guide to southern skies?

The Southern Sky Guide by Ellyard and Tirion. Maybe not as in-depth as you'd like, but still great with mag 6.5 charts. Bargain for around $25. Most telescope shops that sell astronomy books should have them.