TheSkySearchers

Posted: **Fri Jul 23, 2021 5:05 pm**

notFritzArgelander wrote: ↑Thu Jul 22, 2021 7:28 pm
Don Pensack wrote: ↑Thu Jul 22, 2021 6:43 pm To be honest, it's hard to see a difference in magnification <30x between eyepieces.
That means the 15mm and 12mm are essentially duplicates.
And you certainly don't need a 20mm for in between the 26mm and 15mm.
A constant magnification change could be: 24mm, 12mm, 8mm, 6mm, 4.8-5mm, 4mm.
26mm is close enough. I don't think you need the 15mm, though having an "extra" eyepiece is OK.
Yeah, I just don't buy constant magnification change as a good guiding principle. Apart from a 5mm exit pupil being essential for some objects really old folks find that 6mm exit pupil might be a good aperture stop if they can only dilate to 5mm. Fortunately I'm not that old yet and can still find a 6mm exit pupil useful.

I still find exit pupil increments at the long end and focal length increments shorter than 2mm exit pupil to be a better principle for optimizing by object and sky conditions both. Constant magnification increments might make an adequate minimalist set but it won't optimize performance. YMMV.

Constant magnification jumps means the magnification steps between eyepieces narrows by % as you approach high powers. Since the atmosphere is usually the limiting factor at high power, this gives you a way of sneaking up on the night's limit slowly and optimizing the high power. I notice many planetary observers have sets less than 1mm apart in the shorter focal lengths.
And a constant magnification jump at the bottom end assures that when you change magnification, you will see a difference that matters. If you are looking at an object at low power and see it's too small, you want the next eyepiece you pick to provide a realistic increase in size from the previous eyepiece, not just a tad. How many eyepieces do you need at 10x/inch of aperture and lower? I would argue 2 max since 90%+ of all deep sky objects look best above 10x/inch.

I owned about 150-180 eyepieces while I owned an 8" scope, and I found a difference in magnification of about 50x to be ideal. Less than that, and the change wasn't significant enough. More than that, and I wanted an eyepiece in between.
That wasn't a minimal set in that scope because my seeing conditions allowed 300x to be used a lot, so it was a six-eyepiece set I settled on. A lot of the fluctuation in my set had to do more with finding the ideal eyepiece for me at the magnification I wanted.

I have performed the same experimentation with my 12.5" and 4" and find the optimum jumps in both to be fairly even steps as long as apparent fields are similar. Unfortunately, the makers of eyepieces don't seem to shrink the gaps between focal lengths enough at high power to allow the slow crawl up to the seeing limit--you end up having to pick multiple brands and models to do so.

Posted: **Fri Jul 23, 2021 5:26 pm**

Don Pensack wrote: ↑Fri Jul 23, 2021 4:40 pm This might help in looking for eyepieces of a particular focal length:
https://www.cloudynights.com/topic/7583 ... p=10917573

As for selecting by exit pupil, lots of discussions suggest a 40% difference might be optimum. Less than that, and the brightness difference might not be noticeable.
If we start with a 6mm exit pupil, the exit pupil progression is: 6.0mm, 4.6mm, 3.6mm, 2.8mm, 2.2mm, 1.7mm, 1.3mm, 1.0mm, 0.8mm, 0.6mm
Now that may be a bigger set of eyepieces than you want, and it will put the eyepieces really close together in magnification at the low end.

If all your eyepieces have the same apparent field, you can get smooth decreases in field area of 50% between eyepieces by dividing the focal lengths by 1.414
To wit: 30mm, 21.2mm, 15.0mm, 10.6mm, 7.5mm, 5.3mm. We can round off to a set of 30, 21, 15, 11, 8, 5mm, a 6-eyepiece set.
There are many ways to choose eyepieces. And if you buy too few, buy another. Too many? Sell one or more.
One truth: you cannot buy a set of eyepieces today you will find optimum in 10 years. So just take the plunge.

I think I'm doing that. :smile:

Posted: **Sat Jul 24, 2021 12:41 pm**

I've been looking for eyepieces, but I seem to have problems.
I know that pretty much anything from TeleVue works, but they are somewhat expensive, and some focal lengths are missing.
I know that Plössls and orthos from any company with a good name work. It's just that many focal lengths seem to be missing and the eye relief is often quite small.

I think ES 68 degree and wider are supposed to work, but how is it with eyepiece series from Vixen or Pentax?
Or maybe there are series from other manufacturers worth considering?

With the expensive eyepieces, I think, I should consider the eye relief too, because I may have to use glasses in the future.
I have double images in both eyes.

Posted: **Sat Jul 24, 2021 4:12 pm**

turboscrew wrote: ↑Sat Jul 24, 2021 12:41 pm I've been looking for eyepieces, but I seem to have problems.
I know that pretty much anything from TeleVue works, but they are somewhat expensive, and some focal lengths are missing.
I know that Plössls and orthos from any company with a good name work. It's just that many focal lengths seem to be missing and the eye relief is often quite small.

I think ES 68 degree and wider are supposed to work, but how is it with eyepiece series from Vixen or Pentax?
Or maybe there are series from other manufacturers worth considering?

With the expensive eyepieces, I think, I should consider the eye relief too, because I may have to use glasses in the future.
I have double images in both eyes.

So what you need is longer eye relief eyepieces.
Some I can recommend to you that have longer eye relief:
Baader Hyperion in 5, 8, 10, 13, 17, 21, 24, 31, 36mm
Baader Morpheus in 6.5, 9, 12.5, 14, and 17.5mm
TeleVue Delite in 3, 4, 5, 7, 9, 11, 13, 15, 18.2mm
TeleVue Delos in 3.5, 4.5, 6, 8, 10, 12, 14, 17.3mm
TeleVue Plössl in 55, 40, 32mm
TeleVue Nagler in 17, 22, 31mm
TeleVue Panoptic in 27, 35, 41mm
Pentax XW in 3.5, 5, 7, 10, 14, 20, 30, 40mm
APM Ultra Flat Field in 30, 24, and 18mm (and the Celestron, Meade, Altair, and Orion versions as well)
Explore Scientific 92° in 12, 17mm
Explore Scientific 82° in 30mm
Explore Scientific 68° in 28, 34, 40mm
Explore Scientific 62° in 26, 32, 40mm
Explore Scientific 52° in 30, 40mm
Vixen SLV 50° in all focal lengths from 2.5-25mm.

All will work fine at f/6 and longer, though some are better than others. I highlighted my personal favorites, but you may like the others and there are many fine eyepieces I don't mention.

Posted: **Sat Jul 24, 2021 4:16 pm**

Don Pensack wrote: ↑Sat Jul 24, 2021 4:12 pm
turboscrew wrote: ↑Sat Jul 24, 2021 12:41 pm I've been looking for eyepieces, but I seem to have problems.
I know that pretty much anything from TeleVue works, but they are somewhat expensive, and some focal lengths are missing.
I know that Plössls and orthos from any company with a good name work. It's just that many focal lengths seem to be missing and the eye relief is often quite small.

I think ES 68 degree and wider are supposed to work, but how is it with eyepiece series from Vixen or Pentax?
Or maybe there are series from other manufacturers worth considering?

With the expensive eyepieces, I think, I should consider the eye relief too, because I may have to use glasses in the future.
I have double images in both eyes.
So what you need is longer eye relief eyepieces.
Some I can recommend to you that have longer eye relief:
Baader Hyperion in 5, 8, 10, 13, 17, 21, 24, 31, 36mm
Baader Morpheus in 6.5, 9, 12.5, 14, and 17.5mm
TeleVue Delite in 3, 4, 5, 7, 9, 11, 13, 15, 18.2mm
TeleVue Delos in 3.5, 4.5, 6, 8, 10, 12, 14, 17.3mm
TeleVue Plössl in 55, 40, 32mm
TeleVue Nagler in 17, 22, 31mm
TeleVue Panoptic in 27, 35, 41mm
Pentax XW in 3.5, 5, 7, 10, 14, 20, 30, 40mm
APM Ultra Flat Field in 30, 24, and 18mm (and the Celestron, Meade, Altair, and Orion versions as well)
Explore Scientific 92° in 12, 17mm
Explore Scientific 82° in 30mm
Explore Scientific 68° in 28, 34, 40mm
Explore Scientific 62° in 26, 32, 40mm
Explore Scientific 52° in 30, 40mm
Vixen SLV 50° in all focal lengths from 2.5-25mm.

All will work fine at f/6 and longer, though some are better than others. I highlighted my personal favorites, but you may like the others and there are many fine eyepieces I don't mention.

Unfortunately, my tube is F/4. I guess that rules out Hyperion and maybe Morpheus? Maybe ES 52° too?

Posted: **Sat Jul 24, 2021 4:46 pm**

Ok, now I need to look into the TeleVue 2.5X Powermate.

When I first got my scope I picked up the Orion Shorty x2 Barolow. I used it a lot in 2 ways, the normal 2x and then removing the lense and attaching to the EP to get 1.5x. These days when I do use it (rarely) I just use the lense (1.5x) to get me that slight increase for specific views. I still want a good 6mm BCO for planetary though.

Posted: **Sat Jul 24, 2021 5:06 pm**

turboscrew wrote: ↑Sat Jul 24, 2021 12:41 pm I've been looking for eyepieces, but I seem to have problems.
I know that pretty much anything from TeleVue works, but they are somewhat expensive, and some focal lengths are missing.
I know that Plössls and orthos from any company with a good name work. It's just that many focal lengths seem to be missing and the eye relief is often quite small.

I think ES 68 degree and wider are supposed to work, but how is it with eyepiece series from Vixen or Pentax?
Or maybe there are series from other manufacturers worth considering?

With the expensive eyepieces, I think, I should consider the eye relief too, because I may have to use glasses in the future.
I have double images in both eyes.

ES 82 and 68 work quite well in fast DOBs. However, they do have short effective eye relief, impossible to see the whole FOV with eyeglasses on. This is the main reason I sold my ES82s and switched to TV Delites and Pentax XWs which have 20mm eye relief. I believe Delites are well corrected for fast scopes. Pentax XWs are corrected for fast refractors (because they were originally designed to work with Pentax spotting scopes). My understanding that the lower range of XWs 3.5mm to 10mm works well in fast DOBs, but not 14mm and above.

Posted: **Sat Jul 24, 2021 7:18 pm**

Bigzmey wrote: ↑Sat Jul 24, 2021 5:06 pm
Pentax XWs are corrected for fast refractors (because they were originally designed to work with Pentax spotting scopes). My understanding that the lower range of XWs 3.5mm to 10mm works well in fast DOBs, but not 14mm and above.

Newtonians have positive field curvature (FC). Based on the graphs supplied by Pentax, the 14mm and longer XWs have positive FC, which is then added to that of newtonian compounding the visual impact. Whereas the 10mm and shorter all have negative FC which are off-set by the positive FC of the newtonian. That is why the shorter focal lengths are known to work better in newtonians, and precisely why I did not go any longer than the XW10 to use in my dobs. That said, I am not particularly bothered by mild FC. But, I still decided to stay away from the XWs longer than 10mm, and have gone instead to the Ethos (13mm and 21mm). Because of my aging eyes I typically do not go any longer than 24mm and the 21mm Ethos works amazingly in my 17.5 inch.

But, it is my understanding that newtonian FC is related to its focal length and those over around 1200mm have minimal. So it may well be that the longer focal length XWs would work fine with my 10, 12 and 17.5 inch dobs (1250, 1520 and 1987.55 f/l respectively). However, I have already invested in the Ethos at 13mm and 21mm as mentioned above and quite happy with the results.

Posted: **Sat Jul 24, 2021 11:04 pm**

kt4hx wrote: ↑Sat Jul 24, 2021 7:18 pm
Bigzmey wrote: ↑Sat Jul 24, 2021 5:06 pm
Pentax XWs are corrected for fast refractors (because they were originally designed to work with Pentax spotting scopes). My understanding that the lower range of XWs 3.5mm to 10mm works well in fast DOBs, but not 14mm and above.
Newtonians have positive field curvature (FC). Based on the graphs supplied by Pentax, the 14mm and longer XWs have positive FC, which is then added to that of newtonian compounding the visual impact. Whereas the 10mm and shorter all have negative FC which are off-set by the positive FC of the newtonian. That is why the shorter focal lengths are known to work better in newtonians, and precisely why I did not go any longer than the XW10 to use in my dobs. That said, I am not particularly bothered by mild FC. But, I still decided to stay away from the XWs longer than 10mm, and have gone instead to the Ethos (13mm and 21mm). Because of my aging eyes I typically do not go any longer than 24mm and the 21mm Ethos works amazingly in my 17.5 inch.

But, it is my understanding that newtonian FC is related to its focal length and those over around 1200mm have minimal. So it may well be that the longer focal length XWs would work fine with my 10, 12 and 17.5 inch dobs (1250, 1520 and 1987.55 f/l respectively). However, I have already invested in the Ethos at 13mm and 21mm as mentioned above and quite happy with the results.

As Vlad Sacek has explained on his site:
www.telescope-optic.net
specifically, the field curvature diagrams down the page here:
https://www.telescope-optics.net/eyepie ... ions_1.htm
and elsewhere,
When the field curvature of the scope and eyepiece match, it is not additive--we see a flat surface because the eyepiece's field is in "lock step" with the scopes curvature.
It is when they don't match that we see a curved surface:
) + ) = |
| + | = |
) + | = )
| + ) = )
The reason people see FC in the 14mm and 20mm Pentax XWs is because
a. they have short focal length scopes with strongly curved focal planes
or
b. the curvature of the scope doesn't match the eyepieces
or
c. the curvature in the eyepieces is too much to accommodate, diopter-wise.

Posted: **Sat Jul 24, 2021 11:09 pm**

turboscrew wrote: ↑Sat Jul 24, 2021 4:16 pm
Don Pensack wrote: ↑Sat Jul 24, 2021 4:12 pm
turboscrew wrote: ↑Sat Jul 24, 2021 12:41 pm I've been looking for eyepieces, but I seem to have problems.
I know that pretty much anything from TeleVue works, but they are somewhat expensive, and some focal lengths are missing.
I know that Plössls and orthos from any company with a good name work. It's just that many focal lengths seem to be missing and the eye relief is often quite small.

I think ES 68 degree and wider are supposed to work, but how is it with eyepiece series from Vixen or Pentax?
Or maybe there are series from other manufacturers worth considering?

With the expensive eyepieces, I think, I should consider the eye relief too, because I may have to use glasses in the future.
I have double images in both eyes.

Unfortunately, my tube is F/4. I guess that rules out Hyperion and maybe Morpheus? Maybe ES 52° too?
I'll amend the list for f/4:
Baader Morpheus in 6.5, 9, 12.5, 14, and 17.5mm (f/4 is right at the cusp of a no-go for these)
TeleVue Delite in 3, 4, 5, 7, 9, 11, 13, 15, 18.2mm
TeleVue Delos in 3.5, 4.5, 6, 8, 10, 12, 14, 17.3mm
TeleVue Nagler in 17, 22, 31mm
TeleVue Panoptic in 27, 35, 41mm
Pentax XW in 3.5, 5, 7, 10, 14, 20, 30, 40mm
APM Ultra Flat Field in 30, 24, and 18mm (and the Celestron, Meade, Altair, and Orion versions as well)
Explore Scientific 92° in 12, 17mm

You will need a coma corrector at f/4, so the f/ratio of the scope will not be a pure f/4, but more likely f/4.25-f/4.6 with coma corrector in place. That's a good thing, where eyepiece suitability is concerned.

Posted: **Sun Jul 25, 2021 12:16 am**

Don Pensack wrote: ↑Sat Jul 24, 2021 11:04 pm
kt4hx wrote: ↑Sat Jul 24, 2021 7:18 pm
Bigzmey wrote: ↑Sat Jul 24, 2021 5:06 pm
Pentax XWs are corrected for fast refractors (because they were originally designed to work with Pentax spotting scopes). My understanding that the lower range of XWs 3.5mm to 10mm works well in fast DOBs, but not 14mm and above.
Newtonians have positive field curvature (FC). Based on the graphs supplied by Pentax, the 14mm and longer XWs have positive FC, which is then added to that of newtonian compounding the visual impact. Whereas the 10mm and shorter all have negative FC which are off-set by the positive FC of the newtonian. That is why the shorter focal lengths are known to work better in newtonians, and precisely why I did not go any longer than the XW10 to use in my dobs. That said, I am not particularly bothered by mild FC. But, I still decided to stay away from the XWs longer than 10mm, and have gone instead to the Ethos (13mm and 21mm). Because of my aging eyes I typically do not go any longer than 24mm and the 21mm Ethos works amazingly in my 17.5 inch.

But, it is my understanding that newtonian FC is related to its focal length and those over around 1200mm have minimal. So it may well be that the longer focal length XWs would work fine with my 10, 12 and 17.5 inch dobs (1250, 1520 and 1987.55 f/l respectively). However, I have already invested in the Ethos at 13mm and 21mm as mentioned above and quite happy with the results.
As Vlad Sacek has explained on his site:
www.telescope-optic.net
specifically, the field curvature diagrams down the page here:
https://www.telescope-optics.net/eyepie ... ions_1.htm
and elsewhere,
When the field curvature of the scope and eyepiece match, it is not additive--we see a flat surface because the eyepiece's field is in "lock step" with the scopes curvature.
It is when they don't match that we see a curved surface:
) + ) = |
| + | = |
) + | = )
| + ) = )
The reason people see FC in the 14mm and 20mm Pentax XWs is because
a. they have short focal length scopes with strongly curved focal planes
or
b. the curvature of the scope doesn't match the eyepieces
or
c. the curvature in the eyepieces is too much to accommodate, diopter-wise.

Thanks for the more in-depth explanation Don.

Posted: **Sun Jul 25, 2021 7:48 am**

Don Pensack wrote: ↑Sat Jul 24, 2021 11:09 pm
I'll amend the list for f/4:
Baader Morpheus in 6.5, 9, 12.5, 14, and 17.5mm (f/4 is right at the cusp of a no-go for these)
TeleVue Delite in 3, 4, 5, 7, 9, 11, 13, 15, 18.2mm
TeleVue Delos in 3.5, 4.5, 6, 8, 10, 12, 14, 17.3mm
TeleVue Nagler in 17, 22, 31mm
TeleVue Panoptic in 27, 35, 41mm
Pentax XW in 3.5, 5, 7, 10, 14, 20, 30, 40mm
APM Ultra Flat Field in 30, 24, and 18mm (and the Celestron, Meade, Altair, and Orion versions as well)
Explore Scientific 92° in 12, 17mm

You will need a coma corrector at f/4, so the f/ratio of the scope will not be a pure f/4, but more likely f/4.25-f/4.6 with coma corrector in place. That's a good thing, where eyepiece suitability is concerned.

Thanks! That helps a LOT!
My ES HR coma corrector is mentioned to extend the focal length by 6% (-> f/4.24).

Looks like the only 8 mm is Delos. Maybe I'll get 7 mm or 9 mm instead.

Posted: **Wed Jul 28, 2021 11:16 am**

It looks like 8.8 mm ES 82° might be a good option too? At least according to this.
Eye relief 15.6 mm, but cheap enough to be replaced later if needed.

Posted: **Wed Jul 28, 2021 12:09 pm**

Now, that I've been searching for eyepieces to my tube, it made me wonder, what's the idea in that there are so many 114 / 500 (F/4.4) beginner scopes? And other beginner scopes from F/4 to F/5?

Posted: **Wed Jul 28, 2021 1:22 pm**

The 5 mm DeLite arrived. I guess I'm getting ready for the autumn.
Next I'll, probably, get an EP around 8 mm.

Posted: **Wed Jul 28, 2021 2:20 pm**

turboscrew wrote: ↑Wed Jul 28, 2021 12:09 pm Now, that I've been searching for eyepieces to my tube, it made me wonder, what's the idea in that there are so many 114 / 500 (F/4.4) beginner scopes? And other beginner scopes from F/4 to F/5?

Many years ago, the most common f/ratios for that size of scope was f/8 to f/9.
When I got started almost 6 decades ago, that size of scope was f/10 to f/12.
Fashion changes, and shorter scopes cost less to produce and ship.
Weighing less, they can also be used on lighter, cheaper, mounts.
The only negative side effects of the shorter f/ratios are that they demand a higher accuracy in collimation, and, ironically, eyepieces corrected for short f/ratios, which may cost more.

Posted: **Wed Jul 28, 2021 6:21 pm**

turboscrew wrote: ↑Wed Jul 28, 2021 11:16 am It looks like 8.8 mm ES 82° might be a good option too? At least according to this.
Eye relief 15.6 mm, but cheap enough to be replaced later if needed.

ES 82s are indeed good quality EPs and work well in fast DOBs. The only issue is that the top element in the EP is recessed, so the actual eye relief is pretty tight. You can't see whole FOV with eyeglasses.

Posted: **Wed Jul 28, 2021 6:21 pm**

turboscrew wrote: ↑Wed Jul 28, 2021 1:22 pm The 5 mm DeLite arrived. I guess I'm getting ready for the autumn.
Next I'll, probably, get an EP around 8 mm.

Congrats! You will not be disappointed.

Posted: **Tue Aug 03, 2021 2:54 am**

Yeah, I bought a 5mm Delite also. Been trying to get along with a 30 year old Kiyohara 5mm. Coatings are so much improved since then, the images are kind of dim even though the planetary details are comparable.

TheSkySearchers

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