Astigmatism?

duncan · #1 15-01-2008, 03:41 PM

Hi all,
My 12"Dob seems to be well collimated ( everything centred in the cheshire) but in the eyepieces the centre stars are perfect but the outer ones show like a tail. If i move my eye across the eyepiece i can get the tails to dissappear. If i defocus the stars the centre ones are nice and round with the black centre as are the outer ones if i move my eye across the eyepiece.
I'm only using the original eyepieces that came with the Bintel Dob.
Anyone got any ideas?
Perhaps i need a parracor?
Maybe just better quality eyepieces?
Maybe my eyes are stuffed?

Any suggestions are welcome.

rmcpb (Rob) · #2 15-01-2008, 03:53 PM

One word - COMA

Its a problem with faster scopes and your scope, at f5, would be considered fast. The expensive answer is a paracor but better eyepieces will help and most people can live with some coma in the visual world, especially if they are on a budget like most of us.

Cheers

duncan · #3 15-01-2008, 03:59 PM

Thanks for that. At least it's not my eyes.LOL
I've not seen a parracor advertised, any idea how much?
Cheers.

rmcpb (Rob) · #4 15-01-2008, 06:36 PM

About the $400 mark. Check out the Bintel site for more details.

If you are running the GS plossls I would look at trying some better eyepieces before going the paracor way. I find my better eyepieces don't have as much coma as the cheap ones as well as giving more contrasty views.

Cheers

tempestwizz (Brian) · #5 15-01-2008, 07:41 PM

I have two F5 dobs, a 12.5" and a 10". Coma (not astigmatism) is barely evident in my scopes, but I am running purely Televue Nagler and Panoptic eyepieces. If I get ultra-fussy ie no alcohol to blunten my eyesight, I can detect a small amount of coma that is corrected with the inclusion of a paracorr. In all honesty though, the original coma is not really a problem.

I suggest you see if you can borrow a better class of eyepiece to try out first. If it improves the issue, you money may be more effectively spent on a good eyepiece rather than a paracorr.

Cheers

Brian Clements

duncan · #6 15-01-2008, 08:22 PM

Thanks to all who have replied thus far. I have always suspected better eyepieces would go a long way to solving the problem. Once this rain goes away and i'm back at work i will be investing in much better quality eyepieces. Hopefully if i buy a couple at a time i might be able to get a bit of a discount. Naglers and Pentax seem to be the go.
Cheers all.

rmcpb (Rob) · #7 15-01-2008, 08:30 PM

Before you go into buying expensive eyepieces try them!! I always thought Naglers were my ultimate till I tries some and did not like them. I have gone for Panoptics and will keep to the 68 degree field for the rest of my set as I expand it. They are not cheap so make sure you choose wisely.

duncan · #8 15-01-2008, 08:35 PM

Hi Rob,
Fully understand what you are saying. I'll have to do considerable research to find someone close by. Might have to invite someone up from Cairns for a nights viewing if and when the bloody cloud clears.
Cheers

erick (Eric) · #9 15-01-2008, 10:00 PM

Hi duncan

Perhaps the visit to FNQ will have to wait. Aren't you guys under a few feet or water - or is that just Townsville. Not sure I'd want my mirror sinking under floodwaters as I swung the scope to the zenith

Anyway, on topic, I did try putting my "somewhat better than el-cheapo" 30mm eyepiece into a paracorr in my f6 and - nope, no better. Maybe because it was an f6, but probably there is a limit to what you can do with cheaper eyepieces. Invest in better glass and see how that goes before you think of a paracorr. I reckon you'd be happier with a $400 eyepiece than investing that sum in a paracorr. Keep inviting and I'm sure someone will come and visit you with a case full of lovely eyepieces for you to try.

duncan · #10 16-01-2008, 10:19 AM

Thanks for that Erick. Thats my biggest fear, a whole bag of goodies to try and buy,LOL.
Not to much rain now just overcast from here to the end of the visible universe,LOL.
Cheers