Viability of fast Newtonians ?

[Satchmo]

I've started this thread for anyone who wants to continue the debate started re Newtonians in the Sydney Observatory needs your help thread.

In response to Wavytone :

Paracors and modern eyepieces with short focal length and long eye relief solve all these objections and you know that exit pupil is a function of magnification and has nothing to do with F ratio. It is possible with modern eyepieces to obtain the full range of useful exit pupil from 7 to 0.5mm with modern correctors and eyepieces. Computer optimised central obstructions do not exceed 25% for visual use on an F3 compared to 35% for a SCT.

I've observed with a 20" F3.5 over full range of exit pupils and couldn't fault the images. Stars looking almost perfect to the edge of a 100 degree apparent field.

Originally Posted by Wavytone

Actually, no. The mirror had an excellent figure. The problems were:

a) huge secondary mirror - so big that in many eyepieces you see a big black blob swimming in the FoV;

b) range of useful magnification was very limited, largely due to the limitations with suitable eyepieces - many eyepieces won't work well with mirrors this fast, even now.

c) coma and field curvature (at the time I had this, correctors didn't exist) which were a big problem with most eyepieces except the Pretoria.

d) the fact that the fast f/ratio raises the background sky brightness, which basically defeats the point of a larger scope for looking at extended objects (nebulae, galaxies) in average skies. In really good sites with the darkest of sky, this might not be a problem but even in the Blue Mountains west of Sydney it was evident that f/5 - f/7 would have been a much better choice.

In the heart of Sydney the sky glow is much worse - and at Observatory Hill the naked-eye visual limiting magnitude is about 2.5. An f/3 mirror is about the worst possible choice.

[sally1jack (Phil)]

I've observed with a 20" F3.5 over full range of exit pupils and couldn't fault the images. Stars looking almost perfect to the edge of a 100 degree apparent field.[/QUOTE]


Hi Mark,
was this with the new or old paracor?
phil

[Satchmo]

Hi Phil- that was a with a type 1 from memory- using 21 , 17 and 13 Ethos. The 13mm was incredibly quite usable without the Paracor. I hear that the Paracor 2 gives the sensation of a flatter field, but I was very impressed with the view. I think longer focal lengths help with field curvature...may not work so well on an 8" F3.5 etc.

[The Mekon (John Briggs)]

May I ask where the line is drawn for "fast' newtonians? Like you Mark, I have been around astronomy for many years. 40 years ago I would have said an F6 was standard, perhaps bordering on fast, with plenty of F7s and F8s around. Now we appear to have F4.5 as standard. To me an F 3.5 is "very fast" and I am pretty sure I would find the views annoying if using a longer f.l. eyepiece. The longest eyepiece I use is 27mm, even then I find out of focus central spot annoying.

[Satchmo]

Hi John - I'd call fast these days F3 to F3.5. You don't use a 27mm eyepiece with scopes this fast.

With your F4.5 this would be a 6mm pupil about maximum. If you used a Paracor on an F3.5 it gives an amplification of 1.15X in focal length so the new effective ratio becomes F4. A 6mm pupil would then be achieved with a 24mm eyepiece on the F3.5/ Paracorr combo. The secondary shadow is caused by the small image of the secondary in the pupil of the eyepiece. In the case of your 18" sdm this image would be a 1.2mm black dot in the middle of the 6mm eyepiece pupil ( 19% central obstruction ) .

If this dot is blocking your eye pupil then I'd suggest your eye is opening up no where near 6mm and your pupil is cutting off a lot of the light coming from the primary mirror. If you base this test on daytime observation then it would be valid as the daytime pupil is only around 2mm.

[Paul Haese]

I have observed through a mates 22" F3.6 scope and it presents great views in all magnifications. Just some empirical evidence.

[Wavytone]

Low magnification isn't the issue on short f/ratio newtonians.

I'd be interested to know:

1. What is your lowest magnification, ie determined by a 6mm exit pupil (or thereabouts) vs your highest magnification. My guess is your range lowest:highest isn't much more than 1:4 or 1:5.

2. how you're achieving high powers eg magnifications above x1 per mm of aperture. This is more relevant to smaller apertures - trying to use a 60cm f/3.5 scope at 700X obviously isn't sensible. But using 400X on a 30cm f/3.5 has some point, in excellent seeing. And please don't tell me a Barlow...

3. On large apertures the seeing limits the useful magnification to such an extent that I suspect those with apertures of say 60cm never get past lower magnifications on most nights, simply because the seeing is too turbulent, unless you are lucky enough to have a superlative location.

Hence I'd expect most of you don't use much more than 0.3X or 0.5X per mm of aperture.

Mark, I appreciate the temptations of short f/ratios with respect to the implications for tube length, weight, portability, and mounts - but they're less than ideal optically IMHO.

[Satchmo]

Wavy,

My point was that you stated that an F3 to F3.5 telescope was totally unsuitable for a heavily light polluted city sky. Why?

An exit pupil of sub 1mm is achievable with a 3.5mm Televue Delos or Ethos with good eye releif. A Televue Paracor gives about 15% increase in magnification , making an F3.5 an F4 focal ratio. If you want to acheive 1/2 mm pupil which you would rarely use with any scope, the Teveue Powermate amplifiers are optically beyond reproach ( they are not a Barlow design. )

Generalities that long f ratio scopes were better for city skies existed when the best high quality short focal length eyepieces were like a 4 mm Orth with 40 degree field and zero eye relief. Fast scopes often had massive secondaries, which are now obviated by low profile focussers and awareness of true needs for visual observing

Modern focussers like the Feathertouch with 10:1 microfocus make it easy to focus a fast F ratio scope.

I have had no problem using 600X with 20" scopes in good seeing .

Central obstructions can be kept below 25% for visual use even at F3.

I'm not entering into discussion about suitability of various aperture sizes to for general viewing vs average seeing conditions- its not relevant here. I'm simply pointing out that , provided you source a good primary , a faster F ratio can be just as flexible as a long one . If you can achieve a full range of exit pupil that you would use with a longer F ratio scope then the F ratio is irrelevant.

I've spent a few nights with a quality 20" F3.6 with Paracor and good eyepieces and found the images indistinguishable from a 20" F5 ( which I have a lot more experience with ) . The only difference was that I spent a lot more time looking at each object because I was not hanging off a ladder. I have a friend in Perth who had a 20" F5 and converted to an F3.6 and reports no loss of detail on the planets in good conditions.

[alocky (Andrew lockwood)]

I will buy into the limiting useful aperture debate. I use my 25" alongside plenty of smaller scopes in Perth. The atmospheric conditions here are generally average or worse due to the ubiquitous trade winds. Never has a smaller scope been able to operate at anything approaching the limiting magnification of the 25". On a good night I have run it at 800x and wished I had a shorter fl eyepiece.
Now don't take my word for it- as part of the research into the adaptive optics for the VLT the ESO found that resolution is atmospheric limited (ie rarely improves) above 30" or so in good sites, providing your local (ie thermal) conditions were managed.
I use my 25 for outreach events occasionally, and even under heavily light polluted skies a 25" provides brighter and clearer views than anything smaller. I have no idea how some of these myths propagate! I suspect it is more to do with people being too lazy to collimate carefully with the faster scopes, than anything inherent in the design. It's going to look horrible pretty quick at f3.

On the other hand - the ladder is something of a drawback, but at Perth Observatory we have had the public up on ladders since the 19th century without any debilitating injuries.
Andrew.

[Wavytone]

Andrew,

What focal ratio is your scope ? What magnifications are you using, highest and lowest ?

I never suggested larger aperture isn't better. What I do take exception to is the desire for ultra fast f/ratios below f/4 or even f/3, which appear less than optimal for high power.

[alocky (Andrew lockwood)]

Mines f5 - (that used to be fast) so perhaps not as relevant here, but I have looked through a few of my namesake's mirrors in the US and provided they are well designed scopes and very carefully collimated they deliver equivalent views at f3 and less.
Lowest power is around 100x with the termi-nagler, and despite the exit pupil being large than my head, I can still see through it and never been bothered by the secondary shadow. Highest I've used is 800, although on a normal WA night 300 is about it. The real sweet spot is around the 180 mark for deep sky.
My experience is that coma 'tolerance' varies from observer, I'm not bothered by it as much as one my friends, who regards any optical aberration as a waste of photons. I imagine its hard to find something that will be acceptable to everyone.
However, background brightness is largely a function of magnification - I'd challenge anybody to discriminate between two scopes of the same aperture at the same magnification with different f-ratio primaries.
Cheers,
Andrew

[clive milne]

I think a qualifier or two is at least implicit in this discussion, not least being the nature of the optics placed between the focal plane and the eye.

If you try and use plossls in an f3 light cone for example, the astigmatism inherent in the eyepiece is going to render the views less than satisfactorily.

[alocky (Andrew lockwood)]

Right, although if it's implicit we don't need to discuss it here!
I think it's exactly that - the improvement in eyepiece and coma corrector design and manufacturing that has permitted the use of faster primaries, although you can imagine what Stefan's reaction to the view through one of his prized orthos on an f3 newt would be!
cheers,
Andrew.

Originally Posted by clive milne

I think a qualifier or two is at least implicit in this discussion, not least being the nature of the optics placed between the focal plane and the eye.

If you try and use plossls in an f3 light cone for example, the astigmatism inherent in the eyepiece is going to render the views less than satisfactorily.

[clive milne]

Well, yes I agree with Stephan on the point that ortho's do have their place

That place being somewhere other than my focuser(s).

oh, by the way... on a complete off topic tangent, there is/was an issue with one of the relays in your relay box. It's back at SBIG being repaired (under warranty) Bill (the repair man) indicated that since SBIG were taken over by Aplegen, this sort of QC issue is not exactly setting a precedent. Anyway, if you had experienced a lack of 'win' auto-guiding with your G11 when you used it, this is likely to have been the cause.

best
~c

[Wavytone]

So far no-one with a scope faster than f/5 has fessed up concerning min/max magnifications ... Anyone ?

[Barrykgerdes]

Simple/cheap fast scopes have a problem with spherical aberation because the mirrors are not accurately parabolic. This can be improved with correction lenses in the eyepiece but the corrector still needs to be matched to the mirror.

SCT's are basically very fast and have corrector plates to correct this.

Barry

[ausastronomer (John Bambury)]

Originally Posted by Wavytone

So far no-one with a scope faster than f/5 has fessed up concerning min/max magnifications ... Anyone ?

Well my 18"/F4.5 isnt really typical of the new breed of compact faster (sub F3.5) newtonians but I have no trouble running to .5mm exit pupils, in this scope, when conditions are ideal.

I have on several occasions used my 5mm Pentax XW combined with a 2.5X TV powermate for 1025X. It is very rare that conditions are good enough to support this but I can often get to 735X with my 7mm Pentax XW in the 2.5X Powermate. Of course you can't get there all the time but the thermal stability of the optics and the atmospheric conditions are the limiting factor, not the optics themselves.

I have no doubt that a 20"/F3.3 telescope could run .5mm exit pupils under ideal thermal and atmospheric conditions. For instance a 3.5mm Pentax XW in a 2X TV barlow gives a .53mm exit pupil and 950X. A 5mm Pentax XW in a 2X TV barlow will give .75mm exit pupil and 670X.

Of course not all observers can run with .5mm exit pupils and as you get older this ability diminishes slightly. I am 53 years old and can use .5mm so I see the limiting factor to be factors other than the telescope and the optics, namely, the atmospheric conditions and thermal equilibrium.

I haven't factored the paracorr into the equation here as all my scopes have tracking and I do not use the paracorr for high power work as the tracking enables me to keep the view "on axis". If you wanted to use the paracorr for high power views you would just need to adjust the eyepiece/barlow accordingly.

At the low end of the scale, I am happy to push to a 7mm exit pupil on a regular basis. In a 20"/F3.3, with paracorr a 26mm Nagler will give a 7mm exit pupil for 75X and a TFOV of 1.1 degrees. I have on many occasions used larger exit pupils, but that just wastes some aperture IMO.

Cheers,
John B

[Satchmo]

Originally Posted by Wavytone

So far no-one with a scope faster than f/5 has fessed up concerning min/max magnifications ... Anyone ?

I know someone in Perth who has changed from a quality 20" F5 to an F3.6 using it above 600X on good nights of seeing and has noticed absolutely no drop in resolution or contrast over the F5. That guy has retired his 12" Takahashi Mewlon as his reference planetary scope simply because it is being outgunned. Reports from another Melbourne ATMer that their F3.6 is holding its own against longer focal length in the same aperture.

I have not noticed any degradation in the quality of the Airy Pattern with such focal ratios under bench Null test, compared to an F5 they are just more time consuming to make. The current debate in the large mirror scene is over weather large mirrors should be left a little undercorrected. One US optician is making good money correcting the work of another who thinks they should, but whose mirrors just don't seem to be cutting it in the field. I personally believe that a mirror of any size should be fully corrected then thermally managed.

Nick- if you are at SPSP this year perhaps Paul Hatchman can show you through his 20" F3.6 ?

[sally1jack (Phil)]

Mark (satchmo) as you probably know Peter Read is currently working on my scope 28"f3.3 when i get it i'm very happy for you to put through the mill & compare to slower scopes
phil