Thoughts on Solar Flat production

[Macavity]

Thoughts on (Full Disk) Solar Flat production...

I sense we are aware of the usual methods: De-focussing
OR "Cling Film"? Such things don't (known not to!) work
well for FULL disk images. At least it's my experience! 

De-focussing a Lunt 50 helical focuser requires getting a
fair GRIP(!) on things, moves the scope around a lot!

My IDEA: to suspend something in front of the scope
objective, in the fashion of the lightweight Bahtinovs.

Not too keen on clingfilm! I casually wonder if a weak
LENS might work? Remembering the "good old days"
of close-up supplementary lenses, I note such are 
available cheaply in common DSLR filter sizes...

https://www.amazon.co.uk/Polaroid-Optics-Piece-Close-Filter-x/dp/B003UHY7B8/ref=sr_1_8?s=electronics&ie=UTF8&qid=1489928622&sr=1-8&keywords=diopter+lenses

Maybe these would be too powerful? Who knows! 

Aside: Another idea I have is to CROP the FLAT disk 
to *exactly* the same size as the focused solar disk?
Maybe it's the difference in SIZE that confuses most
common stacking programs...

It's been SO LONG since I saw the SUN here? LOL

[Montana]

If you are having problems with a full disc and flats (because of the black areas) sometimes it is much easier to let the full disc drift in your capture. Obviously this can only be done if you have room on the chip to do so, but allowing the sun to drift will mean that the stacking program will then even out any irregularities during stacking and there is no need for a flat.

I hope this helps

Alexandra

[Macavity]

13 hours ago, Montana said:

I hope this helps...

Hi Alexandara. Yes it does! Always good to discuss such things.
Actually, I DO drift the disk to escape the dreaded Newton's Rings.
And, presumably, the dust on the CCD (ooops!) etc. etc.

I suppose my main problem is that, just occasionally, I fail to hit the
"drift button" - Especially during carefully constructed mosaics! 

TODAY (+ One Moth!) I'm expecting my new CAM for H-Alpha.
Nothing exotic, rather "tried and tested" -- CCD not CMOS etc. 
And here's *hoping* (secular praying) re. the absence of NR's!
Heck, it might even work with ONE of my Barlow lenses...

If it works, I may "publish"! Ah, the joys of Solar Astronomy? 

[michael.h.f.wilkinson]

I personally use cling-film for full disks (3-4 layers as a rule), and defocus on the centre of thedisk for my mosaic/close-up work in H-alpha. For Ca-K and WL I have found that the semi-transparent plastic of a bag in which my sons shoes came (from a Clarks shoe shop) works very well indeed. Haven't tried it on H-alpha yet, because I rarely do anything other than mosaics, and defocus works so well there. I haven't used

[Macavity]

NO SUN! So having Started (a random wibble) I'll Finish?
Thought I'd RETRY "Registax 5 Flats" in a most basic way...

Image with a single layer of "Tesco's Finest" (Cling Film):
(Note one or two "Dust Bunnies" on my  "pristine" CCD!) 



The "Light" frame (+ wavelet processing) with NO flat used (Note DUST)

The "Light" frame (+ wavelet processing) using the above flat (NO DUST!):

Flat with significant "Gamma" boost - Note Dust + residual Newton's Rings! 

 
Conclusion: Dust Bunnies seems mostly (only) to impinge
when I Barlow the setup? Even when including the
solar limb, the FLAT inclusion ain't ALL bad?
(And, with a bit of image clipping!)

P.S. I haven't completely escaped Newton's Rings with the
DMK41 Camera! But they are now considerably  reduced!

[Montana]

Easy, clean your chip with a soft brush

Alexandra

[Davey-T]

Just been playing around with your image Chris to even up illumination and there's something strange going on in the background area, did you do any processing other than stacking with flat.

Dave

 

[Macavity]

Hi Dave -- You (anyone!) are always welcome to fiddle!
(Bouncing practical ideas around has to be a good thing?)

ALL the images were presented as .jpg files at HALF size, post 
stacking. The "light" image was also processed via wavelets! 

Avoiding download of GB of .avi files(!) I suppose a possible
way of exchanging data transparently is via .png format?

ALL this talk has made me THINK though! The reality is
"flats" should actually be FLAT!?!? (Even Solar ones?!)

Since I seem to mostly/only need Flats when I Barlow the
image (3x) I should probably just *re-Centre* the sun on
the chip for the flat! There will be plenty of "over-lap"!

I suspect the "edge issues" may arise when the software
tries to multiply/divide/normalise (or whatever!) to a flat
which is in reality distinctly Non-Flat --- at the Edges!  
 
Of course this may not work for FULL disks, but may
perhaps help in this case where some edge is visible!
 

The stacked (unprocessed!) "Light" file (.png)


 

The stacked (unprocessed) "Flat" file (.png)

 

[Davey-T]

I often find full disc flats make the image worse, if there are only a couple of dust bunnies I just clone stamp them , pity you can't do that with NRs, you can mess about with gaussian blur filter which sometimes works for NRs, all a bit hit and miss.

Don't really understand the physics of the NRs, expect you do 

If I use a focal reducer I don't get them, without a f/r they appear, some sort of witchcraft 

Dave

[IanL]

4 minutes ago, Davey-T said:

I often find full disc flats make the image worse, if there are only a couple of dust bunnies I just clone stamp them , pity you can't do that with NRs, you can mess about with gaussian blur filter which sometimes works for NRs, all a bit hit and miss.

Don't really understand the physics of the NRs, expect you do 

If I use a focal reducer I don't get them, without a f/r they appear, some sort of witchcraft 

Dave

They are caused by reflections between a flat surface and a curved surface, typically the sensor cover glass or the sensor itself and the rearmost optical element in the OTA.  As you move across the sensor, the distance between the two elements constantly changes so that you get constructive interference (brightening) where the distance is a multiple of three x half the wavelength of the light, and destructive interference (darkening) where it is a multiple of two x half the wavelength.

Since the curved surface is usually a lens, you get a circular pattern of bright and dark rings.  It's often more apparent when you use a reducer or a barlow since the curved surface and the flat one are close together, but you can get it without between either the rear lens (after the etalon) or even the objective if using a front mounted etalon.  It's a problem for solar since the light is monochromatic and thus interference works, whereas for broadband imaging it isn't as there are lots of different wavelengths falling on the sensor so the effect averages out.

Full disk flats like the ones above aren't really flats and may or may not work well depending on how much gradient you end up with between the edge of the flat disk and the centre.  The 'correct' solution is to keep your sensor clean and use a tilt adaptor which will alter the spacing enough to push the rings mostly out of the field of view, hopefully without ending up with variable focus across the sensor.

[Macavity]

14 minutes ago, Davey-T said:

Don't really understand the physics of the NRs, expect you do 

Who am I to disavow you of that belief. lol

Meanwhile I resorted to printing and measuring the diameter
of the solar disk (School compasses and all that!). Looks like
I'm getting 2x magnification with 3x Barlow a short distance
from the chip --- And not unreasonably so (theoretically!)

The full disk will just about cover the whole chip... The Cling
Film will help! Though the (valuable) point above is not lost
re. inevitable fall-off in brightness across the solar disk!

Will probably invest in a "rocket-blower" or similar. I do
seem to spend a lot of time chasing dust across CCDs.