Louis D

3 minutes ago, Don Pensack said:

Measure carefully, and you can easily get within 0.5° of apparent field.

Only for eyepieces with sharply defined field stops.  For eyepieces that have fuzzy field stops due to being poorly placed or non-existent (barrel defined or field lens defined field stops), you'll be lucky to nail down the AFOV to within a degree.

1 hour ago, vlaiv said:

That is exactly my point - fast phone lens will become small phone lens if its aperture is smaller than exit pupil of eyepiece - it will stop down exit beam because it is simply smaller than that - it is the same case like we have in using eyepieces with very large exit pupils - our eye becomes aperture stop if exit pupil is larger than about 7mm. Similarly if aperture of camera lens is smaller than exit pupil - it will become aperture stop.

In your experience, does an astigmatic eyepiece look sharper when your iris is smaller?  Not in my experience.  The astigmatism in my own eye becomes less noticeable since I'm only viewing through the central part of my cornea, but the eyepiece image does not improve.

19 minutes ago, vlaiv said:

Not sure that I understand this.

Say I have 85mm F/1.4 lens - that lens has aperture of about 60.71mm. This is regardless of where it's iris is.

Exit beam of light from eyepiece is collimated and it has certain diameter of "pencil" - regardless where you position that "pencil" in lens aperture - it will be focused on camera sensor.

All I can say is go out and try afocal projection with that setup.  You'll find out really quickly that you'll only be able to image the very center of the AFOV because you can't get the camera lens in close enough to match exit/entrance pupils.  The objective lens will bump into the eyepiece unless you're using an eyepiece with several inches of eye relief to project that AFOV well behind the eyepiece.  I've been doing afocal projection photography for well over 20 years, and until the advent of small objective lenses with early digital cameras using small sensors in the early 2000s, the method was not very usable except with low power, long eye relief eyepieces.  It's literally no different than trying to see the entire AFOV of an eyepiece from anywhere except with the eye's entrance pupil positioned at the eyepiece's exit pupil.  The cell phone camera replaces the human eye.

19 minutes ago, vlaiv said:

dditional benefit of long focal length lens is that you'll magnify image from eyepiece significantly and lens aberrations will be minimized in comparison to eyepiece aberrations.

Only drawback is that you need to shoot multiple images in order to cover whole FOV of eyepiece and you need to stitch those images together using clever software like Microsoft ICE.

The software will have to get rid of keystone distortion as you point it off axis.  As someone who doesn't do image stitching at all, I have no desire to go down that route.  Another problem can be getting the camera tipped correctly when eye relief is very limited since you want to keep the lens's entrance pupil at the eyepiece's exit pupil as you collect images.  If the camera has any significant width, and if the eyepiece is not a volcano top, the two will bump into each other.  You can back up the camera in this case, but you will capture an even narrower sliver of the AFOV.

19 minutes ago, vlaiv said:

I'm not entirely sure - eyepiece aberrations will be "imprinted" into wavefront of exiting light "pencil" - if you cut away part of that wavefront, you'll effectively throw away aberrations.

I think this only comes into play if the camera's f-ratio is slower than that of the telescope's f-ratio.  Since cell phone cameras are very fast, this is never an issue.  Michael Covington over on CN would be the best to clarify this question.

9 minutes ago, vlaiv said:

How does that work?

What about simple drift timing method?

Put the eyepiece in a telescope, point a flashlight (UK torch) into the telescope, put an eyepiece in the focuser (a diagonal is handy here), and aim the eyepiece at a wall.  Move the light around until you get a nice, sharp circle showing the entire AFOV all the way out to the field stop.

Now, measure the diameter of the projected circle and the distance from the top of the eyepiece to the wall.  Make sure to keep the top of the eyepiece parallel to the wall.  You want a nice circle to measure.

Next, using a white box (or card stock), determine the position of the exit pupil by inserting it between the eyepiece and the wall.  Move the box/card toward the eyepiece until the image circle is minimized.  If there is gross CAEP, this can be difficult to determine.  For most eyepieces, it's fairly obvious, though.  Now measure the distance from the top of the eyepiece to the box/card.  This is the usable eye relief.

Lastly, use the following formula to calculate the AFOV:

2*arctan[(Circle diameter/2)/(Eyepiece to wall distance - usable eye relief)]

You're just calculating the half angle of the projected circle as seen from the exit pupil with trigonometry (tangent(angle) = opposite/adjacent) and they multiplying it by 2.

The drift timing method is a good way of measuring the true field of view (TFOV) of an eyepiece, and thus its field stop, but it tells you very little about its AFOV due to variable magnification distortion across the FOV.

23 hours ago, vlaiv said:

If you want to do this kinds of tests properly - you need large, sharp, fast lens and DSLR as best option.

Unless you can find a DSLR lens with a tiny lens depth wise or are using an eyepiece with a large amount of eye relief, you'll never be able to get the exit pupil of the eyepiece to coincide with the entrance pupil of the lens (generally where the iris resides).  As such, you won't be able to image the full AFOV of the eyepiece.  That's why today's cell phone cameras are so fantastic for this application.  High resolution, wide angle field of view, well corrected, and insanely short distance to the entrance pupil.

23 hours ago, vlaiv said:

Phone cameras being very compact in size - use very small sensors and very short FL lenses. For example iPhone 11 has 26mm equivalent lens at F/1.4. Sensor size is 1/2.55" (crop factor of about x6), so actual lens focal length is about 4.3mm. With F/1.4 - it has aperture size of only 3mm - it can't accept exit pupil of 5.6mm - lens is not wide enough.

This is completely irrelevant for imaging the AFOV to determine eyepiece characteristics.  The image will be dimmer than if it could have accepted all the photons available in the exit pupil, but the entire AFOV will be visible.  It's no different than looking at the full moon with the human eye through a low power eyepiece.  The eye's entrance pupil will be narrower than 5mm because the full moon is too bright to allow it to fully dilate.  However, a low power eyepiece could very well be producing a 5mm or larger exit pupil at the same time.  This has no effect on seeing the entire AFOV which is what we're trying to achieve here.

20 hours ago, Mr Spock said:

I've just measured the field stop. You have to take the nosepiece off to see it as the interior of the eyepiece is a little recessed; and, it's exactly 48mm.

It's not in the insertion barrel?  It must take a lot of in-focus to reach focus with that eyepiece.

23 hours ago, Mr Spock said:

Firstly, the aforementioned LVW 42mm. Some say 72° on the barrel, some say 65°. So which is it? The answer is 'yes'! OK, let's clarify. When you look through it, it definitely looks 72° - between a 65° LVW and an 82° Nagler visually - , but, when you measure it, it's only 65°. Confusing isn't it - read on. The problem is the eyepiece has huge amounts of distortion. The image at the edge is much larger than the image in the centre.
It's only actually 42mm in the centre... So, if you average out the magnification, it weighs in as a 38mm 72° eyepiece. To me that makes much more sense. It's not a 42mm 72° eyepiece, nor is it 65°!

You should be able to directly measure the AFOV using the projection method to within a degree of accuracy.

If you have some eyepieces with known good FS diameter values (such as TV ones), you should be able to extrapolate a conversion factor from the number of millimeters you see on your ruler through the camera lens to the FS diameter in millimeters.  That's how I've been able to determine FS values for all of my eyepieces based on known good values for some of my eyepieces.  I'm usually within 0.2mm of the value from other sources such as Ernest of Russia's measurements.

From the FS and FL values, you can then calculate the eAFOV (effective AFOV) using mathematics (FS/FL*57.3, IIRC).

22 hours ago, globular said:

Most eyepiece manufacturers would put a fs in an eyepiece - rather than relying on the barrel - and typically most don't go over 46 or 47mm to avoid "nasty" effects at the edges.  e.g. my Pentax XW40 has a fs of 46.5 and gives an afov of 67 (rather than the round 70 reported by the marketing gurus for all the XW range).

I've measured my 40mm Pentax XW-R to have a 46.2mm field stop, a 70 AFOV (via projection), and a 66 degree eAFOV (effective AFOV) based on the FS and FL.  I think you are referring to eAFOV rather than projected (perceived) AFOV.  Go back and remeasure your AFOV using the projection method and see if yours isn't also 70 degrees.

Don't feel too bad, it took the Hubble folks a few tries as well to get things sorted:

3 hours ago, badhex said:

I actually wasn't aware of these, thanks - I hadn't really put two and two together that for the very wide 100˚ etc EPs you'd need the 2" barrel. Slightly off topic, but not sure exactly how the crossover scales e.g. I believe it's about 24mm / 68˚, for max TFOV without vignetting in a 1.25", but what happens to the AFOV when you increase FL by say 2mm? Presumably theres a calculation one can do.

Tom Dey over on CN put this graph together to show where each barrel size maxes out:

There is some fudging allowed if you put the field stop above the insertion barrel and accept some vignetting and lots of required in-focus.  It's sort of analogous to how you can use 2" eyepieces with 127mm Maks despite their 27mm diameter rear port.

2 hours ago, a6400 said:

I don't want to use it for DSO AP, i want to use it for visual + VIDEO recording of planets, so then i can process them on a computer.

I know that taking long exposures on Dobsonian is a bad idea.

Without tracking, you'll be having to keep pushing the scope, letting it settle, and then allowing the planet image to drift across the imager.  Doable, but you'll tire of the process fairly quickly.  An equatorial platform will allow tracking for up to an hour before resetting it.

The shorter the Barlow, the larger the effect of placing it in front of the diagonal (that whole 1/FL deal).

If you can screw the optical element directly to the front of the diagonal, you can greatly reduce the magnifying effect.

Sure.  You might need to refocus the reticle due to exit pupil issues.  Give it a try if you've got both.

If you're not getting any interest, try putting OBO in the ad somewhere for "Or Best Offer".  Sometimes, someone (or someones) will make an offer, even a low ball one, that might indicate some interest in your item.

I searched CN classifieds for the following from your ad and got the following ranges of asking prices that sold (no way to know if for that price):

Celestron 23mm Axiom: $125-$150

Lumicon 1.25" OIII filter: $60-$80

Atik EFW2: Impossible to search since you don't specify number of filter positions or size.  The range seems to be from $200 to $425.

Orion solar filter for 10" SCT: $140+

So, your asking prices in GBP would probably be 70% of those prices given the current exchange rate for USD to GBP.  Of course, the second hand British astro market may be more of a buyers or sellers market than the US market affecting asking prices.

At f/10 they will all probably work well out to about 5% or 10% from the field stop.  I'm not sure why you'd want to buy 30mm, 42mm, and 56mm eyepieces, though.  Would it be to cover a range of exit pupils (3.0mm, 4.2mm, and 5.6mm) for nebula filter usage?

I'd recommend getting the 35mm Aero ED if you can find one in stock (TS-Optics sells as the UFL 69° as well).  It, along with the rest of the line (30mm and 40mm) tend to perform better than the GSO Superviews even at f/10.  That, and you'll nearly maximize your true field of view (TFOV) with the 35mm Aero ED's 44.4mm field stop diameter.

3 hours ago, chrispj said:

Ah,  fair enough,  I might look at larger/ increasing the number of teflon pads then. Movement is smooth once it's going, but after being stationary for any length of time the initial movement jerks just enough to be annoying at a higher power...

 

Is the surface smooth or pebbly against which the Teflon glides?  If perfectly smooth, it will have too much static stiction to overcome when it comes time to start it moving.

Make sure you're using virgin Teflon and not mechanical Teflon.  This web page does a good job describing how to modify your bearings.

This material might also make a decent bearing surface for the azimuth motion.  I've seen similar used in some Dobs.  Just make sure the pebbly surface is glossy and not matte.

Both of my Dobs use large Teflon pads riding on Ebony Star Formica (which is discontinued, I believe).  Any pebbly, glossy laminate will work well to provide just the right amount of stiction.  Lazy susans accumulate crud over time, ruining the motion.

As far as non-telescope related items, I'd recommend downloading Stellarium and learn how to use it to know what is up each night as well as when and where to look.

I'm assuming it's a Dob.  As such, there is no automated tracking.  If you really want to get into solar system imaging, I'd recommend buying or building an equatorial mount for it.  There are lots of plans for them out there on the web if you're handy.

Those collimating tools will work fine.  The cheshire is more universal in that it can be used for aligning pretty much everything.  The Alilne is handy to keep the focuser hole plugged and to do a quick check of the primary alignment each time you take it out to observe.  The secondary generally stays put once set, so the cheshire will get used less.

With that Barlow, all you need is a T-ring to mount your camera since that Barlow has a T-thread on top.  I would err toward the lower profile versions to avoid running out of back focus.

You didn't say if you have any eyepieces beyond the beginner 25mm and 10mm eyepieces that come with it.  The 25mm is considered decent, but the 10mm is universally panned.  I'd probably budget for 8mm and 12mm or 15mm BST Starguider eyepieces for medium to medium high powers.

Since you have a 2" focuser, I'd recommend a 35mm Aero ED eyepiece (if you can locate one in stock) to maximize your field of view for finding and centering objects and viewing large open clusters like the Pleiades.

Calling @vlaiv.  He can probably best answer your imaging question vis-a-vis pixel resolution when adding a Barlow.

And when used in a diagonal, the long moment arm of a 2" Barlow or PM with a large eyepiece like the 17mm ES-92 can make the entire telescope turn turtle on an alt-az mount unless a large mass on a long moment arm is hung below it.

The moon is about the same reflectivity as asphalt.  Do you ever get eye damage staring at asphalt at noon?

Here's an actual video of the moon transiting in front of the Earth.  It's not very bright compared to the earth, is it?

Siebert Optics will put together a bespoke 4x Barlow or Telecentric magnifier for you on this webpage.  Just search for 4x repeatedly and try to make sense of his webpage to find them.

The first 10 9mm ES-120 eyepieces were sold without defined field stops, so they actually show as a ~140 degrees apparent field of view.  It's been reported by their owners that the true field of view is roughly equivalent to the 13mm Ethos's TFOV.  Since it has a 22.3mm field stop, this means that the original 9mm ES-120 eyepieces display roughly the same TFOV as a 26mm Plossl.  So, not quite the same as a 40mm Plossl with a 27mm field stop, but getting close and at a much higher power.  So yes, it can and has been done.

3 hours ago, Starslayer said:

Is is the activity at the sun that changes or is it due to our rotation around it giving a different angle?

The active regions on the sun do change over the course of days.  When they are visible is also due to the sun's rotation rate of about 25 to 35 days.  It varies by latitude.  Fastest at the equator, slowest at the poles.