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CFF Classical cassegrains - Any star test available?

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#76 salico

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Posted 06 December 2019 - 07:32 AM

I checked for Coma in my Mewlon 180 C and Vixen LVW 42 EP and wondered: This little distortion annoys people? Must check it in the N22 and N12, but it should be even less...Maybe I m not sensitive enough for this kind of Coma  or know that the FC in my BTs is worse;-)


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#77 Alterf

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Posted 06 December 2019 - 07:40 AM

I checked for Coma in my Mewlon 180 C and Vixen LVW 42 EP and wondered: This little distortion annoys people? Must check it in the N22 and N12, but it should be even less...Maybe I m not sensitive enough for this kind of Coma  or know that the FC in my BTs is worse;-)

I have exactly the same reaction when looking through my CFF 290mm f/13.5 with a Meade 55mm Plossl.  This bothers people?  I can hardly tell, and I hardly ever use such a wide eyepiece; the scope really is meant for narrower views.  Usually, I use a minimum mag of 122x.  I don't notice coma in the Classical Cass any more than I notice it in my 6" f/8 Newt.  It's not an issue really in either scope.  FC in my 80ED refractor bothers me, however.

 

Val


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#78 TG

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Posted 06 December 2019 - 01:25 PM

I have exactly the same reaction when looking through my CFF 290mm f/13.5 with a Meade 55mm Plossl. This bothers people? I can hardly tell, and I hardly ever use such a wide eyepiece; the scope really is meant for narrower views. Usually, I use a minimum mag of 122x. I don't notice coma in the Classical Cass any more than I notice it in my 6" f/8 Newt. It's not an issue really in either scope. FC in my 80ED refractor bothers me, however.

Val


Val, see my post above. CCs have far less coma than D-Ks. The saving grace for Cassegrains is the typically narrow field. Even with our widest eyepieces, a fully illuminated field is rarely wider than 0.5°. Both the narrow field and the illumination drop off result in coma not being seen. If you want to examine things critically, wait for a night of excellent seeing and observation a star at high power. With a D-K it's easy to notice asymmetrical diffraction rings away from field center. I would be interested in finding out what CCs show. I can simulate it but actual visual reports would be great to confirm it.

Tanveer

#79 Alterf

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Posted 06 December 2019 - 02:12 PM

Hi, Tanveer,

 

I can see coma in my own images, actually, even though my field of view is very small (a small camera chip and imaging at >2300mm).  Take a look at this one, for instance: https://polarisb.blo...6-may-2018.html .  It's just quite slight, and it takes a stack of 12-minute exposures to reveal it, but it's fairly obvious.  I can't see it in the eyepiece at all of course, for reasons you mention.  Don't have a D-K handy to compare.

 

Val


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#80 TG

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Posted 06 December 2019 - 06:09 PM

Val, do you use a field flattener? The "coma" in an imaging CC is a mixture of defocus and actual coma + astigmatism. See this post (and following) for what I mean.

https://www.cloudyni...k/#entry9772316

#81 macdonjh

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Posted 06 December 2019 - 06:33 PM

 

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.  

Sorry to quote my own post, but I forgot to edit it to add an image as I intended to do:

 

Parks 3.JPG


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#82 macdonjh

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Posted 06 December 2019 - 06:48 PM

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 µ-250 keeps me happy when I'm in the mood to use a Cassegrain.

 

Tanveer.

I think I knew Dall-Kirkhams have more coma than classical Cassegrains.  My point was: I think CFF is defaulting to producing telescopes with minimal coma rather than faster optics (not that f/15 is fast, just faster than f/20).  As I said, when asked, Catalin was happy to make a corrected system for me which would have been f/11, and did make a native f/15 scope for me, even though the standard offering is f/17.  So CFF is just doing the reverse of what you're suggesting: slow by default, fast(er) by request.

 

I don't know how anyone could be unhappy with a Mewlon.  smile.gif   I'll bet what ever mount you use with that would support a classical Cassegrain from CFF.

 

I checked for Coma in my Mewlon 180 C and Vixen LVW 42 EP and wondered: This little distortion annoys people? Must check it in the N22 and N12, but it should be even less...Maybe I m not sensitive enough for this kind of Coma  or know that the FC in my BTs is worse;-)

I agree with both you and TG, I don't notice coma in my classical Cassegrains, either.  I hope I don't ever go looking for it, either.  From what I've heard, once seen it cannot be unseen.  I came quite close to buying a Mewlon 210 when I was building this scope:

 

Driveway scope 1.jpg

 

Coincidentally, it is built around a classical Cassegrain mirror set made by Tavi and Catalin...


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#83 Daniel Mounsey

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Posted 06 December 2019 - 09:03 PM

If I were worried about some inherent coma then I would just get a corrected mass produced Cat for a fraction of the cost and use it for low magnification views. That doesn't tell anybody how good an optical figure is. 

 



#84 macdonjh

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Posted 06 December 2019 - 11:14 PM

If I were worried about some inherent coma then I would just get a corrected mass produced Cat for a fraction of the cost and use it for low magnification views. That doesn't tell anybody how good an optical figure is. 

I thought about that, too.  But decided I wanted to be the only kid on my block with a 14" classical Cassegrain.  There are lots of C14s around.


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#85 Alterf

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Posted 07 December 2019 - 12:07 AM

Val, do you use a field flattener?

No, but I do use an Astro-Physics CCDT67 Telecompressor.



#86 Reid W

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Posted 08 December 2019 - 07:35 PM

I checked for Coma in my Mewlon 180 C and Vixen LVW 42 EP and wondered: This little distortion annoys people? Must check it in the N22 and N12, but it should be even less...Maybe I m not sensitive enough for this kind of Coma  or know that the FC in my BTs is worse;-)

I just picked up a u210....

 

Out of focus, the coma is easily seen....but in focus, the stars are such specks, I cannot see see it.



#87 ZX12

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Posted 10 December 2019 - 06:05 PM

Hello Vincenzo,

 

The conditions have been poor in my area the last few weeks, so attempts to get any images of a defocused star with the CFF250 have been equally as bad.

 

I did manage a decent shot of the Trapezium despite the conditions.

 

Mike 

Attached Thumbnails

  • Trapezium CFF250.jpg

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#88 Vinny1980

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Posted 11 December 2019 - 01:40 AM

Hello Vincenzo,

 

The conditions have been poor in my area the last few weeks, so attempts to get any images of a defocused star with the CFF250 have been equally as bad.

 

Hi back Mike,

 

thanks for your attempt. 

The defocused star don't require mandatorily steady nights (while the in-focus do). My advice is to give it a try, unless the seeing is really awful. 



#89 ZX12

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Posted 11 December 2019 - 10:35 AM

Vincenzo,

 

Here are some photos through the CFF250 during the last session.

 

Mike

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  • CFF.jpg
  • CFF4.jpg
  • CFF3.jpg

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#90 davidmcgo

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Posted 11 December 2019 - 10:50 AM

Those look really good!

 

Dave



#91 Vinny1980

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Posted 11 December 2019 - 12:28 PM

The star is a little too small to judge conclusively, but Id say the figure dont' show bad aberrations. I cannot see worrying amount of spherical aberration (the secondary shadow seems to have same size on both sides of focus, there are no bright rings on one side). There's some astigmatism (the star looks egg-shaped, rotating 90° from one side of focus to another), accentuated by the sampling you have used, but it is not necessarily in the glass (thermals, mechanical tensions).

If you will reiterate the test, try not using the whole surface of the chip (i.e. activate ROI), defocus a little more and/or use a camera with smaller pixels.


Edited by Vinny1980, 11 December 2019 - 03:58 PM.

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#92 RichA

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Posted 11 December 2019 - 02:23 PM

Hello Vincenzo,

 

The conditions have been poor in my area the last few weeks, so attempts to get any images of a defocused star with the CFF250 have been equally as bad.

 

I did manage a decent shot of the Trapezium despite the conditions.

 

Mike 

Great test object.  Smallest scope I saw all six stars with (under urban conditions) was a 94mm Vernonscope (AP lens).


Edited by RichA, 11 December 2019 - 02:24 PM.

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#93 ZX12

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Posted 11 December 2019 - 02:48 PM

Great test object.  Smallest scope I saw all six stars with (under urban conditions) was a 94mm Vernonscope (AP lens).

Yes, I agree the Trapezium is a great test object visually. It's interesting to see how low of a magnification you can use and still see the E and F stars consistently. 

 

The CFF250 makes it very easy, but the lowest power I currently have is 91x with the 41 Pan.

 

My APM 150 binoculars have performed this down to 66x. while at 47x I have seen the E star but not the F.

 

Mike


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#94 Asbytec

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Posted 11 December 2019 - 06:29 PM

The star is a little too small to judge conclusively, but Id say the figure dont' show bad aberrations. I cannot see worrying amount of spherical aberration (the secondary shadow seems to have same size on both sides of focus, there are no bright rings on one side). There's some astigmatism (the star looks egg-shaped, rotating 90° from one side of focus to another), accentuated by the sampling you have used, but it is not necessarily in the glass (thermals, mechanical tensions).

If you will reiterate the test, try not using the whole surface of the chip (i.e. activate ROI), defocus a little more and/or use a camera with smaller pixels.

Yea, a bit of astigmatism. Was the star centered in the FOV? One possibility is the center of the FOV is toward upper right in the images, if the first image above is inside focus, showing sagittal astigmatism consistent with the image being inside and near tangential focus. If so, nothing to worry about. If the star is on axis, then need to look for another cause as you say.  

 

Which image is inside and which is outside? We need to be sure they are at the same defocus. Presuming so, then I agree the shadow does not look worrying. However, the overall size of the images are different suggesting something going on in the outer zone(s). If the images are the same defocus inside and out, with the first image being inside focus, then it may have a bit of over correction at the edge (or under corrected at the edge if the images are reversed). The innermost ring in the first image looks a little better defined and brighter suggesting the center is focusing short (or long if outside focus). In the second smaller image, the outer ring is a tad better defined and a bit brighter and being consistent with the reverse of the first image. It could be the second smaller image is not quite at the same defocus, too.

 

That's only if the images are the same defocus. If not, then all bets are off until they are. And if not at the same defocus, then a bit more defocus will show a larger second image and a change in the shadow sizes suggesting come correction error at the center. 


Edited by Asbytec, 11 December 2019 - 06:33 PM.


#95 ZX12

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Posted 11 December 2019 - 09:26 PM

The previous images were examples of different defocus to see what is Vincenzo would like to see.

 

I took some more images tonight using Capella with slightly more defocus. I noticed the collimation was slightly off, but it was too cold to attempt perfecting it.

 

The first image is inside focus. 

 

Mike

Attached Thumbnails

  • CFF250 intra.jpg
  • CFF250 extra.jpg

Edited by ZX12, 11 December 2019 - 11:31 PM.

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#96 ZX12

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Posted 11 December 2019 - 09:37 PM

Defocused to the in focus limit.

 

Note blue dot near center.

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  • cff.jpg


#97 Asbytec

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Posted 11 December 2019 - 10:36 PM

The previous images were examples of different defocus to see what is Vincenzo would like to see.

 

I took some more images tonight using Capella with slightly more defocus. I noticed the collimation was slightly off, but it was too cold to attempt perfecting it.

 

The first image is intra focus. 

 

Mike

Thanks, Mike. These images are well beyond 10 waves and lose sensitivity to aberration. But, that far out is good for looking for smoothness and zones, but it's inconclusive to distinguish between seeing and thermals. 



#98 Vinny1980

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Posted 12 December 2019 - 12:42 AM

Mike,

interesting set of images, thank you!

I understood the first images were at different defocus. Of course I presumed the star was on the axis or very close to.

Astigmatism shouldn't be into the inherent aberrations of Cassegrain, which has "only"  coma and curvature of field. So, if it appears, you have some problem. Understanding where could be tricky because astigmatism has several causes, anyway the amount I see is not that bad and many telescopes have some degree of it (including my 4" apochromat Takahashi FC-100D). Id rate it around lambda/4 or less, depending on exact defocus. 

That said, I d like to have some info about the gear. What is the camera you used? Are the images stacked or single frames?

The EF/IF images (coming they from stack or not), should not be stretched in postprocessing, and they need to be taken without changing exposure time, gain and the other settings. The gamma, which introduces non linearity, must be off (=1). Last, despite not strictly necessary, you can put a Green filter on to highlight the dark interference rings. 

To conclude: the first 2 monochromatic images look good, while the third in colour show a smooth surface with no obvious zonal errors. You see that blue dot every time?


Edited by Vinny1980, 12 December 2019 - 07:33 AM.


#99 Asbytec

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Posted 12 December 2019 - 01:39 AM

Vinny, yes, you're right about settings, exposure, etc. If the images are stacked, I'd think they'd give a more true indication of roughness after removing seeing. No real zones seen and smooth enough, it seems. However, to my eye, the two monochrome images as shown suggest less definition inside and more defined outside indicative of a bit of over correction generally. I'd not hazard a guess at PV, though, with different defocus. I do not think the blue dot is related to coma, pretty sure that's not the Poisson spot. Just an artifact. Otherwise the rings would be more skewed, too, and they are not skewed in the previous images. Okay, so a Classic Cass does not have inherent astigmatism but has field curvature. The two are related.

#100 Vinny1980

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Posted 12 December 2019 - 01:59 AM

 I do not think the blue dot is related to coma, pretty sure that's not the Poisson spot. Just an artifact. Otherwise the rings would be more skewed, too, and they are not skewed in the previous images.

If you look carefully or strech the image, you can see the tiny Poisson spot at the very center. 

 

I agree with your caution. Let's wait for other info from Mike. 

EDIT:

 

 

Astigmatism shouldn't be into the inherent aberrations of Cassegrain, which has "only"  coma and curvature of field

I cited by heart from Rutten&Van Venroij Telescope Optics, which i cannot consult now.  But this is is not necessarily true. It is interesting argument to discuss but I dont want to go too much OT. 


Edited by Vinny1980, 12 December 2019 - 07:51 AM.

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