Re: Fw: [APML] IDAS LPS Exposure Question and Problem

[Jerry Lodriguss <lists4@astropix.com>]

>    I've done 2 exposure tests with my new filter, 20, 30, and now the 
> latest of 45 min. using my 105mm lens.  The 20 & 30 min. exposures were 
> very dark so I upped the time to 45 min. and it's still too dark.  I know 
> several of you use this filter and was hoping someone could tell me how 
> much you had to extend your exposures to acquire close to the same 
> density on the slide?  Also, is this magenta cast normal with this 
> filter?  I have uploaded with/without examples to give you an idea of how 
> they look.  These are raw scans with no corrections in prescan or 
> after.  All exposures were using Fuji Provia 400F pushed 1 stop.

Hi Gary,

Take a look here:

http://www.astropix.com/HTML/I_ASTROP/I11/I11.HTM

It shows some examples of the North American Neubla with and without the 
IDAS LPS filter shot on E200 at f/2.8.

Without the filter I went 15 minutes and with the filter I had to go 3x as 
long (45minutes) to get the same density of the sky background.

It does give a slight magenta cast which is easy to correct in Photoshop.

If you are at f/5.6 or f/6, with the Provia 400 pushed one stop, and you 
liked the exposure of 45 min for M31, then you would need to go about 3x as 
long (135 minutes) with the LPS filter to get the same sky background.

Exposing for the same time lets almost the same amount of nebulosity 
through (a little less than 100 percent, probably about 90 percent), but 
cuts the sky background way down.  This is not good for slides because 
scanners had a lot of difficulty with dense dark slides. So you should 
expose a lot more with the filter, I would guess a minimum of 90 to 120 
minutes.

It also will depend on your local light pollution's location, if the object 
is in the direction of the light pollution, and how high it is in the sky 
when you shoot it. Ideally, you should shoot when it transits the meridian 
because that is as high as it can get.




>  My problem you can see in the 2 top images taken at PF.  Stars on the 
> left side are out of focus while the stars on the right look ok.  Is this 
> the film not laying flat in the camera back or does it look like 
> something with my optics?  If it is the former, what is a good way to 
> correct this?
>
><http://www.starryestateobservatory.com/My_Homepage_Files/Page109.html>http://www.starryestateobservatory.com/My_Homepage_Files/Page109.html


You don't say if all 4 of the images on that page were shot with the same 
optical system. IF they were, and you got the star problems on the left 
side in only two of the frames, then it is probably the film not being 
flat. It may be film creep from moisture absorbtion during the exposure, or 
it may just be that the film is not being held flat in the camera.

If you consistently get it in the same place on the film when the camera is 
oriented the same and with the same scope, then it might be the optics of 
the scope, or collimation, or perhaps the camera and film are not 
orthagonal to the optical axis (i.e. the camera may be tilted).

Or it could be a combination of any of the above.

Ain't astrophotography fun?  !  :-)

Hope this helped.

Jerry







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