out of focus concentric diffraction circles

[dizzy2005 (Mike Diskett)]

Doing a star test on my dob, and the shape of the out of focus circle looks good everything concentric, the only thing is I can't make out each individual concentric circle, its as if they are all blurred together.

With my tiny cheap refractor the concentric circles are quite clear, but I cant make out each individual one in my 8 inch dob.

Possibly they are so close together they are all blending together.

Is there something wrong with the dob?

Hope I'm making sense.

Mike.

[Robh (Rob)]

Mike,

From my understanding, the diffraction rings grade into a brighter ring centre then grade out. That is, there is no definite boundary. On a smaller aperture, the rings are larger and the gaps more noticeable. With the 8" DOB the rings are smaller and closer together; therefore not as clearly separated.
Also, are you comparing the rings at the same high power? If you are using the same eyepiece in both scopes there will be a difference in focal length and hence, magnification.
If seeing conditions are not so good, the rings will not be as clearly defined. Have you tested each scope at the same time, in the same seeing conditions? The seeing will also affect the clarity of the DOB's rings more as they are closer together.
Finally, have you checked the collimation by using a laser collimator or cheshire collimator?

Regards, Rob

[bmitchell82 (Brendan)]

Funny you should ask the question.

In my 10" dob i need a 5x power mate to see the clear definition of the diffraction rings.

in a 7" Mak i only had a 20mm ep to see the rings, when i slapped the 2x in it was very distinctive.

From my understanding it is to do with the magnification, though even more to do with aperature. the amount of magnification increases dramatically with bigger aperature to the point that a 5x powermate will only just cut the mustard on a 12" though on a 16" you wouldn't see them unless more maginification was added in.

[dizzy2005 (Mike Diskett)]

I havent tested them side by side, but the magnification makes sense.

The 60(edit away the extra 0) mm refractor was using a stupid ammount of mag, while the 8 inch is only at about 100x (the most magnification I have available at the mo.).

It sounds like the diffraction rings are behaving normally.

I've collimated with a home made site tube, need to buy something a bit more accurate though.

Mike

[Wavytone]

You need about 1.5X per mm of aperture to clearly see the diffraction rings. With an aperture of 7-8" that means 250-300X. On a short focal ratio Newtonian this may be difficult (impossibly short eyepieces), but on an f/15 Mak it is easily achieved as Brendan indicated.

If your refractor was a little 60mm one, it takes just 90X to see diffraction rings. More than that is a waste...