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CFF Classical cassegrains - Any star test available?

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#51 starman876

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Posted 29 August 2019 - 02:49 PM

I don't have the number right now but I remember over 100 to 110mm actually. 

I do have a big wynne corrector that should be about to handle that, but all that weight up front will probably require some major mods.

 

I know Val uses the AP CCD67T reducer. I wonder if the Optec 0.5 or 0.3x would be possible with a small sized chip (doubt it though).

I have the optic reducer.  Would be interesting if it would work with the F20?  I also have the AP reducer.  Will be playing this weekend should the weather cooperate.   You go to love scopes like this.  They are what dreams are made of just when you look at them


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#52 Rouzbeh

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Posted 29 August 2019 - 03:31 PM

Would be very interested. I emailed AP and they said the reducer should work. They said the reducer passes what is on the input side without altering it.
Mine will be the f15 version so ends up as f10.
0.5x would be better but I'm not sure it would be possible.

#53 Alterf

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Posted 29 August 2019 - 03:38 PM

The AP reducer takes mine from f/13.5 to f/8.1. How much reduction depends on the spacing.  I use a pretty small chip with only an 11mm diagonal.

 

Val



#54 Rouzbeh

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Posted 29 August 2019 - 03:45 PM

F8 is much better, typical RCs are f8. Id guess f10 would be painfully slow.

I just ordered a qhy183mm cooled. Mostly for lunar but maybe with a reducer for planetary nebulae.
I'm seen people take amazing shots with modest gear and lucky imaging dso.

#55 starman876

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Posted 29 August 2019 - 08:13 PM

Imagers can only process what is available from the optics.  Can not get something from something that is not there.  You can't make that stuff up smirk.gif



#56 Vinny1980

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Posted 27 November 2019 - 11:44 AM

Just resurrecting this topic which had many visualizations but unfortunately no star test provided. I refuse thinking that with tenth of CFF Cass out there no one shares an image, nor can say what he sees at the eyepiece every time he collimates. This even if I ve read more than one enthusiastic feedback on these scopes here on CN. Don't want to appear caustic or other, but Id curious to know on what the users rely their opinions - expecially the imagers. smile.png  


Edited by Vinny1980, 27 November 2019 - 01:22 PM.


#57 gfstallin

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Posted 27 November 2019 - 08:30 PM

Just resurrecting this topic which had many visualizations but unfortunately no star test provided. I refuse thinking that with tenth of CFF Cass out there no one shares an image, nor can say what he sees at the eyepiece every time he collimates. This even if I ve read more than one enthusiastic feedback on these scopes here on CN. Don't want to appear caustic or other, but Id curious to know on what the users rely their opinions - expecially the imagers. smile.png  

Haha, not caustic at all. Looking through Astrobin, there are a couple CFF CC owners who post images. These aren't star tests, but processed planetary/lunar images. Other CFF owners might not be active on English-language (or Italian-language) forums. One of the CFF owners who posts to Astrobin has his preferred language listed as French, for example. You migjt only be speaking to a small portion of the potential audience. It is also possible CFF sold a significant portion of its CC OTAs to institutions and other organizations which might not normally see and then respond to requests for a star test or other specific images.

 

Even in our relatively small community, it appears garnering the attention of a maximum of ten people is not as easy as it might seem. Of course, even if you get their attention, they would still have to take time out of their already limited observing or imaging schedules to fulfill the image request. It isn't at all surprising you haven't seen a response yet. I suspect you'll see some in time. 

 

George



#58 Vinny1980

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Posted 28 November 2019 - 12:20 AM

Thanks George to have addressed a couple of valid points. I might add a few.

First, it is always very tricky  to judge the optics from post-processed data, albeit images can certainly help. JP Brahic (just to name one) has taken some excellent lunar shot  with his 14",  whose resolution can come only from a well corrected system.

Furthermore, one could have the temptation to examine only the in-focus star (visual observers maybe), and also doing so is not recommended. Not only a steady seeing is mandatory to have a stable Airy pattern, but defocusing the star aberrations become much more obvious. Some, as astigmatism, can be completely hidden in the focused image.


Edited by Vinny1980, 28 November 2019 - 01:31 AM.


#59 Rouzbeh

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Posted 28 November 2019 - 03:49 AM

Before ordering one, I also took at the Facebook users page and contacted a few users who all seemed very happy with their telescopes.

 

I have also been in contact with Catalin of CFF and he has accommodated my requests and updated me during the production phase (currently).

 

I hope to receive mine early next year and will post updates.


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#60 Daniel Mounsey

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Posted 28 November 2019 - 09:33 AM

Imagers can only process what is available from the optics.  Can not get something from something that is not there.  You can't make that stuff up smirk.gif

 

ohlord.gif It doesn't work that way. I'm calling you out on this one. That still does not express the extent or measure of aberration witnessed in a visual image to an experienced observer with trained eyes who understands how to interpret what they're seeing. Planetary imaging can not be used to judge the spherical correction of any optical system unless imaging is being used to judge spherical correction itself by the use of a Fresnel pattern. There's nothing wrong if an enthusiast wishes to acquire a telescope just to take pretty pictures. But don't try to tell experienced observers who know better that imaging is a means or validation of judging visual optical quality, it's not, and your comment is very misleading. The OP requested a Fresnel pattern. How is it that imagers can spend hours processing the heck out of lunar and planetary images, yet somehow can't image a Fresnel pattern. BTW, imagers can make stuff up. They do it all the time. 


Edited by Daniel Mounsey, 28 November 2019 - 11:20 AM.

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#61 danielguo

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Posted 29 November 2019 - 10:43 PM

It's not that easy to get a set of star test images because:

  1. There are probably fewer than 10 CFF CCs in the states.
  2. Not all of the owners are imagers.
  3. Percentage of observing nights suitable for such tests.

For example, I got my CC250 f/15 on June 2019, but since then I only used it for imaging for two nights in my place (I used it more for visual), and due to poor seeing and altitude limits I couldn't do a proper star testing in my place. If I got time to do it from a star party I'll post it up.



#62 Vinny1980

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Posted 30 November 2019 - 01:19 AM

 Planetary imaging can not be used to judge the spherical correction of any optical system unless imaging is being used to judge spherical correction itself by the use of a Fresnel pattern. There's nothing wrong if an enthusiast wishes to acquire a telescope just to take pretty pictures. But don't try to tell experienced observers who know better that imaging is a means or validation of judging visual optical quality, it's not, and your comment is very misleading.

I am mainly a planetary imager, and I think you are too drastic here. While you really don't want to rely uniquely on a processed image to judge an optic, you are not going to capture craterlets on the Moon at the diffraction limit (and sometimes below) if you have lambda/2 spherical aberration.

In the raw image there's all the information you will find in the correctly elaborated image. The art of planetary image processing consists mainly in increasing contrasts to make the features more accessible to the human eye, without creating artifacts and preferably respecting the visual aspect at the eyepiece. This is very difficult to achieve. In this regard visual observation will always be of paramount importance, because while you can be an excellent observer without having done a single image, you cannot be a good planetary imager if you don't spend time at the eyepiece.

 

 

  1. There are probably fewer than 10 CFF CCs in the states.
  2. Not all of the owners are imagers.

And not all CN members are in USA smile.png

As I said from the very beginning, everyone must fastly defocus the star in every session, just  to check collimation. Why no one here can simply report what he sees - this beats me :smile:


Edited by Vinny1980, 30 November 2019 - 06:43 AM.


#63 Daniel Mounsey

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Posted 30 November 2019 - 07:42 AM

It's not that easy to get a set of star test images because:

  1. There are probably fewer than 10 CFF CCs in the states.
  2. Not all of the owners are imagers.
  3. Percentage of observing nights suitable for such tests.

For example, I got my CC250 f/15 on June 2019, but since then I only used it for imaging for two nights in my place (I used it more for visual), and due to poor seeing and altitude limits I couldn't do a proper star testing in my place. If I got time to do it from a star party I'll post it up.

 

I can surely appreciate the challenges but this is why I don't take these reviews seriously. It's like reviewing an Indy car and the driver decides to evaluate it while off roading. That's the problem with a lot of reviews here. There's so many strong opinions from others who have these same problems. 


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#64 Daniel Mounsey

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Posted 30 November 2019 - 07:55 AM

I am mainly a planetary imager, and I think you are too drastic here. While you really don't want to rely uniquely on a processed image to judge an optic, you are not going to capture craterlets on the Moon at the diffraction limit (and sometimes below) if you have lambda/2 spherical aberration.

In the raw image there's all the information you will find in the correctly elaborated image. The art of planetary image processing consists mainly in increasing contrasts to make the features more accessible to the human eye, without creating artifacts and preferably respecting the visual aspect at the eyepiece. This is very difficult to achieve. In this regard visual observation will always be of paramount importance, because while you can be an excellent observer without having done a single image, you cannot be a good planetary imager if you don't spend time at the eyepiece.

 

And not all CN members are in USA smile.png

As I said from the very beginning, everyone must fastly defocus the star in every session, just  to check collimation. Why no one here can simply report what he sees - this beats me smile.png

 

Vincenzo, I appreciate what you are saying. Have a look at post #92

https://www.cloudyni...efractor/page-4


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#65 Alterf

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Posted 30 November 2019 - 01:10 PM

Just pause to note the irony of Dr. D chiding us for delaying a review of a CFF scope.

 

In any event, one reason not to publish a review is the criticism that inevitably follows.  As Daniel says, a lot of what is said on CN is hogwash.  By the same token, there is a lot of expertise here that many of us don't have.  A slip of the tongue, a too-enthusiastic adverb, a technical mistake---any number of things seem to invite criticism and argument both from the learned and unlearned.  Participation here is entirely voluntary, and I find the incentive to volunteer such things in public lacking, though I'm happy to correspond with anyone who asks.

 

Val


Edited by Alterf, 30 November 2019 - 01:12 PM.

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#66 Daniel Mounsey

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Posted 30 November 2019 - 10:25 PM

Val,

If others have some solid observations they'd like to share, I'd be happy to hear what others have to say. 

Regards



#67 ZX12

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Posted 01 December 2019 - 09:26 PM

I received my CFF 250mm CC F15 (quartz mirror) last September and have had it out around a dozen or so nights since.

 

Optically it appears to be of excellent quality, and shows very tight stars once the mirror is thermally stable and seeing is good.

 

Mechanically it is also excellent, and I would rate it equal to any of the top scope manufacturers in quality. 

 

The scope arrived from overseas packed well, but was out of collimation a fair amount at the secondary. It was simple enough to dial in on a star using the precise adjustments since I did not have the Takahashi collimation scope at the time.

Once I acquired the Tak TCS I perfected collimation on the secondary and a slight adjustment of the primary. 

 

I asked Catalin to adjust the primary spacing so a binoviewer can be used easily without needing the GPC. Switching from mono mode to bino mode only requires a 50mm extension to be removed between the diagonal and focuser.

 

My first views of Saturn were very sharp and had the etched appearance that I'm used to seeing with my AP160. The big difference being the exit pupil size and eyepiece necessary to achieve powers above 150x.

 

The negatives I find using this type of scope are:

 

1) Fans do need to be run often in warm weather to keep up with dropping temperatures.

2) Long focal length means the seeing needs to be good or better to enjoy the high powers this scope provides. Not something that happens on a regular basis in my region.

3) The 4 vane secondary support creates a diffraction spike on bright stars and Jupiter/Venus that is tolerable, but takes getting used to after many years of refractor and SCT use.

 

I spend most nights using my scopes visually, but also do some AP with an AP130GT or AP Stowaway. Star tests are done when first acquiring an optic, but I don't spend a lot of time studying it as tight binary stars or planets are more interesting to prove the optics ability.

 

When I set the scope up next time in my observatory, I will take some photos of defocused stars on both sides of focus and post them here for those who are interested in analyzing the optics. Just let me know what you would like to see.

 

Mike

Attached Thumbnails

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  • CFF250.jpg

Edited by ZX12, 01 December 2019 - 10:48 PM.

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#68 Vinny1980

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Posted 02 December 2019 - 03:05 AM

 

When I set the scope up next time in my observatory, I will take some photos of defocused stars on both sides of focus and post them here for those who are interested in analyzing the optics. Just let me know what you would like to see.

 

Mike

Hello Mike,

 

I guess you are not confident with star testing and since you have requested it, I ll write some quick tips. I just apologize if you just know all the following. 

 

- The star hasn't to be too much defocused, around 10 waves should be ok.  This means you have to defocus around 17 mm. This is valid both for IF/EF images which should have roughly the same size. 

- It is better to stack frames to average turbulence, especially if your seeing is not excellent as you say. Images resulting from stack of about 500 best frames should be adequate and sharp looking.

- You should shot at prime focus, so no Barlows are needed. Filters are not necessary as well. 

- The star used mustn't be too bright. One around magnitude 1, well above the horizon, is in general a good choice. 

 

If you have doubts, just let me know. Other users might want to add something I forgot to mention. 


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#69 ZX12

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Posted 03 December 2019 - 06:35 PM

Thanks Vincenzo, I will make an attempt when the weather clears after this recent snow storm.

 

Mike


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#70 Asbytec

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Posted 04 December 2019 - 09:52 AM

I don't remember the formula for 10 waves defocus, so quoted a post from another CN DIY thread below.

As Vinny said, this is true for both sides of focus and not too far out of focus. To do a good star test, we wanna see both sides of focus by about the same amount. Ten waves is good for spherical. Closer in or even in focus for astigmatism, and a little further than 10 waves for zones and roughness. Wag it...

Like others, I've been lurking this thread hoping to see some test results, too.

https://www.cloudyni...10-wavelengths/

"When they talk about 10 wave defocus it does not mean that the focal plane is moved by 10 wavelengths. Instead it means that the focal plane is moved by an amount that causes a maximum 10 wavelength error in the resulting P-V wavefront (P-V means peak-valley).

Rearranging formula 24.1 in the link I provided:

Defocus (mm) = 8 * lambda * f_ratio^2 * number_waves.

So for 10wave defocus at f/8 at wavelength 0.55microns (green light) we have defocus(mm) = 8 * 0.00055 * 8*8 * 10 = 2.82mm which is the result you gave from Aberrator."

The link he refers to is in (Amateur) telescope optics.net.
https://www.telescop...et/defocus1.htm

Edited by Asbytec, 04 December 2019 - 10:21 AM.

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#71 Vinny1980

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Posted 04 December 2019 - 10:27 AM

Asbytec,

 

yes, your formula is that reported among others by Suiter (II ed, p. 78) and it is used also in Aberrator. In my calculation I used the f/20 focal ratio, being the 12" f/20 the main argument of the thread, but Mike actually uses an f/15. The correct result should be around 10 mm in green light (fastly done on cell right now, you can check it). 

Such precision is not necessary for our purpose, though.  


Edited by Vinny1980, 05 December 2019 - 01:32 AM.

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#72 Asbytec

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Posted 04 December 2019 - 10:41 AM

Vinny, thanks for the clarification. Yes, it's not like it has to be exact, or even 10 waves, just close to the same distance either side. Eight to 10 waves is nice, not too far out. I kind of wag it visually scrolling the focuser. It's qualitative mostly, anyway, for a good or bad feel. But, you know imagers and their precision. :)

Edited by Asbytec, 04 December 2019 - 10:47 AM.

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#73 Daniel Mounsey

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Posted 05 December 2019 - 09:04 AM

lol.gif  I think Suiter spent too much time in a cubicle. 


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#74 macdonjh

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Posted 05 December 2019 - 10:15 AM

I looked at this scope seriously last year but thought f/20 did not bring much benefit. f/15 would be more manageable for a modest increase in CO which would have no effect for imaging and be undetectable visually. I wonder why CFF doesn't offer an f/15 by default and build longer versions for who want them.

 

Tanveer.

I've never talked to Catalin about that, but my guess would be CFF are trying to minimize coma (see the company name...).  I remember reading essays on Robert Royce's webpage and he advocated for really slow classical systems.  Dall Kirkhams, in his opinion, shouldn't be faster than f/18 and f/20-f/24 were preferable; classical Cassegrains shouldn't be faster than f/15 and f/20 was preferable.  I think the examples in Texereau have similar focal ratios.  When I was discussing the merits of faster scopes with Catalin I told him I really wanted mine (a 350mm f/15) to be f/10, since that was what I was used to after five years with a C11.  At the time CFF's standard 350mm classical Cassegrain was f/17.  Catalin told me CFF could make an f/11 CC if I would accept a refractive corrector.  After much consideration I ordered my f/15 (as fast as Catalin was willing to go without a corrector) and have been happy since Sept. 2015.

 

My point is, talk to Catalin about what you really want as there seems to be wiggle room in their designs.  He may be able and willing to accommodate what you're after.

 

The G11 I have requires the losmandy plate to be slid into it.  What a pain. I had an old one where it would hook on side and then you just lay it on the mount and lock it down. 

Take a look at ADM, you'll like the tip-in saddle.  I'm saddened that Robin Cassady isn't making things anymore (I hate to see anyone leave a business), but ADM make worthy products.

 

They were nice last night, but after a long day I did not have the energy to set up the scope.  There will be other nights.  Catalin has assured me that this 300mm F20 is a very special scope and on the couple of nights in poor seeing I had it out I believe that.   Catalin also told me they are too busy right now to make a different secondary  to change it to F15.  However, he did tell me there is a gent who is converting his to be able to switch to Newt mode.  That should be interesting.  

I considered a convertible scope, too.  I've always liked the Takahashi CN-212, and bought a Parks HIT 10" because I couldn't find a Takahashi.  The Parks design is compromised in my opinion (the mirrors are too close together), but I'll bet CFF could do a good job optimizing such a design.  Even though the Parks isn't designed the way I would have built it, it is a fun scope to use.  It's neat to be able to switch from f/15 to f/4 and back.  One big design issue, though, is something needs to be able to rotate otherwise the Newtonian focuser will be in unusable positions sometimes.  Rotating rings were the solution for my solid tube HIT, but I guess a rotating UTA would have to be designed for a truss scope.  



#75 TG

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Posted 05 December 2019 - 08:57 PM

I've never talked to Catalin about that, but my guess would be CFF are trying to minimize coma (see the company name...).  I remember reading essays on Robert Royce's webpage and he advocated for really slow classical systems.  Dall Kirkhams, in his opinion, shouldn't be faster than f/18 and f/20-f/24 were preferable; classical Cassegrains shouldn't be faster than f/15 and f/20 was preferable.  I think the examples in Texereau have similar focal ratios.  When I was discussing the merits of faster scopes with Catalin I told him I really wanted mine (a 350mm f/15) to be f/10, since that was what I was used to after five years with a C11.  At the time CFF's standard 350mm classical Cassegrain was f/17.  Catalin told me CFF could make an f/11 CC if I would accept a refractive corrector.  After much consideration I ordered my f/15 (as fast as Catalin was willing to go without a corrector) and have been happy since Sept. 2015.

 

My point is, talk to Catalin about what you really want as there seems to be wiggle room in their designs.  He may be able and willing to accommodate what you're after.

 

 

D-Ks are much worse in terms of coma than CCs. See https://www.cloudyni...k/#entry9772316 which shows a f/12 CC. I've also posted PSF simulations for such a scope. Coma would not be my concern.

 

When I was researching the Classical Cassegrain design, I created a spreadsheet to help figure out various parameters for the design:

 

https://1drv.ms/x/s!...3S70eEV5JUE5dlQ

 

People might find it useful. Just download and change the values in green.

 

The CFF is still on my list but at this point I'm not ready to spend the $$$ for both the scope and the required mount. A ยต-250 keeps me happy when I'm in the mood to use a Cassegrain.

 

Tanveer.




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