Are your eyepieces optimal shortest focal length always the same as your scopes focal ratio

[Voxish]

I just read the following article on another astronomy forum (no, not that one) and wondered what the forum makes of it.

”Since we have a focal length of 1200mm the calculation of the range of useful eyepieces is going to use 1200mm divided by the lower and upper magnification limits. So. . . 1200mm/40 gives the longest focal length eyepiece of 30mm. 1200mm/200 gives me the shortest focal length eyepiece of 6mm.

Now parenthetically, you'll notice that the shortest focal length always turns out to be the same as your focal ratio. 

Interesting as I have a scope with a focal length of 8 (ish) and when observing planets, Jupiter in particular has always provided the most consistent views using a 8 mm eyepiece, that’s x110.

I read endlessly of folks using what seems like insanely high magnifications, I don’t know what to make of them, but it makes me question whether my 5mm (x176) might be a little over the top (had some decent views of Saturn with it mind) but I don’t use it much compared with the 8mm.

Edited December 11, 2023 by Voxish
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[lunator]

Hi Voxish, the magnification used is more affected by conditions rather than focal length.

I have a 1200mm scope and last night I was observing Jupiter at x150 (8mm) as the seeing wasn't very good. A week or so ago I was observing it at x300 (4mm eyepiece) as the seeing was very good.

Paul Couteau who was a professional astronomer that specialised in observing double stars maintained that the "resolving magnification" for tight doubles was twice the aperture. So for my 1200mm F6 newtonian (200mm aperture) I would need a 3mm eyepiece to give x400. 

This would be focal ratio/2. 

I usually don't use that level of magnification as it is a dobsonian x400 would be a tracking nightmare 😁..

Cheers

Ian

[globular]

Eyepiece focal length equal to scope f ratio is just another way of saying exit pupil of 1mm.

So your question becomes "is 1mm the smallest usable exit pupil".

For me, the answer is no - I regularly go to 0.5mm. And with binoviewers even lower on the right targets.

[Voxish]

Which would have been how I usually work since my lowest power which is a 32 mm plossl gives me an exit pupil of 3.7 (I think) and the 5mm gives me 0.6  and I absolutely would not go below that). My 8mm gives me an exit pupil of 0.9, no problems with floaters. At 150x they start to become a problem when observing Jupiter but seem manageable with Saturn 🪐 or the moon 🌙

Edited December 10, 2023 by Voxish

[Voxish]

I find exit pupil to be the most reliable indicator of success of eyepiece choice 

[vlaiv]

Smallest exit pupil is very personal thing.

I tend to be happiest with about 1mm exit pupil.

There are several things that contribute to what people find suits them.

Existence of floaters for example. Floaters tend to be noticed more below 1mm exit pupil. Another thing is visual acuity. Some people have sharper vision than others. Those that do - don't like too much magnification as it makes image look soft. Others on the other hand find that they can see more detail with increased magnification.

It will to some extent depend on target too. Some targets look too dim with smaller exit pupils, but others have plenty of light and don't cause such issues.

 

[John]

In my case the answer is "no". My scopes vary between F/5.9 and F/9.2 but I very frequently use eyepieces with focal lengths of 5mm down to 3mm and occasionally 2.5mm.

 

[scarp15]

A focal length of 1200mm perhaps as you had referred to;  6mm ocular at 200x is generally considered optimum at high power. Yet as mentioned by previous contributors, there are many variables. 

For example, using my F6 / 1200mm F/L, 8" dob such as when observing the moon in steady seeing conditions, it is quite reasonable to increase magnification  beyond 200x and thus create a tighter exit pupil. Whist floaters may become a factor, comfortable eye relief and eye placement, will facilitate relaxed observing. Thus my 3.5mm Delos that yields 343x, at a 0.5mm exit pupil (and 20mm eye relief), is applicable for those infrequent perfect seeing occasions. Therefore the intended subject and seeing conditions determine the ocular focal length / exit pupil.   

[Stu]

As @globular said quite rightly, this comes down to an exit pupil ‘argument’, and what you prefer. This in itself depends on preferred targets, floater visibility and visual acuity.

I also think it depends to an extent on aperture. With large dobs, you often end up seeing limited so, say, a 4mm eyepiece in an f4 400mm dob would give you x400, which is an exit pupil of 1mm and could be worthwhile on a night of excellent seeing.

Take that same principle to, say, a 60mm apo refractor and you limit yourself to x60. This is likely well below the sky limitations and leaves a lot ‘on the table’ as far as the scope is concerned. To my, x120/0.5mm exit pupil is well within the scope’s capabilities and very likely to still be within the sky’s seeing limitations, and will certainly show you more, even if it is just by giving a larger image scale and showing no more true detail.

Likewise a good 4” f7 apo with go to x250 or x300 if asked, and the condition allow, which is getting down to 0.33mm exit pupil and can be challenging floater wise, which is where binoviewers come into play.

[Mr Spock]

For me it depends on seeing conditions. Given excellent seeing I use the same eyepiece, a 3.3mm, in both my main scopes. So, x224 in the 4" and x461 in the 12". If the seeing isn't so good I can use a 7mm in the 12" for x217.

At the lower end I use a 42mm in the 4" for x19 and 30mm in the 12" for x51, This avoids the exit pupil being too much for my ageing eyes.

[Voxish]

Just to be clear the post was from a forum, I don’t own the scope in question.

 

Interesting posts chaps, thanks for your perspectives. I own a very good 4 inch refractor (I won’t mention the brand) but live on the coast which I believe has an impact on seeing. I have found I need to keep things below x120 for Jupiter if I want to keep things razor sharp. Mars and Saturn I have had up to x220 without any image breakdown, just for kicks using a 8mm and a Barlow ages ago. x176 which is as high as I go since I sold the Barlow does me just fine. 

Edited December 10, 2023 by Voxish

[Louis D]

1 hour ago, Voxish said:

live on the coast which I believe has an impact on seeing.

In a good way or bad way?  The Winter Star Party in the Florida Keys is known for very steady seeing conditions.

[Voxish]

Well I was told, by someone on here in a bad way. The North Sea is a couple of miles from my place 

[Voxish]

We are directly under the jet stream here, not sure where Florida sits? 

[Louis D]

17 hours ago, Voxish said:

We are directly under the jet stream here, not sure where Florida sits? 

The WSP location in the Florida Keys is at about 24.6° N, 81.3° W.  It is more or less in the The Straits of Florida between mainland Florida, USA, and the island of Cuba.  Its seeing is influenced by the Florida Current that eventually merges with the Atlantic Current to form the Gulf Stream.  The Florida Keys are generally well south of the jet stream.

Edited December 11, 2023 by Louis D

[Voxish]

Right, I can’t remember who said it to me. Maybe some research is required. 

[Stu]

18 hours ago, Voxish said:

Well I was told, by someone on here in a bad way. The North Sea is a couple of miles from my place 

Depends which sea you are talking about! Atlanttic and North Sea are not renowned for their calm nature!

[Voxish]

The North Sea is a violent, cold and dangerous place. I would have thought that would have an impact on seeing?

Very happy to be wrong on this 

Edited December 11, 2023 by Voxish

[SCANS]

18 hours ago, Voxish said:

We are directly under the jet stream here, not sure where Florida sits? 

Ugh, same here in Nova Scotia. I share your pain. Stormy with gusts 60-90km today so no observing again tonight ☹️ But the upside is that it's +13°C so that's taking care of our recent snow.

[Voxish]

1 minute ago, SCANS said:

Ugh, same here in Nova Scotia. I share your pain. Stormy with gusts 60-90km today so no observing again tonight ☹️ But the upside is that it's +13°C so that's taking care of our recent snow.

What’s your seeing like btw?

Edited December 11, 2023 by Voxish

[Voxish]

We just finished a storm, I might have a small window with Jupiter this evening early doors 

[SCANS]

Do you mean Bortle? I guess it would be 4, but it's quite variable. SQM readings generally 20.8 to 21.4 although some nights down to 20.4. How about you? I use Astrospheric as my main forecasting tool and find that seeing and transparency vary between poor and average, with the two seldom coinciding. Occasionally I've seen above average on one or the other, but never for both at the same time. But lately I haven't given it much thought. If it's even marginally clear and I'm free, I'll go out for a look and just enjoy whatever I can see at the moment.

[Don Pensack]

On 10/12/2023 at 07:19, Voxish said:

I just read the following article on another astronomy forum (no, not that one) and wondered what the forum makes of it.

”Since we have a focal length of 1200mm the calculation of the range of useful eyepieces is going to use 1200mm divided by the lower and upper magnification limits. So. . . 1200mm/40 gives the longest focal length eyepiece of 30mm. 1200mm/200 gives me the shortest focal length eyepiece of 6mm.

Now parenthetically, you'll notice that the shortest focal length always turns out to be the same as your focal ratio. 

Interesting as I have a scope with a focal length of 8 (ish) and when observing planets, Jupiter in particular has always provided the most consistent views using a 8 mm eyepiece, that’s x110.

I read endlessly of folks using what seems like insanely high magnifications, I don’t know what to make of them, but it makes me question whether my 5mm (x176) might be a little over the top (had some decent views of Saturn with it mind) but I don’t use it much compared with the 8mm.

This is the case in places with poor to average seeing.

But, most of the time, the highest power in a scope will be more like 50-60x/inch of aperture, which makes the highest magnification eyepiece 1/2 of the f/ratio or even shorter.

This only applies in certain circumstances of excellent to superb seeing, and often only on double stars or the Moon, where seeing interferes a little less than it does on planets (primarily because planets are small and have a lot of low contrast details).

I'll presume you have a 10" scope.  200x is hardly a limiting magnification in a scope that large.  I have friends who regularly use 300x in their 10" scopes, but they do have good seeing.

Given good optics and good seeing, the limit of high power in a scope is:

--when and where floaters in the eye start interfering with the image.

--when and where the image gets too dim.

For me, the first comes before the second, and I draw the line, with my eyes, at about a 0.65mm exit pupil (about 40x/inch).

But I have seen nights where, on certain objects, I have used 62x/inch and seen a clean sharp image.  It does require superb seeing, though.

Your favorite eyepieces will tend to fall in the 1mm to 4mm exit pupil range on most DSOs, but in neither case is a limit.  I read all the time about people observing objects

with exit pupils as large as 10mm (in refractors) and as small as 0.3mm.

You should try observing some planetary nebulae at 300-400x in your 10".  You'll see a LOT more details.

[Voxish]

I use a 4 inch refractor. I was just out a few minutes ago and Jupiter (it’s still climbing) was just stable at x110. My step up is x176 and it a blur this evening. I can go higher than this on Saturn, Mars, Uranus, the moon and doubles no problem, just not Jupiter. This is a shame as it’s probably my favourite target. The vast majority of my observations are solar system based. I would buy another big dob if I wanted to get into the fuzzy blob game again (unlikely) 

Edited December 11, 2023 by Voxish

[John]

8 minutes ago, Voxish said:

I use a 4 inch refractor. I was just out a few minutes ago and Jupiter (it’s still climbing) was just stable at x110. My step up is x176 and it a blur this evening 

Jupiter rarely seems to benefit from high magnifications. Certainly not as high as can be used on Saturn or the moon. Mostly I use 130x-150x with my 4 inch refractor and similarly with my larger scopes. I've found the seeing here in SW UK quite poor on the last two occasions that I've observed Jupiter, quite probably due to the positioning of the jetstream. Last time out even 112x was pushing it a bit !