Getting back into imaging after a long break

[happy-kat]

The free trial is everything you just can't save but a screen grab can make do while in trial. You'll need to use DSS to generate your stacked fits file.

[Gina]

Thanks but I'll probably stick with what I have already bought rather than learn yet another image processing software.

[Gina]

Been looking at my image capture options :-

1. All Sky Camera - 1.55mm lens covering the whole 180 degrees with ASI 178MM astro camera
2. Vintage lenses ranging from 28mm up to 200mm with ASI 1600MM-Cool astro camera
3. SW Esprit 80ED - 400mm with ASI 1600MM-Cool astro camera
4. SW MN190 - 1000mm with ASI 1600MM-Cool astro camera
5. SW MN190 - 1000mm plus Barlow with ASI 120MC-S planetary camera

The ASC has it's own Raspberry Pi control computer - the rest can use the RPi box shown above except maybe the planetary imager.  The only difference being the focussing rate with different remote focussing on the different systems.

[Gina]

Using lenses with the ASI 1600MM-Cool camera I can avoid guiding but this is only suitable for large DSOs.  For smaller DSOs such as galaxies and M1 I shall need guiding and have to include this in the RPi and INDI driver setup.  The AstroPi3 script I've used to install the INDI drivers also installs PHD2 for guiding so this shouldn't be a problem.  Previously, my guided imaging has been with the Windows system and PHD with ASCOM etc.  Now I have an entirely Linux based setup that covers everything in the multiple software Windows setup.  Instead of needing a laptop and lots of cables, I now have one little Single Board Computer right on the imaging rig that does everything in conjunction with the KStars/Ekos Client software on my indoor computer.

[Gina]

I forget any calculations I might have done before so I shall re-calculate the field of view for each imaging rig and compare those with the sizes of various deep sky objects.  At these very small angles, the FOV in radians is given by the ratio of image sensor to focal length.  The sensor size of the ZWO ASI 1600MM-Cool is 4656×3520 pixels, pixel size is 3.8µm, giving a size in mm of 17.7 x 13.4.  A radian is 180/π = 57.3 degrees.  This gives the formula for FOV as W x 57.3 / F degrees.  Where W is width of sensor and F is focal length of scope or lens.

Thus the FOV is 17.7 x 57.3 / F degrees in width and 13.4 x 57.3 / F in height.  17.7 x 57.3 = 1014  and 13.4 x 57.3 = 768.

Here is a table of FOV for various focal lengths :-

1.  1000mm - FOV ≈ 1.01 x 0.77 degrees
2.   400mm - FOV ≈ 2.5 x 1.9 degrees
3.   200mm - FOV ≈ 5 x 3.8 degrees
4.   135mm - FOV ≈ 7.4 x 5.7 degrees
5.   105mm - FOV ≈ 10 x 7.3 degrees
6.    55mm - FOV ≈ 18 x 14 degrees
7.    28mm - FOV ≈ 36 x 27 degrees

Equating tangent with radians may be a bit out for the shorter focal length lenses but approximate is good enough.

Edited April 13, 2019 by Gina

[DaveS]

Being a lazy little tyke I use the 'scope /  sensor tool in Stellarium or in Astronomy Tools.

[Gina]

Thanks for reminding me of Astronomy Tools - just the job

[Gina]

Edited April 13, 2019 by Gina

[Gina]

It looks like the Andromeda Galaxy might not quite fit onto the image sensor of the ASI 1600MM-Cool camera with the Esprit 80ED telescope.  If this turns out to be the case I would probably use a mosaic of two frames to get it all in rather than go to a lens as the Esprit is well corrected for chromatic aberration and has a very flat field.  Camera lenses are not well colour corrected at the red end of the spectrum.  This doesn't matter for narrow band but galaxies are imaged with LRGB filters and the Luminance channel which is used for sharpness covers the whole visible spectrum.

[Gina]

I had all this FOV stuff worked out before in one of the myriad uber-long threads I've spouted on here but finding it would be like a needle in a haystack, so I guess I'll just do it again.  In any case the whole question of field of view with different focal lengths and which parts of the sky are covered will form part of my talk to our village social group.

I also want to check out if there's anything I can image in widefield and narrow band before autumn as I want to start imaging again ASAP without having to set up extra equipment like guiding.

Edited April 14, 2019 by Gina

[Gina]

Been looking at star charts - CdC - and checked what's in view overnight tonight as an example.  All the interesting stuff is out of view including Andromeda for most of the night.  Not that the sky will be clear tonight with 100% cloud forecast.  The sky looks better for the end of the month as more interesting objects swing into view.  So it seems there might be a chance of something in the next few weeks or so.  Against that, the nights are getting shorter, of course.

[DaveS]

For late summer into early autumn, how about the whole of the Cygnus Loop, really come alive in NB, or the North America / Pelican nebula, that really quite wide.

[Gina]

Yes, I've imaged the Cygnus Loop in the past and the NAN/Pelican.  Orion is full of good stuff too.  I'm certainly planning on more imaging in these areas and in more than just Ha.  You can just see the Cygnus Loop near the bottom of the first image below.  The second image shows the Orion and Monoceros regions with Barnard's Loop, Orion Nebula, Flame Nebula and the Ha region around the Horse Head in Orion and the Rosette Nebula in Monoceros to the left.  These are widefield images with a 28mm vintage camera lens.  I want to cover other nebulae in other parts of the sky too.  I have a whole range of lenses.

Edited April 14, 2019 by Gina

[Gina]

One thing I would like to try is an Ha image of the whole sky or most of it.  Even better would be a three filter NB image.  Whether any of this would be possible I'm not sure.  I guess some experimenting is called for.

[DaveS]

Would you go for a humongous mosaic or use NB filters with your ASC? The problem I foresee with a whole-sky mosaic would be spherical mapping at the edges.

[Gina]

Ha is very faint and OIII is worse and as for SII, forget it (probably).  A well cooled astro camera would seem to be called for so for me that would be the ASI 1600MM-Cool.  I can get down to -20°C by cooling the casing with a Peltier TEC as shown in my ASC and I have an ASI 178MM which is fine with a 1/2" format fish-eye lens but accommodating a filter is a problem.  The ASI 1600MM-Cool can take a filter just in front of the optical window but I don't think a 1/2" fish-eye lens would reach focus.  I'll have to check.  Of course, the image would only be a fairly small circle in the middle of the sensor.

Another possibility might be to capture a mosaic of the sky using the shortest FL standard camera lens.

[Gina]

Crossed posts...  Yes, the curvature would be a big problem with a mosaic.  Mosaics are alright with long focus but not with short - forgot that.  Drat! 

[Gina]

Looks like the ASI 1600MM-Cool will be alright for focus with a 1/2" format fish-eye lens.  The 178 has smaller pixels (2.4µm) so better resolution v 1600 @ 3.8µm.  OTOH the bigger pixels mean better sensitivity.  I can use ROI covering an area of about 1200x1200 pixels, to fit the image.  This compares with 2000x2000 pixels for the ASC thought that is colour so really only 1000x1000 picture elements.

Two screenshots from the specs for the ASI 178MM left and ASI 1600MM-Cool right.

   

Edited April 14, 2019 by Gina

[Gina]

Been looking at the ASI 1600MM-Cool and various adapters plus a 1.25" mounted filter.  The ZWO lenses supplied with the smaller astro cameras are CS mount and definitely won't work with a filter in between.  Nor will the Arecont type 1.55mm FL lenses which are also CS mount.  The Fujinon fish-eye lens is C mount with 5mm longer focus distance and even that won't work with a mounted filter using the supplied adapters. 

Next, I tried taking a filter out of its mount and putting the filter directly on top of the camera window but the lens has a protruding part for its objective element and I'm pretty sure that will touch the filter before focus is reached!  Drat!!  The ASI 178MM has the sensor nearer the front and I think an unmounted filter may work with that.  Distance from sensor to front of camera is 7.5mm for the 178 and 10.5mm for the 1600.  That 3mm might make all the difference.  I'll set up a test rig tomorrow.

Edited April 14, 2019 by Gina

[Gina]

In the fresh light of day (and a fresh brain), I can do some calculations :-

1. For a C mount the flange to focal plane distance is quoted as 17.526 mm. 
2. The objective lens element of the Fujinon fish-eye lens projects 8mm below the flange.
3. Thus focal plane is approximately 17.5 - 8 = 9.5mm.
4. Distance from sensor to front of camera is 7.5mm for the 178 and 10.5mm for the 1600.
5. So the 1600 camera shows an interference of 1mm and cannot be used with the Fujinon lens even without filter!
6. With the 178 the gap will be 9.5 - 7.5 = 2mm.
7. Astrodon filters are 3mm thick   Which is no-go!!!  Interference of 1mm!  BUT a filter moves the focal plane out by 1/3 of the thickness = 1mm

Well, I might just get away with using the ASI 178MM camera with a filter but there is a severe danger of the lens touching the filter.

[Gina]

The alternative might be an M4/3 camera lens which I've read is compatible with the ASI 1600 camera.  Found one on ebay which is almost whole sky (maybe across the diagonal).

UK 7artisans 7.5mm F2.8 Fisheye Lens Manual Fixed Lens For Olympus Panasonic M43

Edited April 15, 2019 by Gina

[Gina]

Googling the spec shows that the "micro four thirds" standard has a flange to focal plane distance of 19.25 mm, only about 1.75mm more than C mount so there still wouldn't be room for a filter with the ASI 1600MM-Cool camera.  There goes another option!!!

[Gina]

APS-C lenses won't cover anything like the whole sky with a 4/3 sensor as in the ASI 1600MM-Cool camera.  8mm FL ones are available for £100-£150 which would give much more coverage that the 28mm M42 lens I already have but probably "fall between two stools".  Seems a lot to pay for experimental imaging in my present financial state.

[Gina]

Set up the ASI 178MM firstly with a Baader 2mm thick filter and got focus fine so tried an Astrodon 3mm thick filter and I can focus with that too with space to spare using the filter unmounted.  Yippee   So what was the matter with my calculations???  If my calculations for the ASI 178 were so far out I wonder about the ASI 1600.

[Gina]

Won't reach focus with 1600 and Astrodon filter.  I'll try Baader.

A little later - nearly but "no cigar"!

This screenshot shows the size of the image circle in the full frame of the ASI 1600MM-Cool.

Edited April 15, 2019 by Gina