Louis D

If I may ask, why is a super high power eyepiece your primary focus when reentering amateur astronomy when it will rarely be useful due to sky conditions on a select few categories of objects?  Are you big into splitting tight doubles?

54 minutes ago, johninderby said:

Meade filled for chapter 11 bankruptcy about a year ago and it looks likely that the assets will be bought by another company, probably Chinese. Hopefully whoever buys Meade will  relaunch it and not just stick the Meade badge on cheap gear.

I think what they're really hoping for is for the antitrust court ordered settlement to be discharged as a debt and then they can emerge from bankruptcy without having to pay it.  My hope is that the bankruptcy court will strip them of the Meade brand and award it to Orion as partial compensation since Meade illegally transferred all their liquid assets out of the US prior to filing for bankruptcy.  If not, it will completely gut the US's antitrust laws.

4 hours ago, John said:

What method does Earnest in Russia use ?

I was wondering that myself.  I can't recall ever reading it anywhere.  His numbers do match up pretty closely with mine on eyepieces we've tested in common.

Here's Jon Isaacs's method, which I've duplicated myself.  It is slightly more accurate than sending the light unfocused through the eyepiece.

The relative photographic method combined with the above technique allowed me to calculate the numbers below by counting pixels and extrapolating the tangential relationships from known good values:

Given magnification distortion in the camera lens, it's probably wise to only apply this technique within a fairly narrow range of apparent fields of view and then cross correlate them with the projection measurements as a double-check on their accuracy.  It's also obvious from the image that it's difficult to determine where the edge of field lies when it gets fuzzy due to a lack of a distinct field stop or has chromatic smearing.

2 hours ago, Andy ES said:

So if I use a 2” diagonal there is no point using more than a 68 degree EP ?

For widest field at 40mm, yes.  If you are willing to give up a bit of true field for higher magnification, you can move up to a 30/31mm 82 degree eyepiece or 20/21mm 100 degree eyepiece.  If you want a larger exit pupil for nebula filter observing, you could drop down to a 55/56mm 50 degree Plossl.  You may end up with all of them after a time as you experiment with what works best for your observing style.

1 hour ago, John said:

Drift timing is probably the most accurate method:

http://www.mikehotka.com/Measuring_field-of-view.htm

 

It's not accurate for measuring apparent field of view.  It doesn't take into account magnification distortion across the field which can vary widely.  The 26mm Meade MWA is an extreme example of this.  This method only gives you the true field of view which can be converted into the effective field stop size once the true focal length is known (probably from exit pupil diameter).

There are several methods to determine the apparent field of view based on projecting light through the eyepiece and measuring the projected circle or cone of light coming out of the eye lens.  The more accurate method is to split the exiting light cone across a sheet of paper and measure the angle with a protractor after tracing the edges onto the paper.  I use the more involved method that requires measuring the eye relief distance from the top of the eyepiece, the distance from the top of the eyepiece to the wall, and the diameter of the projected circle.  Then there's a trigonometric calculation.  I've found it a bit less accurate because it can be difficult to determine the exact eye relief distance and the diameter of the projected circle for certain eyepiece designs.  The formula is AFOV=2*arctan([circle_diameter/2]/[wall_distance-eye_relief_distance]) if you want to go this route.

The best method to project a circle is to put the eyepiece in a telescope and project light into the front of the scope.  Barring that, project light directly into the field end of the eyepiece after blocking it in a miter box to keep it from moving and to keep everything aligned.  Make sure to back the light away from the eyepiece until you get a sharp edged circle projection.  At first, I had the light too close and was getting fuzzy edged image circles as a result.

The exit pupil is where the cone of light exiting the eyepiece is at its narrowest.  Move a card back and forth until the projected circle is minimized and measure that distance to the eyepiece.  This can be difficult for some eyepieces that have a poorly defined exit pupil due to aberrations.

1 hour ago, PXR-5 said:

I took it off for Mars, but I am too lazy to take it off for the other planets.

I take my CC out of my Dob and my field flattener out of my fracs for planets because each contributes a bit of spherical aberration at high magnifications.  They stay in place the rest of the time.

8 hours ago, Zermelo said:

Also sold as Jenolite, the second-most-useful item in the impecunious motorist's toolkit, after WD40. 
It made up 75% of my first car, by the time I had to get rid of it.   

So glad I moved to Texas 27 years ago.  I haven't had a spot of rust on any vehicle since.  It's really odd to see immaculate cars and trucks from the 60s and 70s in daily use around here.  Interior upholstery rots away in the sun here after 10 to 20 years, though.

6 hours ago, Skipper Billy said:

To stop the keels of yachts rusting we use a product called 'Fertan'  https://www.fertan.co.uk/

I don't know what is in it but it converts rust to an inert product. Spray or brush it on - wait until it goes black - wash it off and paint it. Even iron keels immersed in salt water don't go rusty after treatment.

Fertan MSDS:

Yes, much better to have a dedicated Dob for observing and an EQ mounted ED refractor for DSO imaging.  If you want to get into planetary imaging, an SCT or Mak would probably be the way to go.

Mold and mildew are big issues as well for optical surfaces stored outside in humid environments without adequate ventilation.  If you live in a desert climate, this is simply not an issue, though.  First surface reflective mirror coatings are also prone to corrosion in humid environments.  Even professional observatories have to have their big mirrors realuminized every so often.  My 20+ year old mirrors in my Dob look and perform fine after having been stored in a climate controlled house between uses, though.

5 minutes ago, radian said:

How critical is it to avoid slightly oversize exit pupil?

I just bought a GSO 250mm f/5 Dobsonian and a ES 52º 40mm eyepiece for it. It'll have 8mm exit pupil. Is this going to be an issue... or are there more severe issues with this scope-eyepiece combo? I wear glasses so a decent eye-relief is an issue.

As long as the central obstruction doesn't become so large as to cause a distracting shadow, you'll be okay.  You'll just have part of the light from the primary truncated by your pupil, so you won't make full use of your aperture.  You'll also have a more washed-out background making it more difficult to locate faint fuzzies.  If you're just taking in bright star fields, it shouldn't be an issue.

On 08/12/2020 at 08:23, Tiny Clanger said:

That mini mak is cute, is it useful as a tiny travel telescope ?

It's pretty miserable when using much above a 25mm eyepiece due to loads of spherical aberration.  I have it in the PICO-6 livery.  There have been too many reports of this defect to think it's just manufacturing variation.

To get widest true field of view, you'll either need the reducer/corrector for 1.25" eyepieces or a 2" visual back and 2" diagonal and 2" eyepieces with widest possible true field of view such as a 40mm Pentax XW or 41mm Panoptic.  Either way, you're working against a 2000mm native focal length that only grows longer with 2" accessories because of the additional back focus required.  At best, you're looking at well less than 2 degree of true field of view either way.

If you want wide fields of view, get either a short focal length Newtonian or ED refractor.  Either scope will complement the SCT very well.

The 20mm reticle is sharp in the center but miserable in the outer 50%.  It's probably a König design variant if I had to guess.  I've got the Orion version.  See below for a comparison image through it:

A plain Barlow would probably work well for imaging and would give you the ability to vary magnification by varying the distance between it and the imager.  I'm not aware that they add any focal image curvature.

Most eyepieces don't work well for projection because they project a curved image plane.  This isn't an issue for afocal projection where the taking lens has ample depth of field to handle it.  I'm assuming you're attempting projection imaging and not afocal projection.

Here in Texas, we've got "shoulda could" meaning "should have been able to".  We also have "usta could" meaning "used to be able to".  There are many more Texas colloquialisms that sound just plain ignorant to someone born and raised in the upper Midwest where Johnny Carson and many others learned to speak correct and clear American English that sets the national standard.

3 hours ago, Barry-W-Fenner said:

Excellent choice. The 9 is my personal favourite, it gives some stunning mid power views I've only just acquired the 17.5 myself and am still waiting to get a good opportunity to try it out. I have read great things about it!

Let us know if the 17.5mm is actually a 74 degree field instead of 76 degrees as I have heard from at least one report.  I measured my 9mm and 14mm Morpheus(es) to have 78 and 77 degree fields, respectively, so I know there is some variability.  This is way less variance than the variability in the Starguider/Paradigm and HD-60 lines (57 to 65 degrees instead of 60 degrees):

2 hours ago, johninderby said:

And not forgetting loppers for heavier pruning. 😁

Same here. 😁  At least we can agree on some things.

2 hours ago, Tiny Clanger said:

Garden shears over here are things with much longer blades and handles , typically needing two handed use, the sort of thing you would use for light work such as clipping a hedge .

The manual ones are referred to as hedge clippers or hedge shears over here, depending on the region.

2 hours ago, Tiny Clanger said:

In which case one might call them hedge clippers ... but that could be an electrical device

Hedge trimmers are always electric or gas (petrol) powered, so no confusion here.  This is all the more ironic since most Americans refer to the plants they trim with them as bushes rather than hedges.  Hedges are more of a British thing.  We'd just call a hedge a row of bushes more generally.  Very few properties are separated by hedges or hedgerows instead of fences over here.  Bushes are generally planted along fences to make them less ugly, not as a separator in their own right.  They can also be planted around houses as a landscape feature, but I think that's done pretty much everywhere.

Then there's grass shears with a 90 degree offset for trimming grass manually.

Lastly, there are the powered grass trimmers or string trimmers that most Americans refer to weed whackers or weed eaters (the original brand name).  We'll generally say we're weed whacking the yard with one (really, no joke).

This just goes to prove how quickly one comment about yard work can derail a thread.  Apologies to the OP.  Uh, good luck with your RDF to bring it back on topic? ☺️

55 minutes ago, Tiny Clanger said:

Coffee break now, then secateurs to the odd sticky-out branch

It's rare I get hit with a word I've never heard before, but today was such a day.  I had to look up secateurs, what we Americans would call pruning shears.  Do Brits prune unwanted growth with secateurs or is there a verb version of secateurs as well?

If you stay far enough back, the RDF window will subtend a very small angle on the sky limiting any parallax error due to your head bobbing about.  However, it can become more difficult to find the dot in the first place when at a distance.  If this becomes a problem for you, get up close to the RDF, locate the dot, and then carefully back off holding the dot in your vision.

26 minutes ago, woodblock said:

I did an open university maths/physics course a few years ago. When we had face to face tutorials where the tutor worked through various examples there was always a long delay after each one while many of the students queued up to take pictures of the whiteboard. It was a pain in the a**

I wouldn't have any experience with that.  The last class I took was for my Master's program in December 1992.  Definitely no cameras in the room that I can recall.  They would have almost had to have been film cameras at the time.

Back during my undergraduate days in the mid-80s, I did work on campus recording classes for distance learning onto 3/4" VCR tapes that were then sent to various companies where they would be viewed by a class made up entirely of employees.  After a period of time, they were returned, and the tapes were erased via Deguassing coil for reuse and to prevent anyone from viewing a class for free.  I would guess distance learning has changed considerably in the last 40 years as well. 😉

5 hours ago, vlaiv said:

That rule x2 aperture in mm (or x5 aperture in inches)

Make that x50 for inches.  I'm sure just a typo on your end.

In my personal experience, I find x30/inch more realistic for most nights and objects.  The moon's terminator and double stars are the notable exceptions for going higher (high contrast subjects) and planets for going lower (low contrast subjects).

As for the OP's original question, I like to look at exit pupils.  I tend to favor 0.7mm at the high end, and 7mm at the low end.  Given the f/12.7 focal ratio, that would equate to 9mm and 90mm.  You can forget about the 90mm end of things.  It is simply unrealistic to get a large exit pupil in an f/12.7 Mak.  Let's stick with a 32mm Plossl then for a 2.5mm exit pupil and budget for a fast ED refractor or Newtonian for wide field views down the road.  At 9mm, the 8mm BST comes close with a still usable 0.6mm exit pupil.  Your 20mm yields a 1.6mm exit pupil while your 10mm yields a 0.8mm exit pupil.  That 10mm, if it were a decent eyepiece, might indeed be your maximum magnification eyepiece on most nights.  A 15mm BST would indeed split exit pupil sizes at 1.2mm between the 10mm and 20mm.

The more I think about it, the more I think you might be better off with an 8-24mm zoom plus a 32mm Plossl to get you started.  The Celestron/Meade/SW/Astromania/etc. would be a good choice for the zoom and the SW/Astro Essentials/Revelation 32mm Plossl at the long end.  Once you have a better idea of the magnifications and exit pupils that work well for your observing style, you can pick up some wide and better corrected fixed focal length eyepieces.

Given my neck injuries, I can no longer use an RDF/Telrad/QuikFinder above about 30 degrees nowadays on a regular basis.  To work around this, I fitted a green laser sight to each scope which is by far the quickest and easiest way yet that I've found to put the scope on bright targets.  I then center the object in the main scope and do a Skeye app alignment on my cellphone attached to the scope at about 60 degrees to the sky so I can see the screen.  After aligning on several bright stars or solar system objects, I can use the app to locate objects objects close enough that they show up in my widest field eyepiece.  I can't star hop most nights because my skies have gotten so washed out that only alignment stars are still visible naked eye.

I know that laser sights are not popular outside the US for various legal reasons, but they have been catching on quite a bit here among advanced amateurs.  I don't recommend using them at outreach events because they attract little kids like flies who are then completely distracted by their presence.  As far as lasing an aircraft, I just keep my ears open for their distinctive sound and scan the sky before lighting up the laser.

Back to the OP's question, you generally want your finder or sighting device up high enough to be able to get behind it or off the the side of it as with right angle finders.  Put it at the back, and you'll be on the ground trying to use it.  If you're having trouble getting aligned with it because of the tube, put it on a 2" to 4" riser or stalk to get enough clearance for your head.

As far as Telrad vs QuikFinder, the former has zero parallax issues as your head bobs about while the latter shifts a bit relative to the sky.  I've read the QF circles are actually projected at about 6 feet instead of infinity.  The circles are also way easier to find in the Telrad than in the QF.  They're thicker, larger, and for some reason I've never understood, just plain easier to acquire.  Thus, I've been using the Telrad more than the QF, when not using the laser for various reasons, because I can only tolerate the pain of torquing my neck around to look through them for a few seconds at most.  Spending that time trying to locate the circles in the QF is a complete waste of precious neck torquing time for me.  In the Telrad, I can consistently find them almost instantly.  Still, both pale in comparison to the ease of use of a laser, though trying to torque my neck above 60 degrees can be painful in its own right to sight on a high object.  That's where Skeye comes in handy.  Align on lower stars/objects and use the app to navigate to higher objects.  It's not as accurate as my digital setting circles, but the user interface is way more intuitive.  I just need to get in the general vicinity, and I'm good to go from there.

To stay cheap, you could pick up a pair of 23mm 62 degree aspheric eyepieces from ebay.  Once you pull off the rubber eye cups, there's plenty of eye relief for eyeglasses, and almost enough with them in place.  I've found they work really well from f/10 and up.  Unless you have a CAT scope, you'll probably need either a Barlow or GPC/OCS to reach focus, increasing your focal length and focal ratio in the process.  This allows the 23mm eyepieces to perform much better and at a decently high power for planetary and lunar observing.

On the bright side, the Chinese stepped up a few years back and built an even bigger radio telescope of similar design.

Still, sad to see a famous old radio telescope go in such a horrible fashion.