Eyepiece focal length steps - what ratio?

[Andrew*]

In your experience of eyepiece collections, what have you found is the ideal ratio between eyepiece focal lengths? Have you ever felt going from 10mm to 17mm is just too big a jump and you need something in between, or conversely, have you ever felt you have eyepieces too close together that you can't decide which to use (quality notwithstanding)?

What factors affect this? Are owners of uber-wide field eyepieces happy with fewer eyepieces because the FOV "bridges" the gap?

At the moment, I'm planning a new eyepiece line-up and the focal lengths jump up by about 40% each time, e.g. 17-24mm. But I feel that for the higher magnifications, I need tighter-spaced focal lengths to tune the magnification to the seeing conditions, more like 25%, e.g. 4mm to 5mm. But I don't want to end up with less or more eyepieces than I need...

What's your experience?

[Moonshane]

hi Andrew

I don't know if my thread on this helps but here it is.

You'll see I eventually got my 20mm TVP!

cheers

Shane

[rowan46]

I used to have a c5 and 4 eyepieces 20mm 17mm 12mm and 5mm and to be honest i was fairly happy with thatb.But now that I am running on a shorter focal length. I find four focal lengths not enough. I have a 24 for low power 12mm for medium low a 7 and a 5mm I need a 10,9,8. and a barlow and maybe 6 as well. so in answer to your question I think shorter focal length scopes need more eyepieces

[Moonshane]

funny you should say that Rowan as I feel the complete opposite and that long focal lengths need more eyepieces! vive la difference!

[John]

I've found I need more of the shorter focal length eyepieces to give a variety of higher power options, so it's not an even progression in my set:

3.5mm -> 4mm -> 5mm -> 6mm -> 8mm -> 13mm -> 20mm -> 31mm

In % terms the progression, in focal length, is +17% +25% +20% +33% +63% +54% + 55%

I didn't use a formula to arrive at the above preferred focal lengths though, I just tried a lot of eyepieces over a number of years and eventually found certain focal lengths that seemed to "hit the spot" with my scopes. Not very scientific but quite a lot of fun

When I've tried to "do without" one of these focal lengths for a while (most recently 4mm) I've found I missed it so had to fill the gap again.

[rowan46]

funny you should say that Rowan as I feel the complete opposite and that long focal lengths need more eyepieces! vive la difference!

Perhaps it would have been more accurate to say that short focal lengths need more high power eyepieces

[cs1cjc]

For planets one does need close spacing, but otherwise I think a ratio of around 1.4 to 1.6 is a good spacing. I have in my main set ended up with 4.7, 6.7, 8.8, 11, 16, 24, 36 which give ratios of 1.43, 1.31, 1.25, 1.45, 1.50, 1.50. I am experimenting to see if the 8.8 is redundant, giving give a step of from 6.7 to 11 of 1.64, but I have yet to decide.

[Stargazer_00]

I need as many gaps filled as I can squeeze past the wife.

Joking aside though you need more at higher magnifications, so depending on your focal length Ideally have 1mm spacings up to 10mm-12mm and then go for 2-3mm spacings up to 20mm then have 2 or 3 eyepieces from 20mm to 30-35mm. (This is based on an F5-ish scope)

If you have an SCT or other long focal length then you will need to keep the same principle but slide you scales a little. So nothing shorter perhaps than a 10mm as your highest power and work up from there with whatever is available.

Lower magnifications don't really change much in quite large jumps but higher magnifications change a lot with only short jumps.

[RikM]

Mine are spaced at (about) 1.4, all with 68/70° aFOV but I don't really do much planetary observing. This is mainly with an f/4.7 Dob and I use a 2.5x barlow for the shortest two FL.

28 > 20 > 14 > 10 > 7 > (5.6) > (4)

[swamp thing]

I don't do planets so my eyepieces are all deep sky orientated.

I like wide field views, so I picked Naglers for my set. It's also that I find them the most comfortable to use too.

it just so happens that all my eyepieces are 2", so me using a 2" Powermate is no different than others using reducers.

My set (with Powermate) gives me (6,10),12,20, and 31mm (I don't bother Barlowing the 31mm). I will add an 8mm ethos at some point to finish it off.

[Stargazer_00]

Since I recently purchased a new scope with a longer focal length I plan to pad out my lower end in the coming months.

I had a 4.7 , 6.7, 8.8, 11, 14, 18, 24 - i dont count the 6 BGO as it's for specific use

These give me 350x, 246x, 187x, 150x, 117x, 91x, 68x

I intend to add the following to my collection

ES82 - 30mm for 55x

Delos 8mm for 206x

Delos 10mm for 165x

And possibly if I feel the need to keep buying (which I usually do)

Delos 12mm for 137x

Delos 6mm for 275x

And maybe I'll pick up a 100 degree eyepiece, possibly an ES20 100 just so I have a 100 degree eyepiece. 82x

[Mr Spock]

For me the eyepieces I use most go 22, 12, 10, 9, 8.

At low powers anything bigger than 22 gets a brighter background so doesn't really add much - unless it's a large field I want. I sometimes use the 17 for tighter clusters.

The 13 was bought for my former 250mm Newt and is 'inbetween' on the C925 - too high for clusters and not high enough for planets.

At higher powers I like to fit things close between x200 and x300 so I can adjust to the seeing conditions.

[scarp15]

At high, stepping down to medium power in the dob, I use 7mm (x214) 9mm (x167) 13mm (x115) 16mm (x94). The eyepieces are generally a set, share the same 82 degree field and so have no balance issues and are quite parfocal. I tend to switch extensively between them. I also have in between an 8mm and 11mm plossl, which I do not use in the dob and prefer for planetary in the SCT which tracks.

[Stargazer_00]

If someone could come up with a zoom with a constant 82 degree afov for say 6-16mm they'd make a killing. id pay 1000-1200 for that.

[Damo636]

I find a 1.4x (ish) graduation works well for me with my 1500mm fl scope. I have ep's at 5, 7, 10, 14 & 21mm and other than maybe a 6mm for 250x I'm pretty much content with where I'm at.

[John]

If someone could come up with a zoom with a constant 82 degree afov for say 6-16mm they'd make a killing. id pay 1000-1200 for that.

I think Leica might be coming close to that:

http://www.cloudynights.com/item.php?item_id=2390

[andrew63]

Whatever is in the telescope and one in each pocket

andrew

[Damo636]

I wouldn't mind a look through the Leica but at the price it commands it won't be anytime soon.

http://www.apm-telescopes.de/de/product.html?info=3100

[John]

I wouldn't mind a look through the Leica but at the price it commands it won't be anytime soon.

http://www.apm-teles....html?info=3100

1 Leica 8.9-17.8 Zoom = 2.8 Delos at the sale price.

If the image quality is as good as reported perhaps it's a good deal ?

Edit: From a bit more reading it looks like the Lecia zoom is fantastic in F/7 and slower scopes but suffers a bit in edge correction in faster ones.

[Naemeth]

Here are mine, no wonder I feel I have gaps...

2.8, 1.143, 1.875, 1.2, 1.111, 1.3, 1.231

[Damo636]

1 Leica 8.9-17.8 Zoom = 2.8 Delos at the sale price.

If the image quality is as good as reported perhaps it's a good deal

When you look at it like that John perhaps it is.

[Andrew*]

Great contributions guys, thanks. Looks like we're all about agreed on about 1.4x steps, getting a bit tighter towards high power.

The difficulty I have is I'm planning my collection for my current collection of scopes, a 12" f/6 dob (1819mm f/l), an ED120 (900mm) and my TS 90mm (600mm), so a huge spread to cover. At the moment the spread of focal lengths looks something like this:

40mm

24mm (1.67)

17.3mm (1.39)

12mm (1.44)

10mm (1.2)

8 (1.25)

6.2mm (1.29)

5mm (1.24)

4mm (1.25)

3.6mm (1.11)

2.9mm (1.24)

Which is everything I need in all 3 scopes, and probably every scope I'm likely to own in the future.

I'm not really a barlow-user, but I have such a nice Zeiss Abbe 2x barlow it would be a shame not to use it, and I've just got a Klee 2.8x barlow. With a 40mm Aero, 24mm Panoptic, 17.3mm, 10mm and 8mm Delos and those two barlows, I get the above spread of focal lengths. The idea is sticking to 1.25" eyepieces, all of a similar weight and type, I won't mind using the barlows...

[Andrew*]

hi Andrew

I don't know if my thread on this helps but here it is.

http://stargazerslou...yepiece-groups/

You'll see I eventually got my 20mm TVP!

cheers

Shane

Hi Shane, yes, great thread. Kind of set the ball rolling, to be honest, but I was not keen on the idea of having so many eyepieces to juggle with, so that's when I decided to bite the barlow bullet. Already when I have 3 or 4 out at the same time I get stressed out by all the eyepiece caps in my pocket and not having enough room on the eyepiece tray!

I've been making do with a Baader Hyperion zoom up till now, to cover 8-24mm on its own, which I really do love and is almost good enough, but the idea of an orthoscopic 70° FOV is too tempting.

[Andrew*]

3.5mm -> 4mm -> 5mm -> 6mm -> 8mm -> 13mm -> 20mm -> 31mm

In % terms the progression, in focal length, is +17% +25% +20% +33% +63% +54% + 55%

Hi John, that's a really interesting spread. I once had a gap between 12mm and 20mm and always found it too large, but it seems the huge field of the Ethos counters this.

However, when you get to higher power, it's more about the magnification than the true field, so the gaps close up.

[John]

My scopes are not too far off yours Andrew - 12" F/5.3 dob FL 1590mm, ED120 F/7.5 FL 900mm, ED102 F/6.5 FL 663mm

If I spent more time on DSO's then perhaps that 13-20 gap might need filling but my skies are more suited to lunar / planetary / binary observing really.