Understanding LX

[Carole]

Thanks Mac, thanks Robert.

So much to learn!!!

Carole

[Mac]

So much to learn!!!

True, but you will have loads of cloudy nights to study,

[MarkS]

the closer to the North Celestial pole the longer unguided exposure you can take

I never thought of that, but of course you're right.

Mark

[Simon E]

I was outside last night, playing around with camera and settings. I am in the same boat. No tracking so only doing 30sec exposures.

I have gone for something easy and was trying to get a nice picture of Vega with a nice halo around it and it looks ok. Ended up taking 8 x 30sec exposures using a very quick film speed ie: 1600 This way I was hoping to make the images brighter. The pictures did look a lot worse at iso 200-400

I also took 6 darks of 30 seconds on a 1600 setting and then used the deepsky stacker, which is very cool.

Also managed to take 6 x 30sec exposures of the ring nebula, but it was very faint.

I thought of something last night whilst taking the images. I have now got 8 x 30 sec darks in a folder called "dark30seciso1600" which i could use every time now, which will save a bit of time.

Also using imageplus camera control, and talking about focusing, this software has a tab which allows focusing. What you do is take a picture of the sky near the star you want to image and u take a quick temp image. You can then zoom to a star on this image and refocus. You then take another image of the same star and keep taking images of the star adjusting the focuser on the scope as u go, until clear, job done.

The only question I have to ask is "what is a flat image" and how does one take one? and what difference does it make?

Simon

[Mac]

I thought of something last night whilst taking the images. I have now got 8 x 30 sec darks in a folder called "dark30seciso1600" which i could use every time now, which will save a bit of time

In an answer yes.

The only problem with that is those darks are the noise that is produced on your sensor at a given temperature (about 9C last night).
As your sensor cools down aver the next few months, the amount of noise that is produced will also decrease,
You can also take other darks at a later time and temperature, and use them to produce a master dark.

Flat images are just images of a white image (i.e nothing in the picture) so that all the dust and debris on your sensor can be photographed, this is then used to adjust your images.


Dont forget to also take a bias frame, this is similar to a dark frame except its the fastest shutter speed, 1/4000 or 1/8000, these are used to remove the readout noise
from your image which will be present in all of your images.

have a read of this post to explain in detail.
http://photo.net/learn/dark_noise/

[Simon E]

Thanks Mac.

Will take these different frames, will add the temp that the darks were taken.... roughly the temp last night.

si

[Simon E]

Flat images are just images of a white image (i.e nothing in the picture) so that all the dust and debris on your sensor can be photographed, this is then used to adjust your images.

Is this just as simple as putting a white piece of paper in front of the camera and taking a picture, or does the camera have to be on the scope outside, right temp and right film speed etc etc.

si

[Daniel]

Hi Simon, when taking flats you can ignore things like temperature, exposure, ISO etc. If your using a DSLR, I normally take my flats at the lowest ISO possible to cut down on noise and use slightly longer exposures to compensate (still under 5 seconds each) I usually take about 50-100 of them depending on how tired I am.

Daniel
:O)

[Mac]

Is this just as simple as putting a white piece of paper in front of the camera and taking a picture, or does the camera have to be on the scope outside, right temp and right film speed etc etc.

It is as simple a putting a piece of white paper in front of the camera.
but it needs to be attached to your telescope, not necessarily out side as its only a flat field

But you are trying to remove all the dust and debris artifacts from your image train.

So you must use the camera + lens +any filters to take the flat. that way the orientation of the camera + lens's doesn't change

So if you are using 4 filters, red, blue, green and clear, then you must take 4 separate flat frames, one for each of the filters.
so that any dust on each filter can be removed individually.

Mac.

[Carole]

It is as simple a putting a piece of white paper in front of the camera.

Haven't there been discussions in the past about our imagers making a "light box" or something.  I thought this was to do their Flat frames in.  Is this the same thing they were talking about? 

If so, why go to all that trouble if you can just put a piece of white paper in front of the camera, and I assume you mean in front of the telescope which has the camera attached to it.

Carole

[RobertM]

A flat piece of paper will do fine Carole, however you try evenly illuminating a flat white piece of paper held up to your telescope with a torch in the dark whilst trying to take a few images of it.

[Daniel]

The problem is getting an even light source behind the piece of paper, if you've imaged till daylight or taken flats before the sun goes down, you can normally use the sky to provide the light, but shining a light into telescope often  results in an uneven illumination of the field if you don't diffuse the light enough.

one method commonly used is to use a white laptop screen (open up notepad or something) that gives quite an even field, but doesn't work too well for large apetures.

A Lightbox creates a repeatable even field that you can just leave the camera exposing

Daniel
:O)

[Carole]

A flat piece of paper will do fine Carole, however you try evenly illuminating a flat white piece of paper held up to your telescope with a torch in the dark whilst trying to take a few images of it.

I take you point Robert, it was only because of this comment

It is as simple a putting a piece of white paper in front of the camera.

It made it look so simple.

I guess he meant the principal was that simple.

Carole

[MarkS]

If you're using a DSLR, then just take a few images of a white PC monitor once you've brought it indoors after your imaging session - this works really well for both my ED80 and my C11.  I take new flats every couple of months or whenever I see an extra speck of dust appear or (for the C11) after collimating - vignetting does not change from session to session.

If you're using an asto-CCD with various filters then the same approach will probably work but you may be better off to build yourself a light box that you can use in the field.

Mark

[Carole]

Mac,

I made myself a focus mask today like you described above, and tried it out tonight, it worked perfectly.  Many thanks.

Carole