Star test images of new 9.25

[matt]

Guys.

Here's the latest in my 9.25 story. For those unfamiliar with my trials and tribulations this may not make much sense, out of context.

I've "dummied up" a representation of the star tests I saw last night through the new 9.25.

Now, keep in mind these are only representations and are not exact as the images appear in the eyepiece. Both the number of rings and the spacing between the rings is slightly different in reality.

The pic does, however, demonstrate a difference I noticed between inside and outside focus.

I appreciate most scopes, particularly newts and SCTs, rarely show exactly the same star images inside and outside focus.

What I'm hoping you can tell me is whether the difference I'm seeing is anything worth concern.

First of all, the scope had been outside for quite a few hours to come to ambient. However, given Canberra's climate, may not have been precisely at ambient.

Let's assume it was very close.

Inside focus shows a nice set of concentric rings. Outside ring is a bit brighter than the rest which appear evenly illuminated.

Outside shows similar, with that additional brighter ring approximately one third to half way in toward the airy disc.

That's about the only significant difference.

The (dark) space surrounding the airy disc in the outside focus test also appears more extensive. The inside focus seems to have "more" rings nearer to the disc. (NOT SHOWN IN DIAGRAM).

Other than that the scope seems OK. Visually, Jupiter and various DSOs appeared noticeably brighter than my previous (optically flawed) 9.25.

However, this scope only arrived Friday afternoon and as yet the seeing hasn't been good enough to make a comparison of detail.

Also, I suspect, the seeing could have rendered some variation in the inside and outside focus results?????

On first impressions (based on the simulated star test images i've attached).... what do these images suggest?

What does an extra "brighter" inner ring reveal?

Again, many thanks for your feedback:)

[matt]

Latest word is it may indicate a small degree of undercorrection.

Is this acceptable or is it replacement-replacement time?

[Dennis]

Hi Matt

Just tracked down my inside and outside focus images from 18th Feb 2006. The scope had been outside for approx 1 hour when these were taken, and I had a Kendrick heater installed and powered on to combat the dew.

Cheers

Dennis

[matt]

Dennis

Do you think the dew heater had an impact on your star tests?

Yours shows a brighter outside ring on your image on the left, and a few other slight variations.

[Dennis]

Hi Matt

I've not yet given serious thought to the (adverse) affects of the dew heater, as my prime objective has been to keep the optics free from dew and I have sacrificed everything else to achieve that goal.

However, Bird, Mike, DP and others have published some very important findings here, showing the effects of unwanted heat in the system, so I will shortly begin to give this area some serious thought.

When I first posted these images, I think Mr. Ponders did suggest the wavy outer ring was probably the result of the heat from the dew strip?

I guess that I have been pretty lucky in having what appears to be a reasonable set of optics, along with periods of good seeing, so I’ve not had the need to investigate much further.

Cheers

Dennis

[matt]

The aspect of your star test images which really intrigues me is the second bright ring ... inside the bright wavy outer ring.

I too have that in my star test outside focus, but it's not there inside focus???

Curious and curiouser

By the way, which star did you use?

[Dennis]

Hi Matt

I have the famous book by Suiter on star testing. I'll take a look and see if he has an image or description that characterises this pattern.

I'm intrigued; as a young boy, did you pull your toys apart to see how they were built? ;)

Cheers

Dennis

[matt]

Didn't most boys, Dennis?:lol:

I don't remember. It is too long ago.

I suspect I did. The question is whether I was able to put them back together again!:rofl:

(Sorry I couldn't give you better material to work with, Dr Dennis Freud/Jung etc)

[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

[casstony]

I've seen dramatic changes in the star test between a fully cooled and partly cooled scope. You can think you've got a lemon then check again at midnight or so and see very even images.

If the ronchi lines are straight with just 2 or 3 lines showing, and no astigmatism is visible when you roll through focus, you're probably somewhere better than 1/4 wave which is all you can expect from a mass produced sct. I'd be happy with that scope.

Tony

[Dennis]

Hi Tony

Thanks for the input. I have a Stellar Technologies International, Series IV 'Stiletto' Focuser for Film and Digital Imaging – see http://www.stellar-international.com/

This is a modular system that uses a Ronchi-grating to allow you to quickly achieve pinpoint focus using a star. I think the grating in mine has 300lpi?

I have only used it a couple of times with my Pentax *istDS DSLR and the lines have appeared nice and regular as well as parallel, inside and outside of focus, which seems to indicate satisfactory optics.

Cheers

Dennis

[Dennis]

Here is a "star test" of Antares, where the companion can clearly be seen at prime focus on my C9.25, with a much wider split with the Televue x2.5 Barlow and the C9.25.

Of the 1800 frames in each avi, less that 20 looked like the attached files, due to the smearing of the image resulting from the poor seeing of 3 to 4/10.

Cheers

Dennis

[Rigel003]

Hi Matt
Can you give us some frames from a toucam AVI as you move through focus? Here is one I did earlier this year with my C11. Probably would have showed more a bit closer to focus. Extra focal on left and intrafocal on right. The result is extremely dependent on cooling and seeing though.

Something that may be of interest - I've read that SCTs are corrected for spherical aberration only for a specific distance between the primary and secondary mirrors (from the early days when everyone had a 1 1/4" visual back and simple eyepiece). When you extend the optical train with large 2" diagonals, barlows etc. you have to move the primary further forward to focus and need to expect some slight amount of spherical aberration, under or over correction, not sure which.

[UniPol]

Hi Matt,

Earlier this year an acquaintance of mine in Brisbane asked me to look at a used Celestron C11 in Sydney for him and if it was any good to buy it. Everything looked fine and I purchased the scope for him and he subsequently picked it up on his next visit. In the mean time, as one would, I had a great time using it on as many occasions as the weather permitted. To keep my acquanitance informed a series of pictures of the scope and astrophotos were emailed to him. A couple of photos of the in and out focus of a bright star are shown here taken with my 350D through the C11. I can't recall which star or the magnification however they looked pretty good judging by published images of star tests. Needless to say the scope performs very well and he is very pleased with it.

Funny thing about star tests however is that they don't always give a true indication of a scope's performance. I have a number of Unitron refractors which show excellent pin point stars when focused however the star tests show quite different in and out of focus images and a fair bit of chromatic aberration as well as marked spiking on the outer fringes. Perhaps one ought not to look at stars in or out of focus but concentrate on the focused image.
Afterall most of us strive to achieve perfect focus, just look at the number of perfectly good crayford focusers discarded (mothballed) in favour of the more recent 10:1 varieties. While I'm waffling on, why is it that most of of us continually fiddle around with the focus/focusing/focus knob to achieve that "perfect" focus? Is there such a thing as perfect focus?

Cheers, Steve

[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

[Dennis]

Hi Steve

When I am way outside or inside focus, I obtain images very similar to the 2 you have posted. To limit the diffraction rings to just a few around the central maximum, I need to use relatively high magnification and then turn the focuser slightly, by small amounts, inside and outside of focus.

Cheers

Dennis

[UniPol]

Hi Dennis,

Next time I have my 10" LX200 out (puff/pant) I'll try and do the same thing.

Cheers, Steve

[bird]

Matt,

star testing in Canberra at this time of year will require a lot of patience from you - I'd suggest that the only time you're likely to get the scope close to thermal equilibrium is if you set up outside at sunset and then set your alarm for 3 or 4 am.

I've been watching the weather reports around the country and we consistently get the lowest overnight minimums for any capital city, last week we had several nights below zero, even though the daytime temps were reasonable in the mid teens.

Based on the data I recorded from last year I'd expect the nighttime temps to be dropping steadily from sunset through to about 2am, so with a closed tube arrangement like the C9.25 you'd have to allow another hour or so after that for the scope to catch up before you could really get to see the optics properly equalised, i.e. see the "true" state of the mirror.

cheers, Bird

[Rigel003]

Dennis,
Just did a quick Google seach, and this discussion thread suggests that it's optimised for 90 - 100mm behind the visual back.

http://groups.google.com.au/group/sci.astro.amateur/browse_thread/thread/3ccb5bcc09ec82f2/c79889b7f7671ea9?lnk=st&q=sct+spherical+aberration+back+foc us&rnum=1&hl=en#c79889b7f7671ea9

[RAJAH235]

FWIW. Matt, you might just find some extra info in here (http://skytonight.com/search?searchKeywords=collimation&Submit.x=19&Submit.y=8)...HTH..:D L.

[g__day]

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]

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]

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]

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]

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

Cheers

Dennis