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120ST F/5 Achromat. Which eyepieces to get the most from this telescope?

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#1 skookum

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Posted 07 July 2020 - 01:08 PM

I'm upgrading from an 80mm F/5 achromat to a 120mm F/5 Achromat (Orion 120ST).  I would like to get the most from my new telescope.  It has a 2 inch dual-speed focuser to which I will add a 2 inch diagonal.

 

As you can guess from my instrument choice, I want to scan the sky at low magnification, and occasionally zoom in a bit to pleasantly frame deep sky objects (mostly open clusters, brighter galaxies and nebulas, and the occasional globular).  The wider the apparent field of view, the better.  I understand that edge sharpness in fast telescopes is challenging.  Naturally I am willing to sacrifice some edge sharpness in exchange for immersion.  Planetary and lunar viewing through the 120mm F/5 is not a priority for me as I also own a Meade AR5 (127mm F/9.3) which I find satisfactory for planetary/lunar viewing.

 

Here is my current eyepiece collection (all 1.25"):

  • 32mm Meade 4000 Super Plossl
  • 26mm Meade 4000 Super Plossl
  • 18mm Meade 4000 Super Wide Angle (my favourite in my 80mm F/5 achromat, and barlowed in my 127mm F9.3 achromat)
  • 8mm Astro Tech Paradigm ED (on its way, haven't tried it yet. Got it for planetary/lunar in the AR5)
  • Meade 2x Barlow
  • Antares 2x(?) Barlow

To get the most from the 120mm F/5 refractor (and to retain adequate planetary views through the AR5), what eyepieces would you add, subtract, or replace (to / from / in) this collection if:

  1. Money is no object; and
  2. You have a $250 budget?

Since room in the astronomy case is limited, I would like to keep the collection to under 5 eyepieces.  New or used matters not to me.  All I care about is how good things look in the eyepiece.


Edited by skookum, 07 July 2020 - 01:12 PM.


#2 chubster4

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Posted 07 July 2020 - 01:29 PM

Short focus achromats usually have problems with higher powers. You might want to try out someone else's high power eyepieces and barlow in your scope before buying them yourself.   



#3 SeattleScott

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Posted 07 July 2020 - 01:29 PM

You really want to take advantage of the wide field capabilities. The scope is first and foremost a low power sweeper. So get a high quality 2” eyepiece that can handle the F5 light cone. Ideally something like ES 30mm 82 but that is awful heavy and over budget of $250. So maybe an ES 24/82 or APM 30 UFF to go considerably wider than the 32 Plossl without adding too much weight or cost.

I also see you have a gap between 18mm and 8mm. You can barlow your 26mm but a 12 Paradigm would be nicer and more convenient. Plus I suspect your 26mm likely vignettes a little if your 2x barlow is a shorty version.

Scott
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#4 skookum

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Posted 07 July 2020 - 01:40 PM

You really want to take advantage of the wide field capabilities. The scope is first and foremost a low power sweeper. So get a high quality 2” eyepiece that can handle the F5 light cone. Ideally something like ES 30mm 82 but that is awful heavy and over budget of $250. So maybe an ES 24/82 or APM 30 UFF to go considerably wider than the 32 Plossl without adding too much weight or cost.

I also see you have a gap between 18mm and 8mm. You can barlow your 26mm but a 12 Paradigm would be nicer and more convenient. Plus I suspect your 26mm likely vignettes a little if your 2x barlow is a shorty version.

Scott

You are absolutely right.  With the120ST I am only interested in low power sweeping (can zoom in if I want, but with managed expectations).  Indeed, because of the gap in eyepieces I very nearly ordered a 12mm Paradigm along with the 8mm, but chickened out because I have the barlow.

 

24mm seems like a good balance, especially if the AFOV is 82 degrees.  If you don't mind, would you compare your experiences between the ES 24/82 and the APM 30 UFF?  Is there anything which would cause you to choose one over the other?
 


Edited by skookum, 07 July 2020 - 01:42 PM.


#5 Eddgie

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Posted 07 July 2020 - 02:53 PM

These scopes have severe field curvature so the astigmatism from 32mm Plossls is going to quite prominant.  Now that only matters if it matters to you, but with a lot of field curvature, if you want stars to appear reasonably sharp at the edge of a wide field, use a good wide field eyepiece.

 

Also, in the presence of field curvature, the best way to improve the view is to use a longer power eyepiece that gives out the same amount of true field. 

 

For example, rather than use a 30mm eyepeice with 82mm apparent field, you would probably get a better result using a 40mm eyepiece with a 68 degree field. 

 

Rather than use a 24mm eyepeice with an 82 degree field, it would probably be better to use a 35mm Panoptic.

 

Now if off axis abberations don't bother you, then great for you, you can probably be happy with anything.  I have repeatedly seen people post on CN that they only care about the center of the field.  If that is you, then even a cheap wide field might be fine. 


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#6 markb

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Posted 07 July 2020 - 03:15 PM

I have Jaegers 5 and 6" f5s. Smooth polish, excellent figures, decent CA for what they are (superb, but 'purple haze, all in my eyes', sorry Jimi). Field curvature is an obstacle as well, but can be minimized with the right optics.

 

Your ep choices are somewhat similar to what you already have in your ep case, and I assume you are looking to add one ep, not replace a few?

 

The number one add-on for a fast achromat is a Baader LP 495 Long Pass filter, 1.25 is fine for almost everything, but if planning on lots of barrel limited maximum AFOV use it drops in necessity a little due to the low power and diffuse targets, although I personally would still get the 2" for that use. But the 1.25 for your current eps is, in my mind, a must have.

 

Also, IIRC the CA will be noticably worse in the 5" ? IIRC?

 

Before buying any eyepieces, unfortunately.

 

Keep in mind that the 2" barrel eyepieces are TFOV barrel limited at about 33mm at 82 degrees. 27mm at 100 degrees. Lots of combinations, and lots of recommendations, miss this physical limitation.

 

For you, and me, an amazing 4.5 degrees or so, max. 

 

An APM 30mm 70 degree UFF is going to come reasonably close at 3.5 degrees TFOV, still a heck of a lot of sky. I use an older 30mm 82 degree ep on a dedicated Baader MPCC for 4.1 degrees, PM if you want details on what worked for me, probably in the budget with the MPCC, but the UFF 30mm is the safer choice. No personal experience, only the 24mm and below which rock on f5 and f5.5 tubes (I believe the design goal was for the APM big binos which are 5.25 and 5.5 IIRC). I will try the UFFs in the Jaegers, I think I've only used them in the binos.

 

On the $250 budget, the 30mm UFF, and the Baader LP495 1.25 if you can squeeze the budget.. You should just make it, but I did not double check.

 

No budget, Baader Zeiss prism, BBHS mirror or Televue Everbrite, Televue 31mm 82 degree Nagler. or a 21mm Ethos. And the 2" filter. And Clicklocks. (Only minimally seriously- and an impossible to find Chromacorr).

 

I would seriously consider upgrading the barlow at some point, the Televues and even the used classic UO Klee 2.8x or the Dakin (never had one, so reputation only).

 

Just a bit more on the LP495.

 

Wrattens and the other Baaders (Semi APO, Fringekiller etc) are too shallow a cut and hurt your throughput and, unexpectedly but not inexplicably, are simply less effective, by a sizeable margin. The shallow curves let too much blue/violet squeak by.

 

The LP495 color is not an issue at night, and the resolution/contrast increase is big.

 

Agenaastro stocks them normally, Baaders new US dealer.


Edited by markb, 07 July 2020 - 03:28 PM.

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#7 SeattleScott

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Posted 07 July 2020 - 03:22 PM

I only have the ES 24/82 not the APM but the ES does well in my F4 newt with coma corrector. It will probably give a slightly smaller view than the APM, maybe 35mm field stop versus 38 or something like that. The advantage of the ES is greater magnification for zooming in on open clusters a bit, and darker background sky. But as mentioned the greater magnification will likely make field curvature a little worse. I mean I personally wouldn’t go 35-40mm focal length at F5, but 30mm might be a happy medium to tame the field curvature a little and get maybe up to 10% wider view. My impression is the APM is at least as sharp as the ES. More elements with smaller AFOV so the edge distortion will mainly come from the scope.

Scott

#8 skookum

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Posted 07 July 2020 - 07:05 PM

Wow.  These are excellent recommendations.  I have a 1.25" #8 Yellow filter on the way (same order as the 8mm Paradigm ED).  If I am not pleased with the result (and/or when I upgrade to a 2" optical train), I'll probably get the LP495.

 

From what's been written so far, I am considering the following options:

  • APM 30mm UFF: 3.5 degree TVOF, 70 degree AFOV, designed to manage field curvature
  • ES 24mm 82: 3.28 degree TVOF, 82 degree AFOV, field curvature may be more pronounced due to wider AFOV and shorter focal length
  • 35mm Panoptic: 3.97 degree TFOV, 68 degree AFOV, reasonably flat field, $$$!
  • 31mm Nagler: 4.24 degree TFOV, 82 degree AFOV, reasonably flat field, $$$$!!
  • 21mm Ethos: 3.5 degree TFOV, 100 degree AFOV, reasonably flat field, $$$$!!

Notice I haven't considered size or weight.  I have some thinking to do.  Thanks for your input everyone!


Edited by skookum, 07 July 2020 - 07:17 PM.


#9 tony_spina

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Posted 07 July 2020 - 07:19 PM

I recently got the APM 30mm 70 degree UFF for my  120ST.  I highly recommend this combination.  Very sharp eyepiece.  When compared to the ES 28mm 68 degree eyepiece.  The APM wins hands down in sharpness close to edge.  The ES starts getting seagulls at 60% out.

 

The APM  also has better transmission and contrast 


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#10 skookum

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Posted 07 July 2020 - 07:21 PM

I recently got the APM 30mm 70 degree UFF for my  120ST.  I highly recommend this combination.  Very sharp eyepiece.  When compared to the ES 28mm 68 degree eyepiece.  The APM wins hands down in sharpness close to edge.  The ES starts getting seagulls at 60% out.

 

The APM  also has better transmission and contrast 

Bam! Thanks Tony.  I haven't received my 120ST just yet, but it's on the way.  How have you enjoyed using yours?



#11 turtle86

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Posted 07 July 2020 - 07:38 PM

I'm upgrading from an 80mm F/5 achromat to a 120mm F/5 Achromat (Orion 120ST).  I would like to get the most from my new telescope.  It has a 2 inch dual-speed focuser to which I will add a 2 inch diagonal.

 

As you can guess from my instrument choice, I want to scan the sky at low magnification, and occasionally zoom in a bit to pleasantly frame deep sky objects (mostly open clusters, brighter galaxies and nebulas, and the occasional globular).  The wider the apparent field of view, the better.  I understand that edge sharpness in fast telescopes is challenging.  Naturally I am willing to sacrifice some edge sharpness in exchange for immersion.  Planetary and lunar viewing through the 120mm F/5 is not a priority for me as I also own a Meade AR5 (127mm F/9.3) which I find satisfactory for planetary/lunar viewing.

 

Here is my current eyepiece collection (all 1.25"):

  • 32mm Meade 4000 Super Plossl
  • 26mm Meade 4000 Super Plossl
  • 18mm Meade 4000 Super Wide Angle (my favourite in my 80mm F/5 achromat, and barlowed in my 127mm F9.3 achromat)
  • 8mm Astro Tech Paradigm ED (on its way, haven't tried it yet. Got it for planetary/lunar in the AR5)
  • Meade 2x Barlow
  • Antares 2x(?) Barlow

To get the most from the 120mm F/5 refractor (and to retain adequate planetary views through the AR5), what eyepieces would you add, subtract, or replace (to / from / in) this collection if:

  1. Money is no object; and
  2. You have a $250 budget?

Since room in the astronomy case is limited, I would like to keep the collection to under 5 eyepieces.  New or used matters not to me.  All I care about is how good things look in the eyepiece.

 

 

I like the 27mm and 35mm Pan in my ST 120.  Very nice views.



#12 Tyson M

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Posted 07 July 2020 - 07:40 PM

I recently got the APM 30mm 70 degree UFF for my  120ST.  I highly recommend this combination.  Very sharp eyepiece.  When compared to the ES 28mm 68 degree eyepiece.  The APM wins hands down in sharpness close to edge.  The ES starts getting seagulls at 60% out.

 

The APM  also has better transmission and contrast 

Awesome to hear.  I just received this eyepiece and excited to try it out.



#13 Jon Isaacs

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Posted 07 July 2020 - 08:35 PM

What I'd buy:

 

- $250 budget:. The 30 mm UFF gets lots of good reviews and friends find it to be very good.

 

- Unlimited budget: Might as well spend it all. 21 mm Ethos, 31mm Nagler, 35 Panoptic and for good luck, the 41 mm Panoptic.  :). I like these in fast scopes. 

 

On a more realistic note: This scope has a curved focal plane. The TSFlat2 is a 1x field flattener that can be used visually. It's about $200..  

 

Jon


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#14 markb

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Posted 07 July 2020 - 09:50 PM

Good to hear APM UFF 30mm reports, the shorter ones are great in the short refractors and APM 100s and 125 binos. Sure wouldn't turn down a 31 Nagler, but ouch  it'll cost to get the latter.

 

The flat fields really help fast scopes.

 

Darn, another ep to buy, just when I thought I was finished! Really.


Edited by markb, 07 July 2020 - 09:54 PM.


#15 tony_spina

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Posted 07 July 2020 - 09:58 PM

Bam! Thanks Tony.  I haven't received my 120ST just yet, but it's on the way.  How have you enjoyed using yours?

The ST120 is my always outside scope by the backyard door.  It gets used all the time for 10 min views to let's spend a few hours touring around. Typically use it at 20x to 60x.  I have had it about 7 or so years

 

But I also use it at 109x with my other favorite bang for the buck eyepiece that I have had for several years, the Meade 5.5mm UWA that is $99 currently on Amazon.   Truly a great buy... ( strong buy recommend) 

 

I'm also not afraid to put a 4mm Delite for 150x on the moon and planets.  Put a yellow #8 or 12 filter and it will cut the CA around the moon and Jupiter.   For Saturn I don't use a filter, the CA is not bothersome to me. I typically can see 5 moons on Saturn.

 

As far as the optics, mine is very sharp.  Great star test, with nice defined rings. Lots of folks put these scopes down, but the views especially at the low, wide field are fantastic. I have the ST80 if I need to backpack ultra light,  but if there is a little more room this is the scope that goes.

 

It's the scope I use when I need to get my fix and don't have the time to take out the bigger scopes. Its built like a tank and will serve you well. 


Edited by tony_spina, 07 July 2020 - 10:13 PM.

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#16 Tropobob

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Posted 07 July 2020 - 10:44 PM

Skookum, I think that the existing Meade 18mm wide angle EP will be very useful with this scope.  I would give priority to buying a quality EP in the 11 to 13mm range.   After that, let your experience guide U, or better still, try someone elses EPs in your scope. 



#17 skookum

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Posted 08 July 2020 - 02:05 AM

As far as the optics, mine is very sharp.  Great star test, with nice defined rings. Lots of folks put these scopes down, but the views especially at the low, wide field are fantastic. I have the ST80 if I need to backpack ultra light,  but if there is a little more room this is the scope that goes.

 

It's the scope I use when I need to get my fix and don't have the time to take out the bigger scopes. Its built like a tank and will serve you well. 

Man, that's exactly what I hoped for. Mine is coming with an motorized EQ3 (Astroview, RA only).  With any luck, it will be nearly as easy to manoeuvre from the basement as the ST80 on its EQ1.  I expect to operate it like my ST80---well balanced with the friction locks off. But it will be nice to have the option to cinch down and engage the drive when I want to.  I have a few dark sites in mind I'd like to bike to.  Good to know the 120ST sneaks into your backpack from time to time.

 

Skookum, I think that the existing Meade 18mm wide angle EP will be very useful with this scope.  I would give priority to buying a quality EP in the 11 to 13mm range.   After that, let your experience guide U, or better still, try someone elses EPs in your scope. 

I don't doubt that.  By far, the 18mm SWA is the best among my eyepieces (minus the 8mm Paradigm ED, which I haven't tested yet because it's still on its way).  I get a slightly wider TFOV with the 32mm SP of course. But the it's like looking down a straw compared to the SWA.  The SWA is just more immersive.  I rarely use the 26mm SP, except on my way from the 32mm to the 18mm.  I might even sell the 26mm at some point, as it just doesn't get much use in either my  ST80 or my AR5.  I'll wait for the 120ST to arrive before I decide.

 

If you had to suggest an eyepiece in the 11mm to 13mm range, which one would you recommend?


Edited by skookum, 08 July 2020 - 02:07 AM.


#18 Tropobob

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Posted 08 July 2020 - 04:55 AM

 

 

... If you had to suggest an eyepiece in the 11mm to 13mm range, which one would you recommend?

I suggest the Explore Scientific 11mm EP, which also has an 82 degree field.        



#19 Jon Isaacs

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Posted 08 July 2020 - 04:59 AM

From my point of view, you don't buy a 120 mm F/5 to use it with an 11 mm eyepiece. It's about low power, bright fields, it starts with a 30 mm wide field..

 

Jon


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#20 Tropobob

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Posted 08 July 2020 - 05:22 AM

From my point of view, you don't buy a 120 mm F/5 to use it with an 11 mm eyepiece. It's about low power, bright fields, it starts with a 30 mm wide field..

 

Jon

I have done that with a 100mm F5.  It was a curosity to see some many deep sky objects at once, but it was also like looking through a fish bowl.  I could not see any object well, so IMO, more magnification and less distoration are both desirable to see any object well. 



#21 Terra Nova

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Posted 08 July 2020 - 07:17 AM

What about these as a budget eyepiece? (SVBONY Lens 34mm Wide Angle 72° 2" Aspheric Eyepiece)

 

https://www.ebay.com...d-/392863598719
 

Anyone tried one? I have not, but someone did give me one of their budget 1.25" 23mm Aspheric eyepieces to try because I was quite skeptical of them and I found it to give very acceptable views.


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#22 bobhen

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Posted 08 July 2020 - 07:26 AM

I had a 120 F5 and now have a 102mm F5 that I have owned for 16 years.

 

I think 20 to say 80x is where you want to be with magnification for deep sky observing with this scope – give or take. High power for double stars can be very useful. You won’t get true colors but you will get some nice splits.

 

I like the TV Naglers with these fast scopes. They are well corrected at the edge. Yes there will still be slight field curvature but most deep sky objects can be centered within the surrounding field. So you really concentrate on the object in the center not the very edge. If you look at the edge it will show some softness but again you really have to look for it at the edge and at the low powers used and with good well-corrected eyepieces it has never bothered me personally. Yes the Delites might show better edge correction but you give up a little of that “space walk feel”, which is why you get one of these scopes in the first place.

 

Either the 11 or 13 will work well. The 13mm Nagger is one of my favorite eyepieces so…

 

These scopes are inexpensive, fun under a dark sky or used with an image intensifier, very portable and easily used on manual, alt/az mounts for simplicity and low power, sky sweeping. All fun stuff.

 

Bob


Edited by bobhen, 08 July 2020 - 07:27 AM.

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#23 SeattleScott

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Posted 08 July 2020 - 07:54 AM

What about these as a budget eyepiece? (SVBONY Lens 34mm Wide Angle 72° 2" Aspheric Eyepiece)

https://www.ebay.com...d-/392863598719

Anyone tried one? I have not, but someone did give me one of their budget 1.25" 23mm Aspheric eyepieces to try because I was quite skeptical of them and I found it to give very acceptable views.

SVBONY is constantly expanding their product line. First I have heard of this one. Looks a lot like some others but specs are far enough off to suggest it might not be a rebrand. Or it is a rebrand with inaccurate advertised specs. Maybe someone has tried it but maybe not since it seems pretty new. Might get better results posting a separate thread about it.

But most likely it is similar to every other 32-40mm 2” superwide eyepiece under $120. Which means not very good at F5. If it is all you can afford, it can probably get the job done. Might be the same as the 32mm Q70 for 40% less. But most likely there will be a lot of edge distortion.

Seriously, doesn’t that look EXACTLY like a 32mm Q70 except with a chrome barrel instead of black anodized? Yes this is listed as a 34mm 72 AFOV not 32mm 70 AFOV, but it isn’t like no one ever fudges the truth a couple mm and a couple degrees (40mm Swan, cough cough). The weight is a bit more but maybe that is the shipping weight? Or maybe it is just inaccurate? SVBONY isn’t exactly TeleVue so you have to take advertised specs with a grain of salt.

Scott

Edited by SeattleScott, 08 July 2020 - 08:07 AM.

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#24 tony_spina

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Posted 08 July 2020 - 09:05 AM

The specs for the SVBONY 34mm eyepiece says its argon purged, water proof.... now where have we seen that before grin.gif


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#25 gnowellsct

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Posted 08 July 2020 - 09:14 AM

I'm upgrading from an 80mm F/5 achromat to a e

  • Money is no object; and
  • You have a $250 budget?

I find this puzzling as saying that money is no object and then putting a cap on the expenditure is as much to say that money is an object. Which is fine. In real life we worry about expenditure. but one should bear in mind that if one says money is no object there are oculars out there that cost over $1,000 and some members of this group have actually bought them.

All that aside, I personally followed the idea that the best way to improve performance of an eyepiece was to get a better scope. So when I was starting out I upgraded my scopes and left my beginners eyepieces alone. It wasn't until I was at the aperture point that I wanted and had a good mount that I turned my attention to eyepieces. I think it's a good way to go.

In a very fast F5 achromat and you want a quality eyepiece, get the best you can afford in 35 to 40 mm.

There are diminishing returns at the other end of the scale between 5 and 10 mm because chromatic aberration will dominate the view as you increase magnification. You would need a teensy 2.5 mm eyepiece to get to the 0.5 mm exit pup il that is the theoretical limit of this and all scopes.


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