New scope?

[jps]

I've got the bright star blues. I think I'm convinced I will never get good bright star images with my F4 8" Newtonian. When I have everything right like seeing, collimation, thermal equilibrium, good eyepiece, things are improved. No matter what I do I can't get a good image on the 1st magnitude stars.

So, perhaps a Jim Fly set of collimation tools to provide precision collimation. Would this give me what I want?

Or, would that $330 be better of going toward a cheap refractor.

To be able to split most of the bright multiples on a decent seeing night, what would I require?

These are my thoughts(please correct me where I'm wrong).

Magnification required for the job: about 90x
Decent seeing provides: 30x per inch of aperture
Aperture required: 3 inches(I think that's about 76mm).

If this is correct, might I get away with the skywatcher 70mm F10 refractor for $200 from Andrews. The next size up is the 90mm for $400.

Any comments greatly appreciated.

[cahullian]

What eye pieces are you using? I hope your not trying to use a
series 5oo 4 mm ep <<< very bad lol

If everything else is fine you should be able to focus on a 1st mag star.
I have an 8" f6 dob and don't have any trouble.
I see your scope is an f4. I don't know what kind of difference that makes but I'm sure one of our experts can help you out.

Gazz

[ving]

a big fast scope like yours needs good collimation... it makes all the difference. on bright stars with my f6 i can get good.... hang about.

when you say bad bright star images, what do they look like exactly?

you could make an perture mask (dead easy) for viewing bright objects if this is a problem...

[jps]

Hi Cahullian,

My best eyepiece is a celestron 7.5mm ultima. No trouble there.

Can you tell me how often you can get good 1st mag star images? Is it only every now and then when seeing is very good and thermal equalibrium is reached? Or can you leave your telescope outside for an hour to cool down 3 hours after sunset and regularly split bright doubles?

[ving]

an f4 scope is not really a double splitter....
not to say it cant be done.

I leave my scope outside for an hour to cool before use most times. :) as the night goes on i find I can see more.

[jps]

Your comments make me think it's the collimation. Although I have aligned the secondary with a laser and barlow lasered the primary, I have not initially used a sight tube. The reason I have not bought a sight tube is I am unsure whether it will do the trick or if I need the Jim Fly set with the adjustable sight tube(help on this point would be great). Having said that, about two weeks ago I made the decision to buy the Orion from bintel but they had none in stock and suggested I try in two weeks. I think later today I will try again.

What do they look like? Very big fuzzy blobs.

The mask. I put held a piece of cardboard in front of the scope to test the water. It certainly helped in splitting the bright double I was looking at(I don't recall which). Without the cardboard I could see evidence of a double because the image I got was a slightly lobsided blob. With the cardboard I came much closer to actually splitting without splitting.
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[jps]

O.K. So what would be the minimun requirements for a refractor that could split bright doubles on an average night. Would the 70mm F10 skywatcher do the trick? Perhaps the 90mm?

I greatly appreciate these replies!!

[ving]

you can creat your own sight tube with a simple roll of paper or card. I have dont this and ti works perfectly to see if the secondary is aligned properly under the focuser. :)

if you plan on getting another scope, moving from 200mm to 70mm is quite a backward step. if its only for doubles you will see less stars and the dimmer doubles may be beyond the scope...
if you save your money, aoe have a 102mm f/9 on an eq5 ofr about $750 (+postage). i am considering this scope myself just for a grab and go scope

[jps]

.

And just when I was about to call bintel for the Orion. I might try this trick first.



Just had a look at aoe. $750 is too much for me but I agree with the bargain. Now I'm considering the 90mm for $300 from aoe. Bargain, but will it do the job. I'm thinking yes. All I need is performance on bright stars because my Newtonian does a very satisfying job on everything else including faint doubles. I guess when you have a really bright star with a faint close double I might still have a problem.

[RAJAH235]

FWIW. jps, have you read this,>
http://www.iceinspace.com.au/forum/showthread.php?t=6429&highlight=dob+collimation
Might give a few more hints on your probs... :D L.

[mickoking]

G,day jps,

A 90mm Refractor will do a fine job on doubles providing it has a descent focal length/ ratio. When I am viewing doubles I prefer to use my refractor for several reasons including it's better contrast(compared to a fast Newtonian) and better relative performance in poor seeing/ turbulant conditions.