steppenwolf

You know what folks, we have essentially all gone round in a circle or two and yet we still (as far as I can see) haven't reached a satisfactory conclusion. In fact, we have only really agreed on one thing and that is that inserting a filter 'spreads' the light cone 'outwards' (my terms for simplicity) thus moving the focal plane further out and away from the light source and that to achieve focus, this additional light cone length needs to be accounted for. What we haven't ALL agreed on yet is what effect this has on the critical spacing of the sensor from the mounting face of any reducer/corrector in the field. Part of this problem is down to terminology (ain't that often the case?). Sadly, I don't have the final answer although my own current view flies against the advice from QSI - even though I use their camera - and that is simply because I believe that the critical distance requirement of correctors is based on an optical distance viewpoint (that assumes an air-filled void with a refractive index of 1 in the space between the corrector and the sensor) not a mechanical distance viewpoint and if we move the optical 'path' outwards then to me it follows that we need to move the optical 'distance' outwards to maintain the status quo! Not necessarily my last words ......

I tried to discuss this subject with Jane last night - we decided that we are 'incompatible' when it comes to topics like this

Aha, I misread but either way this is a very interesting academic argument and I would love to have a definitive answer but with big guns like Don V QSI not agreeing, that answer may not be forthcoming any time soon!

I agree, Olly, the difference is well within the acceptable tolerance but, one them is right and one of them is wrong and the difference between the two is TWICE the change in light path length! I see both arguments which is why I find this such an intriguing discussion!

Merlin66's argument is compelling (see what I mean about great arguments out there!!). This would seem to confirm the QSI and Optcorp stance but if the light path has now been modified, how relevant is the original 50mm PHYSICAL spacing in the example above? Fascinating stuff....... Sent from my iPhone from somewhere dark .....

It really is, isn't it, Olly!

Here's another link that supports my belief although I confess that the maths behind it leave me rather cold!

This is a confusing subject and I have seen convincing arguments going both ways but I agree with the diagram above! Although I don't use his filters, here's a quote from Don Goldman (Astrodon) who I believe knows a thing or two about filters but maybe hasn't got the price right yet! :- "Camera manufacturers show a smaller optical backfocus than mechanical backfocus because they are measuring the distance from the imaging CCD focal plane. This includes detector chamber window and sometimes coverslips on the detector. HOWEVER, that is not where WE measure backfocus from. We want to know how much space we need to add from the metal back of a telescope or from a field corrector/reducer to the imaging focal plane. This is how we select spacers, etc. So, our starting point is the scope. THEREFORE, the addition of a 3 mm filter ADDS 1 mm [t * (n-1)/n] of backfocus between the scope and your camera (t is the filter thickness and n is the refractive index of the substrate of the filter - typically 1.5) So it ends up being t / 3. 3/3 = 1mm in this discussion. You need to ADD 1mm of space between your scope and your camera. This is often confusing. When you place a filter in a beam of light that converges from left to right, the focus is extended FURTHER right INCREASING the backfocus distance as measured from the scope. All Astrodon filters are 3 mm thick, so the same 1 mm must be ADDED."

This is my understanding too.

The spider vane does indeed have a twist that requires correction to stop that double diffraction. However, an analysis of your image (see below) shows that the diffraction on the right is most likely caused by the bright star Alnilam (the middle of the three Belt Stars of Orion).

Welcome back, Rob, your eloquent and knowledgeable posts have been missed and I'm sorry to hear of your work problems. Your article on Planetary Nebulae is wonderful - thank you for submitting it!