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Question about long focal length eyepieces in small semi fast refractors

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#1 2112_Mike

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Posted 21 November 2020 - 10:23 PM

I recently acquired a nice small 80mm F/7 refractor.  Viewing through it for the first time last night Jupiter, Saturn, Mars and the moon looked very good and pretty sharp.

 

When viewing stars and for example the Pleaides using a 40mm Pentax XL and  Meade 24.5mm wide angle I noticed that I could not quite get the stars in to pin points, pretty close but not quite. Just a little in or out of focus would produce some seagulling effect. If I put in my 12mm Brandon or 9mm UO ortho it seemed to clean up some.

I am suspecting it is seeing condition related. It has been clear but pretty cold here.

Anyway I'm hoping that it is related to seeing as one of the main reasons I got this scope was to use it for low power sweeping.

 

Any thoughts on this would be appreciated.



#2 junomike

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Posted 21 November 2020 - 10:26 PM

Most likely experiencing "Field Curvature" although it could be astigmatism also.


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#3 T1R2

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Posted 21 November 2020 - 10:27 PM

Sounds like astigmatism in your eyes. however to check increase to 100x or so, if its your eye it will get better as the exit pupil decreases.



#4 Spikey131

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Posted 22 November 2020 - 12:04 AM

Do you wear glasses? We’re you wearing them at the time?

#5 2112_Mike

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Posted 22 November 2020 - 12:40 PM

I do wear glasses. I put them on and viewed without them. I need to do a more comprehensive test.  Which I might do tonight. 

I am convinced that yes, it is related to astigmatism in my eye.  I have enough eyepieces to discover the point where the astigmatism is minimal. 

I will be getting my prescription to see what my astigmatism is.

I can then decide whether to live it or go down the (expensive) path of using primarily TeleVue eyepieces with the Dioptrx on lower power eyepieces.

I don't like viewing while wearing glasses.

(I may have to sell my beloved Pentax 40m XL:(...)

 

thanks for the replies and insights...


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#6 russell23

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Posted 23 November 2020 - 06:34 AM

You can use a pair of glasses.  I had to get progressives with my last pair so I also bought a pair of single vision glasses for Astronomy.  You could keep your 40mm XL and the glasses would be less then new eyepieces.


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#7 t.r.

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Posted 23 November 2020 - 07:21 AM

Simply rotate your head (viewing position) at the eyepiece 90 degrees...if the aberration rotated with you it’s your eye astigmatism.

#8 daquad

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Posted 23 November 2020 - 11:46 AM

I recently acquired a nice small 80mm F/7 refractor.  Viewing through it for the first time last night Jupiter, Saturn, Mars and the moon looked very good and pretty sharp.

 

When viewing stars and for example the Pleaides using a 40mm Pentax XL and  Meade 24.5mm wide angle I noticed that I could not quite get the stars in to pin points, pretty close but not quite. Just a little in or out of focus would produce some seagulling effect. If I put in my 12mm Brandon or 9mm UO ortho it seemed to clean up some.

I am suspecting it is seeing condition related. It has been clear but pretty cold here.

Anyway I'm hoping that it is related to seeing as one of the main reasons I got this scope was to use it for low power sweeping.

 

Any thoughts on this would be appreciated.

For most people when the iris is opened much greater than 2 mm the aberrations of the lens are quite severe.  With the 40 mm your scope's exit pupil is 5.7 mm.  So between FC and the eyes aberrations, it is difficult to get pinpoint stars over the entire field or even in the center.  With the 9 mm ortho, the actual field is very small (just look at the diameter of the field stop in that eyepiece) so that FC is practically non-existent.  Furthermore, the scopes exit pupil is only 1.3 mm, which is equivalent to stopping down the eye lens to that aperture with a corresponding reduction in SA, CA and astigmatism.

 

Poor seeing is more of a problem at higher powers than low.

 

Dom Q.

 

Edit:  One more point:  When looking at the moon or bright daytime objects your iris is contracted to its minimum diameter so that even at low powers the view is sharp.  In addition, the reduction of the iris diameter reduces the effective diameter of the objective.


Edited by daquad, 23 November 2020 - 11:51 AM.

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#9 Jon Isaacs

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Posted 23 November 2020 - 02:00 PM

For most people when the iris is opened much greater than 2 mm the aberrations of the lens are quite severe.  With the 40 mm your scope's exit pupil is 5.7 mm.  So between FC and the eyes aberrations, it is difficult to get pinpoint stars over the entire field or even in the center.

 

 

The two eyepieces in question, the 40 mm XL and the Meade 24.5 mm Series 4000 SWA, show significant off-axis astigmatism at F/7. It's true your eyes can make a difference but neither of these eyepiece is comparable to a similar focal length Panoptic. And there's still the f8eld curvature of the scope to consider.

 

There are always compromises and if you want that essentially perfect field of view in a refractor with a 560 mm focal length, the TSFLAT2 plus some fancy eyepieces will get you there as long as your eye is OK.

 

Mike wrote:

 

"When viewing stars and for example the Pleaides using a 40mm Pentax XL and  Meade 24.5mm wide angle I noticed that I could not quite get the stars in to pin points, pretty close but not quite."

 

I suggest leaving well enough alone. The difference between not quite pinpoints and pinpoints is an expensive journey... 

 

Jon


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#10 2112_Mike

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Posted 24 November 2020 - 12:41 AM

Thank you for all of the replies.

I also think, having observed again last night, that the cold air around my eye while trying to observe causing my eye to water adds to the distortion.  When I used my 20mm Meade Ultra Wide and used the adjustable eyecup to seal my eye up while viewing it seemed to really help. Viewing again through the 40mm I realized how good the view was even if the stars are not exact pinpoints.

 

And yes, eyepieces are an expensive journey. I jut ordered a 10mm Ethos....




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