Understanding LX

[Carole]

Hi every-one it's me again on a new learning curve.

Well I've finally got long exposure working having bought a DSLR and a new laptop (plus a few other things) an expensive month!!!  I think I might even have LX working now on my SC3 with the new laptop although I haven't tried it out yet on anything astronomical.

Can you tell me regarding LX is it the length of each sub that it important or the number of subs.

i.e.

a) If I did 2 hours of frames/subs (are subs and frames the same thing?) @ 30 sec subs
b) If I did 2 hours of frames/subs @ 2 minute subs.

Would b) be a much better image?
If so, is the logic the longer the sub the better the image?

c) If I did loads of 30 sec subs (while I am not guiding), will this improve the image?

I'm not quite ready to do guiding yet but I can see I am going to have to quite soon and am gradually getting everything together.

Thanks

Carole

[RobertM]

Hi Carole,

The longer the sub the more signal the camera will record and the better the signal to noise will be (provisos here!) ... but ... by using more subs you will also get better signal to noise.  Jerry Ludriguss writes excellent articles explaining such things with a particular bent on DSLRs.  I recommend you read some of Techniques pages available here: http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM.  That should make you an expert in no time, then we'll be asking the questions

Robert

[Mac]

subs / frames same thing.

If your not guiding, then the 30 sec subs will be adequate, any longer and you will start to get field rotation, (really depends on focal length and what part of the sky your imaging),
The down side is that you wont record the very faint items, as these need longer times to register on the camera.

Catch 22. Longer images, fainter detail, star rotation. There is a balance in there.

As Robert said,

If you take one image of something, then there will be some noise in that image as well as the item your photographing.
By stacking more then one image, the noise in the final image is reduced as this normally a random phenomenon, producing a much better photo.

As your images are 30 seconds long, the difference between the first and last image will have rotated by 30 deg,
but this can be overcome in the stacking. so untill you realy start guiding stick to 30 seconds untill your comfortable.

don't forget to reduce the noise further, you should also take dark frames as well, just cover the camera when you've finished and take as many as you require at the same exposure
as your image file, this will record the noise from your camera, which can be removed in your final image, again producing a better result.

You can always increase this by polar aligning your mount at the start of the night, this may take a liitle more time.
but even taking the time to do this will allow you to increase your image times, maybe even up to 90 secs or more.
before you go down the guiding route.

[Carole]

Thanks Robert and Mac, that has confirm what I thought, just wanted to make sure I had it right.

Love the can of worms BTW!!

I did actually go out on Thursday evening and did my first image, just really to try out the new kit and software (lap top remote control for DSLR).  Learnt a lot doing it but came away with a number of questions, the above being being some of them.  

I did do darks subs but as I was only doing 30 second light subs I sometimes missed switching it off at the right moment (if I popped indoors for something), so when I went to do the dark frames I thought well I've done a total of 12 minutes light frames so far why not do 4 lots of 3 minutes and this gives me more time to make myself a cuppa.

So my next question is:

Do all the light frames need to be the same length.?
Do all the dark frames need to be the same length?
Do the dark frames have to be the same length as each light frame?

I did a mixture and I wasn't sure if that was right.

Thanks for the link to that article Robert, I'll have a look at it.

Carole

[Whitters]

Your dark frames should be the same length as the subs of your light frames. You don't need to do the same total length of time as the sum of all your light frames. If you can do only one dark frame then that is better than none at all. When I take darks I usually take around eight. If you have a temperature controlled camera then you can take them all at the same time once the camera has reached it's operating temperature. If you are using a camera with a cooler but not temperature controlled then take a dark at the beginning of your run, one half way through and one at the end, you can then create a master mean dark to apply to your images.

You can take darks at a different length to your light subs if you also take bias frames, but this is more complex than you need for 30 second shots.

If you take flats then you also need to take a set of darks to apply to the flats, they should be taken at the same time and again be the same exposure time as the flats.

When I started, most of my shots were around 30 seconds exposure, I did not bother with calibration rather spent the time learning the camera and the dark art of focusing.

Don't forget you can use camera lenses too, they are great for wide field and you can get away with several minutes exposure without guiding, the closer to the North Celestial pole the longer unguided exposure you can take. See my latest Cassiopia and North American neb pics, they are 5 minute subs using a 105mm f2 camera lens.

[Carole]

Thanks Paul.

This is a question for those who use DSLR cameras for Astro-imaging.

My Canon 450D has a setting which will automatically do dark frames and subtract them from the image.  I've not actually looked into this yet, but has any-one used it, and is this a good idea to use?

Carole

[Whitters]

Ah! I have the same setting on my D30, and I use it all the time.

[Carole]

Great, that should simplify things a bit.
I've also bought myself amongst the

(plus a few other things)

a remote shutter control which will also do repeated timed subs with intervals, but as it uses the same socket on the camera as the "DSLR to laptop" set up I can't use both at the same time, therefore I intend to use the laptop for setting it all up and then once I am ready to leave it going, switch over to the remote timer, but I never got that far on my first "go". 

Carole

[Carole]

For what it's worth this is what I took on Thursday.  I know there are lots of things wrong with it, but I was pleased to have got this far dealing with new camera, new laptop (VISTA!!), new software, new set up (DSLR instead of SC3), all at the same time also not realising I had the wrong contact lens in for half the evening and wondered why I couldn't read instructions, or see through the scope properly!!!!

I hadn't intended M31 to be my first target, but getting focus with the DSLR was another problem I had (and still need to work out the best way to do), so as this was bright enough to see I did this to test everything out.


Total of 12 mins Light frames & 12 mins Dark frames,  Canon 450D, WO Megrez 72 APO stacked in DSS, slight modification in Photoshop.

Hopefully the only way is UP!!!

Carole

[RobertM]

Hi Carole, I have a 450d too but don't use the pc generally.  I would suggest making yourself a focus mask and using a remote timer (eBay).  Focus on a bright star first then use the timer to record your images to the SD card, that's what I do.  Auto darks are fine to start off with but with long exposures that will take up a lot of time so reconsider in the future when more proficient.

[Mac]

correct me if im wrong, but dosent the 450d have live view?  If so can you not use that to aid your focusing,

Mac.

[Carole]

Hi Robert,

I've got a remote timer, just didn't get as far as using it on my first night (another thing I need to practice - to programme it).

I'm not sure what a focus mask is or how it works, but I'll look into that.

What I actually did was to focus on something bright to start with and then moved to Andromeda and took an image to see how it came out.

Have started reading that site you suggested, it's excellent, will persevere with that.

thanks
Carole

[Carole]

correct me if im wrong, but dosent the 450d have live view? If so can you not use that to aid your focusing

Yes you are correct Mac, but I hadn't worked out how to make it work on the laptop on Thursday.  I've worked it out today, so hopefully next time I can use that.

There were such a lot of new things to deal with, plus my bloody contact lenses stopping me reading instructions, and also trying to read stuff in the dark I did not manage to work everything out all on the first evening. 

But Rome wasn't built in a day and I've learnt a lot even with that meagre attempt and less new things to deal with next time.

Carole

[Mac]

I'm not sure what a focus mask is or how it works, but I'll look into that.

Simple,

Place said mask over the front of the telescope / lens.
When the image is out of focus you will have multiple images of the object,
As you approach focus the multiple images will resolve in to one.

[RobertM]

I would use the two hole version but with triangles centered in the same place Mac has drawn the circles, that'll make it even easier to focus.  There should be one triangle with a vertex to the left and the other to the right (looking at Mac's picture).  Also you will find that there may be a slight red edge or green edge to a bright star in live view, that indicates slight missfocus (I use that for fine focusing with a DSLR lens).  There should be no colour fringing on either side of the star.

Another tip: Live view stays on for 30 seconds which isn't long, so center the focus star first using progressively more zoom then you can quickly go straight to maximum zoom in each new live view session.

Hope that helps
Robert