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Russell Low Power Eyepieces and Celestron SCTs

Eyepieces
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#1 Terry Trees

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Posted 08 May 2021 - 06:59 AM

Hello,

I've been looking for a way to get very low power and wide (true) fields of view with my Celestron 11" and 9.25" f/10 Schmidt-Cassegrain scopes.  There are lots of excellent Cloudy Nights ratings for Russell Optics products in general including his eyepieces in the 56mm to 72mm range.  The Russell Optics website states you should not go below 35x with a scope that has a central obstruction or you will not get the best results..  If I use his 72mm in my 11" it would generate 39x.  If I were to use it in my 9.25", it would be a bit below the 35x limit at 33x.

So, does anyone have any experience using either his 65mm and/or his 72mm XL eyepieces in either the 9.25" or 11" Celestron SCT?

Thanks very much for your help.

Terry Trees



#2 junomike

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Posted 08 May 2021 - 08:13 AM

These will work however they also will have a very large exit pupil (7.2mm).

Is it the larger FOV you crave or larger FOV as a largest FOV can be had with a 40mm SWA (68 - 72°) and will show a larger image which IMO offers more detail.

A 4mm exit pupil is also more reasonable (IMO) unless you have a specific need for a larger exit pupil.


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#3 SeattleScott

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Posted 08 May 2021 - 08:19 AM

Yes the 40mm SWA class is much more popular for SCT for that reason. Same field of view, more magnification, darker background sky. The 72mm could have specific uses like using with OIII filter.

Scott

#4 LDW47

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Posted 08 May 2021 - 08:25 AM

Hello,

I've been looking for a way to get very low power and wide (true) fields of view with my Celestron 11" and 9.25" f/10 Schmidt-Cassegrain scopes.  There are lots of excellent Cloudy Nights ratings for Russell Optics products in general including his eyepieces in the 56mm to 72mm range.  The Russell Optics website states you should not go below 35x with a scope that has a central obstruction or you will not get the best results..  If I use his 72mm in my 11" it would generate 39x.  If I were to use it in my 9.25", it would be a bit below the 35x limit at 33x.

So, does anyone have any experience using either his 65mm and/or his 72mm XL eyepieces in either the 9.25" or 11" Celestron SCT?

Thanks very much for your help.

Terry Trees

My 65mm works great in my refractors but I think there will be very few fellow astronomers that can answer your question to the point of guaranteeing the performance you may want, I think there are just too few in use. Eye positioning is something you will really have to get used, try hard at ! 



#5 LDW47

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Posted 08 May 2021 - 08:26 AM

Yes the 40mm SWA class is much more popular for SCT for that reason. Same field of view, more magnification, darker background sky. The 72mm could have specific uses like using with OIII filter.

Scott

And the 72° FOV helps as well !



#6 macdonjh

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Posted 08 May 2021 - 08:27 AM

Hello,

I've been looking for a way to get very low power and wide (true) fields of view with my Celestron 11" and 9.25" f/10 Schmidt-Cassegrain scopes.  There are lots of excellent Cloudy Nights ratings for Russell Optics products in general including his eyepieces in the 56mm to 72mm range.  The Russell Optics website states you should not go below 35x with a scope that has a central obstruction or you will not get the best results..  If I use his 72mm in my 11" it would generate 39x.  If I were to use it in my 9.25", it would be a bit below the 35x limit at 33x.

So, does anyone have any experience using either his 65mm and/or his 72mm XL eyepieces in either the 9.25" or 11" Celestron SCT?

Thanks very much for your help.

Terry Trees

Terry Trees, welcome to Cloudy Nights.  One of the consequences of having a large exit pupil is the shadow from your secondary mirror may become visible, which is annoying.  Another aspect of the Russell long focal length eye pieces is really long eye relief.  So long I was not able to keep my eye centered over the exit pupil consistently.  The views were nice, when I got them, the I was frustrated often.  

 

One other thing to keep in mind: TFOV is limited by the field stop of the eye piece, not its focal length.  As the others have alluded to: using a 72mm eye piece will give you a larger exit pupil but probably not a larger TFOV.


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#7 GGK

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Posted 08 May 2021 - 09:52 PM

Hello,

I've been looking for a way to get very low power and wide (true) fields of view with my Celestron 11" and 9.25" f/10 Schmidt-Cassegrain scopes.  There are lots of excellent Cloudy Nights ratings for Russell Optics products in general including his eyepieces in the 56mm to 72mm range.  The Russell Optics website states you should not go below 35x with a scope that has a central obstruction or you will not get the best results..  If I use his 72mm in my 11" it would generate 39x.  If I were to use it in my 9.25", it would be a bit below the 35x limit at 33x.

So, does anyone have any experience using either his 65mm and/or his 72mm XL eyepieces in either the 9.25" or 11" Celestron SCT?

Thanks very much for your help.

Terry Trees

The 40mm Pentax XWL has a 46.5mm field stop diameter, which is the largest possible with the 2 inch barrel diameter.  This will give you the maximum TFOV at a 4mm exit pupil.  I use this eyepiece in my C8.  With the longer focal length due to the 2 inch diagonal, I get a TFOV of 1.2 degrees (in my C8). 

 

Increasing eyepiece focal length will increase exit pupil, but you will not get a larger TFOV.  The 55mm TeleVue Plossl, for example has the same 46.5mm field stop diameter, so will have the same TFOV, just at a lower magnification. This will lighten the background sky with the larger 5.5mm exit pupil, but have little impact on star brightness, which can be useful if you are having image brightness issues when using an OIII filter.  

 

I also push the limits of my C8 by using a f6.3 focal reducer with these eyepieces in a 2 inch diagonal.  How that relates to this discussion is that the 30mm Pentax XW in this configuration has a 5.5mm exit pupil and the view is fine.  However, the 40mm Pentax XW with the focal reducer gives about a 7.3mm exit pupil and I see the shadow of the central obstruction.  It's bad off axis, but just barely noticeable with good eye placement.  I use it this way because it is my maximum possible TFOV.

 

Celestron recommends a maximum 35mm eyepiece focal length when using the focal reducer with the standard 1-1/4 visual back and diagonal.  In this configuration, the OTA is close to f6.3; therefore, Celestron is recommending a maximum 5.6mm exit pupil to prevent the central obstruction from creating a shadow in the eyepiece view.

 

In my C8, I can get the TFOV to about 1.95 degrees good and clear, then quickly vignetting to dark at about 2.1 degrees (40mm Pentax XW with focal reducer).  The C8 has a 37 to 38mm baffle tube diameter - much smaller than the larger OTAs.  That might not matter, though, because the focal reducer has a clear optical path of only 41 to 42mm and that is what causes the vignetting in the 40mm XW eyepiece.  I believe the 11 and 9.25 use the same focal reducer as the C8, but am not positive.

 

Again, this is my experience with a C8.  But maybe it will give you some ideas for your SCTs.

 

Gary



#8 Thomas_M44

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Posted 08 May 2021 - 11:48 PM

Hello,

I've been looking for a way to get very low power and wide (true) fields of view with my Celestron 11" and 9.25" f/10 Schmidt-Cassegrain scopes.  There are lots of excellent Cloudy Nights ratings for Russell Optics products in general including his eyepieces in the 56mm to 72mm range.  The Russell Optics website states you should not go below 35x with a scope that has a central obstruction or you will not get the best results..  If I use his 72mm in my 11" it would generate 39x.  If I were to use it in my 9.25", it would be a bit below the 35x limit at 33x.

So, does anyone have any experience using either his 65mm and/or his 72mm XL eyepieces in either the 9.25" or 11" Celestron SCT?

Thanks very much for your help.

Terry Trees

With an SCT, I believe it would probably be best to place a upper limit on eyepiece FL somewhere around 40mm.

 

Remember also: with the 2-inch eyepiece format, A 55mm Plossl (AFOV 50-degrees) gives the maximum *possible* TFOV.

 

With any 2-inch eyepiece of longer than 55mm FL, you can get yet lower magnification, and an increase in exit pupil (which may be a good thing or a bad thing depending upon one's telescope particulars) but *not* any increase in True Field. One will also suffer a proportional decrease of Apparent Field as the eyepiece FL is progressively increased.

 

The only way to avoid a decrease in AFOV, and to increase TFOV, would be to go to a larger Field-Stop system such as 3-inch eyepieces etc --assuming one's telescope could handle a 3-inch system, of course.


Edited by Thomas_M44, 09 May 2021 - 12:03 AM.


#9 LDW47

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Posted 09 May 2021 - 12:17 AM

Hello,

I've been looking for a way to get very low power and wide (true) fields of view with my Celestron 11" and 9.25" f/10 Schmidt-Cassegrain scopes.  There are lots of excellent Cloudy Nights ratings for Russell Optics products in general including his eyepieces in the 56mm to 72mm range.  The Russell Optics website states you should not go below 35x with a scope that has a central obstruction or you will not get the best results..  If I use his 72mm in my 11" it would generate 39x.  If I were to use it in my 9.25", it would be a bit below the 35x limit at 33x.

So, does anyone have any experience using either his 65mm and/or his 72mm XL eyepieces in either the 9.25" or 11" Celestron SCT?

Thanks very much for your help.

Terry Trees

If it hasn’t been mentioned why don’t you contact Gary Russell personally by phone call and explain your situation, he will be glad to give you his knowledge about that issue ! Its the best and easiest way !



#10 Terry Trees

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Posted 09 May 2021 - 04:19 PM

Hello everyone,

 

When I posted this topic/question, I had already spoken to Gary Russell and he had suggested I see if anyone had indeed used any of his very long focal length eyepieces in an 11" or a 9.25" SCT, i.e., what was their personal experience?  I was also concerned about the secondary obstruction and the "black hole" it might create in the field of view of long focal length eyepieces.  Finally, I was concerned about eye relief and if the image would form far past the eyecup.  I experienced that with an Explore Scientific 68*, 26mm eyepiece and had blackouts as my head and eye floated around with no reference point.  I later solved it with the addition of a Televue Eyeguard Extender.  I wasn't considering exit pupil too much because I have not had an issue with it in the eyepieces I have used in the past.  But I'm glad that it was introduced above. 

I kept all that close to the vest so that I would not possibly influence your responses.  I think that approach worked very well because I found your replies to be very educative.

Based on your comments I've decided not to pursue any very long focal length eyepieces.  Thank you for your insights and help.


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#11 BKSo

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Posted 10 May 2021 - 09:16 AM

I have no problem using a 64mm ep with my small scope at 16x. Secondary shadow is visible at daytime, but not under a dark sky. The eye relief was fixed by adding a pull-up eye guard, and some experiments at home to find the perfect position.


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