Is this a gradient?

[mickw]

but this doesn't seem right surely I should be measuring the narrowest gap between the two threads?

You should be measuring between the mating surfaces, so do not include the male thread beyond the knurled ring.
That would make the measurement about 4mm lower - 246mm (or 21mm if you fancy using a decimal point   )

I believe flatteners do have a +/- 5mm leeway in their use - I have been known to be wrong.

Don't forget you need to include any other adapters/rings between the CCD and the flattener when calculating (T ring adapter, filter wheel etc.)

[MarkS]

I believe flatteners do have a +/- 5mm leeway in their use - I have been known to be wrong.

More like 0.5mm!  I guess that was a typo

[MarkS]

One other shortcoming with CCDInspector is that it can report a totally flat field even when the stars towards the corners are totally squiffy in shape.  That's the problem with relying on a single statistic - in this case FWHM.

[mickw]

I believe flatteners do have a +/- 5mm leeway in their use - I have been known to be wrong.

More like 0.5mm!  I guess that was a typo

Oops - stupid decimal points  

[RobertM]

The stars look ok give or take a knats so I don't think it's spacing.  I'd be more concerned that the centre of the flat is offset from the centre of the camera.  This usually means that something is not square to the optical axis.  You can also see that there is some strong vignetting appearing on the left  top/bottom corners of that flat.  There could be a number of reasons for this such as scope collimation, focuser sag or something impinging in the light path.

Do the flats look ok without the reducer ?

Robert

[Ivor]

This is feeling like a eureka moment, I've been really struggling with my kit since getting the ST8300 at least I'm understanding why now.

The CCD to FF gap for the FLT 110 is 73.5mm and the FF adjustment gives me a range between 66 - 86mm.

The back focus of the various bits of kit is as follows:

Camera + Filter wheel 38mm
Extension tube 20mm
Gap between FF lens and housing 10.6mm

This equals 68.6mm, which means I need to add 4.8mm to the FF gauge (78.4mm) to make the true distance 73.5mm.

The adjustable FF is great idea but the lack of documentation makes it very confusing.

I'm not hearing a lot of love for CCDINSPECTOR is there an alternative?

The stars look ok give or take a knats so I don't think it's spacing.  I'd be more concerned that the centre of the flat is offset from the centre of the camera.  This usually means that something is not square to the optical axis.  You can also see that there is some strong vignetting appearing on the left  top/bottom corners of that flat.  There could be a number of reasons for this such as scope collimation, focuser sag or something impinging in the light path.

Do the flats look ok without the reducer ?

I have refractor so there's no collimation. I'd put the issues in the corners down to the subs being taken on two nights but now you comment on it on the flat, I'll check everything is square tonight as part of the test run.

I can't use the the ST8300 without the FF as there isn't enough back focus on the focuser, and I don't know how to calculate the extension tube required.

[MarkS]

I read the SGL thread completely differently.

I understood it like this:
1) 73.5mm is the number you "dial in" as the FF adjustment - it differs from scope to scope.
2) The FF to CCD distance should then be the standard DSLR distance of 55mm - this is the same for every scope. So your 68.6 is already too large.

Mark

[RobertM]

I have refractor so there's no collimation.

That's incorrect although WO might be one of the manufacturers who think they don't need to be collimatable so leave out the adjusters to reduce cost.  Most decent manufacturers include collimatable lens cells.

I still think something is off somewhere as the flat centre is not in the middle however the extra spacing that Mark has identified would give you much more reduction.  That will also have the effect of amplifying any vignetting and would explain the two darker left corners.  Once you have the correct reduced distance then those corner issues will go away and you might not notice the slight off centreing.

An alternative way of getting the FR distance correct - If you know exactly what the new focal length is i.e. 0.8 * 770 = 616mm then you can use plate solving to work out the focal length from a captured image then adjust the focal reducer distance to get a solved image f/l of 616mm (or the equivalent plate scale of 1.81 arcsec/pixel).

HTH
Robert

[MarkS]

This diagram confirms how I understood things:
http://www.williamoptics.com/accessories/images/FLAT4_Layout.jpg

If your CCD is 55mm from the rear shoulder of the FF then you will obtain the necessary 73.5mm CCD spacing from the rear lens element when you have dialled in the 73.5mm distance.

Alternatively you could "dial in" 66mm which gives you an extra 7.5mm (73.5-66) which you could add onto the 55mm giving 62.5mm.  This would preserve the correct 73.5mm distance from CCD to rear lens element.  But your 68.6mm is still too great even so.

So reduce your 20mm extension and then you'll have sufficent adjustment to make it all work.  I'll let you do the maths!

Mark

[Ivor]

Mark, I think we are saying the same thing but I've not articulated it well enough. Your post also made me realise I'd made a mistake in my calculation by excluding a double negative.
The 68.6mm I stated includes the gap between the lens and the front shoulder of the FF and my camera, the camera plus extension tube in my case equals 58mm.

I made this more complicated than it needed to be by working out the inner distances in the FF rather than basing my calculations off the FF rear shoulder (which WO strangely don't do in their diagram)

I've also realised the 10.6mm lens to FF gap stated in the SGL post is wrong it should be 11mm. If I correct the double negative I included in my first calculation I need to adjust the FF to 70.5mm.I anticipate some minor adjustment from this to cater for the filter. I added the measurements to the WO diagram.



It will be interesting to see if this adjustment removes the problems on the left side on the flat as well.

Cheers

Ivor

[MarkS]

Looks like you've got it right now

[Carole]

I find this a real PITA especially with 3 cameras, and 4 telescopes, a flattener or reducer and sometimes a filter wheel so every time I change my set up I have to re-calculate.  I have made a chart.  Normally I put all the pieces together and then measure as this takes into account the thread that gets "screwed in". 

Even when I have the exact measurement recommended I still get elongated stars in the corners sometimes.  I guess swapping cameras and scopes around doesn't help much and one day I'll get it all right hopefully. 

Also I find my images are very slightly offset and this probably

means that something is not square to the optical axis

which might explain the elongated stars in one corner only sometimes, but I can't think how I can get anything more orthogonal, I can't put the camera in crooked......I feel it's to do with only having 2 screws in the draw tube to tighten the camera on, 3 I am sure would rectify this, so not sure what I can do about it.   Maybe if those 2 screws were at 120 degrees to the draw tube LOCK then this could compensate, but I don't think they are.

Am reading this thread with interest.

Carole

[Rocket Pooch]

What you doing Pm?

[RobertM]

What you doing Pm?

She's obviously on thread jacking duty again

[MarkS]

I'm not hearing a lot of love for CCDINSPECTOR is there an alternative?

Not really, so I ended up writing my own.  A prototype exists. The principle is to measure the elongations of stars by assuming they have an elliptical gaussian shape (a big assumption because some optical aberrations give star shapes much more "squiffy" than an ellipse), determining the direction of the major and minor axes of that star's ellipse and then calculating the FHWM in the major and minor axis directions (not the x and y directions as other software often does) and hence determining the elongation.  It is difficult to do this reliably because of various random effects so it only becomes statistically significant when averaged over many stars in an area.
So I do this in the extreme corners of the image, the middles of the edges and the image centre.  It gave a plot like this:



On each detected non-saturated star a "windrose" is plotted to show the direction of elongation and then a larger windrose is plotted to show the average over all stars in that neighbourhood.  Statistics are generated for each rectangle showing FHWM of the minor axis (in pixels) elongation (in pixels) direction of elongation and the number of stars used in the calculation (the more stars, the more reliable the measurement).

Conclusion.  I found this gave pretty reliable results on the whole for the particular aberrations I was seeing but to be honest, no better than simply "eyeballing" the image!  I suppose it helped by confirming with statistics what was already obvious from looking at  the image.

A lot more development work would be required to turn this into a genuinely useful and simple to use tool - I just don't have time to progress it further.

One thing that would make it more useful would be to "subtract" the elongation found in the centre of the image from the elongations found elsewhere (this would be done in a vector arithmetic manner), since the centre elongations are generally caused by guiding issues rather than optical issues and it would be useful to separate out these two effects.

Mark