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Focal reducer vs 2" eyepieces

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#1 hboswell

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Posted 28 January 2021 - 07:04 AM

If I have a .63 focal reducer for my 6se, is there any point in upgrading to a 2" visual back? From a bit of reading I've done it seems the answer is no, but I thought I'd check to see if I'm overlooking something.

 

Thanks,

Harry



#2 peta62

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Posted 28 January 2021 - 07:11 AM

In my opinion the question is what do you prefer. If you like no vignetting stay with 1.25", if you prefer to see wider regardless bit darker edges go with 2". I personally am the latter, I just love to see wide, but I understand the people who are stressing the quality edge to edge.



#3 mlord

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Posted 28 January 2021 - 07:15 AM

If I have a .63 focal reducer for my 6se, is there any point in upgrading to a 2" visual back?

IMHO with a 6se, there is never any point to upgrading to a 2" visual back, except possibly for a stronger connection to the diagonal, or perhaps simply so that one can acquire 2" accessories that will be moved to a larger scope someday.

 

To understand why, just unscrew the existing 1.25" visual back from the scope, and measure the diameter of the opening that remains on the back of the scope.. very small.

 

Cheers



#4 hboswell

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Posted 28 January 2021 - 07:19 AM

IMHO with a 6se, there is never any point to upgrading to a 2" visual back, except possibly for a stronger connection to the diagonal, or perhaps simply so that one can acquire 2" accessories that will be moved to a larger scope someday.

 

To understand why, just unscrew the existing 1.25" visual back from the scope, and measure the diameter of the opening that remains on the back of the scope.. very small.

 

Cheers

This is a factor, since my Starmaster has a 2" focuser. I could share eyepieces.  But it isn't that critical a factor.

 

Harry



#5 junomike

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Posted 28 January 2021 - 07:24 AM

IME I'd use either the Reducer + 1.25" EP's OR the 2" EP's but some have found success with shorter F/L 2" EP's.



#6 wrnchhead

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Posted 28 January 2021 - 09:02 AM

My C8 won't come to focus with the reducer and a 2" visual back and diagonal. And after reading about the effect it has on the correction of the scope and possible aperture loss, I have been content with 1.25 gear. This may be different on a C6



#7 cst4

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Posted 28 January 2021 - 09:12 AM

If you want to use the reducer then I suggest just staying with 1.25" format.  I believe the optimum backfocus for those .63 reducers is around 105mm which many 2" diagonal light paths exceed... add to that field stops in 2" eyepieces that are larger than the SCT baffle and vignetting might become very pronounced when using both a reducer and 2" eyepieces.  I have both a reducer and 2" visual back / diagonal... I really don't know why I've never tried to use them both to see what happens.  Anyway, the reducer provides a much flatter image which is very nice.  However, I still find myself choosing to leave the reducer off and use my 2" accessories because all my other scopes share these accessories.  Even with my widest pieces like my 42mm LVW I do not see any vignetting.  I believe 2" eyepieces can get you a tad wider than 1.25" eyepieces with focal reducer.  It's just a matter of personal preference which way you want to go to extend the scopes range to wider views.


Edited by cst4, 28 January 2021 - 09:14 AM.

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#8 rboe

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Posted 28 January 2021 - 11:17 AM

Not as cheap as the focal reducer but in the end it's the option I use - I use my faster refractor. I moved to refractors for their ease of use and wide field views. But two of my "larger" ones have a lot of weight at each end which brings up mount problems (solved with the Losmandy alt-az mount) so that brought me back to SCT's. Classic case of circular logic. :wron:

 

On the C11 and C8 I have 2" diagonals because I have some nice 2" eye pieces and they all get moved around between the scopes. The C6 came with a Baader 2" quick mount on the back so I got a 2" diagonal for it. I tried to remove it but it has a very very small set screw and I don't have an allen wrench that small to remove it. I think I know why it came with the scope. :)

 

The light cone for all three scope is made for a 1-1/4" eye piece so no advantage to using a 2" eye piece - except you get to use it if you have it. I will say the C11 looks better with a 2" diagonal that with that tiny 1-1/4" - but looks don't help the function. I have a Williams Optics binoviewer and it DOES help, mechanically, to mount the binoviewer on a 2" setup. Just feels more secure. Binoviewers were the bees knees at one time, but like bell bottoms, I've moved on.

 

And I do have a reducer. It gets used as much as the binoviewer.  They are not that expensive so I don't see any good reason to NOT get one. For me the problem is remembering I have it so I consider using it. :p



#9 peta62

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Posted 28 January 2021 - 11:33 AM

... C11 and C8 C6 ...

The light cone for all three scope is made for a 1-1/4" eye piece .....

Are you really sure about C11 ?

I have C9.25, the opening is definitely enough for 2". I do understand the difference between opening and light cone, but I take full frame pictures with it without problem.



#10 rboe

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Posted 28 January 2021 - 12:54 PM

Yes. The real limit is the internal baffle. This topic was beat to death back in the early days of the NexStar11GPS; a couple guys even went so far as to put a couple o-rings inside the baffle to help with contrast. But the whole light train was built for 1-1/4" eye pieces and works very well with them.

 

But as most of us know, you can use 2" eye pieces too. :shrug:

 

If you look at many of the TeleVue 2" Naglers, the skirt is 2", but the actual glass (and secondary skirt) is for 1-1/4".  I have this affinity for 2" 40mm eye pieces; some, like the University Optics 40 and 32mm 2" eye pieces are strickly 2", but the 1-1/4" versions work just as well - usually with less kidney beaning but lacking the huge picture window glass the 2" guys have. Which my eyes can't really take advantage off anyway. So they tend to be used on the refractors.

 

But hey, if you have the interest and money 2" is just fine. It's just my opinion that beginners could do better, in the beginning, to spend money elsewhere and save the 2" stuff for down the road if they are tempted to do so.


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#11 alphatripleplus

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Posted 28 January 2021 - 12:59 PM

Yes. The real limit is the internal baffle. This topic was beat to death back in the early days of the NexStar11GPS; a couple guys even went so far as to put a couple o-rings inside the baffle to help with contrast. But the whole light train was built for 1-1/4" eye pieces and works very well with them.

 

 

+1

 

The C6 and C5 both have the same 27mm internal baffle, compared with about 37mm for the C8.



#12 peta62

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Posted 28 January 2021 - 01:38 PM

Yes. The real limit is the internal baffle. This topic was beat to death back in the early days of the NexStar11GPS; a couple guys even went so far as to put a couple o-rings inside the baffle to help with contrast. But the whole light train was built for 1-1/4" eye pieces and works very well with them.

 

But as most of us know, you can use 2" eye pieces too. shrug.gif

 

If you look at many of the TeleVue 2" Naglers, the skirt is 2", but the actual glass (and secondary skirt) is for 1-1/4".  I have this affinity for 2" 40mm eye pieces; some, like the University Optics 40 and 32mm 2" eye pieces are strickly 2", but the 1-1/4" versions work just as well - usually with less kidney beaning but lacking the huge picture window glass the 2" guys have. Which my eyes can't really take advantage off anyway. So they tend to be used on the refractors.

 

But hey, if you have the interest and money 2" is just fine. It's just my opinion that beginners could do better, in the beginning, to spend money elsewhere and save the 2" stuff for down the road if they are tempted to do so.

Thank you for explanation. Maybe I am really blind towards vignetting, I like 2" where possible/reasonable. I knew even before, there is no way to use more than 1.25" due to optics laws for eyepieces with focal lengths below 20 mm. I have some longer focal length true 2" eyepieces and I do not see any vignetting, I see central obstruction on 50 mm.

 

+1

 

The C6 and C5 both have the same 27mm internal baffle, compared with about 37mm for the C8.

I asked about C11.



#13 DavidWasch

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Posted 30 January 2021 - 03:30 PM

The easiest way for me to work on the question is to start with the maximum FOV of the scope in native mode.

 

FOV=smallest baffle/focal length * 57.3

 

For the C6, it's a bit over a degree before vignetting starts to occur. My 1 1/4" Meade SWA 24mm provides almost exactly that field of view, so there's little benefit to having a 2" eyepiece or focal reducer IMO.

 

The focal reducer comes after the baffle, so it's not able to force a wider view. I don't see any benefit except for astrophotography or EAA.

 

A 2" eyepiece can't provide a wider view either, without vignetting. I could give myself a wider apparent field of view with my 2" ES17 (with a 92 degree apparent field), but that's just stretching the field into my peripheral vision. Using the 2" diagonal effectively increases the focal length of the scope, though (longer light path). So, using the larger diagonal will actually reduce the maximum FOV.


Edited by DavidWasch, 30 January 2021 - 03:34 PM.



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