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FOV on a SW mak 127 for 32mm EP (32mm 1.25in vs 2in)

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#1 Desc82

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Posted 19 January 2020 - 07:48 PM

Hello All,

 

I'm new here. After many years reading your posts, I finally decided to create an account on CN.

 

So here it is. I'm the recent owner of a SW mak 127 (FL 1500, f/12). It came with a 2in diagonal. I also have a 1.25in 32mm Celestron Omni plossl EP, which I can use with this scope using the 1.25in adapter. This EP has a AFOV of 50 deg. as far as I know. This EP was mainly used on my SW 130 reflector before the mak.

 

I'm not an expert, but I know that the f/ratio of this scope makes it limiting regarding FOV when using low power EP. So my question is the following: If I'm using a 2in EP, 32mm 70 deg. WA, will I be able to use the whole FOV, or I will be limited by some vignetting? The EP in question is the Orion Q70 32mm.

 

In short, would I gain any improvement with the Orion Q70 32mm (2in, 70 deg. AFOV) vs the Celestron Omni plossl 32mm (1.25in, 50 deg. AFOV) as far as FOV is concerned?

 

I know this may be tricky to answer based on theory/calculations. Anybody has experience with low power, WA EP on this scope?

 

Any help is appreciated. Thanks in advance.

 

Chris


Edited by Desc82, 19 January 2020 - 08:59 PM.


#2 Eddgie

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Posted 19 January 2020 - 08:58 PM

There will be vignetting, but you won't be able to actually see it.

The vignetting source is the front end of the primary baffle and with all SCTs and MCTs, this baffle vignettes the field of view.

There are two different aspects to vignetting: the fully illuminated field, the diameter of which is the unvignetted are of the field, and everything else, which is the vignetted part of the field.  The thing tough is when the light cone is this slow and the vignetting is quite far away, the vignetting is very "soft." it starts exactly at the edge of the fully illuminated field and gently gets worse to the edge of the field so that by the edge of the field, the brightness is typically about 75% of what it would be at the center of the field and in most cases, you may not even be aware of the vignetting.  Now I do not know your model but this is true of all SCTs and MCTs so while the exact size of the fully illuminted field is not known to us, you can assume it is about 10mm.   Also, I cannot tell you at what point you might start to see harder vignetting, but you would probably not notice it using something like a 35mm Panoptic.

 

So technically, the scope is vignetting even a 32mm eyepiece, but becuase it is very gentle, you simply can't see it, and it should not bother you if you go wider.  How wide?   Not sure. 

 



#3 Desc82

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Posted 19 January 2020 - 09:13 PM

There will be vignetting, but you won't be able to actually see it.

The vignetting source is the front end of the primary baffle and with all SCTs and MCTs, this baffle vignettes the field of view.

There are two different aspects to vignetting: the fully illuminated field, the diameter of which is the unvignetted are of the field, and everything else, which is the vignetted part of the field.  The thing tough is when the light cone is this slow and the vignetting is quite far away, the vignetting is very "soft." it starts exactly at the edge of the fully illuminated field and gently gets worse to the edge of the field so that by the edge of the field, the brightness is typically about 75% of what it would be at the center of the field and in most cases, you may not even be aware of the vignetting.  Now I do not know your model but this is true of all SCTs and MCTs so while the exact size of the fully illuminted field is not known to us, you can assume it is about 10mm.   Also, I cannot tell you at what point you might start to see harder vignetting, but you would probably not notice it using something like a 35mm Panoptic.

 

So technically, the scope is vignetting even a 32mm eyepiece, but becuase it is very gentle, you simply can't see it, and it should not bother you if you go wider.  How wide?   Not sure. 

Thank you for the answer. So would you recommend me to give it a try at 70 FOV? I mean, chances that I end up with significant vignetting, enough to make the view unpleasing, are small?



#4 charlesgeiger

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Posted 20 January 2020 - 12:54 AM

If it were me, I would simply borrow a 2" 32mm 70Degree eyepiece and try it.  You will have a larger field of view.  You might try to borrow a 35mm Panoptic.  If you go much higher (say 40mm), you might see the shadow of the secondary in your image.  

Charlie


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#5 Eddgie

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Posted 20 January 2020 - 08:47 AM

Thank you for the answer. So would you recommend me to give it a try at 70 FOV? I mean, chances that I end up with significant vignetting, enough to make the view unpleasing, are small?

Yes, I would.  

 

If it were incapable of doing a field much wider than 27mm field stop, then there would be no point in equiping it with a 2" focuser.

 

I recommended the 35mm Panoptic as a good choice for this scope because the field stop is 38mm. This is probably very safe in that I doubt that you would see any vignetting because I have used this on small MCTs and SCTs and felt that the vignetting was not enough to be bothersome, but I did not try something like a 41mm Panoptic, which has a 46mm field stop.  That might be pushing it.

 

If the rear baffle opening is 38mm though (same size as C8) then even the 41mm Panoptic would probably be OK.  I could just start to see some harder vignetting at the very very edge of the field of my C8s when using a 41mm Panoptic.  It was right at the very very edge, but there was enough extra field that it was worth it.

 

Once the field stop goes past the size of the rear baffle opening though, vignetting steepens considerably, but again, they eye is not very sensitive to it.

 

Where you will see it the most is if you live under very bright semi-urban or urban skies.  In this case, the field will develop kind of a doughnut profile with the outer part of the field showing as a graduated slightly darker gray circle while the center of the field is a brighter circle (which of course is what vignetting does).  I have had some scopes that showed this even with a 35mm Panoptic, so again, it depends a lot on the rear baffle opening size.  Going a few millimters past this size is not going to change the view much.   Now if the rear opening is 32mm and you try using a 46mm field stop, I think the vignetting would in that case be easier to detect. 

 

I can recommend the 35mm Panoptic though because I have used it on small MCTs and it was very hard to see the vignetting increase.   That does not meany that you won't though. It is kind of a personal thing. I am highly bothered by visible vignetting though. 



#6 Eric63

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Posted 20 January 2020 - 12:41 PM

I have a 2", 32mm 70 degree EP that I use with my 127Mak and I love the view.  From my experience  the vignetting is  barely noticeable, especially at night.  Also, you don't have to spend a fortune on an expensive 2" EP due to the flat field of the Mak.  A decent EP like the the Orion Q70 will work fine.  I have an Old Astro tech Titan EP and I love it in the Mak (not so much in my 6"F5 reflector - but great in the Mak).

 

Eric



#7 Hesiod

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Posted 20 January 2020 - 01:11 PM

In order to be able to use a 2" eyepiece, you will end up by increasing the focal length of the MCT.

I have measured such increase with my model and it is roughly around 200mm (in my case it goes from ca 1270 to ca 1500mm) so you should estimate the field of view on the basis of the actual, increased, focal.

Also, because of the increased focal length, the exit pupil will be slightly smaller too as that you would get with a 1.25" stardiagonal and a 32mm 1.25" eyepiece.



#8 Desc82

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Posted 20 January 2020 - 08:55 PM

Yes, I would.  

 

If it were incapable of doing a field much wider than 27mm field stop, then there would be no point in equiping it with a 2" focuser.

 

I recommended the 35mm Panoptic as a good choice for this scope because the field stop is 38mm. This is probably very safe in that I doubt that you would see any vignetting because I have used this on small MCTs and SCTs and felt that the vignetting was not enough to be bothersome, but I did not try something like a 41mm Panoptic, which has a 46mm field stop.  That might be pushing it.

 

If the rear baffle opening is 38mm though (same size as C8) then even the 41mm Panoptic would probably be OK.  I could just start to see some harder vignetting at the very very edge of the field of my C8s when using a 41mm Panoptic.  It was right at the very very edge, but there was enough extra field that it was worth it.

 

Once the field stop goes past the size of the rear baffle opening though, vignetting steepens considerably, but again, they eye is not very sensitive to it.

 

Where you will see it the most is if you live under very bright semi-urban or urban skies.  In this case, the field will develop kind of a doughnut profile with the outer part of the field showing as a graduated slightly darker gray circle while the center of the field is a brighter circle (which of course is what vignetting does).  I have had some scopes that showed this even with a 35mm Panoptic, so again, it depends a lot on the rear baffle opening size.  Going a few millimters past this size is not going to change the view much.   Now if the rear opening is 32mm and you try using a 46mm field stop, I think the vignetting would in that case be easier to detect. 

 

I can recommend the 35mm Panoptic though because I have used it on small MCTs and it was very hard to see the vignetting increase.   That does not meany that you won't though. It is kind of a personal thing. I am highly bothered by visible vignetting though. 

Indeed, I am subject to light pollution since I live in a suburban area, quite far from the city but still my sky isn't that dark. In fact I'm more concerned by vignetting since I intend to use my mak for terrestrial viewing as well. Therefore the vignetting effect would become very annoying.

 

As for the field stop, according to specification data I found on the web, the EP I'm looking at (Q70 32mm) has a field stop diameter of 40mm. I don't know how this is translated into AFOV (function of the magnification?) but judging by your post I guess this would be far enough.



#9 Desc82

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Posted 20 January 2020 - 09:10 PM

I have a 2", 32mm 70 degree EP that I use with my 127Mak and I love the view.  From my experience  the vignetting is  barely noticeable, especially at night.  Also, you don't have to spend a fortune on an expensive 2" EP due to the flat field of the Mak.  A decent EP like the the Orion Q70 will work fine.  I have an Old Astro tech Titan EP and I love it in the Mak (not so much in my 6"F5 reflector - but great in the Mak).

 

Eric

Glad to hear from someone who actually has experience using that exact scope / EP combination!

 

I also read reviews of people reporting off-axis coma and astigmatism using the Q70 on shorter focal length reflectors. But I suspected this to be attenuated at f/12. I'm now more confident after reading your post.

 

Just out of curiosity, have you ever tried to go wider and/or with EP with even less power? I know that scope is not designed for that (it is in opposition with its purpose), but I'm just wondering what is the limit case.

 

Regards,

 

Chris


Edited by Desc82, 20 January 2020 - 09:28 PM.


#10 whizbang

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Posted 20 January 2020 - 09:51 PM

My experience is the same as Eric's.  A Williams Optics 33mm 72deg eyepiece works great in my 127mm Mak.



#11 Jon Isaacs

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Posted 20 January 2020 - 10:30 PM

Now I do not know your model but this is true of all SCTs and MCTs so while the exact size of the fully illuminted field is not known to us, you can assume it is about 10mm.

 

 

Without knowing the rear port/rear baffle diameter, it difficult to really know much about the vignetting. Being a 5 inch scope, I'd think it was more like a C-5 (1 inch) than a C-8 (1.5 inch)

 

Jon



#12 Desc82

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Posted 20 January 2020 - 10:49 PM

Without knowing the rear port/rear baffle diameter, it difficult to really know much about the vignetting. Being a 5 inch scope, I'd think it was more like a C-5 (1 inch) than a C-8 (1.5 inch)

 

Jon

I'm not sure I get exactly what you are referring as a baffle, but I removed the diagonal and I measured the inner diameter of the rear port. It looks like 1.25in, is it possible?


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#13 SeattleScott

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Posted 21 January 2020 - 12:56 AM

No that’s not the baffle. The baffle tube is inside the scope. Generally 27mm unless bigger in the newer models. I wouldn’t go wider than about 32/33mm with 70 AFOV. Especially if you will be using it for terrestrial, where vignetting is more obvious.

I use a 42lvw with 46mm field stop in my 6” F12, but I have about a 33mm baffle tube.

Scott

#14 Jon Isaacs

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Posted 21 January 2020 - 01:50 AM

No that’s not the baffle. The baffle tube is inside the scope. Generally 27mm unless bigger in the newer models. I wouldn’t go wider than about 32/33mm with 70 AFOV. Especially if you will be using it for terrestrial, where vignetting is more obvious.

I use a 42lvw with 46mm field stop in my 6” F12, but I have about a 33mm baffle tube.

Scott

 

Regardless, it is a restriction in the optical path.

 

jon



#15 charlesgeiger

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Posted 21 January 2020 - 02:57 AM

I have used that 2" Orion 32mm 70 degree in my f/7 80mm apo and it gives poor edges meaning seagulls.  The eyepiece gives great throughput but is not designed for short focal length but I guess you have it covered with your MCT  On axis you should be good and you will have an approximately .2 degree wider field versus having an 1.25" 50 degree ocular.



#16 SeattleScott

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Posted 21 January 2020 - 12:39 PM

Regardless, it is a restriction in the optical path.

jon

By “inner diameter of the rear port” I took that to mean a 1.25” visual back, which would obviously need to be swapped out for a 2” if he wants to use a 2” diagonal and eyepiece. Of
course I don’t know what he was really looking at, and he doesn’t know the terminology to describe it well, so I could be wrong.

If the baffle tube is 32mm wide, that would be encouraging.

Scott

#17 Eric63

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Posted 21 January 2020 - 12:39 PM

Glad to hear from someone who actually has experience using that exact scope / EP combination!

 

I also read reviews of people reporting off-axis coma and astigmatism using the Q70 on shorter focal length reflectors. But I suspected this to be attenuated at f/12. I'm now more confident after reading your post.

 

Just out of curiosity, have you ever tried to go wider and/or with EP with even less power? I know that scope is not designed for that (it is in opposition with its purpose), but I'm just wondering what is the limit case.

 

Regards,

 

Chris

32 mm is about the lowest I have gone with that scope.  Many objects can still be well framed at that magnification, but as you know this is not a wide field scope.  If I want to do wide field observing I take out my reflector or my BT70-45 binoculars. 

 

Eric



#18 Desc82

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Posted 21 January 2020 - 01:06 PM

No that’s not the baffle. The baffle tube is inside the scope. Generally 27mm unless bigger in the newer models. I wouldn’t go wider than about 32/33mm with 70 AFOV. Especially if you will be using it for terrestrial, where vignetting is more obvious.

I use a 42lvw with 46mm field stop in my 6” F12, but I have about a 33mm baffle tube.

Scott

It was the second option (i should always trust my first idea). I tried to measure the inner diameter of that tube as well (not easy to go there without a caliper gauge), and it looked a bit over one inch, so 27mm would make sense.

 

Ok so I will consider 32mm 70 AFOV as being the lowest acceptable limit for the SW mak 127. 

 

Maybe a bit off topic, but regarding the highest magnification I can achieve with it, do you have a recommendation? On the SW website the maximum magnification is clearly stated at 250X. However after some testing (terrestrial viewing) with a 1.25in 15mm Plossl EP combined with a 2.5X GSO barlow (which gives an equivalent 6mm, or 250X I think), the image appeared much darker when compared with the 15mm Plossl alone (like 50%-75% less bright). So I guess the 50X per inch of aperture rule of thumb would not apply in case of a MCT? Or it is mainly because I used a barlow, adding lens into the light path? Or the GSO brand?

 

I would need to try with a 9mm EP to see what it looks like (don't have any at the moment). 



#19 SeattleScott

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Posted 21 January 2020 - 03:15 PM

Magnification decreases brightness with any telescope and eyepiece. You will only use 250x on bright targets like moon and planets, certain double stars. Most DSO are faint and look better at 100-150x through a 5” scope.

Scott

#20 Desc82

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Posted 21 January 2020 - 09:48 PM

Magnification decreases brightness with any telescope and eyepiece. You will only use 250x on bright targets like moon and planets, certain double stars. Most DSO are faint and look better at 100-150x through a 5” scope.

Scott

I didn't expect the phenomenon to be that significant on high power, for daylight use (considering daylight targets as relatively bright). I keep thinking the barlow doesn't help. But using a 6mm EP would probably reduce eye relief too much for my liking.


Edited by Desc82, 21 January 2020 - 09:55 PM.



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