Shortest eyepiece for f10 8" sct

[SubTropicalDave]

Hi all. What is the shortest focal length eyepiece that people are using for f10 8" SCT scope? I have a 7mm TV Nagler giving 287x mag (purchased for an earlier scope) but feel the conditions are rarely good enough for this. What do the experienced users of similar scopes suggest as their routine highest power ep? All your helpful advice appreciated. Cheers.

[Wavytone]

Having had an 8" SCT... the following assumes you have sharp eyesight.

The maximum useful magnification of SCT's is around 1X per mm of aperture, maybe a tad more - and that's in good seeing.

On most nights average seeing will be around one second of arc, worse that the resolution of your scope - so there's no point trying to push it to see diffraction rings - on most nights the seeing wil reduce the Airy disk pattern to mush.

My own conclusion was that a 9mm was optimal in the sense that images remain reasonably sharp and bright, and shows all the details to be had. Going shorter will give a larger image, less sharp, dimmer, and you won't really see any more detail.

7mm is definitely too short.

[AstralTraveller]

Hello and welcome to IIS. Enjoy your stay!

There is a rule of thumb that a scope can support a magnification of 2 x <aperture in mm> or 400 in your case. However this is rarely achieved in practice, certainly not with a 200mm scope. You don't say where you live but if your place is anything like mine, the atmosphere will rarely even support 287x. With my 10" newt and 6" frac I most often work at 175x. The next step up I have is 240x and it mostly is a bit over-magnified. The image gets bigger but loses it's crispness. I'd guess that about 200x would often be the sweet spot for me.

BTW you say you are using a Nagler. These are dang fine eyepieces - I have a few - but not optimal for high-magnification lunar and planetary viewing. For that I use orthoscopics (though there are many other fine planetary ep options). My UO 9mm ortho is noticeably sharper than my 9mm Nagler - it's just like looking through a drinking straw!

[mental4astro]

I would have to differ from my esteemed friend Wavy, :)

I have no problem pushing my C8 to 400X like David says. With good optics of scope and eyepiece, this is the combination I use to pull and sketch wonderful details from the Moon. Certainly finer detail than at 250X with an 8mm eyepiece. BUT, this is totally dependent on great seeing conditions (thermally stable atmosphere). Unfortunately this does not happen very often. 250X is more frequent as the upper most. Higher than 400X and it is a test of exceptional optics, dumb luck and usually a blurry image.

How can you distinguish between poor seeing and excessive magnification? Poor seeing has the image shimmer, come in and out of focus and fine details "dance about" - often this is descibed as the image is boiling. Excessive magnification, the image doesn't move about, but it is soft when at best focus instead of being tack sharp. Can also be a symptom of poor optics, both scope and/or eyepiece. These are all variables that require some time and patience to eliminate one at a time until seeing is the only variable left to contend with routinely.

Collimation of the optics of any reflector occasionally requires tweaking, but quick to identify and correct. While a factor, it is independent of the ultimate optical quality of the scope.

To finish answering your question, an 8mm and 5mm are the shortest eyepiece focal lengths I use with my SCT, which is then seeing dependent on which gets used.

Alex .

[Satchmo]

As a general rule of thumb high quality optics in the 6 to 10" range that are well cooled and collimated should stand a maximum of 50 X per inch of aperture ( ie 5mm on a c8 ) but high contrast objects such as the Moon and Saturn are the usual contenders for that sort of magnification.

That being said if the conditions are that good then 25X per inch will also extract all that can be seen as the diffraction structure in the image ( ie the Airy Disc ) will become visible that power to the trained eye and beyond that is a bit like magnifying news print .

[Tropo-Bob]

I used a 6mm EP to see Sirius B for the first time with my C8 Edge. Otherwise, I have never used anything under 11mm.

[Wavytone]

Ah good point Bob... if interested in close double stars then yes you'll be looking closely at diffraction rings and airy disks and for that 6mm would be fine.

[ausastronomer]

This is incorrect. Normal seeing conditions are generally not 1" of arc or less. Normal "good" seeing conditions for most reasonable observing sites used by amateur astronomers is around 2" of arc. You can experience sub arc second seeing conditions at a number of sites but this is the exception rather than the norm.

The average recorded seeing at the Anglo Australian Telescope site at Siding Springs recorded over many years is 1.8"±0.05" FWHM, with no significant change from year to year.

http://203.15.109.22/astro/manuals/obsguide/node5.html

This site would generally experience seeing conditions at least as good as, if not better than most amateur observing sites. This would have been one of the determining factors in choosing it as the site to build a professional facility, as adaptive optics were well in the future when the site was chosen. Mount Kaputah is one observing site where the seeing more often than not is around 1", but this is certainly an exception not the norm.

Cheers,
John B

[Wavytone]

Mmm... aperture comes into play too though. If the seeing is bad IMHO theres something to be said for putting an aperture stop in front of the scope that reduces the aperture - half or 30% on bright objects. Though I guess they wouldn't stop the 150" down to 20cm.

My little 70mm refractor happily shows a steady airy disk at 150X in quite strong winds and what would normally be considered appalling seeing. That's less than 2" though not by much.

[Satchmo]

I don't think AAO site is a good measure of general seeing - its a notoriously poor site for seeing conditions not being very high and surrounded by many km of hilly country in all directions , so a figure of 2 arc second might be pessimistic for the rest of the country .

If its any value I remember that the prototype stellar interferometer set by Sydney Uni at Lindfield CSIRO found the average size of the air cells deduced by stellar interferometry to be around 150mm at that site . I'm not sure how directly that translates to equivalent seeing but I always found it interesting .

[Wavytone]

The problem with Siding Spring is the prevailing wind is westerly and there's a bunch of good size spiky ridges and peaks west of the mountain. No wonder its turbulent - as any paraglider pilot would know - they would definitely not fly anywhere near it at low altitude as the air would be very bumpy. This translates to poor seeing too.

Air cell size is something distinctly noticeable flying a paraglider - you feel the turbulence through the tips of your fingers much like a bird does through its wingtip feathers, and the trim airspeed is about the same as a wedge tail eagle or sea eagle on glide - 35km/h.

From flying in mountainous regions there are a few things which stood out that I think are relevant to seeing, having had the chance to fly in the afternoon and observe by night on the same occasions at several locations.

1. The turbulence varies enormously depending on the immediate terrain below, and upwind. For example downwind of hilly mountainous terrain will produce a flying experience that is so rough you may be airsick. Similarly these are crap places to observe, seeing-wise. The resons are too complex to go into here but it has to do with the way thermals rise off the ground especially if the lee side of a hill is the sun side, and the upwind side is in shade. This creates invisible "rotors" in the air that are similar to a wave breaking on a surf beach.

2. Yet find a long valley that runs downwind - the King Valley near Bright in NE Victoria is an example - the air over the valley in the late afternoon is laminar - smooth and stable. Observing from the ground below on the same evening shows this air is indeed smooth.

3. A site on a high ridge with a long stretch of clear air upwind will have smooth laminar flowing over it. Gliders use this to ridge soar and for example at Mt Blackheath or Hargraves Lookout in a gentle westerly, flying in this air in late evening is as smooth as a babies bottom to at least 3000 metres altitude. So is the seeing a few hours later.

[Satchmo]

I'd guess that 150mm air cells are going to give a stable airy disc at least some of the time which would imply the potential for 0.75 arc second seeing .

Without a doubt small hills around an observatory are probably not a great idea , so AAO is not ideal .

Wavy- I'm familiar with all those topographic and thermal effects you speak of as I'm a long time glider pilot , but probably wouldn't feel some of them as you would in your `jellyfish' . Had some very nice ridge soaring last year at Barrington Tops with a 5km run .

[SubTropicalDave]

Well thanks all for your considered advice on this issue. In summary it seems that, on average the 7mm is too short and going to the 9 mm would likely cover "best possible seeing conditions". That is particularly true as much of my viewing is at sea level on the north coast of NSW. However, I'll be taking the scope to altitude in a weeks time on a camping trip and may just be in a position to test the seeing conditions at Mt Kaputar and Warrumbungles for my self (weather permitting)!

The other issue raised by AstralTraveller was the most suitable eyepiece design (taking into account cost) at the shortest focal length used in my scope (or anybody's for that matter). Do other peoples also find the orthoscopics ideal for high mag given the objects are concentrated in centre field. Or does that make location of objects too difficult?

Cheers

[Tropo-Bob]

The 6mm I used on Sirius B was a Tak Ortho. Its no problem if U find the object using lower magnification.
I generally prefer using either Plossls or Orthos with SCTs.

[Wavytone]

Dave in short the best thing you could do is attend a few star parties and try a heap of eyepieces in your scope to see what you like vs don't like.

The main issues with eyepiece choice are:

1. Magnification how much is enough vs not helpful (at the high end); you've sorted that one;

2. Exit pupil (at the low, wide field end);

3. Whether they produce "Kidney bean" blackouts or are very sensitive to eye position; this is due to spherical aberration at the exit pupil. A few eyepieces were notorious for this;

4. Whether the eyepiece is a good match for the f-ratio (applies to scopes f/5 or faster, not an issue with SCTs at f/10). More importantly many budget 70- degree eyepieces (eg Prostar) work perfectly well in SCTs.

5. Field curvature of the scope vs that of the telescope. You can google the field curvature of Newtonian scopes Vs that of refractors or SCTs and Mak's for yourself. This one is the reason why the humble Plossl - a very simple eyepiece - works surprisingly well in fast Newtonian scopes but is not a particularly good match for an SCT or refractor. Ditto Koenigs. Eyepieces that suit refractors will also suit SCTs and Maks.

Most Japanese eyepieces - particularly Tak And Vixen - are aimed at refractors because the same manufacturers also sell mostly refractors in Japan, not newtinians.

6. Chromatic correction - specifically lateral chromatic error near the edge of the field of view. In a reflector, Mak or SCT, these scopes are fundamentally reflectors aside from the corrector being used to direct spherical aberration. Off axis stars are not smeared into colourful blobs by the objective, yet if stars are smeared into rainbow streaks. you have a poor eyepiece.

Lastly some eyepieces are specifically aimed at users of fast newtonians to the extent they deliberately include either or both some field curvature, and/or coma. Examples include Edmund RKE's which were designed to match a small f/4 Newtonian, and I have seen two recent eyepiece types which specifically state they include negative coma to suit newtonians. These - and Plossls - are not a great choice for SCTs.

[ausastronomer]

Mt Kaputah is one of the single best observing sites on the planet, due to its altitude combined with dark and transparent skies which is also generally combined with exceptional seeing conditions. That having been said the road up and down the Mountain is not something "Mr Magoo" should attempt after a few beers. The other end of the scale there is around Fox/Franz Joseph Glaciers in New Zealand's South Island which has the most beautiful skies I have experienced for binocular/naked eye observing. They are so dark and transparent combined with high altitude 4th and 5th magnitude stars shine like beacons. Unfortunately due to the topography the seeing would average about 10 arc seconds.

The observing conditions in most areas around Coonababrabran and the Warrumbungles are also pretty good with dark clear skies, but normally you do not experience quite the same seeing conditions you get at Mount Kaputah. That having been said we have observed a multitude of targets at Warrumbungles Mountain Motel and Timor Cottages (now gone) at magnifications between 400X and 800X when conditions have been favourable. Usually targets like the Homunculus and planetaries like the Ghost of Jupiter and the Eight Burst are worthy of close scrutiny at high power in large aperture scopes.

With regard to eyepieces as far as simple eyepieces go I prefer "good" orthos over plossls as they have slightly longer eye relief for an equal focal length and to my eye give an aesthetically cleaner view than a plossl.

Cheers
John B

[MortonH]

Note that the focal length of your SCT is around 2000mm only if you're using a 1.25" diagonal. If you use a 2" diagonal it increases the focal length (and introduces some spherical aberration).

I use a 2" SCT-fitting diagonal on my C8 and that increases the focal length to around 2100mm. If you use an SCT adapter and regular 2" diagonal the focal length gets even longer (and SA gets worse).

It's worth knowing the actual focal length of your setup so you know what magnification you're really getting. This thread on CN explains it well:

http://www.cloudynights.com/topic/478721-c8-actual-focal-length-w2-sct-style-mirror-diagonal/

[SubTropicalDave]

Thats an excellent point MortonH. There are so many recommendations out there to modify the visual back hardware on SCT's. Just one seamingly minor change could invalidate the assumptions for magnification calculation and eyepiece selection. I'll be staying with the standard 1.25" setup for a while, if only just to keep it simple.

[Wavytone]

FUD. Read the whole article.

The difference in SA between a 1.25" vs 2" diagonal is not visible - amounts to maybe 1/20 wavelength.

You have to crank the focal plane out by many cm before it becomes noticeable.

[MortonH]

Yes, which is why I have no concern about using a 2" SCT diagonal. The problems arise when you add things like an external focuser plus normal diagonal or binoviewer, etc.

[Wavytone]

Simplest would be to just try it on a scope. If it works well enough don't agonise about theoretical "maybes"..

[SubTropicalDave]

Are not the theoretical maybe's worth a tiny consideration before the outlay of hundreds of dollars or a hundreds of kms of driving to a star party?

[SkyWatch]

Just a comment about diagonals. When I replaced the standard 1 1/4" Celestron diagonal on my CPC800 with a GSO 2" quartz one, all of a sudden I was getting textbook star images instead of fuzzy blobs.
The image you see is dependent on the weakest link in the system- so it is worth checking the diagonal before blaming eyepieces if you are having problems with sharpness of image etc.

- Dean

[wolfman]

8.8mm is my smallest but i rarely use it i prefer my 14mm on most occasions.