Eyepiece focal length steps - what ratio?

[Andrew*]

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

I'd definitely agree that shorter f/l scopes need more eyepieces in general. In my example, for lowest power in my 90mm I have a 40mm giving a fantastic 4.5° FOV at 15.5x. Can't get enough of it! But the scope is capable of going up to 200x (3mm). Covering a range of 3mm-40mm involves a lot of eyepieces!

However, on my 12" f/6, low power is 45x with a 40mm, and high power is around 230x with a 8mm, so all I really need between is 24mm (76x), 17.3mm (105x), 150x (12mm) and 10mm (180x).

[Andrew*]

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

Nice. That's going to be a yummy collection! Personally though, I wouldn't be able to decide which to put in the focuser. Would be like a kid in a candy shop!

[Andrew*]

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.

That's my issue. I'm too much of a generalist, so I need to cover all the bases! I even do a bit of imaging, so there's another couple of cases of stuff to worry about!

[AlexB67]

From my limited experience so far, I love barlows. I use mine a lot. If there was one item I'd be wanting to spend quite a bit on it would be a quality Barlow, in spite of fiddling about putting them in each time, which IMO is a small price against having to change an eyepiece anyway.

I gather there are a few models in just under 200 pounds range, got one in my bookmarks somewhere on my home PC, that function such that when you put them in, you wouldn't even know there were there in tems of quality reduction, or so I am told in the forum here on varioous threads

In the long run, the more eye pices you have multiplying by two costs nothing extra once you have one. I now got the BST since yesterday. I now also have a nice double the magnification for free .

Athough conditions were poor last night, I gave the BST a spin for the first time last night. I was able to appreciate very quickly that it is good addition, and also with the Barlow that I currently have. With a good quality barlow that will only get better.

I appreciate one can argue the downsides to a barlow versuw the same magnification with eyepiece alone, but if you have that big a range of scopes and eyepieces with wisely chosen gaps, a barlow can fill in a lot the of the gaps at a small cost in the long run.

I know my argument is largely driven by ecenomics perhaps, but I got to be realistic how much I can spend veruss the experience I get in return, the barlow fits the bill nicely when you havent' got a bottomless pit of cash to invest in this stuff. I'd rahter have barlow insteads of having another X amount in my overdraft limit or am I being too sensible ?

[RikM]

I don't know if this is any use, but here's how my eyepiece / telescope combinations work out.

Black is what I currently have. Grey is what I am planning to add.

OTA-EP specs.xlsx

OTA-EP specs.pdf

[Moonshane]

I'd definitely agree that shorter f/l scopes need more eyepieces in general. In my example, for lowest power in my 90mm I have a 40mm giving a fantastic 4.5° FOV at 15.5x. Can't get enough of it! But the scope is capable of going up to 200x (3mm). Covering a range of 3mm-40mm involves a lot of eyepieces!

However, on my 12" f/6, low power is 45x with a 40mm, and high power is around 230x with a 8mm, so all I really need between is 24mm (76x), 17.3mm (105x), 150x (12mm) and 10mm (180x).

it just depends which way you look at it I suppose. for short focal lengths, and assuming a small aperture so slower focal ratio, you have a smaller exit pupil so can use very low powers which are often not possible with larger apertures given the larger exit pupil.

for longer focal lengths, assuming a large aperture and fast focal ratio you are restricted in min. power terms by exit pupil and true field possible. also, every increment of 1mm has a large effect on magnification. e.g. with my big dob (1840mm focal length with paracorr) a 6mm eyepiece gives 307x and a 4mm 460x. in your frac it would be 103x and 155x so you might get away without a 5mm at 124x but I doubt I would. this is one of the primary reasons I have the nagler zoom as you have infinite steps in between 6-3mm.

[Macavity]

Once stuck resolutely to my Hyperion series: (5)... 8... 13... 21 (The Golden Ratio?)!

But, in a quest to reduce amount of kit, now just have: 10mm... 17mm... 31mm (Asp).

Indeed close to 1.7 : 1 in ratio! Based on Root Three, to sound more intellectual?

Have retained a set of TMB planetaries: 8, 6, 4 and 2.5. Difficult to justify the last! But

if I ever get more serious with Planetary observation? These days, concentrating more

on VIDEO Astronomy, remote (indoor!) observing etc. etc. Eyepieces are "old hat"?

[great_bear]

One solution - if you've got a barlow - is for the ratio to be:

(n)th root of (x)

- where x is the power of the barlow, and n is the number of consecutive eyepieces in the set that you'll be wanting to barlow.

- I'm not kidding, either

[Pig]

I find it best to have a good range as the seeing dictates what you will need A good set of binos is not a bad idea either