Inserisco parte della discussione presente sul William Optics Telescope Discussion Group. Ce ne sono a decine, ma penso che questa renda bene l'idea, soprattutto confrontandola con i risultati ottenuti dai possessori di 66Sd + flattener presenti nel nostro Forum. Magari chiederei ai possessori di 66SD+flattener di postare una foto realizzata con questo setup in modo che ognuno possa giudicare il livello di perfezione dell'accoppiata. Resta assodato che senza spianatore questo tele è praticamente inusabile con una reflex digitale mentre con lo spianatore, pur con qualche limite, è possibile ottenere risultati già più che accettabili
Ecco una delle risposte sul problema Field Flattener III (sorry, only in english!
):
Re: [William Optics] Re: FFIII report
Hi Kurt
I am no expert in the field but I have done some paper designs to try to understand how a basic FF works.I have looked at how a simple positive lens can be used to flatten the field and also how a simple negative lens can be used to flatten the field. A positive lens will act as a reducer and a negative lens will increase the focal length. I know nothing about no gain flattener. The performance of a single element flattener is limited by the color error it produces. I have looked on paper at a few doublet FF. WO describes the FFII as a doublet. At least for my simple minded paper study a given design has an optimum scope it will work with. The main parameter is the focal length of the scope. Most doublet scopes of the same focal length have similar field curvature. The glasses used and the size of the air gap can make a difference but not by a huge amount. I thinks the different brands all trend to use similar designs so I would expect all ED doublets of the same focal length to have close to the same field curvature. Though a flattener may have an optimum focal length this does not mean that it will not improve the performance of a scope with a somewhat different focal length. One of the TV reduces is said to work over a 400 to 600mm range. If this reducer did a perfect job of eliminating all field curvature including astigmatism with a 500mm scope, one would expect at 600mm for the field curvature to still be reduced by a factor of 5 (600 is only 20% different than 500). This is a simple minded example but I hope you see the point.
The new FFIII does a good job of reducing the average field curvature of the WOZS80FD to nearly zero, but there is still astigmatism with the reducer. Even with the astigmatism the off-axis performance is clearly much better than without the FF. It looks like 555mm is just a little short of the optimal focal length. With the 80FD the corner stars have a slightly elongated shape (\) at best center focus. At the optimal focal length the shape will be more round with a hint of an X pattern. As focal length increase beyond optimum, the corner stars will start to elongate in the opposite direction (/). Even though the stars will start to elongate they will still be sharper than the elongated image seen without the flattener. If the reducer could also eliminate the astigmatism than the star shape would stay round and become a little blurred when the focal length is not at the optimum.
It is not clear to me what the astro photographic customer is truly looking for in the corners of their images. I think some are looking for the densest star image possible, while others may prefer a round blurry star over a sharper by elongated star. You imagers out there what do you think? For day time use I believe elongation is not an issue and sharpness is the key parameter. This is because during the day we are not taking pictures of point light sources so we do not notice the elongation, we just look for detail.
Now for your question:
"What I am getting at is, should WO specify an optimum and acceptable
focal length for their various flatteners?"
I think they sort of have. Daniel has stated that "on paper" that the FFIII is intended for the 80FD Megrez 90 and Megrez 110 and the 110 triplet, but we will have to see how well they work in real use. This at least gives us a general idea of the range.From users posting review we will know for what scope it works great for what scope the improve may only be modest.
From my report the group knows how the FFIII works with the ZS80FD. Based on the above discussion I would expect the performance with the 90 to be about the same or better than with the ZS80FD. I would expect the corner performance of both the Megrez 110 and the 110 triplet to improve with the FFIII, but to what degree testing will tell. The performance with the Megrez 110 is likely to be better than the 110 triplet due to its shorter focal length. WO has a good return policy so if you do not like the results, you are not out much, a little time and postage. I hope others will post their results to the group. Please provide details so that they can be properly reviewed and expand the corners and the center so we can see the stars in detail.
I hope this helps.
Scott Walker
E' tutto
ciao
v
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