Is this a gradient?

[Ivor]

Morning,

I was advised to add a spacer to the optical path to address elongated stars and I now get a whiter image than I'd normally get, is this is a gradient?



This a stack of 7 x 360secs Blue with the darks frames removed.

[The Thing]

Looks like vignetting to me. Have you used flats as well as darks in this stack?

[Ivor]

I haven't produced the flats yet, that's tonight's job.

[JohnP]

Like Duncan said I am sure flats will get rid of the gradient. Image looks very promising by the way - John

[Ivor]

Thanks, I've not had this problem before it's all a bit new to me.

So on the this basis has the extension tube caused the vignetting?
Is it too long?
How do I calculate the correct length?
Is elongated stars in the corners the lesser of two evils?

[JohnP]

most astronomical processing progs have scripts/ procedures for removing gradients even if the flat doesn't. I would rather deal with gradients than 'dodgy stars'... John

[MarkS]

You definitely need the correct CCD spacing to the reducer/flattener else you will get dodgy stars.  I'll be surprised if that causes a noticeable change in vignetting.

[RobertM]

It's difficult to tell.  Can you post a single stretched but otherwise unprocessed sub?  That should give a good indication of  the source i.e. Whether it just needs flats and/or LP gradient removal.

Robert

[Ivor]

You definitely need the correct CCD spacing to the reducer/flattener else you will get dodgy stars.  I'll be surprised if that causes a noticeable change in vignetting.

e

The spacer was 254mm which I thought was quite large but I'm not sure how to calculate the correct distance and I never got a response from anyone on the SBIG or WO forums.

I've produced some flats tonight I think they're OK, it's first time with the reconfigured light box.

Below is the flat, histogram, file stats do they look about right?

 



I've also attached a single stretched sub



Thoughts?

[MarkS]

Spacing of 254mm?  10 inches?

Remind us of exactly what configuration you have and we might be able to help.

Mark

[Carole]

Is that single stretched sub with or without flat applied?

Carole

[Ivor]

Spacing of 254mm?  10 inches?

A good spot a missing decimal point 25.4mm (I'll double check the length as well)

My kit is as follows:

SBIG ST8300M
CCD Kodak KAF-8300
Pixel Array 3326 x 2504
CCD Size 17.96 x 13.52 mm
Total Pixels 8.3 Million
Full Well Capacity ~25,500 e-
Dark Current 0.002e-/p/s @ -10C
Antiblooming 1000X
Shutter Even-Illumination,
Mechanical Exposure 0.09 to 3600 seconds
A/D Converter 16 bit
Gain 0.37e-/ADU
Read Noise ~9.3e-
Backfocus 0.69 inches (17.5 mm)

SBIG FW8-8300
Holds eight 36mm drop in filters for un-vignetted imaging at any f/ratio with the ST-8300 or ST-4000XM cameras.
Small footprint: 6.3 x 8.1 inches.
Thin design - adds only 20mm backfocus (Total backfocus for ST-8300 with CFW8-8300 is 38mm).

William Optics Focal Reducer & Flattener IV
Lens-to-chip distance : 66-86mm (currently set at 74mm ish)
Power : 0.75x ~ 0.8x
Rotating T-ring adapter
Clear Aperture : 50mm
L 84mm : W 77mm : H 123mm

William Optics FLT110
Aperture  110mm 
Focal Ratio  F/7
Focal Length  770mm
Resolving Power  1.05"
Limiting Magnitude  12.0
Tube Diameter  115mm

I've realised after posting this last night that the flat I've posted is without the extension tube so I'll reproduce them tonight and it will make a useful comparison. Do the stats associated with the flat look about right?

Is that single stretched sub with or without flat applied?

Without the flat.

[MarkS]

Now I understand.  I hadn't come across an adjustable flattener before, where you "dial-in" a different number depending on scope.  So your extension tube is effectively to make your camera CCD to flattener distance the same as if you were using a DSLR with that flattener.

A useful discussion here:
http://stargazerslounge.com/topic/135422-a-question-on-wo-field-flattener-iv-spacing/

Mark

[Ivor]

Right the results of testing tonight speak for themselves, the new flat and the reprocessed blue channel are below.





So hopefully I'll be able to save the data from this now.

William Optics designed the whole setup based around using a canon so I didn't experience any vignette problems (pre ST8300M) before I just thought the flats addressed the dust mites. 

A useful discussion here:
http://stargazerslounge.com/topic/135422-a-question-on-wo-field-flattener-iv-spacing/

Thanks Mark great link, I've been searching for the camera and telescope I should have searched for the flattener as well.

Chris the author pf the thread on SGL, has done quite a lot of research on his site which was very helpful, based on his calculation I need a spacer of 23.9mm, my spacer is 250mm edge to edge



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

To get the measurement completely correct on his kit he measured the curvature of the image

http://www.toinfinityandbeyond.me.uk/astronomy/Blog/Entries/2012/2/24_Distance_setting.html

does anyone recognise the tool he used to do this? I've sent him a message on SGL but but in case he doesn't reply I be interested in knowing how to work this out.

[MarkS]

The tool is CCDInspector.  It measures the FWHM of every star in the image and plots a chart.  The curvature is the max FWHM over min FWHM as a percentage or something similar.  It is not a very stable or meaningful quantity since if you defocus an image slightly then the reported curvature reduces.  However the chart is useful for comparing one frame to another after making an adjustment (at least when it is working well, which often it doesn't).

When measuring extension tubes you need to measure the increase in the light path which is not the overall length of the device since the threads screw into something - giving overlap.