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Are these the longest eyepiece I should get?

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#1 bananas

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Posted 11 July 2020 - 04:11 AM

Wait.  Let me get this straight.  So a 30 mm 52° plossl or a 25 mm 60° Starguider should be the longest eyepiece that will give me the most field of view with my scope?  I think I did the calculations right and my scope max field of view is 3.4°



#2 mac57

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Posted 11 July 2020 - 05:20 AM

Your FOV should be around 2 degrees I think.



#3 Tropobob

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Posted 11 July 2020 - 05:45 AM

Wait.  Let me get this straight.  So a 30 mm 52° plossl or a 25 mm 60° Starguider should be the longest eyepiece that will give me the most field of view with my scope?  I think I did the calculations right and my scope max field of view is 3.4°

Calculation seems correct to me.   The 25mm is probably a better buy as the exit pupil for the 30mm is 7.5mm, which is probably ok if U are under 30.   



#4 cst4

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Posted 11 July 2020 - 05:54 AM

450mm FL / 30mm EP = 15x mag

52o FOV EP / 15x mag = 3.47o

 

450mm FL / 25mm EP = 18x mag
60o FOV EP / 18x mag = 3.33o



#5 Waddensky

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Posted 11 July 2020 - 05:58 AM

Exit pupils are quite large at these focal lengths. Keep that in mind.

 

In the 1.25" range, the ES 68° 24 mm and Televue Panoptic 24 mm give 3.63° TFOV in your scope and a smaller exit pupil.


Edited by Waddensky, 11 July 2020 - 06:12 AM.

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#6 sg6

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Posted 11 July 2020 - 06:14 AM

A long plossl and a Starguider will give almost identical, another "problem" I have withthe plossl is that threre are 30mm and 32mm and 50 degree and 52 degree and they all look the same and I generally suspect are actually the same. Just some marketing person has increased one value to make it appear better.

 

Biggest problem you are likely to have is that the scope is F/4 and that is a fast scope. Looks impressive on paper, sounds good, reality is a pain in the rear.

 

May depend on the reason for a long eyepiece. I have one and it is solely for alignment. I just wanted the widest I could get and the quality of the image was way down the list. If out of focus, fuzzy, blurred, odd shape, anything, but in the view I was happy.

 

When it gets to a 30mm plossl, a 25mm Starguider, 24mm 68 ES the final fields all come out within a fraction of each other. And I suspect if a tolerance were applied all would be in the error band of each other. One catch is the 68 degree ES's cost more.

 

By a paper exercise the 24/68 ES would deliver 0.3 degree more then the 25/60 Starguider, but will I think cost 2.5x more.

 

Starguider seems to be a comfortable eyepiece, plossl may have too great eye relief.



#7 junomike

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Posted 11 July 2020 - 08:09 AM

As Waddensky eluded to, the max FOV is based on the Field Stop which is around 28mm For a 1.25" Eyepiece.

However taking into consideration the exit pupil, the 24mm 68° range would be best IMO or even slightly less FOV (but more magnification)

In a 20mm 68° which would still provide just under 3° (2.90°) but offer a 5mm exit pupil.


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#8 SeattleScott

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Posted 11 July 2020 - 08:51 AM

The Starguider 25 will struggle at F4. Not the best in the series by any means. A Celestron Xcel LX has the same specs but performs well at F4. For 50% more. Or there is ES, TV or APM if you want 24/68 instead of 25/60. More optimal but considerably more expensive.

Scott
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#9 bananas

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Posted 12 July 2020 - 07:47 AM

I was viewing comet Neowise with 20mm 68° and wished the view was wider so I could see land at the same time. 



#10 Waddensky

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Posted 12 July 2020 - 10:32 AM

I was viewing comet Neowise with 20mm 68° and wished the view was wider so I could see land at the same time. 

If your scope has a 2" focuser, you can find wider eyepieces in the 2" range. If not, then the only way to get a wider view is get another scope with a shorter focal length.



#11 Starman1

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Posted 12 July 2020 - 06:42 PM

I was viewing comet Neowise with 20mm 68° and wished the view was wider so I could see land at the same time. 

That would only be possible if the comet were almost ON the horizon.

A typical camera might shoot 30° or more of sky, and NO telescope would give you that.

 

The widest possible views in 1.25" eyepieces are 32mm Plossl, 26mm 62°, 24mm 68°

And, among those, because you have a reflector with a larger secondary, I'd go with the 24mm 68° for a widest field.

 

The calculated true field maximum is about 3.48° at 19x.

 

For most objects in the sky, your 20mm 68° is a better fit, at 23x.  It has a decently wide true field at 2.90°.  It's a rare circumstance to need anything wider, really.


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