Louis D

I realized something based on this discussion.  On many of my eyepieces that lack a defined field stop and instead use the bottom of the insertion barrel as the field limit, it is possible to see slightly more field by peeking around the edge, so to speak, and looking into the eye lens from an angle at the very edge to see what is hiding behind the edge of the barrel on the opposite edge when viewing from the center of the eye lens.  Is this a truncated exit pupil or some other effect?

1 hour ago, SteadyMercury said:

Hiya, newbie so sorry if the questions have already been asked beforehand.

I've been looking at trying out some eyepieces and wondered how the BST performs in a f4.7 dob? (Scope is a dob 250px) I currently have ES 8.8 82° and 20mm, 26mm 62° from ES with a focal extender x2, i have heard some good comments about the BST and was looking at getting a high power EP (5mm) - who better to ask than those using it? 😆

They really don't seem all that different than the ES 62° series, except the price is double for ES 😆

Thank you

I haven't used them below f/6, but I would guess that the 12mm and shorter eyepieces would do find thanks to their fairly strong negative elements extending the focal length seen by the positive section above them.  Certainly the 5mm and 8mm would do fine, with the 12mm starting to look like the 15mm and 18mm do at f/6.

The 15mm to 25mm ones are already showing edge softness at f/6, so I would expect them to show even more edge softness in an f/4.7 Dob.  Wide, low power, sharp fields at f4.7 and faster don't come cheap.

6 hours ago, Ruud said:

Above is how you can calculate the diameter of the eye lens needs to have given the apparent field of view (afov) and the eye relief.

Okay, I missed the point of the second diagram taking into account the exit pupil diameter.  Basically, just a one to one linear offset for EP diameter.

55 minutes ago, Ruud said:

It's impossible if you consider how the exit pupil is formed.

 

I think what @andrew s is saying, what if the edge of the eye lens were masked with a circular aperture mask?  The telescope and eyepiece focal lengths would remain the same since none of the curves changed, and the exit pupil would remain the same, but some of it would have to be vignetted.  Any thoughts on the validity or speciousness of this argument @Ruud?

3 minutes ago, andrew s said:

You won't as that's not the issue. For on axis ray the exit pupil comes through the centre of the lens. For a ray at the extreme of the field of view it comes through the outer edge and if the eye lens is too small it vignettes it just as filters can do in imaging.

Regards Andrew 

Okay, can you quote the geometric equation to ensure this doesn't happen as I've never seen it quoted anywhere?  You're saying that just because you've got a 4mm eye  lens, you may not be able to pass an entire 4mm exit pupil without vignetting which I'll grant you might be the case.  However, what is the required width to ensure that there is no vignetting for a given exit pupil?

On 18/06/2020 at 10:38, Mihai said:

Why does it seem like the US has so many options (at least when it comes to dobsonians) comapred to Europe?

Quite a few have been closing up shop lately like Starmaster and Teeter, so we're seeing less selection as well.

6 hours ago, andrew s said:

It's down to two main factors.

The field of view of the eyepiece and the size of the exit pupil.

The bigger they are the bigger the eye lens needs to be.

Regards Andrew 

Make that these two main factors: apparent field of view (AFOV) and eye relief (ER).  As others have indicated above, the combination of AFOV and ER is generally what dictates the size of the eye lens.  An eyepiece designer can increase the AFOV while keeping the eye lens the same size by decreasing ER or increase ER and decrease AFOV.

Exit pupil doesn't really play into the equation at all as it is strictly determined by the eyepiece focal length divided by the focal ratio of the telescope.  As long as the lens system in the eyepiece is wide enough to pass the entire exit pupil without vignetting or truncation, the actual design has no effect on it.

3 hours ago, sputniksteve said:

Ah man!! I've just realised that Meade still has something in this category? Any thoughts on this vs the SW/Celestron? 

 

https://www.harrisontelescopes.co.uk/acatalog/meade-ng-125.html#SID=1703

The original Meade ETX scopes were made in the USA.  I don't know where they're currently being made since Ningbo Sunny bought them.

2 hours ago, Alfian said:

I came to the conclusion that anywhere north of Watford and south of Sheffield fell into a large no mans land, unknown and generally avoided by folks north and south unless circumstance demanded, in which case it was down to the satnav to provide the knowledge.

I had to look that up.  Isn't that most of central England including Birmingham, Nottingham and Cambridge?

In the US, that which isn't on the east or west coast (95% of the US by land area) is derogatorily referred to as "fly-over country" by folks from the coasts.

1 hour ago, sputniksteve said:

🤔 Well now ...

Avoid ebay for the most part unless you really know what you're doing.

If you have a Craigslist equivalent for local ads over there, it could be good for acquiring large, bulky items like Dobs locally.

3 hours ago, F15Rules said:

So, Steve, you're saying that if you had differently dimensioned facial features, who "nose", you might now have an LVW22 mm pair??😂🤣

Dave

Groan.

7 hours ago, Dannomiss said:

Also located corona.

Hopefully not the virus, either.

What's your budget?  The 70 degree 22mm eyepieces (from Long Perng?) (Olivon 70, AstroTech AF70, Omegon Redline, Arcturus Ebony, Telescope Service Expanse ED, Astromania SWA, Celestron Ultima LX and SkyWatcher SWA) is the best of that line and very reasonably priced.  The Nagler T4 22mm is also quite nice, but tighter on eye relief.  If you want to stick with 1.25" only, the LVW 22mm is probably your best bet again.  If you don't need the eye relief, either 20mm Nagler (T2 or T5) is supposed to be quite nice, but again, 2" only.  The APM HDC XWA 20mm would also fall in this category.

1 minute ago, Dannomiss said:

Finally got my head around it and above the head now makes sense

I kept struggling with the flips until I realized I was having to do the flips in my head.  Holding it over my head and moving it in and out of my line of sight, boom!  Super easy to match up the planisphere with what I was seeing naked eye.

Imagine being in a hot air balloon basket and looking down at a road grid from high above.  You'd want a map of that road grid in the same orientation so you could easily match things up to let your chase crew know roughly where you are in case they've lost sight of you for some reason.

16 minutes ago, Saganite said:

I have had all of the LVW range at different times and I agree wholeheartedly that the 22mm is the 'standout' performer. I would have chosen a pair of these over the 24mm Pans that I have but in a bino, they left no room for my snozzle.

 

I guess I'll have to pick one up some day to compare to my AT AF70 and NT4 22mm eyepieces.  I might even have to pick up a Pan 22mm to complete the shootout.  Lots of folks over here rave about the latter as well.

Check out the weight difference between an 8" f/5 Newt on an EQ-6 and a 10" f/5 Dob.  Roughly 100 pounds versus 60 to 65 pounds.  If you've got to move the scope around to dodge trees and buildings to see various parts of the sky as I do, those extra 40 pounds are a killer unless you're built like a tank.  That, and every EQ mount is really top heavy fully assembled and wants to turn turtle the moment you try to pick it up and move it.  Dobs are inherently bottom heavy and very stable during moves.  If GEM style EQ mounts didn't require counterweights like a split ring Newtonian EQ mount, then the gap would be somewhat narrower.

@Dannomiss You've probably already figured this out already, but the best way to use a planisphere is to hold it over your head, aligning N-S-E-W with the sky, and match the diagram with the sky.  Looking down at the planisphere and then up at the sky repeatedly never worked as well for me.

I look at the 127 Maks this way.  In the US, if you've got $200 to spend on a travel scope (and have a mount of some sort already), a used 127 Mak is the way to go over a new 90 or 102 Mak for similar money.  Why the used/new comparison?  Because so many 127 Maks turn up used over here compared to 90 and 102 Maks, and for not a lot more money.  I'm guessing a lot of newbies buy them new, grow disinterested or want to move up in aperture, and then sell them rather than keep them as a travel scope.  This tends to flood the used market and drive down used prices.  I don't see near as many C5s or C6s turning up in the classifieds, either.  Perhaps they don't sell as well new, or people like to hold onto them longer?

21 minutes ago, John said:

I have some telephone cables that run across my garden. Occasionally I've noticed a new bright diffraction spike on a star only to find that one of those dratted cables has strayed close to the field of view

 

Thankfully, we have buried utilities in our neighborhood, so that's never been an issue for me.  On the other hand, we'll never get gigabit fiber optic internet because it would mean tearing up everyone's backyards to install it.  So, we're stuck with twisted pair DSL and coax cable internet.

Once you get a design like this working, the mount just gets out of the way and you can enjoy observing.

13 hours ago, merlin100 said:

Old thread revisit...

I admire those who move up to the 300P, but for me the diminishing returns are also in the big jump in financial cost and weight.  I can lift my 200P out the back door from my flat in one go, but couldn't with a larger one.  Yes, I could split it, but couldn't justify the cost, especially in such a light polluted area.  Perhaps I'm reading this all wrong, but aren't larger Dobs better suited to dark sky locations and not urban areas?  If so, that would seriously reduce to viewing abilities if I needed to go outside my domicile..

They're also good at resolving more details in planets and globular clusters at higher powers, both of which are doable under light polluted skies.  However, weight and bulk do figure prominently into the trade-off equation.

Nice and false-color free like a Newtonian should be.  So far, it appears to be a winner.

Try focusing on an artificial star indoors in a darkened room for another test (you might need an extension tube for this).  A mirror finish Christmas ornament or glossy ball bearing with a bright flashlight (torch) shining on it from next to you can make for a decent facsimile of one.

6 hours ago, ALZASCOPE said:

Thank you guys just didn't want to surround myself with all wide angle but i will take your advise many thanks

Alan

I have quite a few narrower angle eyepieces and generally only use them when binoviewing.  I just don't see them providing any advantage over premium wide field eyepieces monoviewing.

50 minutes ago, johninderby said:

But the C6 is rather expensive compared to a 5” mak so that is a factor in the decision.

Agreed, even on the used market, the C6 goes for 50% more than the 127 Maks over here.  The OTA is also a few inches longer than the 127 Maks, so compactness relative to a backpack may become an issue for travel.

8 hours ago, Dannomiss said:

Its a skywalker heritage 150p 🙂 My first ever telescope!

I think you might be the first person on SGL to buy one.  We expect a full report when the skies clear. 😉  Until then, you can try out the mechanics and focusing on distant objects while aligning the RDF.

Yep, a well known issue with the BHZ.  It cannot be used with winged eye cups and binoviewers.  I prefer the Celestron Regal 8-24mm zoom for this reason and others.