ICEINSPACE
 Moon Phase CURRENT MOON Waning Gibbous92.5% Moon Phase...
 The Sun Now

 IceInSpace And so the Apo... Semi Apo argument goes on
 Register FAQ Calendar Search Today's Posts Mark Forums Read

#21
15-03-2019, 08:10 PM
 bratislav (Bratislav) Registered User Join Date: Aug 2006 Location: Melbourne Posts: 230
Quote:
 Originally Posted by JA 1. if it has 3 zero crossings on the focus v wavelength curve (3rd order cubic function) then it's a triplet and apochromat and 2. if it has 2 zero crossings on the focus v wavelength curve (2nd order parabolic function) then it's a doublet and achromat 3. if it has 4 zero crossings on the focus v wavelength curve (4th order quartic function) then it's a superachromat So to answer your question: YES, NO and MAYBE
Well, again you are completely and utterly wrong.
A doublet can easily have 3 zero crossings, and there are couple of glass combinations that will allow a doublet to have FOUR zero crossings (see one of my posts above).
Yes, apochromat or superachromat can be done with just two lenses.
#22
15-03-2019, 08:16 PM
 JA ..... Join Date: Oct 2016 Location: Melbourne, Australia Posts: 1,279
Quote:
 Originally Posted by bratislav Well, again you are completely and utterly wrong A doublet can easily have 3 zero crossings, and there are couple of glass combinations that will allow a doublet to have FOUR zero crossings (see one of my posts above). Yes, apochromat or superachromat can be done with just two lenses.
Good I like that . Always a chance to learn, but just so we're clear when was the last time I was completely and utterly wrong (since you didn't mention it previously)? In any event read carefully what I said rather than partially quote it to attempt to make a point. One can hardly be "completely and utterly wrong" when one prefaces one's comments with

Quote:
 Originally Posted by JA Well it really depends on how exactly or inexactly one cares to define an apochromat: Is it defined by 1. the number of lenses in the design or 2. by how well it corrects for aberration (typically spherical and chromatic) ? Some would say that 1. if it has 3 zero crossings on the focus v wavelength curve (3rd order cubic function) then it's a triplet and apochromat and 2. if it has 2 zero crossings on the focus v wavelength curve (2nd order parabolic function) then it's a doublet and achromat 3. if it has 4 zero crossings on the focus v wavelength curve (4th order quartic function) then it's a superachromat Others simply say that the apochromat corrects better than the achromat and leave that as some sort of definition.
Just in case you missed it I was demonstrating two opposing points of view. I thought that would have been clear and that it didn't need further explanation.

So please now join in the conversation and answer Jeremy's original question, instead of relying on the crumbs from someone who tries to. EDIT: My apologies, I just noticed you did now answer his question- Well done. Now that wasn't too hard was it?

XXX

Best
JA

Last edited by JA; 15-03-2019 at 08:42 PM.
#23
15-03-2019, 08:22 PM
 bratislav (Bratislav) Registered User Join Date: Aug 2006 Location: Melbourne Posts: 230
This is a KzFSN2 and FPL 53 doublet (one of Skywatcher ED120/900 variants after a redesign).
3 color crossings. Two lenses.
And plenty of those measured by Wolfgang Rohr, confirming the performance in his lab.

BTW, we'd have very similar curve ("S" curve, denoting three crossings) if we paired KzFSN2 with FPL51, FPL52, FCD-1, FCD100 or FPL55. We can also use some Lantanum glass to have same effect. And don't get me started on Fluorite.
Plenty of doublets with 3 crossings.
Attached Thumbnails
 127.3 KB7 views

Last edited by bratislav; 15-03-2019 at 08:32 PM.
#24
15-03-2019, 08:23 PM
 bratislav (Bratislav) Registered User Join Date: Aug 2006 Location: Melbourne Posts: 230
#25
16-03-2019, 12:27 AM
 Ukastronomer (Jeremy) Registered User Join Date: Jan 2019 Location: Llandysul, WALES, UK Posts: 425
Even more confusing is, is a Semi Apo Filter what it says, or just a fringe killer ???
#26
16-03-2019, 10:29 AM
 brian nordstrom (As avatar) Registered User Join Date: Apr 2007 Location: Perth WA Posts: 4,285
The cheaper ones are just basically a light yellow filter , the more expensive ones eg. Baader ' Semi APO ' do quite a good job , I used one for years on my Saxon 150mm f8 Achro ( yard cannon ) and enjoyed the results before getting a Chromocor made by Aries in Russia , these really work but very expensive .

This set up with the Chromocor compared well to a friends 152mm Astro Physic's f8 Starfire APO ( \$7000 ) on all objects up until about 350x then the APO took the lead as it stayed sharper than my \$1200 Achromat at those high powers .

I did find that the ' Semi APO ' style filters worked well in the 6 inch refractor but dimmed the image quite a bit in the 3 and 4 inch achromat's I had at the time .

Brian.
Quote:
 Originally Posted by Ukastronomer Even more confusing is, is a Semi Apo Filter what it says, or just a fringe killer ???
Attached Thumbnails
 180.0 KB6 views

 Bookmarks

 Thread Tools Rate This Thread Rate This Thread: 5 : Excellent 4 : Good 3 : Average 2 : Bad 1 : Terrible

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Beginners Start Here     Beginners Talk     Beginners Equipment Discussions     Beginners Astrophotography General Astronomy     General Chat     Astronomy Books and Media     Star Parties, Club and Community Events     Celestial and Astronomical Events     Observational and Visual Astronomy         Observation Reports     Astronomy and Amateur Science     Radio Astronomy and Spectroscopy Equipment     Equipment Discussions     Eyepieces, Barlows and Filters     ATM and DIY Projects     DIY Observatories     Software and Computers     Mobile Astronomy Apps     Astrophotography and Imaging Equipment and Discussions Images     Solar System     Deep Space     Nightscapes     Terrestrial Photography IceTrade Classifieds     Telescopes & Mounts     Eyepieces     Accessories     Cameras & Imaging Equipment     Non-Astronomy     Trade/Wanted     IceTrade Archives IceInSpace     IceInSpace Announcements     Website Feedback and FAQ

All times are GMT +10. The time is now 07:26 PM.