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Pentax XW 20 on 8sct or something else for lunar observation

Celestron Eyepieces Moon Observing Optics Planet SCT Visual Equipment Beginner
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#1 Stllrr

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Posted 19 September 2023 - 02:16 PM

I have been trying to figure out which eyepiece to go with for Celestron 8 SCT, f10, with 1.25 diagnol.

 

 

I am wanting something around 18-25mm and has a wide field. I want to achieve super sharp views, as close as I can get, and fit the entire moon in the eyepiece with a little space around it.

 

I have the 25mm plossl that came with the scope and I tried out a ton of other pieces and settled on the Pentax XW10 for planetary viewing. I have been reading about field curvature on the Pentax XW 20mm and I just want to make sure I'm looking for the right eyepiece for what I want to do with my current scope.

 

 

I have a cheap SVBONY 20mm wide angle to try out...just to test the field and magnification and hold me off until I figure which higher end eyepiece to get. Any tips or suggestion to point me in the right direction are greatly appreciated. Clear skies!

 

 

 

 


Edited by Stllrr, 19 September 2023 - 02:16 PM.


#2 mystang52

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Posted 19 September 2023 - 02:25 PM

As I say on other eyepiece questions....when I do reply:   I don't know the technical aspects of EP's, but I know what I like.  I have the Pentax 20mm and I love it ( I also have the 10 and 40).  FWIW, I originally bought the Pentax 23 but returned it.  I felt I had to jam my eyeball right up to the lens.  I've been very happy with the Pentax 20. 


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#3 Echolight

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Posted 19 September 2023 - 02:26 PM

Well, there's the 17.5 Morpheus, 16mm Nagler, some others. Lots actually that are likely better than the SVBony.

 

I don't suppose you have a 2 inch diagonal? Long Perng aka Orion LHD 20mm 80° is very popular. Although I believe a 14 UWA would give just a tiny bit of space around the Moon.

 

The ES68 20mm is supposedly very sharp, by all accounts. 

And it is still on sale for $149.97.

Actually, the similarly highly acclaimed ES68 16mm would just barely fit a full Moon with right at 0.51 degrees in your scope. Also on sale for $149.97.

 

Personally, I like the option to go high power also. And without switching eyepieces.

So.... Baader 8-24 zoom is very sharp. And easy to look through.

You want to just barely frame the Moon, or run the field stop right up against it, this is can't miss either way.

 

Actually, if you want to really see something, for the Moon, specifically the terminator, I barlow the Baader zoom and run it up from 250x to 400x, depending on the atmospheric conditions.

Talk about WOW!


Edited by Echolight, 19 September 2023 - 02:47 PM.

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#4 SeattleScott

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Posted 19 September 2023 - 04:31 PM

100x is pretty low for lunar viewing. I mean it is good for framing the whole moon in the view, but I would want more magnification to dive down into the craters.


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#5 Stllrr

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Posted 19 September 2023 - 04:33 PM

No 2 inch diagonal yet...Im about a month into the hobby so I'm trying to build up a good set of tools I like to use. The SVBONY is just a cheap tester to see how I feel about 20mm eyepieces and to test the view, hold me off until I can make a purchase for a really good eyepiece in that range. I am open to suggestions so I can spend hours on here reading about them lol, but I like those recommendations, from what I read I was under the impression anything lower than 18mm would start cropping the moon. I've only heard good things about explorer scientific eyepieces and I'm trying to stay under 400 dollars.



#6 gnowellsct

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Posted 19 September 2023 - 04:43 PM

The xw 16.5 will give you the same field of view as the xw20, at higher magnification.

They are both excellent eye pieces but these days my hand reaches out more for the 16.5.

Greg N

#7 Echolight

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Posted 19 September 2023 - 04:44 PM

The Moon is full of details. And to see them right, you want to look at a little piece of it at a time.

 

No other object has such high contrast. Or can take such high magnification, and reveal such a high amount of detail.

 

 

Looking at the whole Moon can be serene.

 

But looking at the terminator at unbelievably high magnification is surreal!


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#8 gnowellsct

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Posted 19 September 2023 - 04:46 PM

A quick way to check field of view equivalence is just multiply the focal length in millimeters times the apparent field of view.

20x70=1400
16.5x85=1402
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#9 Stllrr

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Posted 19 September 2023 - 05:08 PM

The xw 16.5 will give you the same field of view as the xw20, at higher magnification.

They are both excellent eye pieces but these days my hand reaches out more for the 16.5.

Greg N

Thanks for the tip, follow up dumb question...will that 16.5 achieve framing the entire moon in the eyepiece? I'm not worried about getting up close to see the surface as I have my 10mm for now, I just want to see the landscape and be able to look around in the eyepiece at all the detail. I'm looking to build my eyepiece collection with above average/excellent eyepieces and I really like my Pentax XW 10. I kind of wanted to keep my eyepiece collection the same, but I don't mind having different brands for different magnifications.


Edited by Stllrr, 19 September 2023 - 05:09 PM.


#10 Starman1

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Posted 19 September 2023 - 05:13 PM

Some eyepieces in the 20mm range (Greg's point is valid, though--a 16.5mm x 85° would be a better moon viewing eyepiece, though slightly outside your price range).

Some good examples of eyepieces that you would like in the 20mm +/- 2mm range that are in your price range:

 

APM Ultra Flat Field 18mm 65° (1.25") 1170 by Greg's shorthand.

Pentax XW 20mm 70° (1.25") 1400

Explore Scientific 20mm 68° (1.25") 1360

TeleVue Delite 18.2mm 62° (1.25") 1128

TeleVue Panoptic 19mm 68° (1.25") 1292

Orion Expanse 20mm 66° (1.25") 1320

 

If you convert to a 2" rear diagonal, then also consider:

APM XWA 20mm 100° (2") 2000 by Greg's shorthand

Astrotech XWA 20mm 100° (2") 2000

Astromania SWA 22mm 70° (2") 1540

 

In your 8" SCT, look for eyepieces of at least 51° apparent field at a 20mm focal length to see the entire Moon in the eyepiece.

My recommendations for eyepieces in your scope:

40mm in 2" (32mm Plössl for largest possible field in 1.25")

20mm in 1.25" or 2"

13-14mm

10mm

8mm

Use a 1.5-2.0x Barlow to achieve any higher powers than that.


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#11 Stllrr

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Posted 19 September 2023 - 05:37 PM

Some eyepieces in the 20mm range (Greg's point is valid, though--a 16.5mm x 85° would be a better moon viewing eyepiece, though slightly outside your price range).

Some good examples of eyepieces that you would like in the 20mm +/- 2mm range that are in your price range:

 

APM Ultra Flat Field 18mm 65° (1.25") 1170 by Greg's shorthand.

Pentax XW 20mm 70° (1.25") 1400

Explore Scientific 20mm 68° (1.25") 1360

TeleVue Delite 18.2mm 62° (1.25") 1128

TeleVue Panoptic 19mm 68° (1.25") 1292

Orion Expanse 20mm 66° (1.25") 1320

 

If you convert to a 2" rear diagonal, then also consider:

APM XWA 20mm 100° (2") 2000 by Greg's shorthand

Astrotech XWA 20mm 100° (2") 2000

Astromania SWA 22mm 70° (2") 1540

 

In your 8" SCT, look for eyepieces of at least 51° apparent field at a 20mm focal length to see the entire Moon in the eyepiece.

My recommendations for eyepieces in your scope:

40mm in 2" (32mm Plössl for largest possible field in 1.25")

20mm in 1.25" or 2"

13-14mm

10mm

8mm

Use a 1.5-2.0x Barlow to achieve any higher powers than that.

Thank you, very informative...it can be hard navigating the forums to find this info and making sure it's up to date. I still have research to do on barlows, but appreciate that input as I haven't even looked into them to get better magnification.

 

I see your recommendations and will be looking into those, but if I had to pick the XW20 or XW16.5 for what I want to see which should I go with? I'm wanting to keep my collection minimal...maybe 3-5 eyepieces. I have the 25mm plossl and 10mm XW already so the 16.5 would make sense right?



#12 PJBilotta

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Posted 19 September 2023 - 05:48 PM

The benefit with the 20mm is that, like your 10mm XW, it is 1.25". It's also parfocal with the 10.

I do love my XWs, but I do see significant field curvature in the 20. Significantly superior, for close to the same price, is the 17.5mm Morpheus, which also gives you a bit more magnification. It's a great lunar performer - sharp, excellent contrast, and just a little soft at the very edges. I generally prefer it to the 20 XW.
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#13 Starman1

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Posted 19 September 2023 - 06:14 PM

Thank you, very informative...it can be hard navigating the forums to find this info and making sure it's up to date. I still have research to do on barlows, but appreciate that input as I haven't even looked into them to get better magnification.

 

I see your recommendations and will be looking into those, but if I had to pick the XW20 or XW16.5 for what I want to see which should I go with? I'm wanting to keep my collection minimal...maybe 3-5 eyepieces. I have the 25mm plossl and 10mm XW already so the 16.5 would make sense right?

The 16.5mm 85° Pentax is $479.95 and 2" diameter.

TeleVue's 16mm 82° is $369 and 1.25" and renowned for its incredible sharpness.  It has only 10mm or eye relief, though, probably just *barely* longer than your eyelashes, so definitely not glasses-compatible.

At 127x, it's beginning to get to be an excellent spread from a 25mm (81x - 127x = 46x).  The next jump, keeping the same spread would be to a 11-12mm focal length.

If the 10mm is the bedrock, the next longer focal length should be 14mm, then 20mm (you can see why Pentax used that spread).

 

Perhaps you should be looking into 2" visual backs and 2" diagonals.  Then you could get some truly wide fields at low power.

 

One other option for the 8" SCT: add the f/6.3 focal reducer/field flattener to convert your scope to f/6.3 (1280mm focal length).

Then you could have the big fields with 1.25" eyepieces and skip the expense of 2" eyepieces, 2" diagonals, 2" visual backs.

A 32mm Plössl or 24mm 65-68° eyepiece could yield a 1.21° true field, where now your 25mm yields only 0.61°!

A nice set of 1.25" eyepieces at f/6.3 could include: 26mm, 13mm, 8.5mm, 6.5mm, 5mm--all small and light eyepieces.

You could even go with all-Pentax: 14mm, 10mm, 7mm, 5mm and add an Explore Scientific 26mm 62° for maximum field or an APM 24mm Ultra Flat Field 24mm 65° or an ES 24mm 68°.

Or, a set of Baader Morpheus at 14mm (measured at 13.9mm), 9mm (measured at 8.9mm), 6.5mm (measured at 6.7mm), 4.5mm (measured at 4.8mm) and add the 24mm wide field for low power/widest field.

The focal reducer costs about the same as a moderately-priced eyepiece.  And, it can easily be removed to use the scope at f/10.

I think you'll get used to the wider true fields, though, and better edge correction.


Edited by Starman1, 19 September 2023 - 06:17 PM.

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#14 Inkie

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Posted 19 September 2023 - 06:15 PM

The Moon is full of details. And to see them right, you want to look at a little piece of it at a time.

 

No other object has such high contrast. Or can take such high magnification, and reveal such a high amount of detail.

 

 

Looking at the whole Moon can be serene.

 

But looking at the terminator at unbelievably high magnification is surreal!

I'll echo this series of statements.  In my experience, a first view with low power, say enough that the moon nearly fills the FOV (field of view) is very appealing, but with some knowledge of the lunar geography, you'll soon have some favourite sights that you will want to visit, and to see even better than last time you viewed it (we all chase that dream).  This means the very best optics, the best seeing, and some pretty stiff magnification.  We're talking about 150-300 power, with the first number almost always going to yield decent views in modestly good seeing.  So, yes, by all means, get a nice 17mm eyepiece, but also get one south of 10 mm.  I have, as my signature states, all the way down to 4.7, and it's a very fine eyepiece.  But, I know from experience that those smaller mm's can only be used two-five nights a year...mebbe. 


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#15 Stllrr

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Posted 19 September 2023 - 06:18 PM

The benefit with the 20mm is that, like your 10mm XW, it is 1.25". It's also parfocal with the 10.

I do love my XWs, but I do see significant field curvature in the 20. Significantly superior, for close to the same price, is the 17.5mm Morpheus, which also gives you a bit more magnification. It's a great lunar performer - sharp, excellent contrast, and just a little soft at the very edges. I generally prefer it to the 20 XW.

Thank you for the feedback, I had to google par-focal to make sure I knew what you were talking about lol. I don't mind having to focus as I just got a tri-bahtinov mask to help when I have to perform collimation, it hasn't arrived yet, but I am excited to try it and see how my scope is currently aligned. That's another recommendation for the morpheus and what I'm trying to view with my current scope so I know what rabbit hole I'll be diving into later.



#16 Stllrr

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Posted 19 September 2023 - 06:28 PM

Also...would these eyepieces sizes be good for the upcoming solar eclipse? I'm supposed to get 73% coverage where I'm at in California.



#17 Starman1

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Posted 19 September 2023 - 06:38 PM

Also...would these eyepieces sizes be good for the upcoming solar eclipse? I'm supposed to get 73% coverage where I'm at in California.

You will need a white light solar filter for the scope, like a Thousand Oaks glass filter, or a Baader Film Solar filter.

The turbulence in the daytime air usually means low powers, meaning 25mm and longer eyepiece focal lengths.

And if you have a finder scope, leave the cap on.

It helps to have a sun finder on the scope:

https://www.highpoin...BRoChbEQAvD_BwE

 

But, that stuff would be great to view the sun any time.

For a 73% eclipse, what you need is a naked eye filter, like:

https://www.rainbows.../eclipse-safety

https://www.rainbows...e-solar-viewers


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#18 gnowellsct

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Posted 19 September 2023 - 07:03 PM

Thank you, very informative...it can be hard navigating the forums to find this info and making sure it's up to date. I still have research to do on barlows, but appreciate that input as I haven't even looked into them to get better magnification.

I see your recommendations and will be looking into those, but if I had to pick the XW20 or XW16.5 for what I want to see which should I go with? I'm wanting to keep my collection minimal...maybe 3-5 eyepieces. I have the 25mm plossl and 10mm XW already so the 16.5 would make sense right?


The xw20 is a very good eye piece. It is even an excellent eyepiece particularly if you're not trying to pop it into an f 4.5 dob.

The xw 16.5 is a spectacular eye piece such as there are few of.

Both eye pieces give a field of view of 0.7°, which is 42 arc minutes. The full moon is 30 arc minutes so you will be able to fit it in and if you have tracking keep it in.

You probably won't be observing the full moon though. You are more likely to be observing it during the phases when you can really see the craters. So let's say it will be anywhere from 25 arc minutes down to a mere sliver in most circumstances.

The full moon of course shows very few features.

Greg N
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#19 ewave

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Posted 19 September 2023 - 07:17 PM

my vote goes to the Baader 17.5mm Morpheous

 

It will fit the entire moon at medium high power in your 8" SCT. 

 

Here, try this field of view calculator with many configurations:

 

https://astronomy.to.../field_of_view/


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#20 Dave Bush

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Posted 19 September 2023 - 07:48 PM

As I say on other eyepiece questions....when I do reply:   I don't know the technical aspects of EP's, but I know what I like.  I have the Pentax 20mm and I love it ( I also have the 10 and 40).  FWIW, I originally bought the Pentax 23 but returned it.  I felt I had to jam my eyeball right up to the lens.  I've been very happy with the Pentax 20. 

See you have an Evolution 6, so do I.  Do you detect any excessive field curvature with that 20mm XW, meaning, with a star in focus in the center are stars near the edge also in focus?



#21 Dave Bush

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Posted 19 September 2023 - 07:55 PM

The benefit with the 20mm is that, like your 10mm XW, it is 1.25". It's also parfocal with the 10.

I do love my XWs, but I do see significant field curvature in the 20. Significantly superior, for close to the same price, is the 17.5mm Morpheus, which also gives you a bit more magnification. It's a great lunar performer - sharp, excellent contrast, and just a little soft at the very edges. I generally prefer it to the 20 XW.

Curious, I know you're using it on a C8 and mine's a C6 but still, did you try it with the r/c?  I know it had field curvature on a C8 at native f/10 but I never tried it with an r/c.



#22 Stllrr

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Posted 19 September 2023 - 07:58 PM

my vote goes to the Baader 17.5mm Morpheous

 

It will fit the entire moon at medium high power in your 8" SCT. 

 

Here, try this field of view calculator with many configurations:

 

https://astronomy.to.../field_of_view/

That link is excellent! Thank you.



#23 gnowellsct

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Posted 19 September 2023 - 09:53 PM

Also...would these eyepieces sizes be good for the upcoming solar eclipse? I'm supposed to get 73% coverage where I'm at in California.



Assuming you have a full aperture solar filter on the C8 you would be fine. The sun is the same diameter as the Moon to within an arc minute or two. So whatever fits a full moon fits a solar eclipse.

For a full disc solar eclipse I probably wouldn't use any telescope at all. It's so mind blowing that's best without an instrument.

Greg N
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#24 Stllrr

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Posted 19 September 2023 - 11:06 PM

I went with the Baader Morpheus 17.5

 

I hear a lot of recommendations for them and it seems like an excellent line, I probably should have went with 9mm Morpheus instead of my Pentax XW10, but we will see what the future holds and I still have to look into barlows. Thanks everyone for the recommendations and tips for picking future eyepieces.

 

Clear skies!


Edited by Stllrr, 19 September 2023 - 11:23 PM.

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#25 PJBilotta

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Posted 21 September 2023 - 08:28 PM

Curious, I know you're using it on a C8 and mine's a C6 but still, did you try it with the r/c? I know it had field curvature on a C8 at native f/10 but I never tried it with an r/c.


Yes - Personally, I see the field curvature in the 20 (and the 14) both with and without the R/C. The R/C helps a bit, but it's still visible.

Many are the times I have prayed for Pentax to redesign the 10 and 14 to correct this - an XW2! We're they to, the XWs would be simply the best truly complete line available, from 40mm all the way down to 3.5mm.
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