Is the additional cost worth going too a triplet or for the average layman does the ED provide as good an image especially when attaching to a DSLR for astrophotography. Or on the other hand does a ED with 53 glass beat a triplet with 51 glass. I'm looking too but either an 80mm ED or triplet and I would like the best value for money scope. Budget $700-800
I think the general consensus is that the Synta ED doublets that use the FPL-53 reach the visual criteria for 'APOness’ when it comes to color correction.
However I think that it is pretty impossible to get full correction with a Doublet. The ‘new’ 80 / 127mm is a triplet that uses FPL-51 glass and it is not marketed as an ‘APO’ as such. I do not think it is a true APO either but again visually it may reach the ‘APOness’ criteria. There does however appear to be more QA variation with the ‘low end’ triplets entering the market which is to be expected at that price point.The above scopes do not reach the same CA or SA correction compared to an AP, TEC or Tak or even a TV APO triplet. I think that a very good 'low end’ ED or triplet is great for visual, and fair for photography. For photography a 'high end' - read expensive - APO is best and the aperture is not as important for most objects.
This is just my opinion and others will vary.
Below is an interesting quote from Roland Christen. [APO lens maker].
The question being:
“The FPL51 -FDC1 would be more difficult to achieve color correction compared to the FPL53”
Reply: It has to do more with focal length than color correction. You can get almost perfect color correction with FPL51 in a 130mm size if you make an F15 doublet. The same perfect correction can be achieved in an F12 doublet using FPL53 because the dispersion number if higher by some 20%. In order to get an FPL51 glass 130 F7.5 lens with the same perfect color correction, you simply place two F15 doublets back to back and you have an F7.5 lens. If the inner elements are the FPL51, you can fuse them together (glue them) and you have a 4 element lens that is totally indistinguishable from a triplet where the inner element is one piece instead of 2 pieces glued together. So, now we have designed a 3 element FPL51 lens of F7.5 focal ratio with essentially perfect color correction. Can it be beat by an FPL53 lens? Yes, using this more advanced glass will result in either a faster focal ratio of around F6, or a lens of F7.5 with less spherochromatism. Since sphero-chromatism is the least noticeable defect in an optical system to the beginner, then it seems smart for a manufacturer to make triplets using FPL51 instead of FPL53 (or Fluorite).
Rolando
Last edited by chris lewis; 29-05-2008 at 08:46 PM.
Hi – the S.V Nighthawk is not the ED version it is a older well corrected achro. It actually is a great scope - built like a tank and very well finished with a very smooth 2 in. R and P. focuser. There is some CA on bright objects mainly on high mags. It can go to 150x on steady nights and gives sharp contrasty images. The WO ED66 is the Petzal design and has the FPL-51 glass – it really is a ‘semi APO’ and is labeled as such. It is about half way between the Nighthawk and my now sold Orion ED80 / ED100 when it comes to CA correction. The achro S.V.Nighthawk and W.O. ED66 Petval are no longer made.
The Skywatcher Equinox [same as Orion EON] ED120 - which replaced the two Orion ED's - has a F.L of 7.5 also gives remarkably CA free images. This scope is capable of AP and produces flat images without the need for a focal reducer. I much prefer the planetary visual images through the ED120 then the Meade 8 in SCT. It gives 85-90 % performance as my friends TV127 APO which is over 3x' s the price.
