Imaging a comet - Anyone know much about this?

[Peje]

As title,

My mission this summer is to image one of the brighter comets coming through, 21P/Giacobini-Zinner, with the plan to mainly use this as a practice run for a brighter one coming in the winter, 46P/Wirtanen. I have been reading about how to get PHD2 to track a comet, it looks like there are two main methods:

• Enter the RA / Dec rate manually - I tried this back in 2013 for another comet and failed miserably.
• Use the rate training option - Centre the comet in you imaging FoV, Start guiding, wait for a period of time, recentre the comet in your imaging camera FoV using manual mount command, repeat for a while, finish process.
   

• Option 3 - Use EQASCOM Custom Tracking with data from web source. (See 5th post for youTube video link)

Entering the RA / Dec rate would be my preference but actually figuring out what it is presents a problem, I don't know how to do it. Back in 2013 I installed CdC and used a tutorial to determine the rates but I assume I did something wrong as I ended up with streaky stars and comet, the worst of both worlds. Rate training feels like it could work but it might be somewhat less accurate.

I'm curious what others have done to get good images? Ultimately I plan to take images of the comet and stack them, then combine with an set of stacked images of a nearby area of sky (probably one the comet recently passed though) and blend both images together for a best of both worlds final image.

Look forward to hearing any suggestions.

Pete

[tooth_dr]

You could guide on the comet nucleus itself?

[Davey-T]

Some of the brighter ones I've guided on the comet nucleus, DSS has a comet stacking mode that just aligns on the comet, then you can stack one for stars and combine them.

I always use colour camera either DSLR or CCD as trying to align colour planes on a fast moving comet is just one more complication you can do without.

Regarding predicted brightness I think SPM said something like " the only thing you can predict about comets is their unpredictability "

Dave

[Peje]

8 minutes ago, tooth_dr said:

You could guide on the comet nucleus itself?

I think that is going to be more difficult, especially with dim targets. Also I read that as targets get brighter they will become saturated so there is a narrow window to guide on them [It was my first option though]

[Hallingskies]

If you run your EQ6 under EQASCOM then this may possibly help...

https://www.youtube.com/watch?v=vTQFoPC_i3s

[carastro]

I have only done one comet but it came out OK.

I found it using co-ordinates and then I think I did lots of short exposures (I tried 1min and 2mins) as I think it was too faint to guide on the comet.  

Although DSS does have a feature for stacking comets I found it didn't work so I ended up stacking the subs manually in Photoshop, lining all the subs up on the head of the comet.

Taking individual subs also enabled me to make an animation afterwards. 

Carole 

[Tim]

It's a tricky one. When you guide on the comet, or track at the comets speed, you get the trailing stars, which are then awkward to register and stack properly, depending on your imaging focal length.

The best comet pictures I have seen are taken at short focal length, under dark skies, with lots of short exposures.

My mount will track a comet itself if I upload its orbital parameters, but even with longish exposures of 15 minutes on the comet itself, it is not easy to get a nice trailing tail and decent background.

 

[Peje]

1 hour ago, Hallingskies said:

If you run your EQ6 under EQASCOM then this may possibly help...

https://www.youtube.com/watch?v=vTQFoPC_i3s

Excellent, will get a look at that shortly.

2 minutes ago, Tim said:

It's a tricky one. When you guide on the comet, or track at the comets speed, you get the trailing stars, which are then awkward to register and stack properly, depending on your imaging focal length.

The best comet pictures I have seen are taken at short focal length, under dark skies, with lots of short exposures.

My mount will track a comet itself if I upload its orbital parameters, but even with longish exposures of 15 minutes on the comet itself, it is not easy to get a nice trailing tail and decent background.

 

I'm wondering how short an exposure I can get away with, are we talking LOTS of 1 min exposures or even less?

This will definitely be one where I'm going to be babysitting the gear so I don't mond a fairly manual process.

[coatesg]

I've done it once using this guide here: http://www.astrofriend.eu/astronomy/tutorials/tutorial-comet-autoguiding/tutorial-comet-autoguiding.html

This may be the method you talked about using CdC? it seemed to work well for me (though there is a slight "blip" in one frame if you look carefully at the brightest stars - but prob unrelated). Result here from Feb (PANSTARRS C/2016 C2) using 10x180s subframes at 1583mm fl.. I think it pretty much worked out as per the instructions, though not sure how much difference it would have made in this case with the movement rate.

Noted to myself that dithering should really be off for this kind of thing (I think this is shown as the "walk" in the hot pixels) - I don't think that helped me out much here in working out if it did what I thought it should (and it was reasonably low in a poor direction for me so some subs were quite poor). You also need all the telescope parameters correctly set in PHD to allow it to correctly work out the pixel scale on the guider or it won't do the tracking.

I can't recall how I stacked - possibly using the Comet Alignment option in PI...? 

 

 

[Peje]

12 minutes ago, coatesg said:

I've done it once using this guide here: http://www.astrofriend.eu/astronomy/tutorials/tutorial-comet-autoguiding/tutorial-comet-autoguiding.html

This may be the method you talked about using CdC? it seemed to work well for me (though there is a slight "blip" in one frame if you look carefully at the brightest stars - but prob unrelated). Result here from Feb (PANSTARRS C/2016 C2) using 10x180s subframes at 1583mm fl.. I think it pretty much worked out as per the instructions, though not sure how much difference it would have made in this case with the movement rate.

Noted to myself that dithering should really be off for this kind of thing (I think this is shown as the "walk" in the hot pixels) - I don't think that helped me out much here in working out if it did what I thought it should (and it was reasonably low in a poor direction for me so some subs were quite poor). You also need all the telescope parameters correctly set in PHD to allow it to correctly work out the pixel scale on the guider or it won't do the tracking.

I can't recall how I stacked - possibly using the Comet Alignment option in PI...? 

 

 

That was the guide I used last time, no doubt I did something wrong. I also stacked it using the Comet Alignment option in PI, my probablem was my comet wasn't round but the process still worked.

The first comet I mentioned is available at the minute but at mag 16 I might struggle to see and recentre it with any exposure not running into 3+ mins

[Peje]

2 hours ago, Hallingskies said:

If you run your EQ6 under EQASCOM then this may possibly help...

https://www.youtube.com/watch?v=vTQFoPC_i3s

Ah I've watched that before. My worry would be that unless the PA is bang on it will drift; that being said, my PA is pretty darn tight as I'm on a pier.

Adding it as an option 3.

[tooth_dr]

I tracked the stars here, but stacked on the comet using DSS, Comet Lovejoy 2015.

[Hallingskies]

2 hours ago, Peje said:

Ah I've watched that before. My worry would be that unless the PA is bang on it will drift; that being said, my PA is pretty darn tight as I'm on a pier.

Adding it as an option 3.

A combination of options 2 and 3 (or manual guiding in PHD) and 3 might work well as the modified tracking rate should take out most of the non-sidereal drift.

[Owmuchonomy]

I have had a go at a couple.  I found it fairly easy compared to DSO imaging.  As Carole and Dave point out, DSS can make it easy, although I see Carole had a problem with it. Colour is the way to go. There are options in the software which let you decide to stack as normal or stack on the comet.  I found by far the best option was to stack a normal.  Comets can be 'relatively' bright so shorter exposures are OK for some and so I would forget guiding and try without.  Here is Catalina in 2015 in Virgo taken on my 60D via an Altair Astro 102 triplet at f/7.  There is a very small stacking artefact in the comet core but I can live with that.  Taken on my HEQ5 Pro (unguided).

 

[happy-kat]

This is how you could do it using DSS to stack comet images. (grabs from DSS 3.3.4)

After loading all the images into DSS ensure to select Check all images wanted.

Then select Register checked pictures (1 on the last image below).

On the resulting dialog do not select Stack after registering.

Now that all images are registered select the first light in the sequence.

See last image.

On the menu on the right hand side select the comet icon (marked as 2 on the last image). For my light it is showing the circle where I had previously saved the comet's position.

Using your mouse wheel you can zoom in and out to get a better view for finding the comet.

Once located I hold the shift key and carefully position the mouse pointer in the middle of the comet and then pick the mouse left click button.

Now select the save icon on the same little menu.

Repeat for all Lights (this is what I do).

Now select Stack checked pictures and then Stacking parameters now select the Comet tab and chose what you want and fiddle with any other settings of choice on the other tabs.

And wait... takes a while.

[happy-kat]

This thread is a link to another method shared by Tonk.

Link here

[Peje]

2 hours ago, Owmuchonomy said:

I have had a go at a couple.  I found it fairly easy compared to DSO imaging.  As Carole and Dave point out, DSS can make it easy, although I see Carole had a problem with it. Colour is the way to go. There are options in the software which let you decide to stack as normal or stack on the comet.  I found by far the best option was to stack a normal.  Comets can be 'relatively' bright so shorter exposures are OK for some and so I would forget guiding and try without.  Here is Catalina in 2015 in Virgo taken on my 60D via an Altair Astro 102 triplet at f/7.  There is a very small stacking artefact in the comet core but I can live with that.  Taken on my HEQ5 Pro (unguided).

 

That looks really good, can you give some more capture info?

Exposure Length per sub? How many subs?

[Peje]

7 minutes ago, Peje said:

That looks really good, can you give some more capture info?

Exposure Length per sub? How many subs?

Looking at the comet alignment workflow it seems i just need to shot exposures where neither the stars or comet are non-round.

I had completely forgotten about the fact I use a mono camera, will have to break out my nikon for the comet images.

[Davey-T]

Comets can be very variable in their speed depending on their distance from Earth, some hardly move from one night to the next, others that I've tried to image in RGB with a mono ccd    ( only because it was already mounted and I didn't want to disturb it ) have clearly moved from one filter to the next @ 2500mm focal length.

Dave

[carastro]

Yes I did find using a DSLR was easier.  However I took some luminance on another night.  Removed the stars and laid it over the top which brought out the tail a bit more.

This is 10 x 1min DSLR (comet Lovejoy)

12 x 2 mins DSLR:

This is the DSLR image with some starless luminance laid over it (remember this was manual stacking as the comet mode in DSS did not work properly, it would stack a couple and chuck the others out:

[celestron8g8]

That first one is nice but that last image is a winner !! Is there a way you can remove the stars in that last one and over lay another image like you did that first one ? 

[carastro]

I suppose that is possible.  I use Straton to remove stars.

Carole 

[Buzzard75]

Just a heads up, C/2017 S3 will be visible to the naked eye in early August. It will be up very early in the morning just before sunrise so you'll have to get up really early.

http://www.skyandtelescope.com/online-gallery/faint-comet-c2017-s3-panstarrs-may-become-a-naked-eye-comet/

Another target for you to potentially practice on.

[celestron8g8]

I took this single image of Comet Lovejoy using my XSi D450 and Canon 70-200mm f/4L at 200mm and 800 ISO and f/5.6 for 120-secs . I guided with a 25mm IR while the camera was piggyback my C8 on an old EQ5 German mount . This was the only decent one with hardly any star trails . 

 

[geoflewis]

Hi Pete,

I haven't done any comets for a few years, but I've tried unguided, as well as guiding on either comet nucleus or a nearby star.

This image of C/2104 Q2 Lovejoy was taken in Jan 2015, as unguided (but tracked using NEQ6) 1 min subs with my DSLR + Nikon 180mm prime camera lens wide open at F2.8. If I recall correctly I first aligned and stacked on the comet nucleus using a sigma median combine which loses the star trails, then blended that with a 2nd stack aligned on the stars also using median combine to lose the comet.

To give an idea how fast some comets move against the background stars please see the below gif of Comet C/2013 US10 Catalina comprising 32x2 minute subs, so just over 1 hour duration. This image was taken through my 4" APO guiding on the comet nucleus.

....and a static image from the same 32 subs using the double stack method as described for Lovejoy - note that trails for some of the brighter stars are still showing...

Good luck, they are fun to do.

Cheers, Geof