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What kind of AFOV do I need for 250x?

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#1 ScottRrtx

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Posted 31 March 2020 - 05:43 PM

I'm contemplating future purchases (nothing better or more fun to do, right?) and I'm struggling with EPs in the 5mm range. I'm currently planning on getting a 10" dob with 1250mm length, end of year. So at 250 magnification, I've heard people look at glob clusters there. Maybe there's other DSOs in that range. Planets and the moon could work there. I'd like to get a Delite 5mm down the road, but that is somewhat of a limited AFOV. Do you think I need a 70° or 82° or 100° EP for DSOs? It seems at 250x, a 100+ AFOV might not be necessary. But that still leaves a lot of choices.

 

 



#2 SeattleScott

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Posted 31 March 2020 - 06:01 PM

At 250x you are generally looking at small targets, so it isn’t so much an issue of fitting targets in the field. The issue is manually tracking. The higher the magnification, the faster objects drift out of view. So one could argue that you especially need 100 AFOV at 250x. Realistically with a little practice, most people can track pretty well with just 50 AFOV at 200x magnification. But more is certainly better. So it becomes a trade off of minimum glass, maximum contrast eyepieces with 42-52 AFOV versus ultrawide or hyperwide designs that typically give up a smidge of contrast due to their complex design, but allow you to stare at the target longer before nudging your scope. The Delite just might be about the best compromise, with performance reportedly approaching the very best planetary eyepieces, but comfortable eye relief and decent AFOV. But I don’t own one so that’s just what I hear. Otherwise get an ultrawide or hyperwide for maximum drift time, knowing you aren’t getting quite the best possible contrast.

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#3 ShaulaB

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Posted 31 March 2020 - 06:03 PM

If I may ask, why 250X? With my 10 inch Dob, all the Messier objects (DSO's) can be enjoyed at half that magnification or less. Globular clusters are very pleasing even in smaller scopes at lower magnification.

 

Are seeing conditions good to excellent where you live? Where I live, good or better seeing is rarely possible outside of a few months in the hot humid summer.

 

It is one of those laws of physics things that objects are brighter in the eyepiece at lower magnification.

 

Thanks, just wondering why a person would latch onto a magnification number.


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#4 Jon Isaacs

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Posted 31 March 2020 - 06:10 PM

If I may ask, why 250X? With my 10 inch Dob, all the Messier objects (DSO's) can be enjoyed at half that magnification or less. Globular clusters are very pleasing even in smaller scopes at lower magnification.

 

Are seeing conditions good to excellent where you live? Where I live, good or better seeing is rarely possible outside of a few months in the hot humid summer.

 

It is one of those laws of physics things that objects are brighter in the eyepiece at lower magnification.

 

Thanks, just wondering why a person would latch onto a magnification number.

 

Objects are brighter at lower magnifications but stars are constant in brightness and increasing the magnification increases the contrast so for open clusters and globulars, higher magnifications can reveal more stars.  250x is a 1mm exit pupil in a 10 inch, it's good for planets and a variety of objects.  For small bright planetary nebulae, higher magnifications are often productive and if the seeing is good, more can be used on the planets and double stars.

 

At 250x, a wider field can be nice but even a 60 degree eyepiece like the 5mm Paradigm provides about a minute for an object near declination 0 degrees. 

 

Jon


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#5 rkelley8493

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Posted 31 March 2020 - 06:33 PM

At ~250x, I'm usually using something in the 70º range. For my SCT, I use the 10 XW more than the 10 Ethos. However, I prefer the 10E in my refractors [for mid range magnification]. In my refractors, it's the 3.5 Delos that gives me about 250x. When I'm using that high of a magnification, it's usually on small, bright [or sorta bright] objects like Saturn, Mars, planetary nebulae, & globular clusters, and viewing comfort usually trumps field of view at that level [for me anyways].



#6 eyeoftexas

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Posted 31 March 2020 - 06:35 PM

One suggestion: Explore Scientific 82° 4.7mm.  That would give you 266x and ca. 0.3° TFOV (exit pupil = 1mm).


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#7 russell23

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Posted 31 March 2020 - 06:37 PM

I'm contemplating future purchases (nothing better or more fun to do, right?) and I'm struggling with EPs in the 5mm range. I'm currently planning on getting a 10" dob with 1250mm length, end of year. So at 250 magnification, I've heard people look at glob clusters there. Maybe there's other DSOs in that range. Planets and the moon could work there. I'd like to get a Delite 5mm down the road, but that is somewhat of a limited AFOV. Do you think I need a 70° or 82° or 100° EP for DSOs? It seems at 250x, a 100+ AFOV might not be necessary. But that still leaves a lot of choices.

I've used 60 deg AFOV eyepieces for the Moon at magnifications of 250x to 525x with refractors and found that manual tracking was not a problem - both hand and with slow motion controls.  It really just depends upon how smooth the mount is.  If there is any stick or backlash then there can be problems. 

 

For the price of a 5mm DeLite (which is excellent) you can also get the 5mm (4.7mm) APM XWA with 110 deg AFOV.  So there are options at both ends of that spectrum.



#8 havasman

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Posted 31 March 2020 - 06:43 PM

My favorite eyepiece in that range is the TV 4.7mm Ethos SX whether I'm using an XT10i or 16" f4.49 Starmaster with full go-to/track capability.

 

I recently observed a faint small galaxy that had a ~bright star nearby and tried a 6mm Tak Abbe Ortho (44' AFOV, minimal glass) and a 9mm Tak Abbe Ortho w/ and w/o TV 2x Barlow to shrink the field so the star was outside it. Magnifications were 233, 350, 446 and 466x with exit pupil ranging from 1.7 to 0.9mm. The advantage gained was that the bright star was out of the field and the observation was accomplished. Outside of that there was NO VISIBLE ADVANTAGE gained from using the "minimalist glass" orthos instead of the much more complex hyperwides. This held when the Barlow was added to the 9mm TAO. Complex modern eyepieces use very advanced glass and coatings combinations unavailable when the less-glass-is-best-glass mantra was formed. Complex modern eyepieces are excellent performers. The advantage in length of relaxed observing time as an object crosses the field of an undriven scope far outweighs everything else. 

 

Some years ago Alvin Huey, an advanced galaxy observer with much big Dob experience, published his findings with minimalist glass eyepieces for high mag fine detail observing. He found them considerably more effective and recommended them. Careful reading of his paper shows he ONLY found that true in large and very large aperture premium Dobs. The effect became relevant > 24" apertures.

 

So, for extended objects I say go for the widefields/super widefields/hyperwidefields. I had a very good ES82 4.7mm sample some years ago I used in my XT10i and recommend them as one of the best in that line. Likewise I think the TV 5T6 Nagler is one of the best Naglers. The Meade Series 5000 82 deg 5.5mm has experienced fans among the forum members. My most accomplished observer friend swears the Delos are best. I like both the Ethos SX a whole lot. Any of them will work very well from under dark Texas skies where we have the huge advantage of almost always being out from under the jetstream so we can use high and very high mag more often than many.


Edited by havasman, 31 March 2020 - 06:44 PM.

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#9 sjc327

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Posted 31 March 2020 - 06:46 PM

Another option is a Meade 5000 UWA 5.5mm. 

 

It's not quite 250x in your scope, but it does have an 82 degree field, gets good reviews and is not that expensive relatively speaking.


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#10 BillP

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Posted 31 March 2020 - 06:51 PM

I don't find manual tracking at 250x much of an issue with any AFOV.  But target acquisition is sometimes a real bear when the AFOV is small.  The performance of the Explore 82 eyepieces is phenomenal and the price is great.  Only drawback is same with Nagler T6s in that the eye relief is not tight but not generous for eyeglass wearers.  So if you do not wear glasses then would heartily recommend the 4.7mm ES82 with 13.6mm eye relief for about $169.  I have used these and they are great.  Now ES also has a long eye relief version in 4.5mm.  I have no experience with this and it has eye relief of 15.9mm for $199.  Not sure an extra 2.3mm more ER is that great of a difference though.



#11 russell23

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Posted 31 March 2020 - 06:52 PM

Another option is a Meade 5000 UWA 5.5mm. 

 

It's not quite 250x in your scope, but it does have an 82 degree field, gets good reviews and is not that expensive relatively speaking.

A very good option.  I used one for a long time and kind of miss it.



#12 BillP

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Posted 31 March 2020 - 06:56 PM

A very good option.  I used one for a long time and kind of miss it.

Forgot about those.  Just as good view from these as the ES82s but without the argon purge which is not a necessity.  And for $99 at Agena Astro a real steal for that kind of performance (they have the 20mm, 14mm, 8.8mm and 5.5mm all on a great sale).  I used to have a full set of the 1.25" Nagler T6s, Meade 5000 UWAs, and Explore 82s.  All put up near identical views so go with the best price point.


Edited by BillP, 31 March 2020 - 06:59 PM.

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#13 Astro-Master

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Posted 31 March 2020 - 08:41 PM

If all these choices are making your head spin, let me just say that the largest globular clusters like M13 need a true field of view around 23' or more to frame the cluster nicely in the field of view.

 

The 5mm Delite at 250x will only have a FOV of 14.88', a bit tight for the largest globulars, and with only a 1mm Exit Pupil a dimmer view.

 

Using my 10" scope for over 20 years I found that the big globulars looked best in the 160x to 200x range with 175x being the sweet spot.

 

A 7mm Nagler at 178.5x  would put you in the middle of the sweet spot, with a wider field of view of 27.5' and an Exit Pupil of 1.42mm for a brighter view and a more useful power on most DSO's, IMHO.

 

I good used 7mm Nagler on CN will cost less than a new Delite if money is tight.


Edited by Astro-Master, 31 March 2020 - 08:45 PM.

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#14 sanbai

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Posted 31 March 2020 - 09:02 PM

An advantage of large AFOV for dim and difficult targets is that you will find more stars of reference to look for them. Also, more stars increases the chances to focus accurately if you have to exchange the eyepiece, without the need of moving out of that field.

This is irrelevant for bright objects, of course.
If you have the budget and the space, I would go minimum with a 70 degrees AFOV. Only if it's for bright objects I would consider less AFOV if there is any other advantage to consider within the budget.

#15 turtle86

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Posted 31 March 2020 - 10:13 PM

My favorite eyepiece in that range is the TV 4.7mm Ethos SX whether I'm using an XT10i or 16" f4.49 Starmaster with full go-to/track capability.

 

I recently observed a faint small galaxy that had a ~bright star nearby and tried a 6mm Tak Abbe Ortho (44' AFOV, minimal glass) and a 9mm Tak Abbe Ortho w/ and w/o TV 2x Barlow to shrink the field so the star was outside it. Magnifications were 233, 350, 446 and 466x with exit pupil ranging from 1.7 to 0.9mm. The advantage gained was that the bright star was out of the field and the observation was accomplished. Outside of that there was NO VISIBLE ADVANTAGE gained from using the "minimalist glass" orthos instead of the much more complex hyperwides. This held when the Barlow was added to the 9mm TAO. Complex modern eyepieces use very advanced glass and coatings combinations unavailable when the less-glass-is-best-glass mantra was formed. Complex modern eyepieces are excellent performers. The advantage in length of relaxed observing time as an object crosses the field of an undriven scope far outweighs everything else. 

 

Some years ago Alvin Huey, an advanced galaxy observer with much big Dob experience, published his findings with minimalist glass eyepieces for high mag fine detail observing. He found them considerably more effective and recommended them. Careful reading of his paper shows he ONLY found that true in large and very large aperture premium Dobs. The effect became relevant > 24" apertures.

 

So, for extended objects I say go for the widefields/super widefields/hyperwidefields. I had a very good ES82 4.7mm sample some years ago I used in my XT10i and recommend them as one of the best in that line. Likewise I think the TV 5T6 Nagler is one of the best Naglers. The Meade Series 5000 82 deg 5.5mm has experienced fans among the forum members. My most accomplished observer friend swears the Delos are best. I like both the Ethos SX a whole lot. Any of them will work very well from under dark Texas skies where we have the huge advantage of almost always being out from under the jetstream so we can use high and very high mag more often than many.

 

Another vote for the 4.7mm Ethos.  It’s super-sharp and has a nice big 100 degree AFOV, for the best of best worlds.  Another good alternative would be the 5mm Nagler—very sharp and still has 82 degrees, and has the advantages of being lighter and less expensive than the Ethos.



#16 ScottRrtx

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Posted 31 March 2020 - 11:16 PM

Interesting suggestions. The widest FOV ones appear to have less ER. I don't wear glasses, so I don't know how important that is. Some people say too much is bad for maintaining exit pupil. I'm not sure where I stand. I do have some long ER eyepieces. Sometimes I think the image is "blacking" out, sort of, while peering into them, but I still end up using them for hours.

 

I already have a Morpheus 6.5, so 5.5 (Meade UWA) doesn't really push much deeper like a 5 would. I've heard people complain of EOFB in the Morpheus 4.5, so I haven't been considering that one. The APM XWA, suggested above, interests me. The 5mm APM doesn't seem to haven't gotten a ton of reviews though. The Ethos is nice, but I'm thinking a cheaper price than that. Speaking of that, the ES 82 sounds interesting with a great price. Wonder how comfortable that one is? Also, the Nagler 5mm T6 and Pentax 5mm XW are in the running. Surprised no on mentioned the Pentax 5, but I know there's various Pentax models that people use. I'm seen mixed comments on the Pentax 5 XW.

 

If I do plan on getting a 5mm Delite, specifically for lunar, maybe a 70-72 degree EP would be too redundant with the Delite.


Edited by ScottRrtx, 31 March 2020 - 11:57 PM.


#17 ScottRrtx

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Posted 31 March 2020 - 11:33 PM

 Any of them will work very well from under dark Texas skies where we have the huge advantage of almost always being out from under the jetstream so we can use high and very high mag more often than many.

Yes, I'm in Texas, so hopefully high mag works for me. Right now, with my small scope and light pollution conditions, I'm not doing anything higher than 50x, because nothing really gets better above that.


Edited by ScottRrtx, 31 March 2020 - 11:33 PM.


#18 25585

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Posted 31 March 2020 - 11:39 PM

Either a Baader Morpheus 76° 4,5 or 6.5mm will give a very immersive view. They are on sale right now.



#19 Tank

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Posted 01 April 2020 - 06:37 AM

If you have no tracking I would suggest larger FOV better

Best Buy is a Meade 5000 5.5 WP
Es 4.7 decent too

Best Eyerelief I would say Delos
5 XW 70degree is great
4.5 Morp 76 degree
Delite

Best out there Ethos 4.7

#20 junomike

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Posted 01 April 2020 - 06:53 AM

I mainly use 100° for Glob's in the 200X - 300X range as it allows for a higher magnification with the same FOV (as a smaller AFOV eyepiece).

AN 82° or 70° will work also, but I find those constricting since going to the 100°.



#21 sanbai

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Posted 01 April 2020 - 07:18 AM

I compared yesterday the Morpheus 9 vs the ES82 8.8. The first was the winner due to the comfort; optically I could not really discern differences (inexperienced). May be the moon was a tad better with the Baader. It may be different with the ~5mm offerings

I have the Meade 5.5. For me is to much magnification in the SCT to give an opinion on quality. However, yesterday with the Moon the reflections (including that of my eye) were quite bad compared to other eyepieces. That may not be a problem with other bright objects, and not at all for dim ones. It's still a great eyepiece for the price.

The APM xwa 5mm is actually a 4.7mm. Some EPs are mislabeled (depends on brand label APM, Lunt, ... )
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#22 bjkaras

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Posted 01 April 2020 - 02:40 PM

I use a 4.8mm Nagler for my 10". It gives me about 260x, with an 82 degree field and an exit pupil of about 1mm. For most DSOs I use lower powers though. I would say my most used eyepieces are a 13mm Nagler and a 24mm Widefield.



#23 russell23

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Posted 01 April 2020 - 08:09 PM

I just got in from a few hours of looking at the Moon.  I used a number of 60 - 62 deg AFOV eyepieces mostly at magnifications from 160-270x and even at 270x I could leisurely study the Moon as it moved across the field. 


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#24 ScottRrtx

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Posted 01 April 2020 - 09:05 PM

I think after reading comments here, I am going to pencil in a 60 degree for lunar and a 100+ degree for general viewing. hmm.gif



#25 areyoukiddingme

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Posted 02 April 2020 - 08:10 PM

I think after reading comments here, I am going to pencil in a 60 degree for lunar and a 100+ degree for general viewing. hmm.gif

That sounds like a good strategy. I find I usually prefer to view the moon with a relatively narrower field of view. Usually a zoom with about 58-76 degrees apparent field.

 

But for deep sky, I like 100 degrees most. While you can get good drift time out of 60 degrees, it's very nice to have context and more field for finding and staying on target.




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