Funny star shapes

[glennbech]

I have finally started to look through my telescope after many years of imaging. 

Last night blue skies arrived on time with a package with new eyepieces. Strange?! Anyhow...

When I tried to focus bright stars- they did not get round and pointy but had a tip protruding from center to 11 o clock. This effect was enhanced when I moved out of focus one way. Moving out of focus the other way gave more the shape of a 6 line cross. The pattern did not seem to flicker of change over time.

Hard to explain:-) Is my refractor out of whack? 

The question feels stupid. I have had the scope for 8 years but never looked through it with a decent eyepiece :-)

[Dave In Vermont]

I'd try a different eyepiece for a start. If it's still happening - something is wrong with the scope. Perhaps the objective came loose? Another way to check the new eyepiece would be to place it loosely in the focuser, and then turn it around. Does the flare and other strangeness turn, too?

Please keep us informed on tracking down the cause.

I'm sure you'll get it sorted. Those are nice telescopes!

Dave

[glennbech]

Problems from bad seeing would not last over minutes right? Can I debug a problem with a refractor by looking at a defocused bright star, like I could with my old reflector?

[John]

Star testing is a good way to determine how well your refractor optics are performing. Good, steady seeing conditions are needed and a reasonably high magnification - something around 180x - 220x would be OK for your 90mm scope.

Polaris is a good star to choose because it's bright enough to clearly show the characteristics of the objective lens and it also stays relatively still of course !

Here is a chart of the sort of images you might get at focus, inside focus and outside focus. The viewing conditons will affect the extent to which you can see these images and the scope needs to be cooled down of course. If viewing conditions are unstable then those are more likely to blame for poor results than the objective lens. You can also perform the tests without a diagonal if you can bring an eyepiece to focus without one. This will eliminate any issues being introduced by the diagonal, which sometimes is the case:

The "ideal" is to have an image that is close to identical in terms of form and brightness inside and outside focus but this is not often the case from the scopes that I've tested !. Often the ary disk and diffraction rings are better defined on one side of focus than the other. Sometimes this is down to the viewing conditions and sometimes the optics.

It's a complex topic though and worth looking into further before jumping to conclusions regarding the optics of your scope.

 

 

[Chris]

Hi, 

Which eyepieces are you using? Your Meg90 is fast so will show up the failings of modestly priced eyepieces. 

My ES68 24mm was terrible in an ED70 but great in longer refractors and pretty much everything else to be honest

Fast refractors can be tricky to please when it comes to eyepieces.

 

[DevonSkies]

Could also be astigmatism in your eyes. Try observing with the other eye - does the pattern look different?

[triton1]

Interesting thread I am sorry to butt in what causes "turned down edge" and "zones" not heard of these before 

regards Jonn

Manuel he say 'I know nothing'

[Chris]

14 minutes ago, triton1 said:

Interesting thread I am sorry to butt in what causes "turned down edge" and "zones" not heard of these before 

regards Jonn

Manuel he say 'I know nothing'

I think it's to do with errors in figuring the obejctive lens at the manafacturing stage. I'm fairly sure zones are localised errors in the figure of a lens, but I'm not sure about turned down edges are specifically?

[glennbech]

Lol. I made a drawing the other day of a pattern that I thought was related to a problem with my Zeiss. I was going to ask you guys about it.

 It's in the diagram under the label "perfect optics" :-)