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9.25 SCT, it ain't no planet scope

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#51 Asbytec

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Posted 22 October 2018 - 06:34 AM

:waytogo:

I agree. Getting good planetary views requires a decent scope and the C9.25 is a decent scope. It requires care and attention to detail.. It requires stable seeing .

And then it requires a skilled and patient observer. It is my belief that the biggest differences in what is seen is not in the optics but in the skill of the observer. I look at what others like Norme see with what they have and I know that I'm leaving something on the table. It's not my equipment that's lacking, it's plenty good enough.. It's me, I have room to grow, to improve, to become a better observer.

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#52 Jon Isaacs

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Posted 22 October 2018 - 07:17 AM

Since the beginning of summer: Jupiter, Saturn, Mars, Uranus.

I'm in Ohio, so it's the elevation they are from this location, pretty low.

The somewhat out of focus planet is surrounded by a wobbling--well--smudge the same color as the planet, like there is a semi-opaque filter of the same color as the planet over it. The smudge is about twice the with of Mars when I Iooked last week. I don't remember how big the smudges were on the other planets.
10mm, so 235x.
I've seen the GRS in my 8" Dob, my 5" refractor, and possibly my 12.5" Dob. It's not a difficult target when well-positioned.

No, it isn't. It describes my experience pretty well.

So far I have no evidence that this scope is an excellent planetary scope, though it does fit on the AVX mount which was a motivation for buying it.

 

Something isn't right. .

 

Did you buy it new ?

 

When you say "C9.25 Ain't no planetary scope",  that's a general statement about C-9.25ss and one might expect a generalized discussion. 

 

"I'm having trouble with my new C-9.25" gets you some help. 

 

Something to think about anyway .

 

Jon


 

#53 luxo II

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Posted 22 October 2018 - 07:54 AM

I'm sorry to say this, but you have what in my country is called a "lemon". From the star test images I'd say its garbage - more like Lambda/2, maybe Lambda/3. The PTV figure of "Lambda/6" is hogwash and I'd say that test result is either fudged, or has been copied and pasted from another scope (this is not uncommon).

 

My guess is on bright stars this scope does not produce a diffraction pattern - just a blurry blob.

At Lambda/6 the diffraction pattern should be discernible but not perfect.


Edited by luxo II, 22 October 2018 - 08:26 AM.

 

#54 skyjim

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Posted 22 October 2018 - 08:05 AM

If your C9.25 is a Faststar model you should be able to remove the secondary housing and look at the secondaries mirror, if the scope was used to you at purchase someone before may have placed smudges on that mirror and its would not be the first time that even it came from the factory like that cause Celestrons QC has never be first rate to begin with. I had this same issue with a C9.25 I bought back in 2004, a USA built OTA but I had to very carefully remove the front corrector plate in which I marked close to the exact position of it before removal, once removed I was able to clean the smudge off the secondary mirror which was very obvious and re installed the corrector. It worked very good after that but still had the tell tale focus shift that many SCT's from that era had to begin with but at least the optics were OK. I ended up placing a nice JMI focuser on the rear of the OTA cause at that time Moonlite and Feather Touch were not yet making anything and from what I remember a decent planetary scope but cool down times were way to long and ended up selling the scope.  


 

#55 luxo II

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Posted 22 October 2018 - 08:19 AM

....but cool down times were way to long ...

The solution for that is known - insulation - not cooling fans. The focus shift... live with it, or buy a better scope. And no, you can't buy mine.


Edited by luxo II, 22 October 2018 - 08:28 AM.

 

#56 AxelB

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Posted 22 October 2018 - 01:14 PM

First that scope needs to be collimated at at least 50x per inch (5mm eyepiece or less). You’ll fine tune collimation with a focussed star. Most night you won’t be able to do this because of bad seeing. Make sure you’re not aiming above rooftops or other sources of heat. When the seeing is good, you should see a complete circle around your star.
A good idea is to chose a collimation star close to where you plan to observe. That way you’ll optimize your collimation at that particular angle.

Then of course there’s a reason why so many observers trade light and resolution for the greater contrast of an unobstructed apo refractor... On most night where I live under the jet-stream, a 5" apo would show all there’s to see.
 

#57 TG

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Posted 22 October 2018 - 01:52 PM

I'm sorry to say this, but you have what in my country is called a "lemon". From the star test images I'd say its garbage - more like Lambda/2, maybe Lambda/3. The PTV figure of "Lambda/6" is hogwash and I'd say that test result is either fudged, or has been copied and pasted from another scope (this is not uncommon).

 

My guess is on bright stars this scope does not produce a diffraction pattern - just a blurry blob.

At Lambda/6 the diffraction pattern should be discernible but not perfect.

Not sure, who are you responding to?


 

#58 photoracer18

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Posted 22 October 2018 - 02:13 PM

At the 2005 Mars opposition I had perfect planetary weather conditions and my older C-9.25 was setup next to my D&G built Jaeger 6" f15 and both scopes reached above 400x before breaking down (Maryland by the way).


 

#59 Deep13

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Posted 22 October 2018 - 02:29 PM

Something isn't right. .

Did you buy it new ?

When you say "C9.25 Ain't no planetary scope", that's a general statement about C-9.25ss and one might expect a generalized discussion.

"I'm having trouble with my new C-9.25" gets you some help.

Something to think about anyway .

Jon


I bought it second hand from someone im Louisiana.
 

#60 dscarpa

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Posted 22 October 2018 - 04:10 PM

 My one of the last USA made not a Fastar C9.25 equals my IM715D mak with 1/8 wave optics for lunar-planetary with very good seeing and beats it when excellent because I can use  200X more in it.  I'd peg the SCT at 1/6 wave. Both cats have Crayfords. If alignment is off performance is bad and that big CO seems to make it more seeing sensitive than my IM715D and WO ZS110. It holds collimation well but not as good as the mak or frack. Seeing has to be good to collimate. David


Edited by dscarpa, 22 October 2018 - 04:16 PM.

 

#61 Asbytec

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Posted 22 October 2018 - 05:04 PM

I'm not an optician. Will you tell me what I am looking at?

Primarily the Strehl ration at 0.96, which is very good. It means the scope is putting almost all of the light where it's supposed to be focusing most of it into the Airy disc and some into the rings (which is unavoidable due to diffraction of the aperture and the obstruction, primarily, and some minor aberration.) A Strehl ratio of 1.0 is perfect (and a pipe dream), somewhere down toward 0.8 is acceptable. So, 0.96 is about as good as we can expect a good scope to be. No scope is perfect.The test was done in green light where the eye is most sensitive.

 

The PV wavefront error is Lambda/6 or 1/6 peak to valley in green light. I believe this is the peak deviation in the wavefront from perfect, not an average deviation. So, this says the maximum deviation is 1/6th PV wave, which is better than some diffraction limited criteria. The RMS is also better than other diffraction limited criteria and is used to compute the Strehl ratio. The intra and extra star test images are smooth, with the extra focal image showing a bright inner ring. This is an indication of some undercorrection where the central zones focus a little long, but that's not a big deal. The edge has more surface area and contributes more to the image than the center. The shadow of the secondary is darn close to being the same size on each side of focus. Both of these indicate a very good level of correction. Plus, the Ronchi lines are pretty straight, the straighter the better. 

 

TG was being sarcastic, this is a very good sample C9. As was mentioned above, the C9 has a slightly slower primary mirror which makes correction a tad easier. More forgiving. So, the tendency seems to be they have a slightly better chance of being well corrected. 

 

Above, you spoke to the 10" Dob operating well under your seeing conditions, but the C9 seems to suffer, regardless. This speaks to the C9 as having some unique problem of it's own. Thermal equilibrium comes to mind, right off, as why the difference when both scopes are operating in the same climate on the same night. The 10" Dob likely has better optics, as I gather from your post, but it is also obstructed. The differences in the obstruction are not likely causing this level of degradation in the image unless you are a connoisseur of exquisite optics and can tell the difference in your seeing conditions. But, even then and at such high standards and expectations, I cannot swear the C9 is really under performing. It may be performing as advertised, but the OP is just not happy with the image. But, if it is performing as advertised, then thermal management comes to mind, again. Even if the C9 is not as good as TG's sample above, you should still be getting nicer images than what is implied. So, unless this is another C9 sucks thread implying the scope is just incredibly poor optics near Lambda/2 (as a rule for all SCTs), I doubt the optics are the cause unless collimation is off. 

 

SCTs are notorious for thermal problems. I have seen thermal plumes in the star test well into the evening. The problem is the interior mass of the primary and it's cell retain heat and conduct it slowly causing a slight temperature differential as the OTA begins to cool. You'll see a heat plume rising up from the center (appearing to rise near the shadow of the secondary) of the defocused star test as the massive mirror and supporting structure radiate residual heat into the interior of the OTA. This is enough to turn otherwise sharp planetary images into mush. The same thing will happen in the 10" Dob as the mirror cools. So, both scopes need to be thermally stable. The SCT needs some help getting there. My guess is, even after sitting outside for a while, the problem is likely thermal. A quick check for the tell tale thermal plume in the defocused star will nail it or eliminate it as the cause. If you haven't done so already, I'd rule that out conclusively before worrying about the optics.

 

It could be you have a very poor sample, but it's outside the peak of the bell curve and not indicative of all C9's out there. Not enough to say C9's are, in general, poor planetary scopes. 


Edited by Asbytec, 22 October 2018 - 05:32 PM.

 

#62 Cpk133

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Posted 22 October 2018 - 06:39 PM

Since the beginning of summer: Jupiter, Saturn, Mars, Uranus.

I'm in Ohio, so it's the elevation they are from this location, pretty low.

The somewhat out of focus planet is surrounded by a wobbling--well--smudge the same color as the planet, like there is a semi-opaque filter of the same color as the planet over it. The smudge is about twice the with of Mars when I Iooked last week. I don't remember how big the smudges were on the other planets.
10mm, so 235x.
I've seen the GRS in my 8" Dob, my 5" refractor, and possibly my 12.5" Dob. It's not a difficult target when well-positioned.

No, it isn't. It describes my experience pretty well.

So far I have no evidence that this scope is an excellent planetary scope, though it does fit on the AVX mount which was a motivation for buying it.

Ok Uranus doesn't really count for much, but it should have a well defined sharp edge when focused under good seeing.  The same applies to all planets.  I was hoping for a bit more detail in your Jupiter observations.  When they're visible, the small white ovals in the SPR are a good challenge.  GRS Jr, aka oval BA should have been relatively easy last summer.  You should easily see delicate festoons, lots of turbulence behind GRS, variation in tone within the GRS, some nice dark storms in the NEB, lots and lots of details.  What do you see in the frac?  What do stars look like?  Do you know how to do a star test?  At warm up mags (I like to start with an 12.5mm ortho at 188x, I know right away if the seeing is decent.  At that power, Jupiter is fully color saturated and there's lots of etched detail.  Festoon bases have a blue tint.  Smaller detail becomes evident around 235x.  I do almost all my planetary observing with a binoviewer.  Ive compared my C9 to a number of nice scopes.  Most recently, a 10" f10 Lockwood "planet killer" under good seeing conditions.  The newt was better, sure thing, but it wasn't as much of a difference as I was expecting.  If I had to own one for the rest of my life, I wouldn't trade.  The PIA factor was much bigger than the difference in the view.  If I lived next to Chas and had perfect seeing, I might change my mind.  The blur you're describing just isn't right.  I've had really pleasingly views of Mars (previous opposition) with a nice sharp edge, albedo features, limb haze, polar cap with dark border and much to my surprise, when I defocused, there was a big heat plume.  It just goes to show, seeing seeing seeing.  Where I live here in MI, seeing is best at sunset and it varies quite a bit.  We do get excellent seeing, it just doesn't last for a long time (usually). 


 

#63 Cpk133

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Posted 22 October 2018 - 06:43 PM

Do you see a defocused smudge on the lunar limb?


 

#64 skywolf856

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Posted 22 October 2018 - 09:43 PM

What is Metaguide?

Check this:  http://www.astrogeek...liss/MetaGuide/


 

#65 Deep13

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Posted 22 October 2018 - 11:11 PM

Do you see a defocused smudge on the lunar limb?


No, I don't.
 

#66 BarrySimon615

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Posted 23 October 2018 - 07:35 AM

I bought it second hand from someone im Louisiana.

I would be curious to know who you got it from.  You can reply to me privately about this.  The reason I ask is because I purchased a C 9.25 back in about 2002 or so.  It was a package deal with a CG5 mount which I quickly sold.  I kept the scope for a few years but eventually sold to a friend because I was not using it much and I felt it gave me nothing more than what an Intes 67 Deluxe gave me (the Intes is a 150 mm f/12 with a Sitall primary, it was a very good scope and I did a lot of photography thru it for the 2003 Mars opposition.  As I said the C 9.25 was I think well within the normal range.  I generally found that magnification on a good night could be pushed with good results up to about 335 x  (a 7 mm eyepiece).  The friend I sold it to also had a Tak CN 212 which he felt had better images but the C 9.25 was close and a lot less expensive.

 

The purchaser has health problems and he has sold a lot of stuff so I was just curious to see if it may be the same scope  (I am in New Orleans, LA and the purchaser lives in Metairie, LA).  In any event, when collimated and under our steady south Louisiana skies, it is a good scope.

 

Note - while I am a refractor fan, the Schmidt-cassegrain has it's advantages.  While you cannot get the same magnification per inch out of a SC scope like you can with a good refractor, be it apo or achro, because you have greater aperture, you can often get to very respectable magnifications with greater resolution.  A Tak TOA 130 at 75x per inch will get you to a very respectable 382x, my current Meade 12" with an 8 mm Delos will get to an almost identical 381x, which is only 32x per inch, but magnification is magnification and a Meade 12" SC tube assembly is a lot less expensive than a Tak TOA 130 tube assembly.

 

Collimate well and observe on decent nights and the scope should do well.

 

Barry Simon


 

#67 Deep13

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Posted 24 October 2018 - 03:07 AM

When I wrote "Louisiana," suddenly occurred to me that thereis no way it could have made that trip without messing up the collimation. Huh, seems obvious now.
 

#68 WyattDavis

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Posted 24 October 2018 - 04:13 AM

Don't feel bad. It has happened before and after a whole lot more care and attention was applied...

 

https://www.newscien...-mirror-fiasco/


 

#69 Magnetic Field

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Posted 24 October 2018 - 04:28 AM

Don't feel bad. It has happened before and after a whole lot more care and attention was applied...

 

https://www.newscien...-mirror-fiasco/

 

Minor flaw compared to what will come to us once  the James Webb Space Telescope has been flown into space.


 

#70 Jaimo!

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Posted 24 October 2018 - 08:13 AM

Let me see if I can "Cliff Note" this... 

 

After 3 pages and 69 posts of C9.25 bashing, it turns out that the scope was shipped from Louisiana to Ohio, over a year ago, and has never been collimated. foreheadslap.gif

 

Jaimo!


 

#71 WyattDavis

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Posted 24 October 2018 - 08:23 AM

Let me see if I can "Cliff Note" this... 

 

After 3 pages, and 69 posts, of C9.25 bashing, it turns out that the scope was shipped from Louisiana to Ohio, over a year ago, and has never been collimated. foreheadslap.gif

 

Jaimo!

Sounds like it might be the case. May I offer that it is still a valuable entry here on CN? Certainly seems like a basic ah-ha was generated, which is a good thing. Also, new observers come here to CN with having just purchased new equipment, and it is really valuable to be able to read content like this from those with more experience and learn. It is easy to say "just read the instructions," but I for one was often when I started (and am still sometimes) a bit overwhelmed with learning/doing everything it takes to really get a setup dialed in and performing well. I took several key, basic points away from this discussion. Thanks to the original poster for starting the topic!


 

#72 BarrySimon615

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Posted 24 October 2018 - 01:23 PM

When I wrote "Louisiana," suddenly occurred to me that thereis no way it could have made that trip without messing up the collimation. Huh, seems obvious now.

This is the collimation procedure that I really like:   http://www.asterism....als/tut14-1.htm

 

Simple, straightforward and has worked for me every time.

 

Let us know what you think about the C 9.25 once you have it dialed in.

 

Barry Simon


 

#73 Asbytec

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Posted 24 October 2018 - 02:06 PM

This is the collimation procedure that I really like:   http://www.asterism....als/tut14-1.htm

 

Simple, straightforward and has worked for me every time.

 

Let us know what you think about the C 9.25 once you have it dialed in.

 

Barry Simon

To add to Barry's suggestion, it's best to collimate as close to focus as seeing will allow. Avoid those huge donuts with the secondary shadow. That's really just a view of the scope's mechanical parts (sic), not (really) the optical diffraction we want to collimate. And when you are that far from focus to see the huge donut, collimation is not very precise. You want to see the diffraction pattern close to focus to collimate it effectively. Put the Poisson spot in the center of the diffraction ring(s) according to the chart in the link above. Then, if you will, check it in focus to see if the in focus star image is throwing light to one side or another and that the first diffraction ring is visually uniform to the extent you can tell it is. If the in focus pattern tends to throw light in one direction more than all others, the best we can tell given the seeing, tweak it out a tiny bit. That is about as good as you can get it visually given the seeing. But, it should be fine at this point. 


Edited by Asbytec, 24 October 2018 - 02:07 PM.

 

#74 Deep13

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Posted 24 October 2018 - 09:19 PM

Let me see if I can "Cliff Note" this...

After 3 pages and 69 posts of C9.25 bashing, it turns out that the scope was shipped from Louisiana to Ohio, over a year ago, and has never been collimated. foreheadslap.gif

Jaimo!

I don't know what you mean by "bashing." I only described my expeience, including what looked like a concentric defocused image to me.
 

#75 Jaimo!

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Posted 24 October 2018 - 09:37 PM

I don't know what you mean by "bashing." I only described my expeience, including what looked like a concentric defocused image to me.

No you were not bashing, I was referring to the overall tone of many of the responses.  

 

Jaimo!


 


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