FLT 98 and Flattener/Reducer IV spacing - thoroughly puggled

[Steve 1962]

Hi

I hope someone can help me with this because I seem to be going round and round.

I've got a WO FLT98, RFIV and a Canon 1000d, and I'm trying to work out the correct setting for the RFIV (it was three marks on the barrel - 66, 76 and 86).

I get very few nights out, so I want to make sure that I'm in the right ball park with this setting, so that I can spend as liitle time as possible refining it in the garden.

The WO website states that the RFIV spacing for the FLT 98 is 76.8mm - so on my first (and only(so far)) night out with the scope I set the scale on the RFIV to about 10% of the way between the 76 and 86 setting.....and didn't get a particularly flat field according to CCD Inspector. So I've been doing some research and here's where I've got;

Flange Spacing EOS Camera = 44mm
Thickness of T mount = 10mm

Therefore the spacing of the face of the T mount to the EOS chip is 54mm.

Spacing for RFIV and FLT98 = 76.8mm

The difference between the camera spacing and the RFIV spacing is therefore 76.8 - 54 = 22.8mm.

So if the spacing of the RFIV is measured from its face which bears on the T mount, I need to add 22.8mm of additional spacer between the RFIV and the camera - which I can't think is correct.

So maybe the published spacing is measured from the rear element of the RFIV? Maybe that's why WO have posted this on their website: http://www.williamoptics.com/accessories/images/FLAT4_Layout.jpg

This shows that they've used the end of the T tread as their base line for measurement and the face of the RFIV is 4.5mm "in" from there, with the centre of the rear lens element being 24.1mm and the edge of the rear lens element being 16.6mm from the same point when set at 76mm.

Since the T mount butts up against this face of the RFIV, it seems logical to use this face as the baseline for measurement so, at a setting of 76mm, the rear lens element is..

24.1 - 4.5= 19.6mm at the centre and
16.6 - 4.5 = 12.1mm at the edge

..from the face of the RFIV.

Ok, so do we use the centre measurement or the edge measurement?

If it's the centre - then to acheive the 76.8mm we need an extra 22.8 - 19.6 = 3.2mm, so need to set the RFIV to 76+3.2 = 79.2 on the RFIV scale.

If it's the edge measurement, then we need 22.8 - 12.1 = 10.7mm so the RFIV needs to be set to 76+10.7 = 86.7, but the RFIV won't go past 86.

So, (sorry this has taken a while!) to achieve a distance of 76.8mm am I correct in saying that the RFIV should be set to 79.2 (i.e. about a third of the way between the 76 and 86 setting)?? If this is right, what's the point of the numbers on the RFIV scale?

That's made my head hurt - can anyone help please?

Thanks

Steve

[DaveS]

Hi Steve

I do know, from what I have been told, that with the FLT98, the spacing for the AFR IV is very critical.

I use the AFR IV, with the FLT110, where it is a little less critical. However, with my scope, it needs to be 76mm of back spacing, that is to say 76mm from the optic in the AFR IV to the face of the CCD in the camera.

For me, with a filter wheel and OAG in-line, it was 'touch & go' as to whether or not I could actually achieve this. Its easy enough to add spacing, but not reduce it.

Fortunately, I did mange it, with nothing to spare, and it all works very nicely indeed.

As I understand it, the scale on the side of the AFR IV takes into account the usual back spacing that is achieved when the reducer is used with a DSLR, which adds around 55mm to the internal distance of the reducer. So, in theory, if you require 76mm of back-spacing, you set the scale to 76mm, and when the camere is attached, there should be 76mm between the optic in the reducer, and the face of the CCD. Well, that's the theory anyway.

Any distance between the markings on the scale, you will have to interpolate (guess).

I actually measured the distance betweed CCD and reducer.

Both Steve Richards and Per (both members here), have the FLT98 and the AFR IV, so will be able to tell you just how they set things up.

Steve, usually looks in every day, but Per in en-route to Olly Penrice's place in France.

Dave

[Steve 1962]

Thanks for your reply Dave.

Seems a real minefield - I'll keep on playing!

Lucky Per!!

Steve

[Dennis]

I don't own any of the stuff you have mentioned but any reference to measurements taken from an actual glass surface must be wrong. Obviously there will be a difference due to the curvature of the glass but I cannot believe that any manufacturer would expect you to place a measurement tool against the glass surface.

Tak talk about 'metal back' distance which is generally at the bottom of any male thread at the end of the scope/reducer. This would be equal to the (usually black anodised) surface that surrounds the rear element. At least you can measure from this surface without risking damage to the glass.

Dennnis

[Steve 1962]

Hi Dennis

I quite agree - that's why I'm puggled!

I think the only answer is to experiment - but it does make one wonder about the point of the scales on the RFIV, the distance quoted on the WO website and the drawing showing distances to glass. I think that's all just confusing things.

Steve

[Whirlwind]

Hi

There was a bit of a discussion about this on Stargazers Lounge a few weeks back (apologies for the link to another forum)

http://stargazerslounge.com/imaging-dis ... acing.html

Based on what you have noted - using a standard t - thread DSLR setup having the dial around 76.8 on the reducer should get you about there. However because of the FLT98 being a bit more critical in getting the spacing arranged, your own unique set up may mean you need to tweak the settings slightly (for example if you are using an internal light pollution filter that will change the settings you need slightly). I use one the AFR IV with a QSI540 so smaller chip, but with the spacers it gives me a spacing of 55.2 to the attachment plate and the field looks almost completely flat for the chip.

Thanks

Ian

[Steve 1962]

Many thanks Ian - that's a really useful link.

So to summarise; in a ideal world the setting on the RFIV would be 76.8 for the FLT98 and the scale on the RFIV is worked on the basis of a standard 55mm EOS flange spacing from the back flange of the RFIV - so because I'm using a CLS CCD filter, my spacing should be within 1 or 2 mm either way of 76.8 on the RFIV scale.

I'm printing out Buzz's scales as I type this.

Thanks again for your help - I'll post how I get on.

Steve

[Coco]

I cant believe that in this new century of telescope manufacturers companies like William Optics don't produce accurate line drawings in say .pdf of known scope/flattener configurations available on there website, surely they have this data?

[Steve 1962]

I agree entirely Coco - I spent over £2k on this kit and I've spent the only two nights that I've had free trying to "calibrate" it - just rediculous

[DaveS]

Does this help Steve?

Its a WO drawing, and one of my own for my particular setup.

For my FLT110 WO state a back spacing of 76mm, and as you can see, I am only just able to achieve this.

It was a very close run thing that I was going to be able to use the AFR IV.

Dave

FLAT4_Layout.jpg

AFR-IV_OAG_FW.jpg

[Steve Richards]

I don't use a DSLR camera with my FLT98 and FRIV but I do simply add spacers to my CCD camera to simulate the distance of a DSLR camera lens mounting face to sensor (nominally 44.5mm) plus the thickness of a standard 'T' mount (10.0mm) - total 54.5mm - and set my FRIV to nominally 76.8 and I get good star shapes.

[perfrej]

The idea of using the base parameters is the way to go (and I did):

1. Just accept that the markings on the flattener assumes 55mm from the T-thread to the sensor.

2. Check you equipment and calculate the difference.

3. Establish a constant to add to the indicated value

My setup is:

Scope -> FF IV -> SX Wheel -> SBIG ST8300M

SX Wheel is 29mm, SBIG has 17.5mm to the sensor. That makes 46.5, which differs to 55 by -8.5 mm. If I could just set the flattener to 76.8+8.5 = 85.3, all would be fine. If it doesn't stretch that far, just add spacers and redo the calculation.

[Steve 1962]

Hi Steve, Per and Dave

Thanks very much for your comments - I think I've got my head around it now!

I'm now just waiting for a clear night when I'm not working to have a "play". I'll post my results when I get something worth sharing.

Regards

Steve