A question on WO Field Flattener IV spacing

[buzz]

I have a question about setting optimium distances on the adjustable WO FF IV. There is of course a suck it and see approach but there should be a scientific approach too!

There is a published optimum setting for my WO FLT110 scope for the FF IV but my Starlight Xpress spacing is shorter and T-extension tubes are either too short or long.

The question is - is it better to set the correct setting on the field flattener and get as close as possible on the extension tubes - or - set the field flattener setting to the actual sensor spacing?

regards

Chris

[ollypenrice]

I don't understand the question - sorry.

The published distance is between FF and chip. Add about 1mm for the optical effect of a filter. Ideally keep your filterwheel next to the camera to avoid vignetting, which will happen if you push the filter too far up the wider part of the light cone.

Olly

[steppenwolf]

There is a published optimum setting for my WO FLT110 scope for the FF IV but my Starlight Xpress spacing is shorter and T-extension tubes are either too short or long.

As I am sure you already know, that setting assumes a DSLR camera and, therefore, assumes 55mm (nominally 45mm for the camera and 10mm for the 'T' adapter). It is quite acceptable to add up your CCD spacing (probably 17.5mm for an SX CCD) plus the nearest extension tube to get close to 55mm (a 20 + 15 to give 35mm extension or 2 x 20 to give 40mm extension) which gets you to within 2.5mm too short or 2.5mm too long and then dial in the 2.5mm difference on the FR IV.

[buzz]

Hi Steppenwolf, yes I realised it had the nominal 56mm flange to sensor distance of an EOS in mind (+ the 10mm rear FFIV flange to rear optic distance) for the 66mm setting on the FFIV.

I think you have answered my question; get as near as possible to the preferred distance and then set the FFIV dial to the actual distance rather than the published one. (allowing for the 10mm)

Did you arrive at that by inspiration, trial and error or WO direction?

Olly - the FFIV has an adjustment setting from 66-86mm, which moves the internal optics - clearly WO had an optical equation in mind that the relative spacing of triplet/doublet - FFIV - sensor optimised the field flattening. They publish settings figures for each of their scopes but it is hard to space the sensor at their exacting figures, hence the question. It will of course be possible to focus in the middle with most combinations, but the edge quality is the issue.

kind regards

Chris

[steppenwolf]

Hi Chris,

WO recommend a setting of 73.5mm for your FLT 110 and this assumes a total spacing of 55mm from the sensor to the CCD mounting face of the FR IV. With the examples I suggested above, if I was 2.5mm too short, I'd reset the dial to 76mm and if I was 2.5mm too long, I'd set the dial to 71.0mm.

Did you arrive at that by inspiration, trial and error or WO direction?

I am working on the basis that the FR IV is simply a 'standard' focal reducer/flattener with an internal moving lens cell. This has the effect of giving you an adjustable extension tube so that you can make up for both variations in the optical requirements of different telescopes and compensate for spacing inaccuracies when using a CCD camera rather than an industry standard DSLR camera and 'T' mount.

[buzz]

Hi Steppenwolf - the penny has dropped - of course, you are right, the entire optical assembly moves so I can dial in the extension correction needed from the 73.5 mm starting point. I have to look at the issue from the opposite direction. Logic wins the day.

Shame WO instructions are non existent.

Many thanks

[steppenwolf]

Perhaps I should write them a manual!

[buzz]

I'm done with writing - my last book was 560 pages long with over 1000 illustrations. It was published by Focal Press and Adobe changed InDesign 3 times in the time I took to do the 2nd edition

Way Beyond Monochrome (www.waybeyondmonochrome.com)

I am tempted to do take some narrow band monochrome images and then print them on a silver halide fibre print!

Chris

[steppenwolf]

Lot of wonderful images are taken in monochrome so you are talking to the converted here.

[Whirlwind]

Hi All

I might be going mad, but I've always interpreted the diagram as showing that your spacing should always be around the 55-56mm mark from the thread of the reducer (give or take filters and a bit of variation) and that the recommended distance is for each scope type at this distance?

For example:-

If you have an FLT110 the recommended distance is 73.5mm. So at that setting the internal distances from the lens to beginning of t-thread is (24.1 - (76-73.5)) - 4.5 or 17.1mm. Hence with 56mm + 17.1 giving 73.1mm (i.e the same as your setting).

If then say you changed the setting to 86 on the dial then the lens to t-thread is 34.8-4.5 or 30.3mm. Hence with 56mm you get 56 + 30.3 giving 86.3mm (i.e. the same as your setting).

However if say your camera assembly was 53.5mm then regardless of what setting you put the lens setting at you would still be 3.5mm out. E.g if you put your setting for the flt 110 at 76mm instead then you would have 24.1 - 4.5 + 53.5 = 73.1 (i.e you are still about 3.5mm out).

Hence I assume that the reducer requires a camera back focus set up of 55-56mm and that the settings are used to match to telescope focal length.

Does that make sense

Ian

[buzz]

Hope this helps - I have written this up on one of my web diary pages as a resource to others on how to calculate the setting.

WO Flat 4 Spacing

and a precision scale too on the same website - carefully done in Adobe Illustrator and made into a PDF file.

http://www.toinfinityandbeyond.me.uk/astronomy/Diary/Entries/2011/11/5_Entry_1_files/WO%20F%20IV%20scale.pdf

[Whirlwind]

Ok, I see what you are saying and that regardless of the setting on the reducer the back focus should be 73.5mm (for a FLT110); whereas I assume the 73.5mm is the recommended distance that should be set on the reducer and you match the your total back focus distance to that setting (which always means then your camera set up has to have a back focus of around 55-56mm). This is how i use it with my FLT98 (but at 76.8mm)

My only concern with your method is that you have a lens to chip distance of 73.5mm but your dialled in setting is 77mm. On the WO website it specifically refers to the flattener as having a "Lens-to-chip distance : 66-86mm". Having a setting on the flattener (that is referred to on the WO website as Lens to chip) that then doesn't actually relate to the actual lens to chip distance in a correct configuration doesn't really seem to make sense to me

Thanks

Ian

[buzz]

Ian , absolutely - that is why I have made a new scale, linked in my prior post. Cut this out and attach with glue or sticky tape to the FFIV - this then reads out in glass to flange distance, which you add to your CCD/filter path distance to sensor.

regards

Chris

[buzz]

To follow up on my investigation - I tried a number of different FFIV settings, +/- 2mm from the theoretical setting for the FLT110-

I'm getting curvature figures from 30-12% but some tilt too, which may be flex. Interestingly, the best tilte and curvature figure is at the low end of the test range.....that got me thinking....

The published 73.5mm setting for the FLT is the OPTICAL distance. In my optical path, I have a filter and a sensor cover glass -

The Baader filter is 2mm thick and has a refractive index of about 1.5 - the sensor window is possibly the same. My optical path is too long.

Back to the drawing board, repeating my test over a new lower range.

Chris