Burgess 24mm Modified Erfle & 10mm Ultra Mono

[WilliamPaolini]

Have been assessing these two relatively new eyepieces for the past few months. Attached is article I put together with my impressions.

[Wavytone]

Hmm.

But you paired the mono with an f/7 scope plus a Barlow, which defeats the point of using the mono. May as well have been looking through an ordinary 5mm such as a Plossl.

Would have been better with a longer focal ratio scope such as an SCT or mak - with no Barlow - and compare with other 10mm eyepieces with small numbers of elements and air glass surfaces elements such as a plossl, or ortho.

[WilliamPaolini]

Not really as it is often more about execution, especially when the group count is low. So as example, for accepted high performing planetary eyepieces like a ZAO, it is 4 elements in 2 groups. The Pentax XO is 5 elements in 3 groups. The Monocentric in question is 4 elements in 2 groups. No Plossl out there that I've had my hands on, and I've had many, has ever risen to even close to the likes of a ZAO, CZJ Ortho, Pentax Ortho, Monocentric, AP-SPL, XO. Execution and backscatter in the base design make big differences.


One of the contrast robbers in any design is the backscatter between the elements. The Abbe design as example is quite low in backscatter...much less than a Symmetrical design (remember that production "Plossls" are really Symmetricals which have much inferior spots to what the true Plossl is able to achieve). The 5XO design is actually very Plossl-like in the upper 4 elements, but Pentax went to some extremes taming the backscatter not just between the groups, but also between the upper cemented groups as well. The Monocentric design is about the lowest until you get to singlets. The very distant spacing of the Barlow's doublet from the Monocentric assembly really negates most of any backscatter between the Monocentric and the Barlow due to the extended distance (and assuming good baffling and a quality Barlow). If you've ever take some of the premium lines of planetaries like ZAOs, AP-SPLs, Pentax Orthos, etc, and Barlowed one of the longer focal lengths so it was equivalent to a shorter focal length of the same line, you would be hard pressed to see any impact at all, again assuming a high quality Barlow build and optics. I have done this on many occasions with these best-in-class planetaries. Plus when you look at specialty designs like the Vixen HR series, essentially the same concept with a 3 lens group up top followed by an internal doublet Barlow. So as long as you can keep that upper group minimized then adding and properly baffling and coating to lower doublet Barlow is always a winner. https://image.jimcdn.com/app/cms/image/transf/dimension=960x10000:format=png/path/s044a9bf44fdefbd6/image/id76b0830c4ef42b2/version/1523592297/image.png


But all the theory aside field results always are a trump card. So where it counts, in the field, the combo of the Mono wit the Barlow was a planetary winner. Was easy to see at the eyepiece on Jupiter, Mars, and Saturn that the Barlowed Monocentric easily kept up with and in some areas beat the venerable XO. Proof for me was in the seeing.

[N1]

Bill, interesting thread. Would be nice to see a direct comparison of the 10mm Mono with a TMB Supermono or ZAO of the exact same FL.

[Wavytone]

I appreciate that well Bill, hence why I am toying with the idea of making a ball eyepiece (one element, 1 group) sure the field is small, and an off axis Newtonian (just one reflecting surface). Already have the mirror :)

[WilliamPaolini]

Sorry guys but I updated the article in post #1 with a final test that turned out interesting enough that I felt it worth the update. Basically the following is added:


Finally, the rectilinear distortion in the Modified Erfle is extremely low. When placed in a daytime spotter scope straight lines in the off-axis, even right up to the field stop, stayed straight. I then wondered how the true field of view (TFOV) might differ in this eyepiece compared to something like an Explore Scientific 24mm 68° or a Tele Vue 24mm Panoptic, both of which have a significant amount of rectilinear distortion. Having an Explore Scientific 24mm 68° on hand I compared it to the Burgess 24mm Modified Erfle and discovered that the TFOV of both the 61° Modified Erfle and the 68° Explore Scientific were almost exactly identical! So although the Modified Erfle visually shows a smaller AFOV, its TFOV is almost exactly the same as the wider Explore Scientific 68° with its added rectilinear distortion to maintain more controlled off-axis star points.


Also added comment about how the elements and groups of the Ultra Mono with Barlow is still on par with other planetary eyepieces like the XO. Your comments here on that I felt were important points to add. Thx!

[WilliamPaolini]

I've made two of those for me. Yes the on-axis that is sharp is about 10 degrees...but the contrast in those 10 degrees is unbelievable!!

[Wavytone]

Bill how does the modified Erfle compare with traditional Erfles and modern variants such as SWA, all of which have strongly curved focal planes ?

[WilliamPaolini]

Alas I no longer have any of those in the eyepiece stall. I do have the 2" Explore 25mm 70 degree eyepiece which is 5 elements and probably an Erfle, but it has been specially adapted to be used on a spotting scope so no longer can be used in standard focusers.

[WilliamPaolini]

I did just get an opportunity to compare the Burgess Modified Erfle against an ES/Bresser 70 degree Series 25mm. Centering the eyepiece on a prominent star in M45, the off-axis stars were moderately tighter in the Burgess at the same off-axis TFOV points. Maybe appearing 20%-30% tighter -- not much but noticeable. Contrast, brightness, and other common attributes seemed on-par.

[bigjoe]

Bill..Hi..
Purchased this Burgess 10mm Monocentric after your review and it is by a margin BETTER at teasing out faint secondaries than ANY other eyepiece I have, or can remember; better too than my wonderful Tak Abbes and lovely SLVs...QUITE remarkable in this regard, and very low scatter also... only downside is the very narrow FOV and 2 opposing indentations in the field stop though!
PS:There now $212 aus posted here from Burgess Optics.
bigjoe

[bigjoe]

Added some pics POST down.. to give an idea of MONO EP Size etc.
Its sensational at CONTROLLING SCATTER and sharp in all the Apos and Maks I tested it in; same good seeing , same Tak diagonal , same objects, same location, same time..No Moon. ..Narrow field will not suit wide field junkies though !
Perfect double star splitter..even barlowed!!

PS:Easily shows polar cap and Syrtis on Mars at under 10" in the 150mm SW MAK and split Alnitak and Eta Orionis MORE so than the Wonderfully sharp Taks and Sharp 10mm SLV...Inserting the pitiful 3 element SW Saxon super 10mm, made the difference even more so.
bigjoe.

[WilliamPaolini]

Yes. I commented on those indents I believe in my write up. Appears to be damage during assembly as if the spanner to install the field stop accidentally missed the surface groves and slipped into the field stop opening. Unfortunate but a cosmetic thing only tends to go unnoticed after using the eyepiece for a while.

[N1]

I was able do a very quick comparison of the 10mm Burgess vs. 10mm TMB Supermono the other night on Moon, Planets and general Milky Way through 2" f/10, 6" f/10 and 8" f/6. I did not have enough time to really evaluate any difference in on-axis performance (none was immediately obvious), but what did jump out at me was the stronger field curvature of the Burgess. At f/10, the TMB was usably sharp almost to the edge without refocusing, and the Burgess was not. The other thing I noticed was ergonomics. The eyecup on the Burgess won't stay folded down so it had to come off for the EP to become as comfortable for me to use as the TMB.

To be continued.

[Ant0nio]

Burgess now have the 6mm Ultra Mono advertised, any updates on these? I'm tempted to go for this instead of the 6mm Tak Abbe Ortho I had my heart set on.
Cheers,
Tony