Star test images of new 9.25

[g__day (Matthew)]

So do Celestron actually publish a quality standard, so that if your scope falls below it its treated as a return? Do they publish a minimum specification that the scope must pass to be deemed reasonable?

I wish I had that with their CG5 mount cause I found wierd behaviour if you set lattitude and longitude by hand (I guess it mess up time zone and the motors keep colliding - vs select Sydney and all is well). Trouble is Celestron offered no trouble shooting, self check diagnostics or fit for use readiness check. Are they any better for their OTA's?

[casstony]

Both Meade and Celestron imply a standard of 1/4 wave peak to valley through use of the term "diffraction limited" in their advertising.

Tony

[Don Pensack]

Originally Posted by Dennis

Hi Graeme

Thanks - that is very interesting information regarding SA, as I have just added a flip mirror unit to the JMI motofocus on the C9.25, which further extends the back plane to a total of approx 6 to 7 inches, or 15 to 18 cms, from the bare position.

Hmm, one night I must capture some star test avi's with the SCT in the OEM bare configuration, then add the JMI motofocus and finally add the Meade flip mirror and see what results I get.

Cheers

Dennis

The contemporary SCT design is only diffraction limited with one set distance between the corrector and primary. Moving the primary mirror from this perfect position influences more than spherical aberration--it also changes the f/ratio. Essentially, as the back-focus distance (the distance the focal plane is moved back from the back of the scope), the longer the f/ratio, and the greater the spherical aberration.
For the purely visual observer, paying attention to that would result in seeking out the shortest visual back available (some 1-1/4" ones are up to 1/2" shorter than others), and using a 1-1/4" star diagonal to shorten the back focus. I would note that this will be of greater importance to the lunar and planetary observer than the deep-sky observer.
For the deep-sky observer, the elimination of vignetting would be equally as important, and for this, the use of a 2" adapter tube with a refractor-style 2" diagonal would help (or the equivalent of the Peterson Eye-Opener), but not the thread-on SCT diagonals, which all have smaller openings. The best I've seen in answering both back-focus and vignetting issues is the TeleVue Shorty adapter and Shorty EverBright Diagonal. The use of both together results in the smallest back-focus distance available in a 2" diagonal, and the least amount of vignetting as well.
Don Pensack

[matt]

Thanks guys

The jury is still very much out on this scope.

I'll not make any decision until at least I'm sure I've got the scope stabilised and at ambient. Thanks for your advice Bird, re: Canberra's o'night temps.

The demo (computer) images I posted were based on my scope being put outside around 4-30pm... and then the star test around 7pm. Probably not ideal given that, as Bird points out, Canberra's temps will keep falling for quite a few hours into early morning.

I'll be setting the alarm and having a pre-dawn look.

I'm also waiting on the arrival of my Ronchi test eyepiece.

Just wanted to share initial impressions.

Again, thanks for all your help.

[Dennis]

Hi Matt

Do you have access to a 100m long grass paddock? If so, there is a paragraph in the Suiter book that describes how to make an artificial star for testing, but recommends placing it 100m away for optimum results. The pinhole through which the artificial star is observed should be less than 0.3mm dia to ensure it is a point source and not an extended disc.

Ground thermal currents would be something to contend with, even over a paddock.

Cheers

Dennis

[Dennis]

Originally Posted by JohnG

Hi Dennis

Have a look at page 184 and 268 of Suiters and let me know what you think, looks like a bit of undercorrection to me.

JohnG

Hi John

Hmm, must have missed this one first time through. Not too sure of the interpretation based on those examples in Suiter. I'll try to take some cleaner images and then go back to the book for a more detailed comparison.

Cheers

Dennis

[Satchmo]

Originally Posted by Dennis

Hi John

I'll try to take some cleaner images and then go back to the book for a more detailed comparison.

Dennis

Unfortunately the book only shows simple cases ie spherical aberration. A depressed or raised zone around the secondary area can have the same effect superficially on th extra focal discs in the star test even though the rest of the mirror is good. A ronchi grating can help distinguish overall Spherical Aberration from more localised defects if you are not experienced at star testing.

Mark

[Dennis]

Originally Posted by Mark Suchting

Dennis

Unfortunately the book only shows simple cases ie spherical aberration. A depressed or raised zone around the secondary area can have the same effect superficially on th extra focal discs in the star test even though the rest of the mirror is good. A ronchi grating can help distinguish overall Spherical Aberration from more localised defects if you are not experienced at star testing.

Mark

Thanks for the cautionary words of wisdom Mark, I guess sometimes a little knowledge can be dangerous in this area.

Cheers

Dennis