Louis D

1 hour ago, Highburymark said:

Interesting discussion elsewhere in this thread about what constitutes a plossl and what doesn’t. From the images posted earlier, is it right to conclude that the true plossl design is asymmetric, and the TV patent symmetric? 

More or less, the original Plossl design was asymmetric while the current usage has mostly come to refer to symmetric designs.  I saw mostly because there are the 5 element "super" Plossls out there that are even more remotely related to the original Plossl design.  Meade marketed a line of 60 degree, 5 (and in one case, 6) element Plossls (their 5000 series) a few years back just to muddy the Plossl waters even further.

At f/12 in my 127 Mak, the cheap 30mm 80 degree Widefield clones do quite well across the field as is visible in this image (Agena UWA, 2nd from bottom):

The ES-82 at the bottom would be equivalent to the 31mm Nagler since it is basically a Chinese clone of the design.  The UWA eyepiece has the advantages of no CAEP or SAEP, less magnification distortion across the field, longer eye relief, more compactness, and much lighter weight than the ES-82/Nagler equivalents.  For under £70, it might be worth a look.

On 06/07/2020 at 10:22, Stu said:

.....I’m a refractorholic.

Hi Stu! 😄

On 06/07/2020 at 10:22, Stu said:

ScopeTech STL-80A Maxi 80mm f15

Zeiss Telementor II f13.3

TAL 100r f10

Vixen 102 PST Mod f10

Vixen FL102s f8.8

Takahashi FC100DC f7.4

TS 72mm f6

Televue Genesis f5

Are these the first 8 steps towards recovery from refractorholism?  Only 4 more to go.  Question is, what will they be?

You'll really enjoy the Delos 10mm.  I've had mine since it was introduced almost a decade ago.  It's super sharp across the field and easy to view through.

1 hour ago, michael.h.f.wilkinson said:

For observing I much prefer having a range of individual EPs, not so much because of optical quality, but due to ergonomics. Especially for planets, I really like to switch directly between EPs (preferably parfocal ones too) so I do not lose critical focus, or only have to adjust minimally.

I'll sometimes leave my GSO 2" ED Barlow with the TV PBI in the focuser to double my scope's focal length all night and use longer focal length eyepieces at higher powers just to change things up a bit on some nights.

And then there's the equally mysterious Antares Speers-Waler Series 4 eyepieces which are also fairly recent, similar in specs and price to the ES-82 LERs, and also have a dearth of reports/reviews of them.

8 hours ago, Dannomiss said:

I have thought about the barlow question though. if my FL of scope is 750mm and I use a 5mm I piece I can get 150x Magnification.... however I could also use my 10mm EP and 2x Barlow and get 150x magnification. So my question which is similar, why would you do either? WOuld you the 10mm + barlow give you a more comfortable viewing experience or would going straight to a 5mm be better?

Its a good question 🙂

Also: I have read something on here about barlows and that with the barlows where you can take the lens of the end - if you then attach that to an Eyepiece, its not 2x now but approx 1.5x. However it doesn't work with my new BST starguider lens as it need a little extension to separate the EP lens and the barlow lens. Just a thought.

It really depends on the eyepieces.  If I've got a 5mm Huygens (not so far fetched, I've got an H6mm) and a 10mm Delos (I do) and a quality 2x Barlow (such as Tele Vue, Meade 140, GSO ED, etc.), then I'd definitely use the 10mm with the Barlow any day over the 5mm Huygens.  It will be more comfortable, sharper, etc.

However, let's say I've got a 5.2 Pentax XL (I do), I'll use it because the integrated factory solution of positive and negative design elements in it works better than the Delos/Barlow combination, it's more compact, and it will come to focus in all of my scopes.  Longish Barlows that I prefer work best in Newtonians and other scopes that don't utilize diagonals.  Shorter Barlows that do work well in diagonals have more aberrations than longer ones in my experience.

The trick of attaching the Barlow lens element to the bottom of an eyepiece works best when the field lens is well up inside the eyepiece barrel.  When they're near the bottom as in your Starguider, the field lens is intercepting the diverging Barlow rays way too soon which leads to a host of aberrations.  That's because the Barlow lens has a designed working distance related to its focal length, and you don't want to get too far inside of that distance for best performance.  It also doesn't help that the lower elements of the Starguider are acting like a Barlow in their own right, so now you're adding even more divergence to the system with that Barlow lens element.  This trick works best with positive only designs in my experience; and even then, it is hit or miss.  Often, the field of view is constricted and the outer field is distorted.

That matches up well with this CN comparison of the edge correction of each.  I've linked the images below:

30mm Pentax XW:

30mm APM Ultra Flat Field:

What I'm wondering is how the Pentax XW 40mm stacks up against the Meade 5000 SWA 40mm that I've been using since the great SWA blowout sale.  I picked it up for $125 and am wondering how much better the $400 Pentax 40mm would be.

I'd be interested in hearing your preferences among them.

2 hours ago, sputniksteve said:

What are you clamping it to on the OTA, and what adapter is holding the phone?

I think it's clamped to the bottom of one of the truss poles.

Here's a collection of longitudinal aberration plots for various achromatic, semi-APO (ED?), and APO objectives analyzed by Vlad on his optics page linked above.  Note that the horizontal scale varies from plot to plot:

One of the takeaways is that there is no one singular definition of an achromatic, semi-APO (ED?), or APO objective.  Depending on the design and glass choices, the chromatic performance can vary quite a bit across the field of view within each group.  However, it is clear that each step up in general correction yields an improvement.

4 hours ago, John said:

Thanks Louis.

The number of "crossings" seems to be important. This one appears to show 4 and nearly a 5th. What is the significance of that ?

The classic definition of achromatic is 2 crossings and apochromatic is 3 widely spaced crossings.  Superapochromatic is 4 or more.  However, many APOs have one or more crossings in the infrared rendering them less useful.  Here's a diagram from Wikipedia showing the various objective types:

Crossings refer to wavelengths of light that focus at the focal plane of the objective.  In a reflector, all wavelengths focus at the focal plane by comparison.

1 hour ago, John said:

I wish I knew how to properly interpret diagrams of this type

I believe this shows the distance in millimeters each wavelength of light actually focuses in front of or behind the focal point as you move away from the optical axis (bottom) to the edge (top).  Thus, about 70% out from dead center to edge produces a nearly color free image when in focus.

1 hour ago, Bego said:

Louis can you please recommend me a Goto telescope, on my budget. 450 to 500

Thank you for helping me out. X

I strictly use manual scopes.  I have Sky Commander DSCs (Digital Setting Circles) on a couple of them for push-to operation.  As such, I'm in no position to recommend any GOTO mounts.

@Dr Strange thinks highly of the Skywatcher AZ-GTi mount.  However, it appears to be out of stock everywhere.  I would contact him for further GOTO advice if you are absolutely set on a GOTO mount.

Personally, I would probably go the Dobsonian route as others have suggested above and attach a cell phone via an accessory arm as @Dannomiss did in this thread to allow for rough, push-to operation while you learn the night sky using a planisphere app.

Here's a nice comparison of various objective designs done by Vlad Sacek on his Amateur Telescope Optics polychromatic PSF page:

As you can see, an FPL-51 triplet has roughly the same correction as an FPL-53 doublet.  Also, a moderate focal ratio FPL-53 triplet comes very close to the ideal refractor objective.

These two pages on semi-APO and APO refractor objectives and design are also a good read.

Most folks who need to wear eyeglasses at lower powers (larger exit pupils) due to astigmatism can get by without them at higher powers (smaller exit pupils) as this chart from Tele Vue's Dioptrx page shows:

8 minutes ago, AstroMuni said:

Disadvantages for Go-To are mainly price and you need a source of power to connect to it.

Add trying to get a successful alignment.  There are lots of requests on SGL for help getting alignment to successfully complete.  This assumes you can see enough bright stars to do an alignment.  Due to buildings and trees, I often have difficulty finding enough alignment stars for DSC alignment.

Another issue is the often poor reliability of the handsets.  They were designed to a low price point which means that durability wasn't high on the priority list during design.  Again, I've read plenty of threads about handset issues.

1 minute ago, rl said:

Having just bought a secondhand 14mm Delos to replace a 15mm Celestron 82 degree, you would be welcome to try the 15mm on a sale or return basis if that helps. I use f/4 or f/4.5 scopes and it's not the best but it should work a lot better at f/8. 

Can you confirm the Luminos's viewing comfort without eyeglasses?

The Luminos is supposed to be fairly comfortable without eyeglasses thanks to it's large eye lens being mounted close to the top:

I measured the usable eye relief of the Panoptic 27mm to be 14mm, which is about how it feels with my eyeglasses.  Remember, the design eye relief is measured from the center of the eye lens, and it is deeply concave on this eyepiece and slightly recessed.  The usable eye relief is measured from the top of the folded down eyecup.  So you've got about 1mm for the folded eyecup, 2mm for the eye lens edge recession from the metal lip the eyecup attaches to, and 2mm of eye lens concavity.  By comparison, the Pentax XLs and XWs advertise 20mm of eye relief and deliver 18mm of measured, usable eye relief, so much closer to advertised than the Panoptic.

Part of my problem wearing eyeglasses with eyepieces is the bridge of my nose is 14mm deep, so I need a few millimeters more than that to accommodate my eyeglasses.  I suppose I could try using a monocle. 🧐

I use a TSFLAT2 in my short refractors to flatten the inherent field curvature of them.  Even then, the slight field curvature relative to the APM is obvious.  The Panoptic is a fine eyepiece, and there weren't a lot of other options at that focal length at the time I bought it.  I now have a wide range of eyepieces between 26mm and 30mm; and the Panoptic sees very little scope time now, mostly owing to its lack of usable eye relief and fairly tight true field of view relative to its competitors.

28 minutes ago, jetstream said:

Anyway, the spot diagrams show why I like orthos. The TV "plossls" are well executed but still limited by their design, like all others.

Although they don't look too good even at f/6 at the edge in my experience, I've found.  By f/12, they clean up nicely.  I can't imagine what they look like at f/4.  I'll stick with my modern widefields for my non-tracking scopes.

I've used my Panoptic 27mm in a variety of scopes for 21 years.  It is extremely sharp in the central 50%.  It remains very sharp in the remaining 50%, but slight astigmatism and field curvature starts to creep in.  It's very minor, but enough I switched over the the APM UFF 30mm over the last year or so.  The APM is also significantly more than 10% wider due to having much less edge of field magnification.  It is also usable with eyeglasses while the Panoptic is too tight on usable eye relief to use comfortably with eyeglasses.  I've also scratched eyeglasses on the Panoptic's exposed eye lens retaining ring.

5 hours ago, markse68 said:

I think this one must be an erfle type as it has 80+deg afov. Not sure if that’s possible with a symmetrical?

Look at the field lens end and see if it is near to the bottom.  Also look to see if the field stop is visible below it.  Most likely it has an internal field stop and is a negative-positive design like many more modern eyepieces.  Military eyepieces were often fairly cutting edge for their time because cost was not much of an issue.

2 hours ago, Stardaze said:

I do plan to add a 127 mak and for that, the 1.25" will be needed.

Get a Mak to SCT thread adapter, a 2" visual back, and a 2" diagonal and you won't have to duplicate expensive eyepieces on the 127mm Mak.