Eyepiece 101 and selecting eyepieces

[SimM]

I'm looking for some help with getting a balanced set of useful eyepieces... for me it's the most difficult thing to manage.

I realise that there is a lot to know about eyepieces. I'm confused by some of the terms like back focus, AFOV, field stops, vignetting and differences between 1 1/4" and 2" eyepieces/diagonals when considering what eyepieces to get.

I have a new equatorial mount and an existing F3.5 10" newtonian (lots of coma) with an EdgeHD 800 OTA.

My collection of 1 1/4" low grade eyepieces are not very exciting and aligned more to the newtonian scope requires me to give eyepiece selection some thought. Originally I was going to put the newtonian on the mount (requires some work) and replace my eyepieces with a view to eventually being able to use them with a 2" diagonal. Later I planned to buy the EdgeHD 800. However the difficulty of selecting eyepieces and my interest in the SCT has lead to buy the SCT and consider buying eyepieces for the SCT now that will also be useable on the newtonian.

I wear glasses and have astigmatism and my previous dislike of small exit pupils e.g. 4mm eyepieces attracts me to some Baader Hyperian eyepieces - they mostly have 20mm eye relief.

The complete range is 5/8/10/13/17/21/24mm, so approximately 400x, 250x, 200x, 150x, 120x, 100x and 85x magnification.

In the UK I can't see myself reaching for the 5mm 400x as seeing conditions good enough to use it would be very rare... I'm drawn to combinations of:

1. 8/13/17/24mm e.g. 250x, 150x, 120x and 85x
2. 8/13/21/24mm e.g. 250x, 150x, 100x and 85x
3. 10/17/24 e.g. 250x, 120x and 85x

I can see value in everything except the 5mm - can this one really be useful e.g. detail on moon or splitting stars in all but best conditions?

Some of the terms like back focus or actual focal length being longer or F/10 really being F/10.5 mean nothing to me.

If the field stops are less than 1 1/4" (28mm is 28.5mm) is there any value gained by switching to a 2" diagonal for the same lens (probably not) and only a benefit for longer/ultra wide eyepieces?

Is vignetting in an eyepiece likely with any of these with a 1 1'4" diagonal?

A 2" diagonal is a lot bigger and the light path is longer, but for the OTA and an eyepiece, the focal length is somehow increased for reasons I don't understand.

I can see value in all the eyepieces except for the 5mm but having all of them becomes either costly or requires too much swapping, so 3-4 as a starter set would be preferable?

Later I will add a 2" diagonal and some longer/wider eyepieces.

Simon

Edited January 18, 2020 by SimM

[John]

Hi Simon,

Would I be right in thinking that you will be using a coma corrector with your F/3.5 10 inch newtonian ?

Picking eyepieces that will perform well in both an F/10 Edge HD 800 and such a fast newtonian is going to be challenging I think. The Edge 800 is not too challenging but the newtonian certainly will be !

 

 

[SimM]

45 minutes ago, John said:

Would I be right in thinking that you will be using a coma corrector with your F/3.5 10 inch newtonian?

I should use one because there is lots of visible coma however I probably won't get one because the main intent is to use new eyepieces e.g. Baader Hyperium with the F/10 scope where it's not critical.

With the newtonian (that I find unwieldy and too heavy), I have some ancient Televue Plossl EP that work quite well.

I can see that it would be nice to get all the above EP (except the 5mm) but I think later on I might start to regret getting them too soon. There is also the Aspherics as well and a 2" diagonal (Celestron have a new on that I like)...

Simon

[John]

The Hyperions will be fine in the F/10. They will not cope at all well with F/3.5, coma corrector or no coma corrector though.

 

[Louis D]

4 hours ago, SimM said:

If the field stops are less than 1 1/4" (28mm is 28.5mm) is there any value gained by switching to a 2" diagonal for the same lens (probably not) and only a benefit for longer/ultra wide eyepieces?

It's mostly a benefit for really heavy eyepieces like the 12mm ES-92.  It's field lens could probably fit within a 1.25" barrel, but hanging over 2 pounds of eyepiece on that small barrel and small diagonal  would seem iffy.

4 hours ago, SimM said:

Is vignetting in an eyepiece likely with any of these with a 1 1'4" diagonal?

No, though there is the possibility of vignetting with a 2" diagonal and a widest field eyepieces like the 41mm Panoptic because the rear baffle is likely around 38mm in diameter if it is similar to older Celestron 8" SCT designs.

4 hours ago, SimM said:

A 2" diagonal is a lot bigger and the light path is longer, but for the OTA and an eyepiece, the focal length is somehow increased for reasons I don't understand.

An SCT focuses by moving the primary mirror forward and back.  To throw the focal plane back far enough to clear the optical path length of a 2" diagonal, the primary mirror must be moved forward a much smaller amount thanks to the magnification factor of the secondary.  This then causes the overall focal length of the telescope to grow.  It also induces some spherical aberration because the scope is designed with a 1.25" diagonal's path length in mind, so now the scope's working focal length is slightly off-spec.  Thus, for best planetary performance, swap out the 2" diagonal for a 1.25" diagonal.

4 hours ago, SimM said:

The complete range is 5/8/10/13/17/21/24mm, so approximately 400x, 250x, 200x, 150x, 120x, 100x and 85x magnification.

I'd recommend a 35mm Aero ED SWA for widest field viewing.  They work pretty well even at f/6, let alone f/10, and have just enough eye relief to work with eyeglasses if you push in a bit.  This would give you 57x.

I wouldn't go much above 200x in an 8" with such a large obstruction.  You're down to a 1mm exit pupil at that power.  I find I can't go much below 0.7mm with any sort of comfort due to floaters in my eye.  Technically, that would be 285x with a 7mm eyepiece, but seeing most nights probably won't support that much magnification.

3 hours ago, SimM said:

There is also the Aspherics as well

A wee bit overpriced for the amount of astigmatism and chromatic aberration they show in the outer field.  At 30mm, I'd get the APM UFF instead and the 35mm Aero ED instead of the 36mm.  With an SCT, you could also get the 56mm Meade Plossl to open up your exit pupil to 5.6mm for viewing nebula with a nebula filter.

[SimM]

I like the Baader Hyperion eyepieces because they have good eye-relief. For an 8" SCT the 5mm is perhaps too short as seeing conditions rarely allow 400x but the other EP are a good selection of sizes 8mm - 24mm. Their boxed smaller set of 4 EP are 5, 10, 17 and 24mm of which I would only make use of are three: 24, 17 and 10 giving 200x, 120x and 85x. I can also see value in the other three EP that are 21, 13 and 8mm giving 250x, 150x and 100x. I looked at the resulting AFOV at astronomy.tools and it would be nicer to buy them all except the 5mm! The larger boxed set is also very tempting because the closer spacings can be better matched to seeing conditions. Perhaps the 5mm could also be used to split binaries or to aid 2/3 star alignment (almost as good as having a reticle eyepiece)?

Although I don't need a 2" diagonal until I buy one or two really wide/long eyepieces e.g. the 35mm Aero mentioned or APM UFF or similar mentioned, the supplied Celestron 1 1/4" diagonal has some drawbacks. Eyepieces are screwed in which is fiddly and prone to damaging them. The diagonal is also held on with screws which might not be the most secure mounting. If I did look at a 2" diagonal it would be one with an integrated SCT attachment so that the light path through the bigger diagonal was about the same as a 1 1/4" with the 7cm visual back. Celestron have two, the XLT coated one (compression ring with a screw to secure the eyepiece 93527) and a better one with a dielectric diagonal with true twist locks (93573). Compared to the 1 1/4" equivalent version this would be more future proof.

Of course with the shorter SCT connector the eyepiece is also closer to the back of the SCT - but I think it would be OK. Perhaps unscrewing the SCT coupling isn't ideal but using a push fit on a long visual back with screws is also not ideal too? A 2" diagonal would be nice and I really would be interested to know if the focal point is retained when switching to this type e.g. the length of the 1/4" visual back is optimised for the SCT and the more direct SCT coupling compensates for the bigger 2" diagonal - the blurb says "using the SCT adapter provides a solid and secure connection whilst limiting additional optical distance" - suggests that it's "close" rather than exactly like the "optimal" focal length e.g. with a 1 1/4" diagonal and "long" visual back.

It's suggested that it's better to use the 1 1/4" adapter rather than risk putting a Baader Hyperion EP directly into a 2" diagonal and risking touching the mirror. The adapter doesn't interfere (vignette) the smaller (1 1/4") eyepieces.

Simon

Edited January 17, 2020 by SimM

[rwilkey]

Hi Simon, the Baader Hyperions will work well in your f/10 but not in the Newtonian f/3.5, as John refers above.  Have you considered the BST StarGuiders which work well in most scopes, they also have 16mm eye relief which should be good with spectacles, found here: https://www.firstlightoptics.com/bst-starguider-eyepieces.html

[Louis D]

54 minutes ago, rwilkey said:

Hi Simon, the Baader Hyperions will work well in your f/10 but not in the Newtonian f/3.5, as John refers above.  Have you considered the BST StarGuiders which work well in most scopes, they also have 16mm eye relief which should be good with spectacles, found here: https://www.firstlightoptics.com/bst-starguider-eyepieces.html

Unfortunately, due to the design of the eye cup, they only have 12mm of usable eye relief in reality, except for the 25mm which is usable with eyeglasses at 18mm of ER.  I compared the AT Paradigm (BST SG equivalents) to the Meade 5000 HD-60 line in this thread.  I'll repeat my table of eye relief values here.  The HD-60s are much easier to use with eyeglasses.

[SimM]

The search for EP goes on... astronomy.tools has a good tool for estimating AFOV etc.

Thanks for all the help. I'm leaning towards some combination of Hyperion but I'm also eyeing up a more concentrated set of Morpheous with perhaps one Hyperion.

I'm not too bothered with eyepieces for Newtonian. BST StarGuiders from FLO are not expensive but they don't have sufficient eye-relief for my tired old eyes!

My choice for Morpheous would be three eyepieces: 17.5mm (116x) + 14mm (145x) + 9mm (226x).
The 6.5mm (313x) wouldn't be viable for UK sky conditions.
The 12.5mm (163x) is perhaps too close to the 14mm (145x).

For Hyperion it would be 5 eyepieces: 21mm (97x) + 17mm (120x) + 13mm (156x) + 10mm (203x) + 8mm (254x).
The 5mm (400x) wouldn't be viable for UK sky conditions.
The 24mm (85x) has a simliar AFOV as the Celestron Plossyl 40mm (51x) that's in with the OTA).

So 3 Morpheous EP would provide a cleaner and wider FOV compared to 5 Hyperions which are more tightly grouped and overall slightly less expensive. I could add the 24mm Hyperion to either setup. If I was adding it to the Hyperions then I would simply buy the 7 eyepiece set as it's no more expensive than 6 EP to get a case.

The 4 eyepiece set has 3 EP that I could use e.g. 10mm, 17mm and 24mm as the 5mm isn't really viable.

Later I might get a 2" diagonal and one/two wide field eyepieces but that's for the future.

Simon

[SimM]

Decision made/ordered/anticipated... I had some more thoughts and decided to get B-P Hyperion eyepieces and an (ideally BH) case. The target OTA is my EdgeHD 800 and not my Newtonian. I won’t be buying a 2” diagonal just yet. The “thought process” for me isn’t entirely straightforward and perhaps I have a tendency to over think things a bit too much?

My “need”is satisfied by 8mm, 13mm and 21mm (high) (medium) (low) and coincidently about 3/4 x F/10, 1 1/4 x F/10 and 2 x F/10, that’s sometimes suggested as a “guide”.

I also “want” a wider view satisfied by 24mm (wide) which has an exit pupil 2.4mm (for my tired “old” eyes). For the moment I don’t want to replace my diagonal with a 2” one, so a true low wide TFOV EP isn’t a priority.

Depending upon UK seeing conditions I can appreciate that the 10mm would sometimes give better viewing than trying to use an 8mm so also adding this to the “mix” would be worthwhile. I can’t see a reason for getting the 5mm as it is about on the seeing limit of 3/4 x F/10 x 2/3 and in the UK not worth the trouble. The 17mm would be quite nice (a Morpheus 17.5 would be even nicer).

Excluding the 5mm I found it cheaper to buy 5 eyepieces and a case but add the 17mm later it becomes more expensive than the full kit. Common sense says buy the 5 eyepieces (and avoid the temptation) later to add the 17mm or 17.5mm. My credit card reveals something else because (clearly I can’t justify that) it shows that I bought the full set.

I don’t plan to use all the eyepieces in an evening. More likely I will pick a low, medium and high that’s skewed a bit lower, normal or higher, if that makes sense, especially as UK skies are very variable too.

Simon

[Don Pensack]

On 15/01/2020 at 15:01, SimM said:

I'm looking for some help with getting a balanced set of useful eyepieces... for me it's the most difficult thing to manage.

I realise that there is a lot to know about eyepieces. I'm confused by some of the terms like back focus, AFOV, field stops, vignetting and differences between 1 1/4" and 2" eyepieces/diagonals when considering what eyepieces to get.

I have a new equatorial mount and an existing F3.5 10" newtonian (lots of coma) with an EdgeHD 800 OTA.

My collection of 1 1/4" low grade eyepieces are not very exciting and aligned more to the newtonian scope requires me to give eyepiece selection some thought. Originally I was going to put the newtonian on the mount (requires some work) and replace my eyepieces with a view to eventually being able to use them with a 2" diagonal. Later I planned to buy the EdgeHD 800. However the difficulty of selecting eyepieces and my interest in the SCT has lead to buy the SCT and consider buying eyepieces for the SCT now that will also be useable on the newtonian.

I wear glasses and have astigmatism and my previous dislike of small exit pupils e.g. 4mm eyepieces attracts me to some Baader Hyperian eyepieces - they mostly have 20mm eye relief.

The complete range is 5/8/10/13/17/21/24mm, so approximately 400x, 250x, 200x, 150x, 120x, 100x and 85x magnification.

In the UK I can't see myself reaching for the 5mm 400x as seeing conditions good enough to use it would be very rare... I'm drawn to combinations of:

1. 8/13/17/24mm e.g. 250x, 150x, 120x and 85x
2. 8/13/21/24mm e.g. 250x, 150x, 100x and 85x
3. 10/17/24 e.g. 250x, 120x and 85x

I can see value in everything except the 5mm - can this one really be useful e.g. detail on moon or splitting stars in all but best conditions?

Some of the terms like back focus or actual focal length being longer or F/10 really being F/10.5 mean nothing to me.

If the field stops are less than 1 1/4" (28mm is 28.5mm) is there any value gained by switching to a 2" diagonal for the same lens (probably not) and only a benefit for longer/ultra wide eyepieces?

Is vignetting in an eyepiece likely with any of these with a 1 1'4" diagonal?

A 2" diagonal is a lot bigger and the light path is longer, but for the OTA and an eyepiece, the focal length is somehow increased for reasons I don't understand.

I can see value in all the eyepieces except for the 5mm but having all of them becomes either costly or requires too much swapping, so 3-4 as a starter set would be preferable?

Later I will add a 2" diagonal and some longer/wider eyepieces.

Simon

Read this:  https://www.handprint.com/ASTRO/ae1.html

And the 4 parts that follow.  You will understand a lot more than the basics by you reach part 3 or 4.

[JOC]

I was scanning the thread titles and saw Eyepiece 101 and thought, 'my word someone's bank balance has been hit hard!!!'

[John]

20 minutes ago, JOC said:

I was scanning the thread titles and saw Eyepiece 101 and thought, 'my word someone's bank balance has been hit hard!!!'

William Optics did actually have a 9mm 101 degree eyepiece:

That extra 1 degree makes all the difference !

[JOC]

31 minutes ago, John said:

William Optics did actually have a 9mm 101 degree eyepiece:

LOL

Mind you I was thinking more in terms of someone actually owning 101 different EPs!!

[Louis D]

3 hours ago, John said:

William Optics did actually have a 9mm 101 degree eyepiece:

That extra 1 degree makes all the difference !

Sort of like in Spinal Tap: "These go to eleven."

[Louis D]

3 hours ago, JOC said:

LOL

Mind you I was thinking more in terms of someone actually owning 101 different EPs!!

There are quite a few folks who have many eyepieces.  Jim Barnett and Tamiji Homma on CN both report having hundreds of eyepieces, though each has thinned the herd over time.  I'm sure there's some folks on SGL that have hundreds as well.  I have just over 60 myself.