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CFF Classical cassegrains - Any star test available?

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#101 ZX12

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Posted 12 December 2019 - 09:23 AM

All the images I've taken are RAW singles with no post processing using a Canon 60Da at ISO 400 and typically around 4 seconds exposure time.

No filters were used and all are in color. The blue dot only appeared on the photos taken at the limit of inside focus.

 

My AP skills are beginner level so bear with me in trying to get the desired results.

 

So far, the sky conditions have not been good enough to see faint stars as anything more than a blurry dot visually.

 

Tonight's forecast looks decent so I will try again from 10-20mm on each side of focus. 

 

Mike

 

 


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#102 Axunator

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Posted 12 December 2019 - 10:21 AM

Despite the risk of being called "amazing speculator" or "armchair theorist" (I am not referring to other posters of this particular thread), I will mention that classical Cassegrain (paraboloid primary, hyperboloid secondary) DOES have off-axis astigmatism - in addition to coma and field curvature, of course. Which, in fact, becomes the dominant aberration at sufficiently large field angles (depending on the f-ratio of the system), because coma increases linearly and astigmatism exponentially with the distance from the optical axis.

 

You don't have take my word on it, check: https://www.telescop...d_aplanatic.htm

 

And I think this has implications for this thread, because it is possible that astigmatism seen in the defocused images on the previous page may be due to collimation/tilting issues, and not necessarily present problems in the glass.


Edited by Axunator, 12 December 2019 - 10:26 AM.

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#103 Vinny1980

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Posted 12 December 2019 - 01:20 PM

Mike,

I measured your image, they are around 150 pixel in size which corresponds roughly to 10 waves defocus.waytogo.gif

Four seconds of exposure seem a little too much tough. It would be better to stay much lower to freeze turbulence, but I never used your reflex so cannot be of much help here.

Axunator, I do appreciate your contribution. Evidently coma is dominating around axis - and I just couldnt remember astigmatism shape in the Rutten/Van Venrooij ray spots, nor in my own ATMOS simulations in the first pages of this thread. The authors state, literally, that no Cassegrain could be free from astigmatism - and with this justice is done to their very useful book. smile.png

Of course you can have astigmatism on axis if there's some misalignment, but I expected to see it with coma. Since the latter is very modest in the images, I tend to think to some other issue somewhere else ( but I could be wrong here).


Edited by Vinny1980, 12 December 2019 - 03:09 PM.


#104 Asbytec

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Posted 12 December 2019 - 08:18 PM

 

Of course you can have astigmatism on axis if there's some misalignment, but I expected to see it with coma. Since the latter is very modest in the images, I tend to think to some other issue somewhere else ( but I could be wrong here).

Yea, coma is not apparent (to my eye) in the first images as shown above. Axial collimation looks very good. On axis and collimated coma and astigmatism should not be present for all intents and purposes, yet astigmatism is apparent. It is probably due to somethign else rather than field angle. Good point. 


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#105 ZX12

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Posted 12 December 2019 - 08:50 PM

Here are some images from tonight's session. ISO 1000 was used to shorten exposure time.

 

Seeing was fair, but transparency deteriorated quickly so exposure time was increased from one to two seconds in the last two images. 

 

Mike

 

 

Attached Thumbnails

  • CFF inside.jpg
  • CFF outside.jpg
  • CFF inside Alph .jpg
  • CFF outside Alph.jpg
  • CFF inside Capella.jpg
  • CFF outside Capella.jpg

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#106 Asbytec

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Posted 12 December 2019 - 10:05 PM

Mike, your Capella images look pretty darn good, and they appear to be close enough to the same defocus either side. Close enough to be telling. I cannot work with the blue Alph images. Outside appears too much further from focus than inside for my liking.

However, your defocused Capella images and the shadows are close to the same diameter (upon visual inspection), with a very slight amount of over correction as evidenced by some alternating slight variation in the inner and outer ring brightness on both sides of focus. Inside, the inner ring is a tad brighter indicating the center is focusing short, outside the outer ring is a tad brighter indicating the edge is focusing a bit long. The shadow is a tad larger inside focus which is normally associated with under correction, so not sure what to make of that other than it does not appear significant in any case. (Possibly the affect of a slight zone near the center, sometimes associated with a bright inner ring and large shadow, or the size of the obstruction).

I'd argue for a correction significantly and assuredly better than 1/4th and likely even better than 1/6th PV of primary spherical. How much better than that is anyone's guess, I am just sure it's not 1/4 waves PV LSA and appears to be better than 1/6 waves PV LSA. Based on those images, inside and outside are closely similar and /could/ be as good as 1/8 waves PV LSA. I don't normally go out on a limb like this, however your Capella images look very good in terms of correction. Saying that is not evidence of the actual PV of combined aberrations, as astigmatism may have a larger value, rather just an approximate read on spherical correction. Generally and very slightly over corrected with some minor deviations from a smooth over correction across the surface. Totally normal. No optic is perfect.

There appears to be a shallow over corrected zone as evidenced by the darker ring(s) inside focus between the bright inner and outer rings, and a little brighter ring(s) between both bright diffraction rings outside focus. Not enough to worry about, certainly. Astigmatism still evident, though, but no significant and noticeable coma to my eye. At least not to the level of pretty good collimation you seem to have attained. (Interestingly, astigmatism appears closely oriented with your spider vanes...hmmm).

That's my read on it, others may differ...but I'd suppose, astigmatism aside for the time being as it may be fixable, we'd agree your Capella images indicate a very good optic. Enough to say you should be very happy with the images, especially if the source of astigmatism can be isolated and corrected.

Edited by Asbytec, 12 December 2019 - 11:53 PM.

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#107 Vinny1980

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Posted 13 December 2019 - 12:43 AM

Mike,

 

while I greatly appreciate your efforts, I must say the tests show too much variability to be really conclusive. I understand your difficulties as visual observer - but the star test must be conducted in controlled conditons to assure repeatablity and hence consistency. In such conditions, it's very difficult to state something safely. The long exposure used could smear everything - while one must rely first of all on brightness profile to estimate LSA (Lower Spherical Aberration).

The only thing which seems sure it's a certain amount of astigmatism, probably coupled with some spherical aberration. Both not worrying and maybe fixable acting on aligment/spacing. Probably good/very good optics, hardly excellent IMHO.  

In defense of the manufacturer, I think it's fair to remember the difficulties in making that kind of mirrors. An f/3 parabolic alone could scare many opticians, without mentioning the small, hyperbolic secondary which must be obscenely difficult to figure properly. 

Other users are encouraged to share their opinion, as I don't want to monopolize the topic. 


Edited by Jaimo!, 15 December 2019 - 10:27 PM.

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#108 Daniel Mounsey

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Posted 13 December 2019 - 08:11 AM

Nice post Vincenzo, I agree.

 

Mike (ZX12), you also deserve to be commended for making great efforts to help address Vincenzo's OP, so thank you for trying. waytogo.gif

 

Just one thing I wanted to say regarding the Trapezium image for others who may be reading into this thread at the side lines. I think it's fine if others wish to take nice pictures and split doubles with imaging if that's what they enjoy. Imaging can be a rewarding accomplishment and thus, my following comments are not meant to take away from Mike's nice picture.

 

The downside that I wish to address with imaging doubles though is that splitting them in images can't be used as testament of optical quality, in fact, in some cases just the opposite is true. That's not necessarily meant to imply any solid conclusions here regarding Mike's CFF as Vincenzo has already explained. 


Edited by Daniel Mounsey, 13 December 2019 - 08:16 AM.


#109 ZX12

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Posted 13 December 2019 - 08:28 AM

One of the issues with getting equal images is the focuser does not have enough travel to adjust from one side to the other and get the star far enough outside focus.

 

I have to loosen the extension, slide it out and then measure, which also shifts the stars position slightly. The cold weather is not helping the process.

 

It might be awhile before the skies are clear again for any more attempts.

 

Mike


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#110 Asbytec

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Posted 13 December 2019 - 09:41 AM

One of the issues with getting equal images is the focuser does not have enough travel to adjust from one side to the other and get the star far enough outside focus.

 

Mike

You did a good job with your Capella images. Close enough. it does not have to be exact because this is a subjective test, so long as it's close enough to focus and not loosing too much sensitivity. A best guess based on the available images. After all, we're observers and not interferometers. 

 

Vinny summed it up nicely. 

 

The only thing which seems sure it's a certain amount of astigmatism, probably coupled with some spherical aberration. Both not worrying and maybe fixable acting on aligment/spacing. Probably good optic, hardly excellent IMHO.

 

The image below is an Aberrator simulation of 0.3 waves of astigmatism only at 10 waves defocus using your scope parameters. It's not exact, but it gives some idea of how much astigmatism might be present in the images above using a model of your scope's parameters. Maybe Vinny can better model the amount of astigmatism present, potentially flirting with the diffraction limit for astigmatism. Try this on for size, some of it may apply to your case. 

 

Astig.jpg


Edited by Asbytec, 13 December 2019 - 09:42 AM.


#111 ZX12

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Posted 13 December 2019 - 04:08 PM

Thanks Asbytec, that is an excellent article by Mike Lockwood.

 

I have noticed thermal issues playing a large part in distorting the star image with the scope and combined with poor seeing it has not allowed me to really get the most out of the optics.

 

Last night I adjusted the primary mirror and noticed an improvement in the appearance of the focused and defocused star. 

 

Mike


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#112 Asbytec

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Posted 14 December 2019 - 01:54 AM

I have noticed thermal issues playing a large part in distorting the star image with the scope and combined with poor seeing it has not allowed me to really get the most out of the optics.

Mike

Mike, there are some slight thermal affects in your star test images. They do not look particularly severe on the night you captured them. They are tame enough to read through. As Lockwood explains, a warm mirror can add some amount of over correction. It may, but I doubt it due to the minor thermal activity seen, account for the slight over correction seen in the star test. However, if so, then it means the correction will likely improve as the mirror stabilizes.

You're right, as I'm sure you know well, the larger mass of primary mirror should be thermally stable with the environment for the best images as the mirror retains its form and conducts little or no heat to the air. Due to the modest conductivity of optical glass, thermal stability can take a while. During cooling the mirror tends to warm the air near it's surface. This warming causes a variance of air density near the primary surface and is the source of the slow moving caustic lines (like the bottom of a swimming pool on a sunny day). We see this in a defocused image, and only slightly in your images.

So your mirror appears to have been stable enough, IMO, on that night. I've seen worse and surely you have, as well. The good news is the warm boundary layer can be blown away with fans to some extent, depending on the tube design, as the mirror cools. I presume your fans were running during your session.

Edited by Asbytec, 14 December 2019 - 02:17 AM.


#113 Daniel Mounsey

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Posted 14 December 2019 - 06:49 AM

Open tubes are notorious for thermals due to the high rate of heat transfer with unstable temperatures. 


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#114 Alterf

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Posted 14 December 2019 - 12:31 PM

My CFF has a closed tube.  With three fans running, thermals more or less disappear.  I was shocked when I first realized this.  It is one my favorite features.

 

Val


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#115 Peter Besenbruch

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Posted 14 December 2019 - 05:48 PM

My CFF has a closed tube.  With three fans running, thermals more or less disappear.  I was shocked when I first realized this.  It is one my favorite features.

I think Daniel was thinking along the lines of: Open tube=no corrector plate. Closed tube=SCT or Maksutov. There are other terms floating out there: Solid vs. truss tube.

 

I own a closed tube and insulate; basically the opposite approach. It's good that fans work so well on your scope. It's nice to "get down to business" fast.


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#116 Asbytec

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Posted 14 December 2019 - 06:09 PM

I think Daniel was thinking along the lines of: Open tube=no corrector plate. 

I thought he was talking about truss tubes and heat from external or structural sources getting into the light path. hmm.gif


Edited by Asbytec, 14 December 2019 - 06:11 PM.


#117 Daniel Mounsey

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Posted 14 December 2019 - 08:44 PM

I think Daniel was thinking along the lines of: Open tube=no corrector plate. Closed tube=SCT or Maksutov. There are other terms floating out there: Solid vs. truss tube.

 

I own a closed tube and insulate; basically the opposite approach. It's good that fans work so well on your scope. It's nice to "get down to business" fast.

Correct. 



#118 Daniel Mounsey

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Posted 14 December 2019 - 08:45 PM

 

My CFF has a closed tube.  With three fans running, thermals more or less disappear.  I was shocked when I first realized this.  It is one my favorite features.

 

Val

It’s still the same issue.

 

It’s obvious in Mike’s images that thermals are far from under control.


Edited by Daniel Mounsey, 14 December 2019 - 08:47 PM.


#119 ZX12

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Posted 14 December 2019 - 11:41 PM

Once the Sun has set and daytime temperatures have been less than 60 degrees, it typically takes 30 minutes of running the fans to get a decent image with the scope initially.

 

With the colder weather we've had recently, I can open the observatory and be viewing within 10 minutes of fan operation and the mirror looks like the above photos.

Not completely rid of thermals, but the image quality is decent depending on the seeing.

Large drops in temp require the fans to be used again, but they are very effective in keeping up with the change. 

 

I've only had two sessions back in October that had good seeing combined with stable temperatures, and the image quality was excellent.

Stars were round with no sign of astigmatism when racking focus either side.

 

Mike

 

 



#120 Asbytec

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Posted 15 December 2019 - 02:45 AM

Stars were round with no sign of astigmatism when racking focus either side.

Mike


That's interesting as astigmatism is evident in the images above. If you do not always see it, then it likely means the reason we see it now is transitory and can be isolated and fixed. What about astigmatism in focus?

There is some thermal activity seen in your images, but not enough to dissuade a star test on the mirror as seen and presuming your primary mirror is near ambient. Nothing compared to my own mirror at set up and modest ambient temperature differential. Looks as though your secondary might be contributing a bit, too.

#121 Vinny1980

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Posted 15 December 2019 - 05:17 AM

Once the Sun has set and daytime temperatures have been less than 60 degrees, it typically takes 30 minutes of running the fans to get a decent image with the scope initially.

 

With the colder weather we've had recently, I can open the observatory and be viewing within 10 minutes of fan operation and the mirror looks like the above photos.

Not completely rid of thermals, but the image quality is decent depending on the seeing.

Large drops in temp require the fans to be used again, but they are very effective in keeping up with the change. 

 

I've only had two sessions back in October that had good seeing combined with stable temperatures, and the image quality was excellent.

Stars were round with no sign of astigmatism when racking focus either side.

 

Mike

Did you optimize the spacing from primary to secondary or you are using the factory setting?

Is it possibile to adjust secondary transversally?

About visual estimations, let me to report an anectodal story. I examinated recently a Dall-Kirkham, defocusing the star accurately searching for aberrations: the optics looked good to me. Then I attached my ASI174MM to take a shot of the out of focus patterns.

The telescope came definitively back to the optician.


Edited by Vinny1980, 15 December 2019 - 05:20 AM.


#122 Daniel Mounsey

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Posted 15 December 2019 - 12:44 PM

Mike,

Is your observatory insulated and vented? Is the floor paved? Is it a dome with a door or a roll off? This plays a huge role. Check out your temps.

https://forecast.wea...tType=graphical


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#123 Vinny1980

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Posted 05 February 2020 - 04:07 AM

I resurrect the topic once again, since I dont' want definitely to open another thread to discuss about what should be a good argument, at least from academic point of view.

Im not optician nor I ever grinded a mirror, so basically I am not basing myself on my own experience. In my opinion (and not only mine) the mainly difficulty in doing a Cassegrain resides into the small secondary, and you can find this stated on various sources. In particular, Malacara (in his Handbook of optical design, par. 14.2.2) gives a very interesting explanation, showing how the main of the 5 Seidel aberrations for the Cass (i.e. spherical and coma) are separately present into the primary and secondary mirror and need to be corrected into each of them. I ve always refused the statement "the secondary amplificates primary defects" and I have a strong theoretical support for doing so. 

I was curious to know the opinion of the CFF mirror optician, Octavian Stanescu, whose point of view is a little different. According to him the most important thing is to have a good parabola, then making the secondary is relatively easy. He's not alone in his opinion, though. I ve heard more or less the same from at least another optician, so I wonder where the truth really is, assuming there's actually a gap between theory and pratice here. 


Edited by Vinny1980, 06 February 2020 - 01:12 AM.


#124 ZX12

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Posted 05 February 2020 - 07:09 PM

Vinny,

 

 I noticed this thread back up and almost forgot about it due to so many other pursuits. 

 

 Shortly after the last session I had with the CFF250 back in December I started looking into the astigmatism that was apparent in the photos I posted here and found the cause.

After collimating with the TCS, I made the mistake of fine tuning the secondary shadow on a star using the secondary adjustments when it must be done with the primary adjutments only.

 

Once we have clear skies again I will take a few photos and post again.

 

Mike


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#125 starman876

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Posted 07 February 2020 - 06:34 PM

soon it will be spring and I will be taking my SCT out and do some viewing.   Have not even had a chance to do the DPAC.  Where has all the time gone since my last post on this topic?




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