great_bear

I used to enjoy playing with this Drift Alignment Simulator but of course these days the Adobe Flash player has been stripped out of all the main web browsers. 

Is a non-flash drift alignment simulator available anywhere?
(maybe even as a tool in a smartphone app?)

38 minutes ago, johninderby said:

Yes that was the one. SW 150PL with the OO mirrors fitted. 

Oh... I am genuinely sorry to hear that you are missing it.

If it's any consolation it is much-loved and is the most-used scope I have, due to requiring near-zero cool-down time (I never used that fan, ever).

I ordered it a present today - a Berlebach Planet tripod.

Overkill, no doubt - but the AZ4 makes it dance around for about ten seconds every time I touch the focuser which is maddening - even when I owned (and later sold) one of those heavy duty T-mounts similar to the Sky-Tee. I considered turning it into a dob, but I have high fences, so it's better at home to have it mounted high on a tripod.

The only thing I'm not keen on is the Finder arrangement, which has never worked well for me. I'm considering moving the finder back to its original position, and replacing the Telrad (which was always too much in-my-face for my liking) with the more compact Rigel one, so the Finder arrangement would then be as per my 8" Dob (which I bought for use at the Baker Street Irregular Astronomer's meets at Regent's Park in London)

So it would be something like my dob above, anyway - not definite yet, I might need to experiment.

Anyway John, I'm sure that I mentioned this at the time, but if I ever consider selling that 6"/F8, you can be sure that I would certainly offer you first refusal on it.

Rgds, Jeremy.

On 31/01/2021 at 23:54, johninderby said:

I had a 6” f/8 1/12PV that was exceptional. I was an idiot for selling it. Also an 8” and a 14” 1/10PV that I couldn’t fault.

Was it this one, John?

Like all these comparisons at this level we are talking about very small "differences" rather than better or worse and personal preferences are probably stronger motivations than any actual performance differences.

- at this level though it's not about performance but usability*

Tele Vue really nailed this with Delos - creating a consistent customer experience across the range, whilst keeping performance as high as reasonably possible.

* Declared interest - I am by trade a Customer Experience Consultant, so my comments might in some respects be considered a foregone conclusion  

Naglers have suffered over recent years from not being "the latest and greatest thing", with sales also being cannibalised from other in the Tele Vue lineup

I remember a review from a well-known observer that basically stated the William Optic / Sky-Watcher Nirvana UWAs were more-or-less as good - but now, a few years down the line - with the benefit of experience I really doubt that many would agree with that sentiment.

The prices on the used market make them very, very tempting EPs...

Nice post Shane - I'd add that the moon is a weird thing inasmuch as the dark patches *do* seem to emit some strange character of light with some eyepieces but not with others.

> There was a very slightly (spherical?) aberration

Not SA, no - "spherical aberration" is a kind of fuzziness (regardless of position in field) that occurs when the outer parts of the lens area are in use and are not focussing in the same place as the central zones of the lenses are (and for reference - in case anyone cares - "spherical aberration of the exit pupil" is yet another, completely unrelated, eyepiece issue...)

Yay! This could be a very significant experiment if all goes well!

More effort than it's worth. At the end of the day, we all know which scope in our arsenal gives the best views for particular objects and we just choose appropriately. We may not know exactly why one thing works well and another does not.

Sorry, but that's not good enough!

We need to find out because:

• You can't be a master of your tools if you don't know why one tool sometimes outperforms another
  
• We owe a debt to our forebears to advance telescope science for the next generation of observers
  
• Forum polls like this deserve a quality answer
  

maybe I should do a head to head with my 6" f11 and 16" f4 stopped down to 6.7! f11.

Definitely! - and see if you can do a 6" F5 at the same time for reference!

Observing a better image in a small compared to large scope doesn't tell you why the difference is there. This is why this is an unsuitable test. Stopping down is a more controlled experiment since everything else is held constant.

True, but it's less helpful - because people can argue (quite rightly) that you're not replicating the phenomena under the same conditions which they're reporting it under.

Better still, is to sort it out once and for all by having large and small telescope side-by-side, under the kind of "seeing" conditions commonly reported, and then after confirming such a difference, investigate further by stopping down the big scope to match the smaller and see whether the two scopes now show identical images. You'd want to have the stopped-down large scope match the F-Ratio of the smaller scope - since the dreaded "F-Ratio Conjecture" hot potato will make an appearance at some point in the conversation

My own standpoint on this - for what it's worth - is that the statement "Smaller scopes perform better in bad seeing than large ones" is inherently flawed anyhow, since how do you know that the observer making such a statement is technically able to detect the difference between actual bad seeing, and poor thermal management in the larger scope?

I agree that stopping down presents a strong case for detecting the latter - but only if you've got a small telescope of equivilent aperture and focal-ratio to act as a reference.

If as a scientist, I cannot reproduce observations of others, which concerns a claim running counter to scientific theory...

But there's nothing asserted here that's counter to scientific theory here.

No-one's deputing optical science - that larger aperture has higher resolution - the contention is that smaller telescopes are observed to be less susceptible to poor seeing than large ones are and under such conditions produce better images.

Using a stopped-down version of the same scope has the advantage of limiting the difference in views just to the aperture effect, to the exclusion of others.

But you're missing the key point: People don't say "smaller aperture (alone) is less susceptible", they say "smaller aperture telescopes are less susceptible". So to reproduce the phenomena as described you need a smaller telescope not merely reduce the aperture on a big one. Doing otherwise means you're already assuming the cause (e.g. atmosphere, when it may be internal tube currents for example).

The first step is reliably reproducing the observation. Next would be quantifying it, then some hypotheses, and then finally, some test measures to prove which hypothesis (if any) is correct.

But - as I'm sure you'll agree - the amateur astronomy community seems to have a bit of an aversion to testing & reproducing such observations in an objective manner, prefering instead to trade pulled-from-the-air theories about what "might" be going on...

S&T even ran a piece on it a while ago: Four Infamous Telescope Myths - Visual Observing - SkyandTelescope.com

I don't give much credance to Gary Seronik's opinion. A couple of comments from me here:

S&T(GS): "I call this assertion a myth for two reasons. First, my own observations do not bear it out."

Wow!! How flimsy is that! What a totally self-centric viewpoint! If he doesn't observe the phenomena, why not investigate those who do? There's plenty of people who do observe this - maybe he's doing something different? This reasoning is philosophically unsound, being classed as "Argument from self-knowing" i.e. (from Wikipedia) "Arguments from self-knowing take the form: If P were true then I would know it; in fact I do not know it; therefore P cannot be true."

S&T(GS): "Second, no one has proposed a plausible mechanism for it."

Oh dear... That's a shameful thing for anyone with a scientific mind to say, it's "Argument from ignorance".

As Carl Sagan says in The Fine Art of Baloney Detection (chapter from The Demon-Haunted World) "This impatience with ambiguity can be criticized in the phrase: absence of evidence is not evidence of absence"

S&T(GS): "my principal telescope was an optically good 12½-inch f/5 reflector. I built a 5-inch-diameter off-axis mask that could quickly be placed at the front of its tube[...] Not once did the reduced-aperture view show greater detail than the full-aperture view."

Maybe so - but it's not quite the same as actually having large and small telescopes side-by-side and directly comparing the views is it?Maybe the alledgedly better views in the smaller telescope were due to another reason - such as reduced turbulence within the tube due to its different geometry?

He'd be doing astronomy a far better service by investigating why people are reporting such a phenomena rather than effectively saying "I've not seen it, so this magazine officially declares it a myth."

I'm personally not asserting an opinion either way - but what I am saying is that Gary Seronik's article has no credibility on this issue since it makes no philosophically sound assertions.

I think the question the OP should be asking is not which scope to buy for planetary viewing, but which scope to buy for planetary viewing first.

Like I always say; when choosing your first scope, plan to end up with two - you will anyway!

So by implication, are all those of use who have them and enjoy using them wasting our time then

Up to what job ? - it's a hobby - we do it for enjoyment don't we

Why, the job of being a planetary scope for serious study of course!

(the subject of this thread)

Now, people may treasure their small APOs for any number of reasons, and that's all well and good - but that's not necesarilly helpful to the observer looking to select the optimum instrument for planetary observations.

I like Astro-Physics' scope designer Roland Christen's take on it here <click> (very short, but well worth a quick read)

Note that when it comes to refractors, Roland says:

"Refractor, 7" to 9" Apo, F8 to F11"

- so even though he is in the business of designing and selling APO telescopes, even Mr Christen himself does not try to convince us that today's small APOs are up to the job!

Many myths also stem from comparisons between say a TEC 140 and some cheap 1/4 PV 10 Newtonian

Agreed - you rarely see best-of-breed comparisons discussed - especially since well-made Newtonians are so under-appreciated.

There's a 6", F11, 1/12th Wave P.V. Newt that's languishing in the For Sale section here on SGL - an absolute bargain. Yet it seems to have attracted little interest.

Not looking to do any imaging, hence tracking is not required

Tracking scopes aren't specifically for imaging - they're for planetary visual too. At 240x magnification, Jupiter doesn't stay in the view for very long!

OK - so on a dob you can use widefield eyepieces and track manually - however, widefield eyepieces that work well at dob-like focal ratios are very expensive. On a tracking scope, you can use a world-class, minimum-glass planetary eyepiece like a TMB supermono with ease - and it will cost you less than a quality widefield.

Focal ratio is not relevant it's the size of the obstruction that matters.

I think this is oversimplifying things a bit.

All eyepieces perform better in just about every respect at longer focal ratios, and also it's easier to make more accurate lens surfaces - for both eyepieces and telescope objectives - at longer focal lengths.

Just to clarify something stated on this thread.

You can only estimate the Apparent Field of View (aFOV) with maths, and in many cases the maths gives you an answer that's quite badly out (I had a long discussion on a Cloudy Nights thread a while back with David Knisely and Jon Issacs about this).

The best way to measure the Apparent Field of View of an eyepiece with any degree of accuracy, is to shine a torch through it backwards, projecting a big circle of light on the wall. Measure the diameter of the circle and the distance from the wall and... Well, I'm sure you can do the maths!

I'm aware that there are of course limitations to telescopes, and that everybody probably wishes they could push their scope just that little bit more to bring out the detail they're after.

Sure - and I've certainly been surprised how high my 130P can go when expensive eyepieces are used.

Regarding the branding of those TMB eyepieces then - I take it I'd be safe buying the 'un-official' pieces?

Yes - the official TMB ones may look smarter - but I've not seen any compelling evidence of any optical quality differences (I've had both).

It's definitely a good idea to try all three and see which you like.

All good questions.

The limit of scopes for resolving detail is little over 30x magnification per inch of aperture, which roughly equates to an exit-pupil of 0.75mm. In summary, if you multiply your scopes focal ratio by 0.75, that's the length in mm of your highest-power eyepiece.

Now, what happens beyond this is very much down to the quality of the scope, but - in short - although the image gets bigger, you won't see any more detail per se. Nonetheless, this is in some ways a "purist" view, and some people prefer to go higher if the view is still pleasing in their scope. If you're observing double stars, you tend to push higher anyway, since you're not after a sharp view, just looking to detect if you can see both stars.

With regard to the "official" spec of 260x, that's just what all manufacturers say - i.e. 2x the aperture in millimeters - but it's a fact that the resolution of fine detail tops out at more like 1.3x the aperture in millimeters.

It's important to understand that optical resolution is a complex subject, and as such "maximum magnification" only has meaning in relation to specific object types and conditions - see my other post here <click> for info.

Regarding the TMB differences - oh what a legal can of worms that one is! Basically the "TMB Planetaries" are the "Official" version of that eyepiece design, and the "TMB Designed" ones are eyepieces that the original factory claimed it was legally entitled to make and sell without TMB's permission. It's really an issue for the law-courts to decide, since none of us are privvy to what was written into their business contract!

Not if you're using the "stock" eyepieces

Since one only needs about 3 to 5 eyepieces (depending on the scope), many people forgo a Barlow entirely and just buy modern, long-relief eyepieces of the required focal length.

Eyepieces are a terrible slippery-slope really. You can spend one heck of a lot of money if you're not careful. That's why it's good to purchase second-hand, so that if you don't get on with one, you can sell it at cost - since different telescopes respond to eyepieces differently.

- oh - forgot to say that if the Barlow that comes with the 130M is not the "Deluxe" one, then it's really not much use for anything. Seriously! - I'm not being mean or anything - the SkyWatcher "Standard" Barlow that ships with some scopes is really, really poor!

Consider a TMB Designed eyepiece from Sky's The Limit at £36 <click>

I'd recommend you try out both a 5mm and 6mm - keep the one you like and send the other back. Sky's the Limit have no problem with this so long as you let them know in advance it's what you intend to do.

The 5mm will give you 180x magnification in your scope, which may be too much, and the 6mm gives 150x magnification.

Hi! Opticstar has these but they seem to be out of stock for the moment.

Ascension Eyepiece Cases.

At £4 and £5 per bolt case??? They've gotta be kidding!

Some companies give these away free when you buy an EP

Scopes 'n' Skies do three for £5 or nine for £15 here <click>

That's a good start, £10 doesn't sound too bad, it's a lot cheaper than a new case.

Was £10 a pricing error?

- that Jessops link comes up with £1.95 for that foam now...