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9.25 SCT, it ain't no planet scope

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#1 Deep13

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Posted 20 October 2018 - 01:28 AM

I bought a 9.25 about a year ago to supplement and eventually replace a DIY 5" f/12 refractor. I've had it out serveral times, always for several hours. Defocused, a bright star looks like a compact disk. In focus on a planet, it's always a smudgy. It's like a thermal problem, the scope is always out for hours. The 5" still delivers sharp, though not especially bright images. And the 8" Dob still gives remarkably sharp images.
 

#2 otocycle

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Posted 20 October 2018 - 01:48 AM

Well, the C9.25 will never be as sharp as a 5" f/12 refractor....mine sure isn't.   But if it is cooled, with good collimation it should perform much better than "smudgy".   Star test at higher power on a less bright star (say Polaris) and see what it's doing inside and outside of focus.   Also check to see if collimation is changing as the scope changes position (mirror flop).


 

#3 Deep13

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Posted 20 October 2018 - 01:56 AM

Well, the defocus is concentric, like a silver circle with a black dot in the middle, each side of focus.

 

Suppose it's mirror flop. Is there a cure for that?


 

#4 otocycle

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Posted 20 October 2018 - 02:10 AM

Concentric defocus is great, but what are you seeing for the star test diffraction rings for even light distribution.  Are the rings evenly bright out to the edge, or is there a difference from center to the edge ?

 

Mirror flop and/or focuser image shift can be improved a bit with inspection, fastener tightening, and baffle tube lubrication.   Some owners prefer to lock the mirror down and use an aftermarket focuser, especially for imaging.


 

#5 dweller25

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Posted 20 October 2018 - 02:43 AM

I collimate my SCT as you have described but then finish off at x400 at perfect focus. I see a tiny, perfect airy disk around stars when this is done.

 

I also use 2 layers of Reflectix sheet around my OTA for thermal stability.


Edited by dweller25, 20 October 2018 - 11:31 AM.

 

#6 DMach

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Posted 20 October 2018 - 03:15 AM

I collimate my SCT as you have described but then finish off at x400 at perfect focus. I see a tiny, perfect airy disk around stars when this is done.

+1 on this. I learned the hard way that the "slightly de-focused star" method wasn't accurate enough for planetary work.

 

A high-power, in focus star test will tell you for sure. Even though I thought my collimation was good, the high-power star looked more like a copyright symbol the first time I tried this!


 

#7 Deep13

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Posted 20 October 2018 - 04:13 AM

I said defocus looks like a silver circle. I should have said a silver disk, like a CD with a black disk at center. There are no rings, just a uniform silver disk.

Also, would mirror flop mean that the image is sharp some of the time?
 

#8 otocycle

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Posted 20 October 2018 - 04:21 AM

Mirror flop means that the image would likely be sharp if the scope is well collimated for a set position....say pointed towards the object of interest (planet).   The more the scope moves from the original collimation position the more likely it is to be impacted by mirror flop if it is a factor.   Does the scope have a lot of image shift when focusing ?

 

Try collimating close to the planet of interest and see if there is any improvement.

 

Has the corrector plate ever been removed or mis-aligned ?


 

#9 davidc135

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Posted 20 October 2018 - 07:21 AM

One more call for the high power star test.  David


 

#10 Eddgie

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Posted 20 October 2018 - 07:53 AM

I bought a 9.25 about a year ago to supplement and eventually replace a DIY 5" f/12 refractor. I've had it out serveral times, always for several hours. Defocused, a bright star looks like a compact disk. In focus on a planet, it's always a smudgy. It's like a thermal problem, the scope is always out for hours. The 5" still delivers sharp, though not especially bright images. And the 8" Dob still gives remarkably sharp images.

Your experience matches mine.   

 

There was (is?) an urban legend that the C9.25 is somehow "special" but stacks of iterferometer tests testify that it is a scope that is rarely anything special optically.  The quality varies quite a bit, and the chances of getting a really good one seem to be about the same as getting a barely passable one.

 

Coupled with the large obstruction, you get a scope that is in my own experience, a very poor planetary instrument and near the top of my list of telescopes to avoid for that application.  I know the SCT crowd will hate me, but I have owned pretty much every SCT model made, and the C9.25 to me was simply the worst overall in terms of performance per inch of aperture.  The fact that a good 5" Achromat would equal or best a C9.25  on planets does not surprise me at all.

 

My top recommendation to people that want to do more serious planetary observing is a cheap 8" or 10" dob with a mirror that has been refigured to a high Strehl ratio.  This will be a hard scope to beat on planets even with a $10,000 Apo. 

 

Anyone that has had the chance to  view planets through a really finely made 8" or 10" premium mirror reflector on a night of even pretty good seeing will tell you that they offer extremely detailed planetary views and views that are far more colorful than high power views with smaller instruments like a 6" Apo (and yes, I have owned one of those too, and a very good Astro-Physics one at that). 


Edited by Eddgie, 20 October 2018 - 07:53 AM.

 

#11 mvas

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Posted 20 October 2018 - 07:58 AM

I said defocus looks like a silver circle. I should have said a silver disk, like a CD with a black disk at center. There are no rings, just a uniform silver disk.

Also, would mirror flop mean that the image is sharp some of the time?

Always finish the focus in the same direction = Counter-Clockwise.

Turn Counter-Clockwise when collimating and use HIGHER POWER

Turn Counter-Clockwise when viewing planet.

Mirror flop issue ( if present ) is solved

 

Add ...

a) Reflectix and/or

b) Anti-Dew Strap and/or

c) Dew Shield

Heat / Thermal issue solved.

 

Finally, some mirrors are just better than others


 

#12 mvas

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Posted 20 October 2018 - 08:05 AM

 

My top recommendation to people that want to do more serious planetary observing is a cheap 8" or 10" dob with a mirror that has been refigured to a high Strehl ratio.  This will be a hard scope to beat on planets even with a $10,000 Apo. 

 

What do you consider "High Strehl Ratio" ?

Greater than 0.90 ?

 

I have looked through a Meade 'Research Grade' 12.5" f/6 at 350X and the view was phenomenal.

I am not sure what the Strehl Ratio was, but it had be very high


Edited by mvas, 20 October 2018 - 08:09 AM.

 

#13 bobhen

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Posted 20 October 2018 - 08:37 AM

1. Ohio seeing is rarely great and the planets are low. Not the best combination.

 

2. You need to be aggressive about cooling: fans (Tempest or a cat cooler), insolation or letting the scope cool for “hours” are steps to take. Check a defocused star. If you see heat plumes, the scope is not ready for high power viewing.

 

3. Collimation is important. Learn how to do it. However, if your collimation is off just a tad it will still be perfect somewhere in the field of view. Just move the planet around to the spot where the scope does come to collimation. The scope will give the best views it can. The views will just be off center until you collimate the scope.

 

4. If you have mirror flop (this is different than focus shift) the scope will appear collimated when pointing say to the east. Then when you flip the scope and point it to the west, the mirror shifts position slightly and the scope goes out of collimation. Mirror flop can only be fixed if you take the scope apart or send it back to Celestron.

 

5. Seeing will limit the resolving potential of all telescopes. On many Ohio nights seeing will limit your 9.25-inch scope so that it will only have the resolving power of your 5” refractor. If you compared a 5” SCT to a 5” refractor on the same night the refractor would deliver better views. Add in the SCT’s large central obstruction, possible thermals, light scatter, less smooth and less well figured optics, etc. and on many nights in Ohio, this could also be what you are experiencing.

 

6. Collimate at high power. Address the thermals. Check for mirror flop. Wait for a really good night. The SCT might surprise you. If you are still not happy and want a larger scope, get a large Newtonian with a custom mirror like Eddgie says in post number 10.

 

Bob


 

#14 Eddgie

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Posted 20 October 2018 - 08:38 AM

What do you consider "High Strehl Ratio" ?

Greater than 0.90 ?

 

I have looked through a Meade 'Research Grade' 12.5" f/6 at 350X and the view was phenomenal.

I am not sure what the Strehl Ratio was, but it had be very high

.95 or higher.  Anything of this quality is in essence optically perfect meaning that there will be no discernible improvement in the quality of the view past this (though most will struggle to see the difference between a .8 and .95 Strhel instrument).

 

Even a barely diffraction limited 12.5" scope will give great planetary views on nights of perfect seeing, but seeing of course is the primary limit to planetary observation with large apertures. 

 

The diffraction of the aperture itself though is what holds back smaller perfect instruments.  They can never deliver contrast better than that allowed by the diffraction of the small aperture, which forever restricts them to relatively large detail.   As the aperture increases, you unlock the potential for smaller, lower contrast detail, but in my own experience, the 8" to 12" range of apertures have the best chance of giving the best view on the most number of nights for the average observer.   Seeing better than 1.5 arc seconds really opens up the potential of the 10" range, and seeing is rarely good enough for a 14" aperture to reach its full resolution potential.   

 

8" Will give more nights where the aperture is working near its full potential, but even on such nights, there may be moments of improved seeing where a 10" or 12" would show more for a brief moment or two.

 

I recommend a 10" f/6 with premium mirror as pretty much an ideal planetary telescope.   There will be many nights where you fully exploit the contrast transfer of this size aperture but it may be that on those nights, the full resolution views will come and go with seeing. 

 

12" Is more limited.  I would say that I get 50 nights a year where I can fully exploit 12".   Most nights, this size is not working at its full potential. 


Edited by Eddgie, 20 October 2018 - 08:39 AM.

 

#15 Foehammer

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Posted 20 October 2018 - 08:40 AM

Have a go at collimating it with Metaguide, make sure it's properly acclimated and give it a night of good seeing. You will be surprised. I've owned 2x C9,25's one very good and one superb.
 

#16 Eddgie

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Posted 20 October 2018 - 08:50 AM

 

I recommend a 10" f/6 with premium mirror as pretty much an ideal planetary telescope.   There will be many nights where you fully exploit the contrast transfer of this size aperture but it may be that on those nights, the full resolution views will come and go with seeing. 

 

 

And to further expand on my recommendation for the best planetary scope, I would throw in a binoviewer and a pair of zoom eyepieces.

 

As I sad above, on most nights, seeing will not allow it to give its full resolution and contrast transfer in a constant state condition.  The full resolution and contrast will only occur when the seeing falls below about 1.3 or 1.2 arc seconds for an aperture of 10".  Suppose though that your night is ranging from 2 arc seconds to 1.5 arc seconds.  On most such nights, there might be brief moments of seeing where you dip down to 1.2 or 1.3 arc seconds.  If you do not have a zoom eyepiece, these few precious moments may be gone before you can enlarge the image in proportion to the improvement in seeing.  With a zoom (or a pair of zooms when using binoviewers), you can immediately increase the magnification to exploit this temporary opportunity (which might happen many times during an hour session). 

 

Otherwise, I would recommend that the observer watch the average seeing over maybe a 5 minute window, then put in the shortest focal lenght eyepiece that they think they will get to use for the best seeing for that session and just sit there and be patient and wait for those brief moments of improved seeing to come and go.  These moments are when the very best detail will be seen, but if the wrong eyepiece is loaded, it may be gone before you can get the right eyepiece loaded. 

 

I used this method with my C14 the first time I was able to resolve surface features on Io and Ganymede.  I had a night of 1.5 arc second seeing, and I loaded an 11mm Televue Plossl which gave about 350x.   During perhaps an hour and a half session, I had three moments of seeing that must have fallen well below one arc second because on three occasions during that session, I was able to see the bright Osiris and the C shape of Galileo Region on Ganymede, and in a similar session, I was able to see the polar shading on Io using the same scope and eyepiece.  It takes patience.  I did not change the eyepiece during any of those sessions and one might note that my exit pupil was 1mm, which is the exit pupil that I think works best for me for the purpose of resolving the lowest contrast detail. 


Edited by Eddgie, 20 October 2018 - 08:51 AM.

 

#17 bikerdib

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Posted 20 October 2018 - 08:53 AM

I said defocus looks like a silver circle. I should have said a silver disk, like a CD with a black disk at center. There are no rings, just a uniform silver disk.

Also, would mirror flop mean that the image is sharp some of the time?

I believe you are not doing a correct defocus collimation.  What you are seeing is simply a reflection of the secondary in the lightpath because you are either using too low a power when doing it or you are just defocusing TOO much.  You need to use a high power eyepiece that will give you around 200X at least.  As suggested, find a fairly bright star near the planet you want to observe and bring it to focus.  Now slowly turn the focuser counter clockwise until the star starts to form a small disc (I do counter clockwise like finish focusing to make sure you are pushing the mirror instead of pulling which can allow the mirror to still move after turning the focus knob).  Only defocus enough that you see a disc with a black circle in the center surrounded by small disc rings.  This us what you need to do collimation.  If you continue to defocus from this point, you will get the image you describe.  You can play with this a little to get the image to a size that lets you see it best but never defocus so much that you loose the rings in the bright part of the image.


Edited by bikerdib, 20 October 2018 - 09:00 AM.

 

#18 Richard Whalen

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Posted 20 October 2018 - 09:16 AM

What eyepiece are you using when checking collimation? I check my 8" MCT at 50x per inch for out of focus patterns, 100x per inch for examining infocus airy disc and ring for brightness and width variations. Seeing must be steady, and scope must be cooled for best results. What might appear to be collimated at 200x might not be so much at 800x. If your optics are just average or poor though you might be limited in what magnification you can use effectively.

 

Also star must be dead centered for best results.


 

#19 carolinaskies

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Posted 20 October 2018 - 09:17 AM

Sounds like only the first step in Collimation if you're looking at a 'cd' type image. At that defocus you can only get a rough centering of the secondary. for any planetary work you need to get a very fine centering because you're picking up details on planetary that are much closer to the resolving limits of the telescope. This holds true for splitting close binaries as well.

Mirror flop in this discussion has almost zero effect because that is generally an issue with astrophotography and the actual movement of the star not a change in collimation.

There is far more evidence of fine performance of SCTs for planetary despite a few individuals whose eyesight is apparently superior to the masses.

I would suggest as others have to really fine-tune the colimation and also insulate the OTA to reduce tube current issues. That should resolve your dilemma.
 

#20 otocycle

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Posted 20 October 2018 - 02:57 PM

Keep at it with proper cooling and collimation...the C9.25 is not a refractor, but it is capable of excellent planetary views.   I have been using my C9.25 more on Mars (instead of 4" - 6" refractors and C6) now that its elevation is increasing and size is decreasing.   (C9.25 is brighter of course, and has higher resolving power...but will never compete on contrast.)  At lower planet elevations, the C9.25 is at a disadvantage for average seeing conditions. 

 

This may seem like blasphemy, but when I have Mars in the FOV using a ZWO CMOS camera, I will tweak the Bob's Knobs collimation screws ever so slightly to see if there is any improvement.   Call me crazy !

 

I agree that a binoviewer plays to the C9.25's strengths and gets the most out of the scope for planetary viewing in my experience.


 

#21 jgraham

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Posted 20 October 2018 - 03:31 PM

I use a centered, out of focus star to rough-in the collimation. A camera is a great help in fine tuning the alignment using a slightly out of focus star, but the final touch is looking at the diffraction rings of a star focused with a Bahtinov mask. I do a lot of double star splitting and precise collimation and acclimation are needed to get the best performance out of any scope. In a scope with a central baffle also look for a thermal plume coming off of the central shadow while looking at a centered, out of focus star. If your Cat doesn't have built-in fans a cat-cooler can be very effective.

 

With great hesitation I bought a 10" f/6.3 Meade SCT just out of curiosity given their long production run, yet spotty reputation. At first I was disappointed with the soft image using my usual relaxed approach to collimation. I then dedicated an evening with a camera to really zero in the alignment, and I'll be darned, it blossomed into a fine scope. I have since bought two more; another 10" for imaging and an 8" for visual and EAA. I have since 'fixed' a lot of Cats just by taking the time to do as good a job as I can with the alignment, and the ol' always focus turning the knob counterclockwise trick.

 

I would not give up on a Cat until I was absolutely sure that it was precisely collimated and well acclimated. However, sometimes it just t'ain't gonna happen.

 

Food for thought.


 

#22 mich_al

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Posted 20 October 2018 - 04:55 PM

2. You need to be aggressive about cooling: fans (Tempest or a cat cooler), insolation or letting the scope cool for “hours” are steps to take. Check a defocused star. If you see heat plumes, the scope is not ready for high power viewing.


YES, and not just at the start of a session. Every time I take a break the cat cooler goes back in, temps fall all night long and the 9.25" won't keep up.
 

#23 DMach

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Posted 20 October 2018 - 05:00 PM

Here's a great article/guide on collimation with examples of what the patterns should look like:

 

http://www.astrophoto.fr/collim.html

 

I do a high-power star check before each imaging session, partly because (as a bonus) it allows you to do a more objective measure of seeing by using the Pickering scale:

 

http://www.damianpea...m/pickering.htm


 

#24 Asbytec

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Posted 20 October 2018 - 05:13 PM

I said defocus looks like a silver circle. I should have said a silver disk, like a CD with a black disk at center. There are no rings, just a uniform silver disk.

Also, would mirror flop mean that the image is sharp some of the time?

As others have said, that's too far from focus for accurate collimation. You need to defocus slightly as seeing will permit, enough to see the Poisson spot and a few rings. Then, finish off in focus. I normally see if there is a tiny bit of light being thrown off to one side more so than the other, then tweak collimation until the in focus pattern is not only concentric, but any stray light thrown off by seeing is also uniform all around. 

 

It's true a 5" refractor will be sharp, and the C9 will be obstructed. All that is true, for what it is, and the subject of much debate. Regardless, planets should be sharper than implied by being "smudgy". 

 

Here's what I see in a well collimated and thermally stable 6", obstructed, off the shelf CAT (color and contrast are enhanced for easy viewing). You should see bunch more in a C9. 

 

Jupiter 6 Feb 1330UT Final.jpg


 

#25 photoracer18

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Posted 20 October 2018 - 05:37 PM

The only thing "special" about the C9.25 is it was originally a Vixen design and its optical formula is closer to F2.5 and 4x verses the normal everyday SCTs formula of closer to F2 and 5x. All this really gives based on similar quality is a wider flat field and of course a longer tube for its diameter. Its been suggested that the white tube one that Ed Ting owned that is/was so good was the original prototype submitted to Celestron by Vixen. I used to have one that was pretty good, subjectively speaking, an original CG-9.25 model, the one with the Losmandy made G-9 mount and the crappy aluminum tripod carrying it. Nice thing was the flat adapter plate in between the 2 that allowed you to immediately mount it onto an SCT tripod for a much sturdier mount.


 


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