Eyepiece advice for an 8 inch Dob.

[Neutrinosoup]

I’ve only ever owned refractors and have a nice range of eyepieces for them…(I think) all are 1.25 size

TV Delos 17.3, 10, 6 and 4.5

TV Panoptic 24

TV Nagler 13

I’ve just ordered an 8 inch dobsonian to use with my 13 year old daughter and suspect I need to add to my eyepiece collection at the low power end?

1) Do I want 2 inch eyepieces for a dobsonian? And if so which ones? Or do I just want one additional TV Panoptic eyepiece? e.g. 35mm

2) Are there any midrange eyepieces (2 or 3) that I could get for my daughter to use when I’m not around? (She’s always a bit nervous handling the Delos eyepieces because she knows how much they cost). What I don’t want to do is get poor eyepieces for her though.

When I ask her what she prefers using she likes parfocal eyepieces and likes not having too many choices! 
 

Thanks

Niall

 

[Voxish]

Baader Zoom?

[Carbon Brush]

Another vote for a zoom EP. Not necessarily Baader then it can be your daughters affordable zoom😁

BST starguiders are affordable and give a lot of view for the ££.
I don't have enough experience of these to comment on parfocal. But a couple of parfocal rings won't cost much.

 

[Louis D]

If you want a widest field view without breaking the bank, or dealing with a lot of weight, the 40mm Lacerta ED is very good.  I've found it to be just a bit less sharp at the edges than my 40mm Pentax XW.  The Lacerta appears to still be available from this dealer where I bought mine.  Here's my review of it:

Keep in mind, if you live in light polluted skies, the sky background will be very washed out at that power/exit pupil.  However, the Pleiades and other large star clusters will look very nice in it.

[Neutrinosoup]

11 minutes ago, Louis D said:

If you want a widest field view without breaking the bank, or dealing with a lot of weight, the 40mm Lacerta ED is very good.  I've found it to be just a bit less sharp at the edges than my 40mm Pentax XW.  The Lacerta appears to still be available from this dealer where I bought mine.  Here's my review of it:

Keep in mind, if you live in light polluted skies, the sky background will be very washed out at that power/exit pupil.  However, the Pleiades and other large star clusters will look very nice in it.

Back garden is Bortle 3 (rural south Scotland). I could travel for darker skies but don’t usually bother.

[Don Pensack]

Then a compromise: the 30mm 70° UltraFlatField from United Optics.

It's available for £179 from FLO as a Stellalyra Ultra Flat Field:

https://www.firstlightoptics.com/stellalyra-eyepieces/stellalyra-30mm-ultra-flat-field-2-eyepiece.html

Magnification won't be too low, so the field won't be completely washed out.

The step to the 17.3mm is a good jump up in magnification.

It's one of the finest 30mm eyepieces out there today.  I find it sharper than every other 30-31mm eyepiece except the 31mm Nagler.

The true field is 34.4% wider than the 24mm Panoptic.  In a typical 8" dobsonian, 1.73°.

[Louis D]

10 hours ago, Neutrinosoup said:

Back garden is Bortle 3 (rural south Scotland). I could travel for darker skies but don’t usually bother.

That is nice and dark by suburban standards.  I'm in Bortle 7 or worse in most directions.  I would think either a 30mm or 40mm would work well under your skies.  I actually use both regularly, so it's not an either/or situation.  The 40mm (Lacerta ED, Pentax XW, ES-68) is best for really big objects or groups of objects and for centering smaller, but brighter ones before swapping to a higher power.  The 30mm UFF is nice for framing slightly smaller objects or groups of objects at a higher contrast level, higher power, and rendering them more sharply while using a smaller, lighter eyepiece that typically balances better when swapping with higher powered eyepieces.

[bosun21]

Another vote for the 30mm UFF👍.

[Voxish]

On 20/12/2023 at 23:31, Don Pensack said:

Then a compromise: the 30mm 70° UltraFlatField from United Optics.

It's available for £179 from FLO as a Stellalyra Ultra Flat Field:

https://www.firstlightoptics.com/stellalyra-eyepieces/stellalyra-30mm-ultra-flat-field-2-eyepiece.html

Magnification won't be too low, so the field won't be completely washed out.

The step to the 17.3mm is a good jump up in magnification.

It's one of the finest 30mm eyepieces out there today.  I find it sharper than every other 30-31mm eyepiece except the 31mm Nagler.

The true field is 34.4% wider than the 24mm Panoptic.  In a typical 8" dobsonian, 1.73°.

Good call

[Mr Spock]

The 30mm UFF is brilliant in my 12" SL Dob.

[bingevader]

Maybe have a play first, and see if you think you are missing anything?

Ah, she/you are worried about the expensive EPs.

None of my EPs were terribly expensive, and they all work well in my 8" dob.

Is it the 1200mm?

If so, I find the 6mm is super on the best nights, but if the sky isn't quite there, then I need to drop back a couple of mm.

You might get away with it at Bortle 3, if your skies are still, as well as clear.

Is she's not using the Nagler either!?  Then something in the middle would be good for the the DSOs.

I like the 2° EPs for framing DSOs at shorter FLs and for the drift time at the longer FLs.

Edited December 22, 2023 by bingevader

[Don Pensack]

2 hours ago, bingevader said:

I like the 2" EPs for framing DSOs at shorter FLs and for the drift time at the longer FLs.

A note: the field stops of 100-110° eyepieces (and anything narrower in apparent field) fit inside a 1.25" barrel at any focal length shorter than 14mm.

So you don't get any wider a field with a 2" barrel below that point.  2" may be convenient, or support a large heavy eyepiece more safely (e.g. 9mm 120°), but to all intents and purposes, 1.25" eyepieces can be as wide as they get

when the focal length is shorter than 14mm.  If the goal is nice "framing of DSOs at shorter focal lengths", one needn't use a 2" eyepiece to attain the necessary field.

[bingevader]

Hi Don,

Have I got my terminology the wrong way round then?

I was thinking short FL = Wide FOV?

Apologies for the confusion if I have.

[bosun21]

Shorter FL equates to more magnification. The FOV is dependent on the design of the eyepiece itself. As Don said 14mm and below can be accommodated by 1.25" eyepieces showing the same FOV as 2" eyepieces. 

[Spile]

I’m going to recommend the zoom as well. Although I have and really like the Baader Hyperion IV, user reviews on here and CN suggests the Svbony equivalent is fantastic for the money.

[Don Pensack]

17 hours ago, bingevader said:

Hi Don,

Have I got my terminology the wrong way round then?

I was thinking short FL = Wide FOV?

Apologies for the confusion if I have.

An eyepiece can have any apparent field of view you want, from 20° up to 100-120°.

The True field on the sky that you see in the eyepiece is dependent on an inside focal plane diameter called the Field Stop.

Field stops get smaller with shorter focal length.

Once the field stop easily fits inside a 1.25" barrel, it is no longer necessary to make the eyepiece in a 2" diameter.

And as the apparent field gets larger, at any given focal length, the field stop grows as well.

With currently-produced eyepieces, the transition point between 2" and 1.25" is:

41-40mm at 40° apparent field.  41mm and longer will be 2" eyepieces because the field stop will be too large to fit in a 1.25" barrel.

33-32mm at 50°

27-26mm at 62°

25-24mm at 68°

17-16mm at 82°

14-13mm at 100°

So you don't need 2" eyepieces to get a large apparent field and large true field with shorter focal length eyepieces.

[Louis D]

On 22/12/2023 at 16:02, bingevader said:

Have I got my terminology the wrong way round then?

I was thinking short FL = Wide FOV?

Wide True FOV or TFOV is generally associated with longer FL eyepieces.

On 22/12/2023 at 16:52, bosun21 said:

The FOV is dependent on the design of the eyepiece itself.

Apparent FOV or AFOV is eyepiece design dependent.

Edited December 24, 2023 by Louis D

[bosun21]

1 hour ago, Louis D said:

On 22/12/2023 at 22:52, bosun21 said:

The FOV is dependent on the design of the eyepiece itself.

Apparent FOV or AFOV is eyepiece design dependent.

Edited 1 hour ago by Louis

That's what I said.

[DAT]

On 20/12/2023 at 12:59, Louis D said:

If you want a widest field view without breaking the bank, or dealing with a lot of weight, the 40mm Lacerta ED is very good.  I've found it to be just a bit less sharp at the edges than my 40mm Pentax XW.  The Lacerta appears to still be available from this dealer where I bought mine.  Here's my review of it:

Keep in mind, if you live in light polluted skies, the sky background will be very washed out at that power/exit pupil.  However, the Pleiades and other large star clusters will look very nice in it.

Somewhere else I asked the question about what was the lowest mag ep that could be used in an 8" dob and as expected replies were quite different. Some people suggested respecting the 6mm exit pupil "rule", especially if the observer is of a "certain age". On an f/6 that meant using a 35-36mm, no lower. This would avoid washed out backgrounds and unwanted reflections. Others said you could get away with a 40mm depending on the target (e.g. Pleiades), and that the problems expected would not be too noticeable. I really enjoy my Stellalyra 30mm uff but it cannot frame the Pleiades that well, a fair chunk is missing. I would be curious as to what it looked like in a 40mm from where I am (heavy light polluted area). In my previous scope (130p f/5), the Pleaides was stunning through an es 68 24mm  (x27).

[Mr Spock]

I use a 30mm UFF in my 12". It gives an ideal exit pupil of 6mm.

However, I also use my 42mm LVW with success. It's 7.6mm exit pupil so loses a bit of light around my old eyes, but not much more than 7x50 binoculars. It will pick up nebulae in the Pleiades in my Bortle 6 skies.

[John]

Over the years I have found my 21mm much more use for observing fainter DSO's than my 31mm under my bortle 5 skies. The longer FL eyepiece is not often used unless I want the widest possible true field.

The above applied to my F/5.3 12 inch dob and my current F/5.9 8 inch dob.

I have owned 40mm wide field eyepieces but found them more useful with my refractors which have slower focal ratios.

 

[Ricochet]

3 hours ago, DAT said:

avoid washed out backgrounds

This is going to be your main problem given your location in London. Your pupil size won't be the limiting factor, rather how (un)acceptable you find the greyness of the sky at any given exit pupil. You will have to try to judge how your 30mm performs in this regard to decide whether it is worth trying a longer focal length with a lighter background sky.

3 hours ago, DAT said:

I really enjoy my Stellalyra 30mm uff but it cannot frame the Pleiades that well, a fair chunk is missing.

The one downside of the 30mm UFF is that it "only" has a 70° field. If we use the approximation of TFOV = AFOV * FLeyepiece / FLtelescope then the 30mm UFF should have about the same TFOV as my 21E, approximately 1.75°, although I would say the 21E just fits the Pleiades so perhaps that is a sign of the differing distortions in the two eyepieces. Switching to my 28mm Nirvana increases the field to ~1.9° and frames the cluster that little bit more nicely and so for this size of object provides a worthwhile alternative, but as @John says, the shorter focal length eyepiece tends to provide the better view when you don't need the extra field of view. If you do go for a longer eyepiece then check that the AFOV is wide enough that the TFOV does increase over your 30mm and look for reviews that discuss the edge performance as you don't want to be buying an eyepiece where the extra field isn't actually usable, which is quite likely with cheaper eyepieces.

[Ratlet]

I think if you want best bang for buck with eyepieces you can't do better than bst starguiders.  £55 a pop, 60° fov and a nice eyecup setup.  The 8mm and 12mm work really well on my 10" Stellalyra.  Not tried the 18mm, but might work well.  Does it all for less than the baader and if she one she only looses that FL rather than them all.

Those 3 eyepieces will frame a lot of DSO well and the 8mm will handle planetary most nights.

Personally I'd avoid the svbony zoom on a dob.  The fov gets down to 40° which for me is a nip too tight.

What dob did you get?  The lower power eyepieces that come with most are usually okay and recommendations is usually to replace it last. 

Edited December 24, 2023 by Ratlet

[DAT]

2 hours ago, Ricochet said:

If you do go for a longer eyepiece then check that the AFOV is wide enough that the TFOV does increase over your 30mm and look for reviews that discuss the edge performance as you don't want to be buying an eyepiece where the extra field isn't actually usable, which is quite likely with cheaper eyepieces.

 It but would be interesting to see how a tv panoptic (35 & 41mm) or Pentax (40mm) performs. I have managed to get a great view of the double cluster with the SL 30mm uff and an even better view of each cluster separately with a TV delite 18mm, I wouldn't want to loose much quality by increasing the TFOV

[John]

26 minutes ago, DAT said:

 It but would be interesting to see how a tv panoptic (35 & 41mm) or Pentax (40mm) performs. I have managed to get a great view of the double cluster with the SL 30mm uff and an even better view of each cluster separately with a TV delite 18mm, I wouldn't want to loose much quality by increasing the TFOV

A hyper-wide eyepiece such as the APM 20mm 100 will show very nearly as large a true field as the 30mm UFF does but with 50% more magnification. This can be very useful under skies with some light pollution: