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CFF Classical cassegrains - Any star test available?

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#26 Vinny1980

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Posted 14 August 2019 - 03:17 PM

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.

The point - but one could ask directly to them- could be that such an instrument wouldnt be anymore diffraction limited in the whole field. My ATMOS simulations show (assuming an f/3 primary as fixed) that the f/15 would be only few cm shorter, 5% more obstructed, and not diffraction limited on the edge of the field.


Edited by Vinny1980, 14 August 2019 - 03:18 PM.


#27 TG

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Posted 14 August 2019 - 05:19 PM

@Vinny1980 did you account for image plane curvature in your modeling?

#28 Vinny1980

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Posted 14 August 2019 - 11:51 PM

Yes, TG, they take into account all the aberrations (coma and curvature of field in this case). For those not confident with such diagrams, referring to my post #17,  the ray spot for the f/15 is bigger than the black circle, which represent the Airy disc. My simulations are approximated because I dont know the exact values CFF takes for backfocus and fully illuminated field, but the deviations shouldnt be significant.



#29 peleuba

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Posted 15 August 2019 - 12:27 PM

Johann,

 

Ant first light reports?  



#30 starman876

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Posted 15 August 2019 - 06:03 PM

Got out last night.  Really muggy and lousy seeing.  However, the scope performed rather well.  Went from a 40mm eyepiece (150X) on Saturn to 14 MM (250X) and the image got sharper.  That was impressive.  The went to 14mm (428 X) and the image did get a little soft but still was worthwhile viewing.  Cassini easily visible.   Some banding on planet. Cannot wait until a really clear night.  


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#31 danielguo

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Posted 15 August 2019 - 09:37 PM

Two weeks ago when in star party I used my CFF 250mm f/15. Unfortunately Jupiter and Saturn are both pretty low in the sky (< 20 degrees) for me to test the optics. It also gets windy here some nights.

 

Under the condition Saturn still looks pretty nice. Image feels solid and I can see the Cassini division and some bands. Maybe even better than my friend's 14" dob, although I'd say both are limited by seeing.

 

For DSO globular clusters look pretty nice. The largest exit pupil I can get is 3.7mm with 55mm Plossl, not exactly big, but I knew that before buying the scope. I mounted a WO 71mm refractor on its side as a finder.


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#32 starman876

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Posted 16 August 2019 - 05:41 AM

These are impressive scopes.  F20 is a bit challenging, but 100 degree eyepieces really help.  I cannot imagine using an F20 scope without widefield eyepieces.  



#33 starman876

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Posted 21 August 2019 - 01:42 PM

This weekend should be clear enough to do some star testing.   Clear skies are coming my way.   whee.gif



#34 peleuba

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Posted 21 August 2019 - 01:56 PM

This weekend should be clear enough to do some star testing. 

 

Take a picture, would love to see that big boy mounted outside...   And your right, some great skies coming our way here in the mid-Atlantic.



#35 starman876

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Posted 21 August 2019 - 10:05 PM

Take a picture, would love to see that big boy mounted outside...   And your right, some great skies coming our way here in the mid-Atlantic.

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. 



#36 Rouzbeh

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Posted 25 August 2019 - 12:38 PM

After several chats with Catalin I also placed and order for a CFF.

 

They seemed to have a 450mm blank for a project that was never started. I'm currently using a C14 (good example I believe) and figured a 14 or 16" won't be that much different.

 

Its planned to be an f/15.5 system with an f/3 conical primary.

 

Its planned for early next year, so I have a long wait. Need to wait for a batch of 300mm scopes (thanks Vince!)lol.gif

 

I was using the G11 for the C14 and it could cope but the 450mm will be a big fella, so recently got a Titan and think that should be up to the job.

 

Will post updates.


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#37 Vinny1980

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Posted 26 August 2019 - 05:38 AM

The point - but one could ask directly to them- could be that such an instrument wouldnt be anymore diffraction limited in the whole field. My ATMOS simulations show (assuming an f/3 primary as fixed) that the f/15 would be only few cm shorter, 5% more obstructed, and not diffraction limited on the edge of the field.

I quote myself cos I asked Catalin of CFF to stress the differences among the f/15 and f/20 300 mm Cassegrain, here's what I get:

 

_______________________________________________

f20 version

overall length = 1070mm

CO = 75mm (OD of secondary casing) real obstruction 25%

weight = 19kg (for solid mirror)

 

f15 version

overall length = 1020mm

CO = 90mm (OD of secondary casing) Real obstruction 30% 

weight = 19kg (for solid mirror)

 

Field correction is better (theoretically) in the f20 version.

F15 version will still show pinpoint stars in 2” eyepieces.

__________________________________________________

 

I expected the f/15 to be more appreciably lighter than f/20, but since they have truss structure this is not necessarily true. 

I assolutely trust the pinpoint stars in the EP cos the differences in ray spots are not that big, and probably under the threshold of eye detection in most of the seeing condtions, while for imaging one has more stringent requirements. 


Edited by Vinny1980, 26 August 2019 - 05:54 AM.


#38 starman876

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Posted 26 August 2019 - 07:40 AM

I wonder if it were possible to get secondaries that could make the scope either F20 or F15.  Seems the 50mm difference should not be hard to manage.



#39 Rouzbeh

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Posted 26 August 2019 - 07:40 AM

That's interesting, I was told the 450mm f15 would have a secondary in the low 20s.
It could be different as its a bigger primary?
He did say a corrected field of 20-25mm.

"One of our customers took a full frame photo with a 350mm f15 and it had pinpoint stars on it.

Since a 450mm will have longer FL, field will be less curved and aberrations smaller."

Edited by Rouzbeh, 26 August 2019 - 07:41 AM.


#40 Rouzbeh

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Posted 26 August 2019 - 07:46 AM

I know the f15 is an f3 primary.
That is a good question, sort of like swapping barlows but I suspect its not that simple.

I also though about maybe using it as an f3 astrograph. You could maybe swap out to a flat secondary and use a corrector to get a fast Newtonian. Maybe even prime focus.

They use parabolic primaries.

#41 starman876

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Posted 26 August 2019 - 03:04 PM

I sent Catalin an email asking about the chance of changing from F20 to F15.   Or even using the OTA as a newtonian.   Hope I did not upset him too much smirk.gif



#42 Rouzbeh

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Posted 26 August 2019 - 03:57 PM

I know Val has an F13 (or so) and uses an AP reducer to get it down to F8.
Got a great shot of M1 with his cff250.

#43 Alterf

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Posted 27 August 2019 - 12:33 PM

I've been sitting on the edge of this discussion and have talked with a couple of participants.  I have a CFF 290mm f/13.5 Classical Cass.  It's a wonderful scope I enjoy every time I set it up.  Thanks for the kind comment, Rouz.

 

Starman876, did your clear skies not occur?

 

Val



#44 Rouzbeh

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Posted 27 August 2019 - 03:45 PM

I've been sitting on the edge of this discussion and have talked with a couple of participants.  I have a CFF 290mm f/13.5 Classical Cass.  It's a wonderful scope I enjoy every time I set it up.  Thanks for the kind comment, Rouz.

 

Starman876, did your clear skies not occur?

 

Val

Sorry 290mm. I thought it was a very nice M1 image :)


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#45 starman876

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Posted 29 August 2019 - 07:23 AM

I've been sitting on the edge of this discussion and have talked with a couple of participants.  I have a CFF 290mm f/13.5 Classical Cass.  It's a wonderful scope I enjoy every time I set it up.  Thanks for the kind comment, Rouz.

 

Starman876, did your clear skies not occur?

 

Val

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.  



#46 Thandal

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Posted 29 August 2019 - 08:28 AM

<snip>
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.  

 

"Interesting" doesn't begin to cover it!  laugh.gif

 

It will be fascinating hear how that goes, and even more so if the owner provides a detailed description of the process and the results.  ("Pics, or it never happened!" cool.gif  )


Edited by Thandal, 29 August 2019 - 08:29 AM.


#47 Rouzbeh

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Posted 29 August 2019 - 09:00 AM

I did have an email conv with him recently asking if that were possible. I did put together an ATM astrograph a while back so have some experience.

Not sure if he was referring to me as I was asking if it were possible.
He did say the warranty would be void if scope was modified.

I don't have any exact dimensions, but from my estimates the 450mm will need massive secondary to Illuminate even a mid sized chip. Probably the same for smaller scopes as they are f3 primaries.

One foreseeable issue would be the spiders not being designed to support that much weight.

#48 Alterf

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Posted 29 August 2019 - 09:41 AM

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.  

OK.  Just want you to know we would still like to hear a report if it arrives.  I will try to produce one as well.  I'm always surprised and pleased with the scope's performance.  Next time I have it out, I'll take a few notes.  Val



#49 Thandal

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Posted 29 August 2019 - 09:56 AM

<snip>
I don't have any exact dimensions, but from my estimates the 450mm will need massive secondary to Illuminate even a mid sized chip. Probably the same for smaller scopes as they are f3 primaries.

One foreseeable issue would be the spiders not being designed to support that much weight.

 

That's what I was thinking.  A 450mm f3 mirror means a huge lightcone to then be bounced out the side to the ocular.  I calculate it would need about a 90mm (3.5") secondary (presuming the 225mm radius primary + 50mm for a low-profile focuser.)

 

I guess the weight of such a secondary could be carried using a curved spring-steel "spider", (which would also minimize diffraction spikes) but still...


Edited by Thandal, 29 August 2019 - 09:57 AM.


#50 Rouzbeh

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Posted 29 August 2019 - 10:21 AM

That's what I was thinking.  A 450mm f3 mirror means a huge lightcone to then be bounced out the side to the ocular.  I calculate it would need about a 90mm (3.5") secondary (presuming the 225mm radius primary + 50mm for a low-profile focuser.)

 

I guess the weight of such a secondary could be carried using a curved spring-steel "spider", (which would also minimize diffraction spikes) but still...

 

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).




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