Louis D

I took a bunch of images of the field of view (FOV) through many of my eyepieces in my 127mm f/12 Mak for comparison against those I took through my AT72ED f/6 refractor. I haven't had the time to pull together comparison groups for all of them as I did for the images taken with my AT72ED. However, I did pull together one for the 29mm-30mm group since it shows how much marginal/terrible eyepieces can improve in a slow telescope. It also shows how spherical aberration of the exit pupil (SAEP or kidney-beaning) becomes more obvious as black donuts. It also shows chromatic aberration of the exit pupil (CAEP or ring-of-fire) as a rainbow effect. Last, it shows that really good eyepieces become razor sharp. The above images were taken with the high resolution Samsung Galaxy 7 camera at f/1.7. The edge images were taken by pointing the camera straight at the field stop for best sharpness. The above "full width" images were taken with the lower resolution LG G5 super wide angle camera at f/2.4 so center and edge can be seen in a single image. Notice how this camera's smaller aperture is more sensitive to SAEP and CAEP than the Samsung's camera. The above image is from my original group of FOV images taken through the AT72ED which is an f/6 telescope. I included it for ease of comparison. I did not include a "full width" image for the APM UFF when I composited this image since its entire image fit within the Samsung camera's FOV. By comparison, notice how much the Rini, Kasai, and Agena eyepieces improve by going from f/6 to f/12. The ES-82 shows CAEP at f/12 and is sharper while the APM UFF shows some SAEP and has incredible edge sharpness at f/12. The slower Mak seems to show any inherent eyepiece SAEP or CAEP more obviously than the faster refractor. The Mak images all show greater image scale due to its 1540mm focal length versus the AT72ED's 430mm FL. Both sets of images were taken from the same distance, to within a few inches. I will try to add more comparison images to this thread as my time allows it. Each one takes over an hour to edit and composite.

Not really in amateur astronomy. Perhaps $2000 to $4000 on a Chinese APO, but it's generally a crap shoot on most low end Chinese made stuff. As an example, to get the Synta 200p/8" Dobs to work effectively, there are plenty of threads on here and CN detailing how to improve them to make them reasonably usable. Had they bought a Teeter scope to begin with, they wouldn't have had to mess with it. However, that would have been 10x more expensive.

Even in 15x binoculars, I can usually tell that Jupiter is a disk with the Galilean moons as pinpoints on a line on either side of it. Are you sure you were looking at Jupiter?

I noticed everyone seems to use counterweights on the other side of the mount when no scope is mounted over there. Is this done avoid the mount tipping over sideways or because it doesn't do well unbalanced?

I've used rubber O-rings to parfocalize my TV Nagler T4 12mm since it focuses so far out. I even had to add an M48 spacer ring to the 2" skirt to leave enough 2" barrel to lock onto. ebay is a good source for odd sized rubber O-rings. You want to shop by inner diameter (ID) since that will be fitting around the insertion barrel. Parfocalization rings generally have sharp set/grub screws that dig into the insertion barrel. O-rings are harmless to the barrel.

If you could find the right buyer, you might get $200 for it based on sales in the US. The problem is, it is old, huge, heavy, commercial quality, and non-computerized, so it appeals to a very narrow group of buyers into vintage scopes. There might be someone who would want to either turn into a solid tube Dob or just scavenge the optics to make a truss Dob. I have seen some folks put retractable castors on these and wheel them out of their garage onto their driveway for use.

The ES-52 line is really only advantageous at 15mm and below relative to a similar Plossl thanks to their more generous eye relief. Those are all 6 elements in 3 groups eyepieces. Presumably, they all have a negative/positive design similar to the Vixen SLV line at those focal lengths. The 20mm, 25mm, 30mm, and 40mm are all 4 elements in 2 groups eyepieces. Presumably, they are Plossl variants. As such, I don't know what advantage the first three offer over a Plossl. The 40mm is interesting in that it is a 2" eyepiece allowing for a 36mm field stop diameter instead of the usual 27mm FS limit imposed by a 40mm eyepiece in a 1.25" barrel. Does anyone know of any other 40mm 2" Plossls?

You could try M5 set/grub screws if you can find them and then paint them over.

No, don't get that adapter. Get a proper 2" visual back and adapter as noted above. You won't notice much if any vignetting, but bright objects will have an oval aberration on the opposite side as they pass the edge of the rear baffle. I put the Mak to SCT Adapter linked above on mine and then put a standard SCT 2" visual back on that. That way, I can use the same 2" diagonals in both my refractors and Maks. If you already have the SCT threads on the back of your Mak, skip the adapter and proceed straight to the visual back. You'll be able to expand your true field of view from 1 degree with a 32mm Plossl to 1.7 degrees with a widest field eyepiece such as a ES-68 40mm.

The Bresser/Explore Scientific/JOC Dob is very nice as well as the Revelation/GSO Dob. The Skywatcher/Synta Dob is a slight step down mechanically. All are similar optically. Before rushing out to buy an 8" Dob versus a 127 Mak, consider the vast size difference. Where will you be observing from? How far will you be carrying the scope? Do you plan to transport it by hand, car, other? Do you want to learn the sky or simply let electronics point things out to you? What is your budget? What is your weight lifting limit?

I use a 19" plastic tool box to hold my bits and bobs that don't need foam padding like collimators, various adapters, spare counterweights, spare batteries, etc. I use a vintage Tamrac video camera case from the 1990 era to hold my 127 Mak. For my daughter, I use a purple gym bag to hold her 127 Mak and associated gear with bubble wrap around each object. She can leave the closed bag out at campsites and no one thinks to steal it. I use heavy-duty, surplussed Manfrotto photographic tripods for my DSV mounts. They're old, but still have years of life left in them. I've used clamps, L-brackets, and weights hanging in a plastic grocery bag to achieve balance with some Dobs. When the angle gets low, the weight settles on the ground board and no longer pulls the scope downward. I've used PVC plumbing bits to adapt refractor objectives to diagonals. I use laser sights and picatinny rails to attach them, both intended for guns, to aim my telescope. I use multiple Plano Protector 4 Pistol cases to hold lower value eyepieces, binoviewers, and diagonals. I use a Tamrac photographic filter wallet to hold most of my astronomy filters. It lives in the tool box mentioned above between uses.

In the US, sellers generally cross-list on Craigslist hoping for a no-fee local sale and then cancel the ebay ad if sold on CL. Sometimes, they'll suggest searching CL for the item to find their contact information outside of ebay if you're local to them and want to pay cash. On another note, I opened a bank account specifically for Paypal to limit my exposure to fraud issues. I opened it with $50 20 years ago, and due to refunds sometimes going into it instead of back onto my CC, and interest earned, I've got about $150 in it now. Since CC companies pretty much cover all losses due to fraud, I only stand to lose up to $150 due to fraud using PP.

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?

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.

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. 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?

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?

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

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.

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.

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.

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.

Groan.

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.

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.