Imaging doubles

[Ags]

I have a plan to image double stars. It's hard to find good images of these online, and I think my C6 and AZ-GTi could do a good job of it in combination with lucky imaging / lucky tracking techniques. I have yet to make a DSO astrophoto I am entirely pleased with so any tips would be welcome.

Of course with doubles star color is absolutely key and I only have a mono ASI178MM and a very cheap set of dichroic RGB filters. My prior experience with these filters is the blue channel is always incredible weak. When imaging planets I set gain for R to 210, raise it to 260 for G and raise it again to 310 for B to get the same brightness - each gain of 50 is linear doubling of the gain. For DSOs the blue channel is frequently unstackable (bear in mind I am using exposures of no more than 10 seconds). So I am thinking of shooting R and G only and making a linear approximation of B (i.e. if R is stronger than G for a star, then B is weaker than G, and vice versa). 

It might be sacrilege but I am thinking of using a mask to introduce diffraction spikes. Would this offer any advantages in terms of showing star colors?

I did a 2 minute test shot of Albireo a few nights ago which serves as a baseline of awfulness on which I hope to improve. In the picture, collimation is off, focus is not 100% (must make a Y mask), there's thin cloud, and som dew on the corrector plate. It's just a couple of minutes of 4 second subs of Albireo (f6.3), and it gives me hope because the setup splits the double easily, surrounding stars are picked up and there's lots of room for improvement.

Edited June 29, 2021 by Ags

[wimvb]

That's a nice start.

Since you are targeting stars, you probably don't need to worry too much about exposure time. Keep the time short enough so you don't blow out the core of the stars. I would keep the camera gain constant, but maybe increase the exposure time for B (or just take double as many exposures). This will make calibration easier. As far as I know, the ASI178MM has amp glow, and in order to calibrate this out, you need to match the dark frames to the light frames in exposure time, gain, and (if possible) temperature. Having fewer settings to worry about makes this process easier.

Good luck.

[Knighty2112]

Yeah,  been meaning to start to get some images of doubles too at some point this year with my ‘fracs. Must make a note to start at some time in July. Nice image of Abireo. Well done! 👍🏻

[vlaiv]

1 hour ago, Ags said:

Of course with doubles star color is absolutely key and I only have a mono ASI178MM and a very cheap set of dichroic RGB filters. My prior experience with these filters is the blue channel is always incredible weak. When imaging planets I set gain for R to 210, raise it to 260 for G and raise it again to 310 for B to get the same brightness - each gain of 50 is linear doubling of the gain. For DSOs the blue channel is frequently unstackable (bear in mind I am using exposures of no more than 10 seconds). So I am thinking of shooting R and G only and making a linear approximation of B (i.e. if R is stronger than G for a star, then B is weaker than G, and vice versa). 

You should be able to produce accurate star color from only two filters with some math.

Stars emit light that is similar to black body at same effective temperature. You only need good calibration for your filters and you should be able to produce accurate star color with only two of them. This is similar to B-V color index in astronomy that specifies stellar "color index" (do not confuse that with actual color that we see).

Try wavelet sharpening on linear image - that should help you split stars that are at the edge of visual split.

[Merlin66]

If you are going to photograph doubles why not use some "lucky imaging" to actually measure the double pair??

Florent Losse's REDUC program is available to help analyse your double star images. Drop him an email for a copy: florent_losse@yahoo.fr

http://astrosurf.com/hfosaf/

http://astrosurf.com/hfosaf/reduc/tutorial.htm

 

[melsmore]

What @Merlin66 said and look at the  Double Star Imaging group

Edited June 30, 2021 by melsmore
Changed link

[Ags]

My aims are mostly aesthetic, but if I can make meaningful measurements I will have a go!

[Ags]

I made a simple focus mask with a craft knife and thick cardboard. I think it worked pretty well for a purely guestimated mask 😀

Then I worked on collimation and tried to photograph Albireo, but for some reason the AZ-GTi just wasn't tracking well.

I had a go at Polaris instead as that SHOULD have been easy to track but the mount was playing up  (panning in random directions spontaneously).

I'll have to track down the source of the halo around bright stars.

[chiltonstar]

A good start - doubles are fun to image because you don't need perfect viewing conditions, which some of us in NW Europe don't seem to get very often anyway.

I always struggle when trying to reproduce the subtle colour differences that the eye sees - these seem to work better I find with a small frac as the Airy discs are larger and fainter, and don't burn out so easily as with more aperture. Emphasising the colour contrast with PhotoShop helps!

Chris

[Ags]

I am thinking about getting a color camera to back up my ASI 178 MM. The Svbony SV305 (non-Pro) looks like it might do. I don't need super-high USB 3 speeds for double stars, and shooting all the colors in one go is tempting. But also thinking of getting an EQ mount to replace my recalcitrant AZ-GTi.

[happy-kat]

Perhaps this might gives ideas on halos, the thread gets more interesting nearer the end of it.

https://stargazerslounge.com/topic/184511-flocking-a-6se/

 

[kirkster501]

Great minds think alike .  I am imaging some stars as well, not just doubles.   I've done a few of them.  30-60s exposures and 8 - 10 or so in RGB.  Interesting thought from Vlaiv about just using two filters and synthesising the third.  However, with such short exposures and number of subs it's as easy to just take all of the RGB.

[Ags]

I quite fancy giving this €60 camera a go...

8mm diagonal IMX477 RGB sensor, Python scriptable, runs off a Raspberry Pi, has ASCOM driver and easily modded for cooling. It seems to produce credible results:

https://terramex.neocities.org/astro/

Edited July 3, 2021 by Ags

[happy-kat]

Would you bin the images in post processing to match pixel size

The results are interesting I like the globular.

 

[Ags]

I don't know... the Raspberry Pi HQ Cam is something I keep coming back to - I suspect it might work quite well with brighter targets, and it would be fun rigging it up with Peltier cooling.

[ollypenrice]

It might be worth doing a G2V calibration in order to get the correct exposure weighting for your filters.  I'd have thought it would be preferable find your colour by this natural method than by doing it artificially.

It's odd that you found so little colour in Albireo. What post processing software do you use?

Olly

Edit: maybe also consider dithering, not to reduce noise but to move the Bayer Matrix around under the star to ensure proper sampling of colour.

Edited July 4, 2021 by ollypenrice

[CraigT82]

I notice your first image in the thread was stated captured at f/6.3 so you're using a reducer on the C6?

I'd ditch it, it's unnecessary for double stars and may be part of the cause of the halos you're seeing. 

Collimation is key to getting nice complete first diffraction ring around each star. I'd collimate the scope whilst looking at the star you're going to image (as long as the secondary star isn't close enough to it to distort the star image) and use the in focus image of the star, rather than the out of focus doughnut technique, as long as the seeing supports it that is. 

[Ags]

1 hour ago, ollypenrice said:

It's odd that you found so little colour in Albireo. What post processing software do you use?

 

The image is purely mono 😀 Good idea about doing a G2V weighting. My wibbly-wobbly mount takes care of dithering all by itself!

Edited July 4, 2021 by Ags

[ollypenrice]

Just now, Ags said:

The image is purely mono 😀 Good idea about doing a G2V wieghting.

Doh! 🤣

If you move on to ultra-tight doubles which you're struggling to split, you might be able to split them in an Ha filter and use that as luminance.

Olly

[Ags]

Thats an idea - I was also thinking of trying to use IR.

[Ags]

40 minutes ago, CraigT82 said:

I'd ditch it, it's unnecessary for double stars and may be part of the cause of the halos you're seeing. 

The reducer is in place for two reasons - firstly to help with tracking, and secondly to put the double in a richer star field.

[Ags]

I found a nice stash of images of double and single stars on Wikipedia:

https://en.wikipedia.org/wiki/User:Dritter_wiki 

[Sunshine]

This is interesting as I have often thought about how great it would be to image Epsilon Lyrae, I wonder if it can be done with a planetary imaging cam? sure as heck I will try.

[Ags]

Epsilon Lyrae is so tight, I think it is a real challenge. I think lots of aperture is needed and very short exposures.

[chiltonstar]

12 hours ago, Ags said:

Epsilon Lyrae is so tight, I think it is a real challenge. I think lots of aperture is needed and very short exposures.

I find it works ok with a planetary camera, eg with my 180 Mak and an ASI224 MC camera:-

Chris