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How to calculate Maximum Field of View in c6 SCT

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#1 chipotle mg

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Posted 16 June 2015 - 09:46 AM

Can someone tell me what the  maximum telescope field of view without vignetting I can get with a celestron C6-a.   I understand that the max field of view can be limited by the telescope if the eyepiece has a low enough magnification and wide enough apparent field of view. 

 

The reason I am asking is I want to get 1 eyepiece that has the widest field of view for locating an initial object of interest, then use higher powered eyepieces to magnify a smaller area of the selected sky. 

 

Would it be a 40mm or a 32mm per say with what AFOV and how did you get that calculation?

 

My knowledge is limited and I appreciate your help and responses. Thanks



#2 havasman

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Posted 16 June 2015 - 10:46 AM

True Field Of View = EP field stop diameter / Scope Focal Length x 57.3

 

In your scope

A 40mm Plossl @ 43o Apparent FOV yields     38x   4.1mm exit pupil  69 arc' TFOV

A 32mm Plossl @ 50o AFOV yields                   47x  3.3mm exit pupil  64 arc' TFOV

A 24mm 68o eyepiece yields                              63x  2.4mm exit pupil   65 arc' TFOV

 

Smaller exit pupils yield darker background skies.

 

EP field stop diameters can be found here: http://www.cloudynig...e-buyers-guide/


Edited by havasman, 16 June 2015 - 10:46 AM.

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#3 Brian Carter

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Posted 16 June 2015 - 10:54 AM

I have a 24mm 68* eyepiece (the ES68) that I use in my C6, it does have a bit of vignetting at the edge of field.  I can't see it unless I defocus the star a little bit and see that half of it is chopped off, but that means there's only a little bit of dimming towards the edge.  But unless I'm really looking for it I don't notice it.  In my higher power EPs (14mm and lower), I don't see any vignetting at all.

 

I have even used that 24mm along with a focal reducer in my C6.  In that case there is quite a bit of vignetting, but it still isn't awful enough to be bothersome.  I think that people hear about vignetting in SCTs and worry that it is the worst thing in the world, but it is really overstated.  It would be a big deal if you were trying to study a faint fuzzy at the field stop, but no one would do that.  And even if you vignette to the point where you lose half the light, that's only about a magnitude and not significant since it is part of the peripheral framing of the object.



#4 scott_larry_d

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Posted 16 June 2015 - 11:25 AM

True field of view(TFOV) is approximately equal to the apparent field of view(AFOV) of an eyepiece divided by the magnification of the eyepiece/telescope combination(magnification=focal length of telescope/focal length of eyepiece). The two things controlling your TFOV are the telescopes focal length(longer focal length=smaller TFOV) and the eyepieces field stop diameter(larger field stop=larger field of view).

 

Or you can try one of the many eyepiece calculators online such as: http://televue.com/e...LCULATE#results

 

The 24mm Panoptic or 24mm Explore Scientific are both good eyepieces for your purpose. A 32mm plossl would be cheaper and provide the same TFOV. I've found the long eye relief on a 40mm plossl causes blackouts for me but YMMV. Try before you buy if at all possible and don't be afraid of the used market.

 

Have fun eyepiece shopping!

 

 



#5 Eddgie

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Posted 16 June 2015 - 12:15 PM

Can someone tell me what the  maximum telescope field of view without vignetting I can get with a celestron C6-a.   I understand that the max field of view can be limited by the telescope if the eyepiece has a low enough magnification and wide enough apparent field of view. 

 

The reason I am asking is I want to get 1 eyepiece that has the widest field of view for locating an initial object of interest, then use higher powered eyepieces to magnify a smaller area of the selected sky. 

 

Would it be a 40mm or a 32mm per say with what AFOV and how did you get that calculation?

 

My knowledge is limited and I appreciate your help and responses. Thanks

 

If you want the largest possible true field in the 1.25" eyepiece format, then look for the eyepeice with the biggest field stops.

 

The 24mm Panoptic used to be advertised as having the widest possible true field in a 1.25" format, but there are wider field choices.

 

The 35mm Ultrascopic (Also sold as Ultima or Parks, but all about the same eyepiece) has a 29mm field stop.  This gives it about the widest possible field of view you can get in 1.25" format and not really see any vignetting.  It is there, it is just very difficult to see.

 

The Hyperion 24mm has a 28mm field stop.   It is not the sharpest eyepiece off axis but it is not to bad.  

The ES 24 has a 27.2mm field stop, so it offers a tiny tiny but larger true field than a 24mm Panoptic, has similar off axis sharpness, and costs less new than many 24mm Panotics sell for used.

 

I think a lot of people today would rather use an eyepiece like the 24mm Pan or ES and give up a tiny amount of true field for the benefit of having a sharper off axis view.

 

But the 35mm Ultrascopic gives the widest field possible of all the 1.25" eyepecies I have owned, and while the design does indeed vignette, you won't see it at all when looking though it.  This is because the vignetting source is so far away from the focal plane that it is waaaaay out of focus.   And the dimming imposed is simply not enough to see.  It does though have very long eye relief that makes the view hard to hold, but like the 28mm RKE, gives the effect that the stars are floating in the eyepiece opening.

 

The 24mm Hyperion is a nice comprise between price, true field, exit pupil and apparent field.  Cheap too.    



#6 chipotle mg

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Posted 16 June 2015 - 04:33 PM

havasman, brian c, scott larry, eddgie,

 

Thanks so much.  I have to re-read what you wrote and let it sink in for a bit.

 

I was wondering if I could attach a 2" diagonal onto the c6 and get a wider field of view with a 2" eyepiece.  Is that possible  or would the scope just show the same field of view with the 2" eyepiece as it does with the 1.25" eyepiece?


Edited by chipotle mg, 16 June 2015 - 04:33 PM.


#7 Bill Barlow

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Posted 16 June 2015 - 05:41 PM

If you use a 2" eyepiece in a C6, then there will be some light loss/vignetting on the outer portion of the view since the rear cell opening on a C6 is only about 26mm.  The field stops in long focal length eyepieces like a 35mm or 41mm Panoptic are much larger than 26mm, so the light coming through the baffle tube won't illuminate the entire FOV of these eyepieces.  But for some observers, this light loss isn't too much of a problem. 

 

Bill



#8 Jon Isaacs

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Posted 17 June 2015 - 07:02 PM

If you use a 2" eyepiece in a C6, then there will be some light loss/vignetting on the outer portion of the view since the rear cell opening on a C6 is only about 26mm.  The field stops in long focal length eyepieces like a 35mm or 41mm Panoptic are much larger than 26mm, so the light coming through the baffle tube won't illuminate the entire FOV of these eyepieces.  But for some observers, this light loss isn't too much of a problem. 

 

Bill

 

As Bill says, the rear port of a C-6 is about 26mm in Diameter, the field stop of the 35mm Panoptic is 38.7 mm, there is definitely going to be some vignetting.  If the rear port were in focus, then it would limit the field of view with a sharp cut off.  But it is way of focus and it actually works surprisingly well, I became a believer when I used the 31mm Nagler and the 35mm Panoptic in my C-5.. You still won't get the field of view afforded by fast Newtonian but it's a real step up from a 1.25 inch eyepiece.

 

Jon



#9 chipotle mg

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Posted 18 June 2015 - 12:38 AM

Jon & Bill Thanks for the input.

 

I understand tele vue to have the best quality but also higher prices.  I am also getting the idea that 32mm plossl design might be the sweet spot for my scope.

 

Would you guys consider  this 32mm plossl from high point scientific?

 

http://www.highpoint...high-point-pl30

 

would it be a good "finder eyepiece" to start viewing with after using the real finder, but before switching to higher magnification eyepieces?



#10 Eddgie

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Posted 18 June 2015 - 07:50 AM

havasman, brian c, scott larry, eddgie,

 

Thanks so much.  I have to re-read what you wrote and let it sink in for a bit.

 

I was wondering if I could attach a 2" diagonal onto the c6 and get a wider field of view with a 2" eyepiece.  Is that possible  or would the scope just show the same field of view with the 2" eyepiece as it does with the 1.25" eyepiece?

 

 

A typical 2" Visual back and 2" diagonal will raise the focal length of the scope quite a bit.  The scope is only reallly 1500mm when you use the factory visual back and factory 1.25" prism.   It  is more like 1600mm if you use 2".

 

What no one has mentioned yet is the use of a focal reducer, which changes the scope to about 1000mm, and this will work without vignetting.

 

My recommendation would be the f/6.3 focal reducer, a 1.25" prism diagonal, an ES 24mm 68 degree eyepecie, and a Baader Zoom eyepiece.

 

Now you have a very nice, wide well illuminated low power field and a very nice range of magnifications for general use, and if you take off the focal reducer, the zoom makes a great planetary eyepiece because you can adjust the magnification very exactly for seeing .



#11 Brian Carter

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Posted 18 June 2015 - 04:13 PM


havasman, brian c, scott larry, eddgie,

Thanks so much. I have to re-read what you wrote and let it sink in for a bit.

I was wondering if I could attach a 2" diagonal onto the c6 and get a wider field of view with a 2" eyepiece. Is that possible or would the scope just show the same field of view with the 2" eyepiece as it does with the 1.25" eyepiece?



A typical 2" Visual back and 2" diagonal will raise the focal length of the scope quite a bit. The scope is only reallly 1500mm when you use the factory visual back and factory 1.25" prism. It is more like 1600mm if you use 2".

What no one has mentioned yet is the use of a focal reducer, which changes the scope to about 1000mm, and this will work without vignetting.

My recommendation would be the f/6.3 focal reducer, a 1.25" prism diagonal, an ES 24mm 68 degree eyepecie, and a Baader Zoom eyepiece.

Now you have a very nice, wide well illuminated low power field and a very nice range of magnifications for general use, and if you take off the focal reducer, the zoom makes a great planetary eyepiece because you can adjust the magnification very exactly for seeing .

I mentioned it :)

I use a Meade FR on my C6 and like it when I take it out for dark sky viewing. It makes a nice compliment to my dob with the 2-meter FL. But it definitely does vignette a bit with my 24ES68. In focus, it is not really noticeable unless I'm looking for it.

I think that's a better use of money than a 2" diagonal and more eyepieces... That's just me though.

#12 BillShakes

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Posted 23 June 2015 - 08:52 AM

chipotle_mg, for your purposes, a widest FOV for spotting objects with a switch then to higher mags, the 32mm plossl is perfect and low cost.  I've used the Celestron Omni 32mm for this purpose and it works very well.  If I get the RDF well aligned, I can usually find the object in my FOV with the 32mm.  Also this eyepiece gives nice views of some of the tighter clusters such as M36/37/38.  I found some of the cheaper 32mm plossls gave me trouble with annoying "kidney beaning" - a little difficult to hold my eye in position to get a good view.  I found the Omni 32mm did better on that point.  Everyone's eyes are different however and your experience may be different as well.

 

The 24mm ES is also outstanding and, due to its smaller exit pupil vs the 32mm should deliver slightly better contrast.  But for just locating objects, seems overkill.  

 

Try the 32mm plossl because it is relatively risk free - you can find used ones here on CN quite regularly and resell if you don't like it or when you upgrade to an ES68*

 

Good Luck and Clear Skies!

 

BS



#13 tomykay12

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Posted 23 June 2015 - 11:08 AM

I have a GSO 32mm superplossl, and while I haven't tried it in a C6, it gives nice views in an 8" sct, especially for the price. It is currently my lowest powered EP giving 62.5x in this scope. A little lower power would be nice and a focal reducer sounds like an excellent solution.



#14 Jan Owen

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Posted 23 June 2015 - 09:21 PM

This combination will pretty well maximize the field of view with a C-6 without significant vignetting...  It's a Celestron f/6.3 reducer/corrector, some Baader spacers to obtain the correct back focus, a 1.25" Baader T-2 prism diagonal, some more spacers, the Baader 1.25" Baader Click-Lock eyepiece holder, and a 24 mm TV Panoptic.  I can get to about the same field with a 2" Antares diagonal that uses a short throw SCT input collar much like the 1.25" Baader unit, and a 20 mm T-2 Nagler, but this configuration is a little longer in the back focus department than I prefer, so I almost always go with the pictured setup.  Pinpoint stars edge to edge, too. 

 

IMG_6848aa (1365 x 1024).jpg


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#15 Jan Owen

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Posted 23 June 2015 - 09:57 PM

Here's a very similar field-producer, but using 2" components.  It's an Antares 2" short-throw SCT mirror diagonal in conjunction with the Celestron f/6.3 reducer/corrector, and using a 20 mm T-2 Nagler.  The field is virtually the same as the 1.25" setup above, but this setup is a lot heavier, if that matters. 

 

There's also more than an ideal amount of back focus (but theory hasn't worked well for me in developing these setups; much trial & error has), though the images are still pinpoint edge to edge with this (or even with the 35 mm Panoptic), and any vignetting/aperture loss isn't visually apparent with either eyepiece... 

 

In the end, I usually use the 1.25" setup, even though the alternative works well with *MY* C-6...  For a closer look, click on either image...

 

IMG_6315 (1408 x 1056).jpg


Edited by Jan Owen, 23 June 2015 - 10:01 PM.


#16 Jon Isaacs

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Posted 24 June 2015 - 09:46 AM

Here's a very similar field-producer, but using 2" components.  It's an Antares 2" short-throw SCT mirror diagonal in conjunction with the Celestron f/6.3 reducer/corrector, and using a 20 mm T-2 Nagler.  The field is virtually the same as the 1.25" setup above, but this setup is a lot heavier, if that matters. 

 

There's also more than an ideal amount of back focus (but theory hasn't worked well for me in developing these setups; much trial & error has), though the images are still pinpoint edge to edge with this (or even with the 35 mm Panoptic), and any vignetting/aperture loss isn't visually apparent with either eyepiece... 

 

In the end, I usually use the 1.25" setup, even though the alternative works well with *MY* C-6...  For a closer look, click on either image.

 

Jan:

 

Have you tried eyepieces like the 31 mm Nagler or the 35 mm Panoptic with the focal reducer? I see people say the do it but the vignetting would seem to be horrible. How about those eyepieces withou the focal reducer?

 

I tried them with my C5 which has the same sized rear port as the C6.. The vignetting was not the issues, the problems was the SCTs field curvature.

 

Jon



#17 Jan Owen

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Posted 24 June 2015 - 10:27 AM

Hi, Jon!

I mentioned above that the 35 mm Panoptic works fine visually (that there is certainly vignetting, but it's not visually obvious). I haven't used a 31 mm Nagler with this setup, though, so can't talk to that... Given the extra field in the largest Nagler, though, I'd expect that you'd be getting to a point where it might be visible, and the field might also be so wide that it would be beyond the ability of the reducer/corrector to provide enough correction to fully suppress the field curvature at the extreme edge...

Of course, from my perspective, knowing what I know now, I'd see no reason to use the 2" route, with all the extra weight hanging off the back, when I can get there with equal performance, and same field, using the Baader diagonal & spacers, and the 1.25" 24 mm Panoptic... And since I also have the 19 mm Panoptic, and 16 mm Nagler, I have other equally sharp options in the wide field range as well (and if I want to go to a lower magnification beyond the 20 mm T-2 Nagler in the 2" size, I also have the 26 mm Nagler... Heheheh!!! But it has a field stop considerably wider than the C-6 rear port diameter, like the 35 mm Nagler does, so it isn't exactly a match made in heaven.

Edited by Jan Owen, 24 June 2015 - 10:41 AM.

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#18 chipotle mg

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Posted 24 June 2015 - 11:27 AM

Guys thanks for the responses.   Jan, if money was not an issue , then your set up would be uber ideal.  However, if money was not an issue, then a bigger better telescope would be more ideal.  I see you have been in the hobby for awhile and have lots of eyepieces and telescopes.   Your expereince and information is definitly appreciated.  I get the idea that there is more than one way to go about this, but Jan's setup with the 1.25" setup would be the best, albeit more expensive.

 

 

I have a few questions regarding the baader angle.  Is it correct image or upside down?  Can you really tell the difference in using the baader angle vs the stock angle that came with the scope (which is upside down image).    
 

Also, with my loose calcualations it seems a 32mm plossl  with 27mm field stop (generic or televue) with a .63 focal reducer/correcter would give 1.6 degree field.  The 24mm panoptic from televue also has a 27mm field stop as per thier website.  and would also yield a ~1.6 degree field with the .63 focal reducer.   The difference being the magnification in the 24mm panoptic would be greater than the 32mm plossl and perhaps the image would be shaper? in the panoptic due to the quality of the lenes. 

 

I'm leaning towards getting the focal reducer 1st, then considering the 32mm generic plossl (something along the lines of what bill and tom suggested).  Or should I go for qaulity and get the focal reducer and the 24mm panoptic?   The panoptic seems to cost 300 dollars and the generic 32mm plossl seems to cost 30-40 bucks.   a 10x fold difference.   

 

Thanks to Jan I'm no longer considering the 2" route.


Edited by chipotle mg, 24 June 2015 - 11:29 AM.


#19 Jan Owen

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Posted 24 June 2015 - 12:28 PM

My (Baader setup) objective was a little different than yours, which is why I ended up using a more expensive approach (not because I wanted to spend more money, but because I needed to be able to *tune* the back focus). 

 

My objective was to optimize the wide field performance using the Celestron f/6.3 reducer/corrector to obtain the widest field possible in a 1.25" format in that scope, while at the same time using all the correction available from the reducer/corrector at the optimum back focus to deliver the widest possible aberration-free visual field.

 

Getting to the widest possible field with improved correction is easy.  Just use the Celestron reducer/corrector, the standard 1.25" Celestron visual back, the standard Celestron 1.25" star diagonal, and a 32 mm Plossl.  Done. 

 

But, wringing out the highest level of performance available from the Celestron reducer/corrector, in conjunction with the C-6 optical train, requires optimizing the back focus distance...  And to do that requires something like the Baader T-2 system, so that you can add or subtract spacers until you reach the perfect match between the eyepiece correction, the r/c correction, and the telescope's system...  And that may vary somewhat between various sets of accessories, and from scope to scope, as a result of normal manufacturing tolerance variations...  So, it was to enable that degree of tuning capability that brought me to incorporate those extra-cost items...  And I'm quite happy with the results.

 

But for many folks, this level of optimization may not be necessary to meet personal needs, and the amount of trial & error experimenting to achieve a higher level of performance may not deliver enough improvement to justify the work to get there, given your personal perspective on the level of performance that's important to YOU...  And you are the only person that counts in this equation...  I've met my objectives, and I hope what I've shared can help get you pointed closely enough to the right place to let you select what's best for you!!!  Enjoy!!!


Edited by Jan Owen, 24 June 2015 - 12:51 PM.

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#20 n516na

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Posted 27 December 2017 - 03:32 PM

Hi everyone,

 

I am just starting out in astronomy and this is my first post here.  I have been following the posts here for a couple of months.

 

I would like to bring back this old topic.  I also have a C6 with a TV 32mm Plossl.  I thought that was just about the maximum field of view I could get with the C6 due to the opening in the tube. If I add the 0.63 focal reducer will the field of view with the 32mm Plossl increase by (1/.63) or will it remain the same because the limiting factor is the 26mm rear port in the C6?  Would there be any benefit to adding the reducer other than the flatter field?

 

Thanks

Nikolaos



#21 mclewis1

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Posted 27 December 2017 - 03:45 PM

Nikolaos,

 

Yes, with the focal reducer you will see a wider field of view ... but you will also have a some edge darkening (vignetting) where objects towards the edge will not be fully illuminated. Will you be able to notice this or be bothered by this? Maybe, but it's not something that immediately jumps out at you (at least not unless you are specifically looking for it).

 

I use the f6.3 SCT reducer on my C6 quite often with a 32mm Plossl eyepiece. I also have 2" diagonals and eyepieces that I could use instead, but I think 1.25" eyepieces with and without the reducer make more sense on the C6. If I really want a big wide field I use a smaller refractor instead. 

 

I like the quality of the field with the reducer/corrector in place and I use it quite often with my C6.


Edited by mclewis1, 27 December 2017 - 03:46 PM.


#22 n516na

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Posted 28 December 2017 - 12:59 AM

Thanks Mark



#23 MrJones

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Posted 29 December 2017 - 10:39 PM

So, the illumination is not even because you have 2 mirrors. The question becomes what is the max TFOV with 90% illumination or such, along with the changing focal length so use at least 1600mm. As pointed out above the baffle is not in focus and wider TFOV than you might expect can be used. Many have said 1.3 deg and others have said up to 1.7 deg with the C6. People I trust have said the Nagler 26T5 provides a maximum useful TFOV, I've seen this myself, and at 1600mm this is 1.3 deg TFOV. So this is what you want to be thinking as the maximum for the C6 - 1600mm and 1.3 TFOV.




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