How to take deep sky photos without long exposure?

[Ahgii]

Hello, i really want to image deep sky  objects and i dont think my RA motor will do the job, so how do i take deep sky images without a long exposure. I heard that you can take diffrent types of pictures and stack them but i didnt understand much of that, so please help me find a way to make good/decent deep sky photos without long exposure. Also, sorry for being inactive but i haven't found anything worth while to post. Clear skies!

[geordie85]

The simplest answer is take exposures as long as you can. If your RA motor will only allow for 30 second exposures before trailing, take lots and lots and lots of these.

Just remember to re-center your target every now and again 

[steppenwolf]

The dim nature of many deep sky objects means that longer exposures are required to capture detail. The stacking process that you mention does allow you to 'combine' a number of short exposures to produce a stronger single image but only within limits.

If your mount is not up to the job with your telescope, try mounting a standard camera and lens on your mount and capture longer, wide field exposures.

[The Admiral]

Have you looked at the thread "The No EQ Challenge"? This thread is devoted to imaging using Alt-Az mounts, but as a consequence exposures are necessarily short. Whether it's EQ or Alt-Az, the way the images are processed is essentially the same. Don't give up, you'll be surprised what is achievable. In essence, as the previous posts have said, you take lots (100 or more) of say, 30s exposures, and you stack them together in software like Deep Sky Stacker (which is free) and then 'stretch' the resulting image in order to bring out any detail. This can be done in Photoshop or Gimp, or more specialist software like 'Startools', or the more complex and expensive application called 'Pixinsight'. There are others.

The book 'Astro-photography on the Go-Using Short Exposures with Light Mounts' by Joseph Ashley is worth a look at, if you can get a copy.

Ian

[Macavity]

You might also look at the section on Video Astronomy? As an alternative
to long exposures these (very sensitive) cameras internally (or externally)
stack LOTS of short duration frames and output quasi real time images.
The setup need only be steady for times measured in seconds not min/hr!

The net effect is to present an image on a screen that *resembles* what
might be seen by a scope of 2-3x the aperture of the one used...

[newbie alert]

What scope /mount do you use..whats the longest exposure that you feel is comfortable with your setup?

Also depends on the target..ive pulled a large amount of data out of a 4 sec exposure of m42 as it's so bright...a dim galaxy I probably wouldn't of seen anything..

[AKB]

4 minutes ago, newbie alert said:

Also depends on the target..ive pulled a large amount of data out of a 4 sec exposure of m42 as it's so bright...a dim galaxy I probably wouldn't of seen anything..

...although see this spectacular result for M51 (albeit not a very dim galaxy) with 8 second subs

 

[newbie alert]

56 minutes ago, AKB said:

...although see this spectacular result for M51 (albeit not a very dim galaxy) with 8 second subs

 

Great image for lots of 8 sec subs,but why so short? Arguably  don't need any longer but I'm sure the question is aimed with a DSLR in mind..

[AKB]

12 minutes ago, newbie alert said:

Great image for lots of 8 sec subs,but why so short? Arguably  don't need any longer but I'm sure the question is aimed with a DSLR in mind..

CMOS has low read noise, so does shorts well.  Are not many DSLRs CMOS?  Seems to address this OP's subject exactly?

[bobro]

It's a familiar question from someone who has purchased a 130EQ reflector scope and wishes to put it to a particular task. (Moderators - should a new sticky thread be started?)

A starting point : is the mount polar aligned i.e. pointing towards Polaris? May sound simple but not intuitive.

Once polar aligned the simple RA motor needs to run at the correct speed. Again, how to ensure that it is running correctly? 

Next step : different approaches to capturing images e.g. video planetary and individual still images.

Followed by processing the video or individual images.

it can be a good idea to read up on possible approaches - try the book 'Making Every Photon Count', available from the sponsors of this site. It should help with a general understanding.

It's not straightforward stuff but my 130EQ keeps me happy with many hours of DSO imaging.

 

[newbie alert]

1 hour ago, AKB said:

CMOS has low read noise, so does shorts well.  Are not many DSLRs CMOS?  Seems to address this OP's subject exactly?

Oops..Sorry I touched a nerve so it would seem..yes Cmos  sensors are in lots of dslrs..does that mean they have low read noise too?? 

 

[Ahgii]

5 hours ago, newbie alert said:

What scope /mount do you use..whats the longest exposure that you feel is comfortable with your setup?

Also depends on the target..ive pulled a large amount of data out of a 4 sec exposure of m42 as it's so bright...a dim galaxy I probably wouldn't of seen anything..

Well i havent tried deep sky yet, and before i go out of the city next time, but my max time without my RA is, i would say 2-3 secs, and with the RA probably 9-12 secs.

[AKB]

8 hours ago, newbie alert said:

10 hours ago, AKB said:

 

Oops..Sorry I touched a nerve so it would seem..yes Cmos  sensors are in lots of dslrs..does that mean they have low read noise too?? 

No offence taken at all... My fault being too brief, perhaps.   The read noise is only really an issue if you're taking lots of subs and stacking them.  That's why the CCD aficionados like long subs (well, that's one reason.)

[ronin]

6 hours ago, Ahgii said:

Well i havent tried deep sky yet, and before i go out of the city next time, but my max time without my RA is, i would say 2-3 secs, and with the RA probably 9-12 secs.

Would have expected more then 9012 seconds with an RA motor with reasonable polar alignment. Without knowing the mount on your scope it is difficult to say however. It does say EQ I see. I am not sure of the scope itself.

Assuming no polar scope is available then you really have to work on getting any polar alignment as accurate as possible. If you want images in the nature of DSO's then that aspect is vitally important. Even if you do it by "mechanics" you need to get the mount level, as close to level as you can, then you need to angle the axis of the mount drive at the North rotational point - could possibly use a compass but not a magnetic one, simply too inaccurate for what you want. The that leaves getting the latitude scale right - set at your latitude - guess the scale on the mount is not greatly accurate in this area also, likely reasonable but not great.

Do not expect to spend a few sceonds on each, expect say 5 or 10 minutes on each, and then recheck. Even with a polar scope people will take 20 minutes to get it correct.

Now comes the expense: You need a DSLR, an intervalometer, a T-Ring for the DSLR and a nose piece for the T-ring.

Now you need luck: Attach T-ring to DSLR, attach nose piece to T-ring, put nose piece in scope, adjust the focuser to get an "in focus" image - this is the luck bit as in many instances the DSLR sensor and the image plane may not coincide and you have to start altering the scope itself = move mirror up the tube by about 25mm and recollimate.

In a way that is the prerequisites to start getting images, there is more dtail like setting the DSLR and setting the Intervalometer. Oddly once you have the images the stacking is the easy bit - load images into a folder, point DSS at folder, select all images and tell it to stack them. After that is the processing and that is again a big aspect.

One aspect on the purchase of scopes that is becoming a little apparent is that the mount+scope packages that people go and buy and are advertised on retailers web sites are primarily for visual astronomy and the equipment used for that does not cross over to astrophotography.

For visual you might buy a 200P on an EQ5 mount for AP you would buy the same mount - an EQ5 - then put something like a 72mm ED refractor on it, maybe the 130PDS. A much smaller scope. Also do not get mislead by the words "Can attach a DSLR". That says nothing, it does not say it will get an image not does it say it will work well. It just says you can attach a DSLR - super glue or a 6" nail will attach a DSLR.

One possibly easier start, and I would strongly suggest, is get a DSLR with lens and remove the scope from your mount and put the DSLR in it's place. Set it up for say 20 second images and head out for some wide field shots. Less weight on the mount. Gets you used to the set up required for the DSLR, the set up for the Intervalometer, stacking and a bit of processing. And a few months doing that is going to be very worthwhile learning as you will have a much better idea of what to do what you add DSLR to a scope. It is not as simple as camera on scope = get images.