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#1 Newtoastronomy85

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Posted 04 August 2020 - 08:24 AM

Hi everyone, this is my first post, I’m the resident newb here. So I just bought a Skywatcher 250P which is the classic 10” Dobsonian. I’ve done a lot of reading the last 2-3 weeks and prior to purchasing so I’ve figured a lot of things out on my own but like everyone else eyepieces are daunting.

So here’s where I’m at...the telescope is a 1200mm and I totally understand the math involved to calculate magnification. Right now I have a 10/20MM that came with the scope so 60 and 120 and a F/ratio of 4.7

I know 50X is max magnification so 600X
I’ve seen anywhere from 30-40X is generally useable on an average night and in general most nights are average.

I understand eye relief pertains to the distance from the eyepiece and still able to see. I don’t wear glasses so this isn’t an issue but I don’t want something that is so low it hurts your eyes.

I maybe understand focal ratio? From what I read I believe mine would be considered a “slow” telescope that’s inherent with the design and size. So I have some blurry areas on the outside of my scopes view so higher end eyepieces would benefit me?

I see the term exit pupil but this I don’t get and I don’t know how an eyepiece is even rated for this?

As far as my viewing site, it can change regularly if I choose. I live in Colorado so a short drive for me can put me in pitch black and in the mountains.

Right now I’m pretty amazed by what I’ve been able to see with Jupiter, Saturn and the moon although it’s been cloudy a lot lately so that sucks. Originally I thought I would buy a X2 Barlow lens and have a 5, 10, 10 and the 20 but the more I thought about it, it’s pretty redundant and buying a X2 for cheap eyepieces seems dumb.

Sorry this is long, easier to get all the info out first. So I’m looking at eyepieces now and I’m maybe $100-150 on what I want to spend. I’m checking out a 82 degree explore scientific ($159 on sale) 4.7mm right now which I thought seemed a good fit because the FOV is large and since my scope is large it will give me more sky at a higher magnification. The 4.7mm would give me 255X

I don’t know if that FOV is good for a beginner? Or recommended? Any other recommendations on eyepieces that might work? Is 255X too much for average nights or could I go more, closer to 300?

I’d like an eyepiece that’s usable without being overkill. With a 10” I feel like anything over a 4mm would probably rarely be useable?

I’m going to Estes Park for a fishing trip in 2 weeks so I want to order something pronto. Because I know I’ll be disappointed if it’s pitch black and a beautiful night and I’m restricted to 120X.

So in conclusion, I’m looking for the highest useable magnification my telescope can handle on average to very good nights.

Thanks!

#2 tjschultz2011

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Posted 04 August 2020 - 08:43 AM

Hi, 

 

Don't have a ton of time at the moment but I can comment on a few things. Your scope is large enough to also be effective for deep space objects (DSOs). Especially if you have access to really dark skies. For many DSOs a really wide field eyepiece will give a pleasing view. If you're looking at ES eyepieces, I would check out their 82 degree 30mm. It's a close contender to Televue's 82 degree 31mm Nagler which is a fan favorite but very expensive. I would agree that anything over 4mm for your scope will rarely be usable. Even a 6mm might be a good fit. You could also check out the Meade Ultra Wide Angle EPs. I've heard a lot of good things about them and they are relatively cheap on price.  EDIT: I've also heard that their 5.5mm one is generally regarded as the best so maybe that would be a good option for you. 

 

Your scope would actually be considered a fairly fast scope as far as f ratio goes. Though it's somewhat near what many people would consider a cutoff point between fast and not so fast. It's just a measure of how quickly (distance wise) the scope brings light to focus. A faster scope will give better images with higher end and better corrected eyepieces but they aren't required. For me, just upgrading to eyepieces with wider apparent fields of view (AFOV), such as the 82 degree ones you're looking at, was a huge factor in enjoyment. Looking through the standard 50 degree plossls that come with most kits now feels like looking through a straw. You'll be happy with the Meade or ES ones. 


Edited by tjschultz2011, 04 August 2020 - 09:05 AM.

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#3 Allanbarth1

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Posted 04 August 2020 - 09:14 AM

Welcome to Cloudy Nights welcome.gif

 

I'll speak on what I own. Thees are all currently in my ep case and get used regularly. One winner for sure is the Explore Scientific 82° 24mm ep. Also the 14mm in the 82° series is a good ep. Ton's of folks have the 24mm and rave about it. (myself included I own both ep's and the barlow I'll link to below) Both are well worth getting on sale especially in sale. For me they are forever ep's.

 

As far as explaining exit pupil,I understand it myself but don't know how to explain it simply. I'll leave that to someone else. 

 

At F/4.7 that's typically considered a fast scope. 

 

If I were in your shoes I would pick up the ES 82° 24mm ep for sure, especially if it's on sale. The 14mm is up to you but looks like it will fill a gap in your eyepiece arsenal. This site's sponsor/owner Astronomics sells this barlow lens.  I also own it and am happy with its performance. Another plus is that is handles both 2" and 1.25" eyepieces. You'll also get a discount from them if your a member of Cloudy Nights which you are.

 

What ever you decide, just have fun, keep looking up.

 

Clear Skies, Allan in N.J,


Edited by Allanbarth1, 04 August 2020 - 09:26 AM.


#4 SeattleScott

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Posted 04 August 2020 - 09:19 AM

Ideally you want to be able to hit 200, 250 or 300 to match magnification to seeing, but the 4.7 could be good for now. Keep in mind high power planetary viewing is really best from the city. You go to dark skies to see DSO. So get something to go higher than 120x but I wouldn’t worry about not being able to hit 320x two weeks from now. I would want something in between also like a 6.7mm for high power DSO and low power planetary viewing. That would really be more critical for your trip than a max power planetary eyepiece. The Celestron Xcel LX 7mm (really 6.5) is a cheaper option that is very well corrected, just the view isn’t as wide.

Scott

#5 cookjaiii

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Posted 04 August 2020 - 09:20 AM

Exit pupil is the diameter of the shaft of light projected from the eyepiece and into your eye.  Your iris can probably dilate so that your pupil is 5-7mm in the dark (varies with age and individual) so an exit pupil greater than your eye pupil diameter is wasted light.  It can't enter your eye.  

 

Exit pupil can be calculated by dividing the focal length of the eyepiece by the focal ratio of the telescope.  For example, your f/4.7 scope with a 25mm eyepiece will give you an exit pupil of 25/4.7 = 5.3mm.

 

I have read that optimal resolution is obtained with an exit pupil of 1-2mm.  I can tell you that if I use an eyepiece approaching 0.5 -0.6mm, the view becomes very dim and I see the floaters in my eye.

 

You chose an excellent scope.  Have fun!


Edited by cookjaiii, 04 August 2020 - 09:21 AM.


#6 sg6

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Posted 04 August 2020 - 09:21 AM

I know 50X is max magnification so 600X

What makes you say that?

 

That is based on the exit pupil being 0.5mm which is a limit of the eye. It has no relevance to what the scope can deliver, so is therefore not a maximum.

 

Here is another one, that is based on the optics of a scope and the eye: MaxMag

And as you will guess and may even read has a different value.

So now what?

 

Best advice is these nice big numbers are there to part you from however many $$'s they can.

 

Your maximum will depend on how good the Skywatcher mirror is made. So now the reality kicks in: Do Skywatcher make mirrors to equal  say Zambuto?

 

Because that "rule" you have read up on, quoted and appear to believe, says a hand crafted mirror made over many hours and tested at each stage is no better the a mass produced Chinese one from some unknown manufacturer.

 

Next:

I’d like an eyepiece that’s usable without being overkill. With a 10” I feel like anything over a 4mm would probably rarely be useable?

By over a 4mm I hope you mean under a 4mm, and that you really mean "Anything over the magnification of a 4mm"

 

Scope is f/4.7 so fast (not slow), you need fairly good eyepieces and ES should match that aspect.

You will likely find on a dobsonian that the widest possible is usefult to find amything initially.

Widest field in 1.25" format is the 24/68 which gives a magnification of 50x and a field of 1.36 degrees.

Consider that M31 Andromeda is 3 degrees and M45 Pleiades is just over 2 degrees so you will not see all of either when they appear.

 

A field of 0.5 degrees is likely useful so that is around 120x and so a 10mm eyepiece.

Most DSO observing is at the 60x to 80x area.

The 4mm in an 82 degree format would give 300x and a field of view around 0.25 to 0.3 of a degree (tiny bit of sky)



#7 n2dpsky

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Posted 04 August 2020 - 10:16 AM

I have the Flex-Tube version of your scope.   At f/4.7, is not a "slow" scope.   The 50x rule is per inch of aperture, so your max is really 500x not 600x, but I will tell you I rarely get close to that number.   I very rarely use more than 250x, which is a 5mm eyepiece.  That said, because it is a relatively short focal length, your eyepiece collection will probably have more shorter focal length eyepieces than long ones.   This is simply due to the fact that the long focal length eyepieces don't change the magnification all that much.    The jump from a 24mm eyepiece to a 14mm for instance is only a change of about 35x.  Whereas the jump from a 6mm to a 5mm is 50x.     

 

Those new to the hobby usually use too much magnification, which is not required.   On Jupiter or Saturn, I usually end up around 136x-200x depending on how good the skies are.   As magnification goes up, contrast on the planets goes down so it can become more difficult to see features.  Most of my observing is done with 5 eyepieces (see below).  

 

Explore Scientific 68 deg 24mm - 50x

Explore Scientific  82 deg 14mm - 85x

Explore Scientific 82 deg 8.8mm - 136x

BST Planetary 58 deg 6mm  - 200x

BST Planetary 58 deg 5mm - 240x

 

Add a 2x barlow and these eyepieces become a 12mm, 7mm, 4.4mm, 3mm and 2.5mm.    

 

You don't need to spend a couple hundred dollars on each eyepiece.    If you are on a budget, look at the Astro-Tech Paradigm Dual ED eyepieces.  Good eyepieces for the money.   The BST Planetary eyepieces (at Agena Astro) are also quite good for about $55/ea with comfortable eye relief.

 

Hope that helps.


Edited by n2dpsky, 04 August 2020 - 02:10 PM.

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#8 Newtoastronomy85

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Posted 04 August 2020 - 10:53 AM

Hi, 

 

Don't have a ton of time at the moment but I can comment on a few things. Your scope is large enough to also be effective for deep space objects (DSOs). Especially if you have access to really dark skies. For many DSOs a really wide field eyepiece will give a pleasing view. If you're looking at ES eyepieces, I would check out their 82 degree 30mm. It's a close contender to Televue's 82 degree 31mm Nagler which is a fan favorite but very expensive. I would agree that anything over 4mm for your scope will rarely be usable. Even a 6mm might be a good fit. You could also check out the Meade Ultra Wide Angle EPs. I've heard a lot of good things about them and they are relatively cheap on price.  EDIT: I've also heard that their 5.5mm one is generally regarded as the best so maybe that would be a good option for you. 

 

Your scope would actually be considered a fairly fast scope as far as f ratio goes. Though it's somewhat near what many people would consider a cutoff point between fast and not so fast. It's just a measure of how quickly (distance wise) the scope brings light to focus. A faster scope will give better images with higher end and better corrected eyepieces but they aren't required. For me, just upgrading to eyepieces with wider apparent fields of view (AFOV), such as the 82 degree ones you're looking at, was a huge factor in enjoyment. Looking through the standard 50 degree plossls that come with most kits now feels like looking through a straw. You'll be happy with the Meade or ES ones. 

Gotcha, thank you, that is very helpful! My plan when I bought this scope was to be able to see enough with it due to its size so that I didn’t feel the need to get a bigger one in the future, that’s the hope anyways. 🤞 I didn’t realize that dark skies were for DSOs, I thought dark skies were just beneficial overall so that’s good to know. Now about the eyepieces, you use lower power for DSOs? I was of the assumption you needed high magnification to see far away stuff.

 

Welcome to Cloudy Nights welcome.gif

 

I'll speak on what I own. Thees are all currently in my ep case and get used regularly. One winner for sure is the Explore Scientific 82° 24mm ep. Also the 14mm in the 82° series is a good ep. Ton's of folks have the 24mm and rave about it. (myself included I own both ep's and the barlow I'll link to below) Both are well worth getting on sale especially in sale. For me they are forever ep's.

 

As far as explaining exit pupil,I understand it myself but don't know how to explain it simply. I'll leave that to someone else. 

 

At F/4.7 that's typically considered a fast scope. 

 

If I were in your shoes I would pick up the ES 82° 24mm ep for sure, especially if it's on sale. The 14mm is up to you but looks like it will fill a gap in your eyepiece arsenal. This site's sponsor/owner Astronomics sells this barlow lens.  I also own it and am happy with its performance. Another plus is that is handles both 2" and 1.25" eyepieces. You'll also get a discount from them if your a member of Cloudy Nights which you are.

 

What ever you decide, just have fun, keep looking up.

 

Clear Skies, Allan in N.J,

Fantastic, thanks! I’m sensing a trend here I did not realize. I was under the impression I needed more magnification to see far away objects like nebulas etc. 

 

Ideally you want to be able to hit 200, 250 or 300 to match magnification to seeing, but the 4.7 could be good for now. Keep in mind high power planetary viewing is really best from the city. You go to dark skies to see DSO. So get something to go higher than 120x but I wouldn’t worry about not being able to hit 320x two weeks from now. I would want something in between also like a 6.7mm for high power DSO and low power planetary viewing. That would really be more critical for your trip than a max power planetary eyepiece. The Celestron Xcel LX 7mm (really 6.5) is a cheaper option that is very well corrected, just the view isn’t as wide.

Scott

Good to know! I plan on getting these over time and building a set that makes sense and doesn’t become redundant, I will check that eyepiece out. I’m a little bit confused on DSO’s, are you viewing them on low power wide angle lenses or high power, you mentioned high power? Or doesn’t it depend on which ones you’re looking at? 

 

Exit pupil is the diameter of the shaft of light projected from the eyepiece and into your eye.  Your iris can probably dilate so that your pupil is 5-7mm in the dark (varies with age and individual) so an exit pupil greater than your eye pupil diameter is wasted light.  It can't enter your eye.  

 

Exit pupil can be calculated by dividing the focal length of the eyepiece by the focal ratio of the telescope.  For example, your f/4.7 scope with a 25mm eyepiece will give you an exit pupil of 25/4.7 = 5.3mm.

 

I have read that optimal resolution is obtained with an exit pupil of 1-2mm.  I can tell you that if I use an eyepiece approaching 0.5 -0.6mm, the view becomes very dim and I see the floaters in my eye.

 

You chose an excellent scope.  Have fun!

Okay! That makes perfect sense and that’s easy math, thank you! 

 

What makes you say that?

 

That is based on the exit pupil being 0.5mm which is a limit of the eye. It has no relevance to what the scope can deliver, so is therefore not a maximum.

 

Here is another one, that is based on the optics of a scope and the eye: MaxMag

And as you will guess and may even read has a different value.

So now what?

 

Best advice is these nice big numbers are there to part you from however many $$'s they can.

 

Your maximum will depend on how good the Skywatcher mirror is made. So now the reality kicks in: Do Skywatcher make mirrors to equal  say Zambuto?

 

Because that "rule" you have read up on, quoted and appear to believe, says a hand crafted mirror made over many hours and tested at each stage is no better the a mass produced Chinese one from some unknown manufacturer.

 

Next:

 

 

By over a 4mm I hope you mean under a 4mm, and that you really mean "Anything over the magnification of a 4mm"

 

Scope is f/4.7 so fast (not slow), you need fairly good eyepieces and ES should match that aspect.

You will likely find on a dobsonian that the widest possible is usefult to find amything initially.

Widest field in 1.25" format is the 24/68 which gives a magnification of 50x and a field of 1.36 degrees.

Consider that M31 Andromeda is 3 degrees and M45 Pleiades is just over 2 degrees so you will not see all of either when they appear.

 

A field of 0.5 degrees is likely useful so that is around 120x and so a 10mm eyepiece.

Most DSO observing is at the 60x to 80x area.

The 4mm in an 82 degree format would give 300x and a field of view around 0.25 to 0.3 of a degree (tiny bit of sky)

Fantastic info, thanks! As far as the max magnification, yeah it was just the general consensus the Internet feeds you. I get what you’re saying about the quality of the mirror, totally logical, I checked and I don’t think my $600 scope matches their $1,200 mirror lol so the answer would be no. 😂 I’m going to read what your hyperlinked, thanks! 

 

I have the Flex-Tube version of your scope.   At f/4.7, is not a "slow" scope.   The 50x rule is per inch of aperture, so your max is really 500x not 600x, but I will tell you I rarely get close to that number.   I very rarely use more than 250x, which is a 5mm eyepiece.  That said, because it is a relatively short focal length, your eyepiece collection will probably have more shorter focal length eyepieces than long ones.   This is simply due to the fact that the long focal length eyepieces don't change the magnification all that much.    The jump from a 24mm eyepiece to a 14mm for instance is only a change of about 35x.  Whereas the jump from a 6mm to a 5mm is 50x.     

 

Those new to the hobby usually use too much magnification, which is not required.   On Jupiter or Saturn, I usually end up around 136x-200x depending on how good the skies are.   As magnification goes up, contrast on the planets goes down so it can become more difficult to see features.  Most of my observing is done with 5 eyepieces (see below).  

 

Explore Scientific 68 deg 24mm - 50x

Explore Scientific  82 deg 14mm - 85x

Explore Scientific 82 deg 8.8mm - 136x

BST Planetary 58 deg 6mm  - 200x

BST Planetary 58 deg 5mm - 250x

 

Add a 2x barlow and these eyepieces become a 12mm, 7mm, 4.4mm, 3mm and 2.5mm.    

 

You don't need to spend a couple hundred dollars on each eyepiece.    If you are on a budget, look at the Astro-Tech Paradigm Dual ED eyepieces.  Good eyepieces for the money.   The BST Planetary eyepieces (at Agena Astro) are also quite good for about $55/ea with comfortable eye relief.

 

Hope that helps.

That’s embarrassing math, I don’t know why I had 600 in my head. 😑

 

This is great help though, I will look at all of these! I was going to get the flex tube but mike high astronomy has issues getting it in stock so I just got the Classic and decided to pocket the $100. So you use the lower power for DSO’s in dark environments and the higher power for planets and the moon. Right now I notice the restriction of 120X as Saturn has a distinguished ring but now distinguishable rings and I was under the impression from video and photos that I would be able to make out a bit more depth there. 


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#9 csrlice12

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Posted 04 August 2020 - 11:02 AM

Where in Colorado are you?  There are astro club's throughout the state.  Denver Astronomical society has a dark blue site out near deer trail, Colorado Springs has a dark site and hosts Rocky Mtn Star Stare, Grand junction is surrounded by really dark skies.....Denver even has a small brick and mortar astro store (Mile High Astro).  Talk to people in your local club, someone will help you thru it all.  +1 on the ES82 24mm though...look for a used one, someone in a club just might have one for sale.



#10 Newtoastronomy85

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Posted 04 August 2020 - 11:17 AM

Where in Colorado are you?  There are astro club's throughout the state.  Denver Astronomical society has a dark blue site out near deer trail, Colorado Springs has a dark site and hosts Rocky Mtn Star Stare, Grand junction is surrounded by really dark skies.....Denver even has a small brick and mortar astro store (Mile High Astro).  Talk to people in your local club, someone will help you thru it all.  +1 on the ES82 24mm though...look for a used one, someone in a club just might have one for sale.

I live North of Denver around Firestone which is a bit east of Boulder. I will check out the clubs, that’s good info, thank you! Mike high astronomy is actually where I bought the scope! I didn’t want to pay massive shipping charges so I bought it local and tried to support small business at the same time. 


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#11 Newtoastronomy85

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Posted 04 August 2020 - 11:21 AM

I just realized the ES 82 degree eyepieces are very expensive in the 18/24/30 range but they’re also 2” eyepieces. Advantage is even more area to see? More light? 



#12 csrlice12

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Posted 04 August 2020 - 11:22 AM

Maybe sometime we can meet up at the Deer Trail dark site.  I can bring an assortment of eyepieces you can try out (I'll want a peek thru that zambuto though).



#13 Newtoastronomy85

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Posted 04 August 2020 - 11:29 AM

Maybe sometime we can meet up at the Deer Trail dark site.  I can bring an assortment of eyepieces you can try out (I'll want a peek thru that zambuto though).

That would be very cool! No Zambuto though, the other poster mentioned that brand but their mirrors are twice as expensive as my entire telescope lol. But in the world of optics I know premium items collect a premium. 



#14 Newtoastronomy85

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Posted 04 August 2020 - 11:34 AM

Can I ask where you guys shop? I know ES has a website but as far as other online stores to look at?



#15 Starman1

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Posted 04 August 2020 - 11:40 AM

10"?

1200mm focal length?

OK, then, try the following focal lengths for a set:

20mm, 10mm, 6.7mm, 5mm, 4mm (all multiples of 60x)

or

24mm, 12mm, 8mm, 6mm, 4.7mm, 4mm (all multiples of 50x)

or, for a basic set chosen by exit pupil:

24mm, 14mm, 9mm, 7mm, 4.7mm (5mm, 3mm, 2mm, 1.5mm, 1mm exit pupils)

 

With any set, you can reduce your eyepiece purchases by the creative use of a 2X barlow lens.

The good news on that front is that, often, a 12mm eyepiece in a 2X Barlow (effectively a 6mm eyepiece) will outperform a 6mm eyepiece

because the eyepiece is now fielding an f/9.4 light cone instead of an f/4.7 light cone.


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#16 tjschultz2011

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Posted 04 August 2020 - 12:04 PM

I've used Amazon and Agena Astro before but I like to support this site and it's contributors. This site is run by Astronomics so it's good to support them plus you get a discount for being a member here. I've also bought from the poster above (Don at EyepiecesEtc.com). His knowledge on eyepieces is immense and he's very helpful to people on this site including myself. 


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#17 25585

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Posted 04 August 2020 - 12:05 PM

My first 3 eyepieces were a 32mm, 20mm & 13mm with a 2x Barlow. For a long while those, with a nebula filter showed me everything. It was only later I started adding more. But as I got more curious, I began looking for more select things and so got deeper into choices.


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#18 Newtoastronomy85

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Posted 04 August 2020 - 12:10 PM

10"?

1200mm focal length?

OK, then, try the following focal lengths for a set:

20mm, 10mm, 6.7mm, 5mm, 4mm (all multiples of 60x)

or

24mm, 12mm, 8mm, 6mm, 4.7mm, 4mm (all multiples of 50x)

or, for a basic set chosen by exit pupil:

24mm, 14mm, 9mm, 7mm, 4.7mm (5mm, 3mm, 2mm, 1.5mm, 1mm exit pupils)

 

With any set, you can reduce your eyepiece purchases by the creative use of a 2X barlow lens.

The good news on that front is that, often, a 12mm eyepiece in a 2X Barlow (effectively a 6mm eyepiece) will outperform a 6mm eyepiece

because the eyepiece is now fielding an f/9.4 light cone instead of an f/4.7 light cone.

Brilliant! That’s immensely helpful to think of it that way, thank you! I’m looking at your website now, appreciate it. 


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#19 Newtoastronomy85

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Posted 04 August 2020 - 12:37 PM

Thoughts on getting a 14MM 82 degree ES with the Barlow someone linked prior. That gives me 5, 7, 10, 14 and 20 for now and I can save up to get a wide view 24-30mm later on to fill out my lower magnifications and replace the 10/20 that came with the scope for better EPs later on.. 240, 171, 120, 85 and 60 magnification would be what I have. 
 

I imagine the 14 would come in more handy with a X2 Barlow for good nights than a 4.7 would as it can’t be any less magnification. 


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#20 GeneT

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Posted 04 August 2020 - 01:27 PM

In my opinion, Explorer Scientific eyepieces are for the most part excellent value for the money--excellent quality for the price. For planetary viewing, I like eyepieces in the range of 70 AFOV, and I like a lot of eye relief--minimum of 15mm, with 19 or 20 even better. For deep sky objects, I like the 100 AFOV eyepieces, however 82 will also provide a nice wide field. I need to wear glasses, but not to view since, I have no astigmatism. If you need glasses to view through the eyepiece, then I recommend 19 or 20mm of eye relief. 



#21 n2dpsky

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Posted 04 August 2020 - 01:43 PM

Gotcha, thank you, that is very helpful! My plan when I bought this scope was to be able to see enough with it due to its size so that I didn’t feel the need to get a bigger one in the future, that’s the hope anyways. I didn’t realize that dark skies were for DSOs, I thought dark skies were just beneficial overall so that’s good to know. Now about the eyepieces, you use lower power for DSOs? I was of the assumption you needed high magnification to see far away stuff.

 

Fantastic, thanks! I’m sensing a trend here I did not realize. I was under the impression I needed more magnification to see far away objects like nebulas etc. 

 

Good to know! I plan on getting these over time and building a set that makes sense and doesn’t become redundant, I will check that eyepiece out. I’m a little bit confused on DSO’s, are you viewing them on low power wide angle lenses or high power, you mentioned high power? Or doesn’t it depend on which ones you’re looking at? 

 

Okay! That makes perfect sense and that’s easy math, thank you! 

 

Fantastic info, thanks! As far as the max magnification, yeah it was just the general consensus the Internet feeds you. I get what you’re saying about the quality of the mirror, totally logical, I checked and I don’t think my $600 scope matches their $1,200 mirror lol so the answer would be no. I’m going to read what your hyperlinked, thanks! 

 

That’s embarrassing math, I don’t know why I had 600 in my head.

 

This is great help though, I will look at all of these! I was going to get the flex tube but mike high astronomy has issues getting it in stock so I just got the Classic and decided to pocket the $100. So you use the lower power for DSO’s in dark environments and the higher power for planets and the moon. Right now I notice the restriction of 120X as Saturn has a distinguished ring but now distinguishable rings and I was under the impression from video and photos that I would be able to make out a bit more depth there. 

I use the magnification that suits the object.  There is no rule.   The size of the object is what is important.  I observe many small DSO at the same magnification as some planets.   Small globulars or planetary nebulae also do well with a 8-14mm eyepieces in your scope.  Double stars might require even more magnification.    Wide field views of open glusters or extended objects benefit from lower power.     In your scope, you're limited to about 1.3 degrees true field of view unless you want to buy an eyepiece that costs as much as your scope.  Then you might get 2 degs.    It's not worth it in my opinion.  A 68 deg 24mm or a 52 deg 32mm plossl yield about the same field of view.

 

Saturn can be tricky.  You want it high in the sky, which for now is well after midnight.  The Cassini division should be pretty easy in your scope, as well as the c ring, but the mirror takes time to cool down to ambient air temp and the skies need to be stable.   You can see some banding on the planet itself and see several of its moons.  That's about most of it.  Jupiter has many more features to see, such as the equatorial bands, the great red spot and small festoons (visible in moments of excellent seeing).   You can catch transits of Jupiter's moons that will cast their shadow on the surface as the moon passes in front of the planet.   That's neat.  High power views of the planets require excellent optical collimation, so as a new newtonian owner, start reading about collimation before you attempt to change anything on your scope.    This is something you will have to learn at some point.   Most collimation requires no tools and can be done on a med bright star, but leave that for now.  Just start reading up and getting your mind wrapped around the process. 

 

I forget to cover exit pupil which is simply the diameter of the shaft of light leaving the eyepiece.   This is calculated by dividing the aperture in mm by the magnification.   Your scope with a 5mm eyepiece yields 240x.  With a 254mm aperture, that gets you 1.06mm.   Much below this and you'll start seeing floaters, which are objects in the fluid in your eye.    Some folks feel good down to .5 mm exit pupil and other start seeing floaters around .8.  Some of this depends on the object.   You'll see them more on planets than double stars.  On the other end of the exit pupil spectrum, you don't really need an exit pupil larger than about 7mm as an adult pupil can't dilate much more than that (there are exceptions).   In your scope, that's achieved with a 33mm eyepiece.  I don't really get 7mm anymore and I observe a lot from the suburbs where it isn't so dark, so the 5 mm exit pupil I get with my 24mm works just fine for me.


Edited by n2dpsky, 04 August 2020 - 01:59 PM.


#22 areyoukiddingme

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Posted 04 August 2020 - 01:59 PM

Hi everyone, this is my first post, I’m the resident newb here. So I just bought a Skywatcher 250P which is the classic 10” Dobsonian. I’ve done a lot of reading the last 2-3 weeks and prior to purchasing so I’ve figured a lot of things out on my own but like everyone else eyepieces are daunting.

So here’s where I’m at...the telescope is a 1200mm and I totally understand the math involved to calculate magnification. Right now I have a 10/20MM that came with the scope so 60 and 120 and a F/ratio of 4.7

I know 50X is max magnification so 600X
I’ve seen anywhere from 30-40X is generally useable on an average night and in general most nights are average.

I understand eye relief pertains to the distance from the eyepiece and still able to see. I don’t wear glasses so this isn’t an issue but I don’t want something that is so low it hurts your eyes.

I maybe understand focal ratio? From what I read I believe mine would be considered a “slow” telescope that’s inherent with the design and size. So I have some blurry areas on the outside of my scopes view so higher end eyepieces would benefit me?

I see the term exit pupil but this I don’t get and I don’t know how an eyepiece is even rated for this?

As far as my viewing site, it can change regularly if I choose. I live in Colorado so a short drive for me can put me in pitch black and in the mountains.

Right now I’m pretty amazed by what I’ve been able to see with Jupiter, Saturn and the moon although it’s been cloudy a lot lately so that sucks. Originally I thought I would buy a X2 Barlow lens and have a 5, 10, 10 and the 20 but the more I thought about it, it’s pretty redundant and buying a X2 for cheap eyepieces seems dumb.

Sorry this is long, easier to get all the info out first. So I’m looking at eyepieces now and I’m maybe $100-150 on what I want to spend. I’m checking out a 82 degree explore scientific ($159 on sale) 4.7mm right now which I thought seemed a good fit because the FOV is large and since my scope is large it will give me more sky at a higher magnification. The 4.7mm would give me 255X

I don’t know if that FOV is good for a beginner? Or recommended? Any other recommendations on eyepieces that might work? Is 255X too much for average nights or could I go more, closer to 300?

I’d like an eyepiece that’s usable without being overkill. With a 10” I feel like anything over a 4mm would probably rarely be useable?

I’m going to Estes Park for a fishing trip in 2 weeks so I want to order something pronto. Because I know I’ll be disappointed if it’s pitch black and a beautiful night and I’m restricted to 120X.

So in conclusion, I’m looking for the highest useable magnification my telescope can handle on average to very good nights.

Thanks!

I've highlighted on part that I didn't see addressed (sorry if I missed it).

 

At F4.7 you are going to see coma in any eyepiece at the edges. Better eyepieces are likely to seems a little better because they will lack astigmatism which adds to the coma to make tings quite hairy.

 

Given this is something you have noticed, you may also want to consider a coma corrector. The GSO can be little finicky to get the eyepieces spaced right (apparently), but should do a good job of cleaning up the edges even in inexpensive eyepieces. Also be aware that it has a magnification factor too--from memory about 10%.

 

The rule is the lower the power and wider the field, the more of the mirrors coma will be seen. So you may find it's less of a concern to you viewing planets at relatively high power.

 

For now, I'd be aiming around 300x as a maximum. Also, I'd think about how low I want to go. Without a coma corrector, F4.7 means that a 30mm gives a it over a 6mm exit pupil which should be good for most people. So I'd tend to look around there too.

 

That will be a 2" format eyepiece. It doesn't have special properties, just think of a 2" as looking through a 2" diameter tube, and how that allows you to see a wider piece of the sky. Other than that, there's no advantage to 2". 

 

If 30 is where you start as a lower power, as stated, you can think about 50-60x steps up in power from there to plan your focal lengths. 



#23 Newtoastronomy85

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Posted 04 August 2020 - 06:49 PM

I use the magnification that suits the object.  There is no rule.   The size of the object is what is important.  I observe many small DSO at the same magnification as some planets.   Small globulars or planetary nebulae also do well with a 8-14mm eyepieces in your scope.  Double stars might require even more magnification.    Wide field views of open glusters or extended objects benefit from lower power.     In your scope, you're limited to about 1.3 degrees true field of view unless you want to buy an eyepiece that costs as much as your scope.  Then you might get 2 degs.    It's not worth it in my opinion.  A 68 deg 24mm or a 52 deg 32mm plossl yield about the same field of view.

 

Saturn can be tricky.  You want it high in the sky, which for now is well after midnight.  The Cassini division should be pretty easy in your scope, as well as the c ring, but the mirror takes time to cool down to ambient air temp and the skies need to be stable.   You can see some banding on the planet itself and see several of its moons.  That's about most of it.  Jupiter has many more features to see, such as the equatorial bands, the great red spot and small festoons (visible in moments of excellent seeing).   You can catch transits of Jupiter's moons that will cast their shadow on the surface as the moon passes in front of the planet.   That's neat.  High power views of the planets require excellent optical collimation, so as a new newtonian owner, start reading about collimation before you attempt to change anything on your scope.    This is something you will have to learn at some point.   Most collimation requires no tools and can be done on a med bright star, but leave that for now.  Just start reading up and getting your mind wrapped around the process. 

 

I forget to cover exit pupil which is simply the diameter of the shaft of light leaving the eyepiece.   This is calculated by dividing the aperture in mm by the magnification.   Your scope with a 5mm eyepiece yields 240x.  With a 254mm aperture, that gets you 1.06mm.   Much below this and you'll start seeing floaters, which are objects in the fluid in your eye.    Some folks feel good down to .5 mm exit pupil and other start seeing floaters around .8.  Some of this depends on the object.   You'll see them more on planets than double stars.  On the other end of the exit pupil spectrum, you don't really need an exit pupil larger than about 7mm as an adult pupil can't dilate much more than that (there are exceptions).   In your scope, that's achieved with a 33mm eyepiece.  I don't really get 7mm anymore and I observe a lot from the suburbs where it isn't so dark, so the 5 mm exit pupil I get with my 24mm works just fine for me.

Good to know, thank you!! I'm trying to narrow down some candidates to add. It's difficult because I don't want to buy everything at once but you want to feel prepared for every situation. I still need to learn the night sky though!

 

I actually just learned about the red spot on Jupiter this week but I cant see it on 120X. I can see the bands around the planet that it's known for and some of the moons, 3 I think is what I've seen. But I would love to take a closer and larger look of them! As far as collimation, I read up on that as well prior to purchasing. I already have a laser collimator and actually adjusted the mirrors the other day so they were aligned, fairly easy and intuitive process. I do appreciate you bringing this up though!

 

Thanks for the detailed explanation! I'm starting to get what you guys are saying.


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#24 Newtoastronomy85

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Posted 04 August 2020 - 06:51 PM

I've highlighted on part that I didn't see addressed (sorry if I missed it).

 

At F4.7 you are going to see coma in any eyepiece at the edges. Better eyepieces are likely to seems a little better because they will lack astigmatism which adds to the coma to make tings quite hairy.

 

Given this is something you have noticed, you may also want to consider a coma corrector. The GSO can be little finicky to get the eyepieces spaced right (apparently), but should do a good job of cleaning up the edges even in inexpensive eyepieces. Also be aware that it has a magnification factor too--from memory about 10%.

 

The rule is the lower the power and wider the field, the more of the mirrors coma will be seen. So you may find it's less of a concern to you viewing planets at relatively high power.

 

For now, I'd be aiming around 300x as a maximum. Also, I'd think about how low I want to go. Without a coma corrector, F4.7 means that a 30mm gives a it over a 6mm exit pupil which should be good for most people. So I'd tend to look around there too.

 

That will be a 2" format eyepiece. It doesn't have special properties, just think of a 2" as looking through a 2" diameter tube, and how that allows you to see a wider piece of the sky. Other than that, there's no advantage to 2". 

 

If 30 is where you start as a lower power, as stated, you can think about 50-60x steps up in power from there to plan your focal lengths. 

I haven't seen it, but I read about that being an issue so I was aware. Thanks, that is good info identifying the lower power based off the exit pupil. I'm looking through different magnifications trying to identify what makes the most sense for me. Don't want to go crazy but want to buy EPs I want to keep.



#25 csrlice12

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Posted 05 August 2020 - 11:32 AM

Oh, going crazy is half the fun!


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