Dobsonian Astrophotography

[Dboots (Duncan)]

Hi,

I have been toying with astrophotography with a 14" goto dob and Canon 60d. I know that a dob is not the best tool for astrophotography but I have to use the tools I have.

I am now looking at stacking for Deep Sky Objects. Can you please let me know:

1. is it better to use high ISO (stacking will eliminate the noise) or should a use a lower ISO (stacking will amplify the faint image)?

2. What would be the best (longest) length of exposure to minimize any rotational effects from the dob?

2. should I process the photos before stacking or process the stacked result (or both)?

Please let me know if these questions have been asked before and, if so, where I can find the answers.

Also, I have attached a single 20 second exposure picture I took of the Lagoon Nebula as a starting point for future comparison.

Thank you in advance,

Duncan

[rustigsmed (Russell)]

Hi Duncan,

Great start with the dob photography
You can get some nice results with shorter exposures particularly with the brighter deep space objects

In my experience sticking to 20 seconds was the most stable for good snaps. Although I could probably have used 25 secs comfortably...

I would use iso 3200 most of the time and some times iso 1600... I experimented with 6400 but wouldn't recommend it. 20 secs @ iso 3200 was the sweet spot for me.

Also don't forget to take atleast 10-20 darks.

Process after stacking.

are you using a coma corrector?

Keep up the great work

Rusty

Ps I was going to attach some pics I had taken on my dob but manage attachments doesn't seem to be working. if you look on my flickr page you will see plenty - although note the more recent ones are on my eq8.

[rustigsmed (Russell)]

Could seem to get it to work now for some reason.
exposures vary from 2 minutes up to over half an hour.
Hopefully it gives you a bit of inspiration.

Good luck!

cheers

rusty

[Dboots (Duncan)]

Hi Rusty,

Thanks for the info. I have been using a much lower ISO (around 800) so I will crank it up with my next attempts.

I do not have a coma corrector. The scope is f/4.5 and I use the 2" for the t-ring. Is there any particular corrector that you would recommend?

I am new to the stacking. What are "darks" and how do I take them?

Finally, what would be the minimum number of images for a good stack?

Also, they are great photos. Any info on the exposures, number of images etc?

Duncan

[rustigsmed (Russell)]

no worries Duncan,

you will see a huge difference with more subs and using darks. then it comes down to processing which takes a lot of practice. Have you access to photoshop?

a dark is simply the same as a light sub - except with the lens cap on (so comes out dark). it records some of the noise that your camera creates. In DSS you select open darks and select them all. it will automatically subtract it (from the light subs) for you when processing. note that you need to use the same time and exposure eg 20 secs @iso 3200.

you are meant to do them around the same temperature so good to do on the same night. although a lot of people will use a 'master dark' for each season to save time (DSS will create a master dark [a stack of your darks in one picture] automatically if you have enough).

As many subs as you can get is the best rule, although I would say the magical minimum number of subs stacked would be 16.

don't be afraid to experiment a bit with times and iso on different targets.

you will have to click on my flickr page (see my signature) I have most of the details listed when you click on the pics.

A lot of people use the baader MPCC (multi purpose coma corrector) - it does a good job.

cheers

[Dboots (Duncan)]

Hi Rusty,

Thank you again for all of your help. I'll have to give it all a go on the next clear night.

I am having problems with bright stars being blobs rather than pin-points. Is this just a coma issue or is there a way of reducing the size of the stars in post-production?

Duncan

[rustigsmed (Russell)]

No worries Duncan,

out of my images below I haven't touched the stars - with the exception of the tarantula nebula where I did mask the stars in post production.

Yours probably is not coma, coma is most evident the further away you are from the middle of the field of view. the stars end up looking like comets rather than circles.

other suggestions perhaps you have some dew forming on the optics? or perhaps the scope isn't collimated too well, or it could be normal star bloat which happens on brighter stars.

if you post a pic and point out the blobs we should be able to find the cause.

cheers

[jjjnettie (Jeanette)]

20 seconds is about the longest you can go. Use ISO 1600 or ISO 3200 and take as many as you can. Fill the card.
If you have a barlow, you can also do some planetary. Capture video of the planet and process it in Registax.
The Jupiter images I took with a non tracking dob, Saturn with a tracking dob. Both were 10".

[Dboots (Duncan)]

Hi All,

thank you for your input. I see I have a lot to learn. I have attached an attempt at a stacked image. I was having trouble uploading images so I had to shrink it down. This is only 4 images with 4 darks so not a lot of detail but my main query is about the "blob" starts at the edges. this a coma issue that may be fixed with a coma corrector or can I fix this in post production?

Thanks,

Duncan

[jjjnettie (Jeanette)]

Nice one. There's some coma around the edges, but if you have photoshop you can fix that with the Lens Distortion filter.
Catch some more subs and your image will really start to pop.

[rustigsmed (Russell)]

Originally Posted by Dboots

Hi All,

thank you for your input. I see I have a lot to learn. I have attached an attempt at a stacked image. I was having trouble uploading images so I had to shrink it down. This is only 4 images with 4 darks so not a lot of detail but my main query is about the "blob" starts at the edges. this a coma issue that may be fixed with a coma corrector or can I fix this in post production?

Thanks,

Duncan

great start Duncan

yep coma corrector is the way to go. as jjjnettie has mentioned lens correction can help out post production but can be a little tricky when the coma isn't even (notice the bottom left of your pic hasn't got much compared to the top right). a lot of people crop out the edges to avoid the worst sections.

already making some quick progress some more data and you'll have something to hang on the wall.

cheers

rusty

[1kmodem (Will)]

Nice pics and info! helps me out too