Re: Some Recent Wide Field Galaxy Images With My Trusty AP900


Robert Chozick
 

Really nice  images Dean. I love my FSQ106 but the only disadvantage is how often you have to focus. I use the same temperature points for focusing.  I also think the Min tool in Photoshop is a great tool. 

Roland have you ever made a Petzval?  

Do the F5 catadioptric scopes hold focus better than refractors or is it about the same?

Robert 


On Mar 3, 2020, at 8:39 AM, Dean Jacobsen <deanjacobsen@...> wrote:

On Mon, Mar 2, 2020 at 10:45 PM, Michael Hambrick wrote:
Hi Dean

I would be curious to know if you are using any special focusing techniques to get such sharp images.

Hi Michael,

No, nothing special.  I don't even use a motorized focuser.  I just use a Bhatinov mask and monitor the temperature.  If the temperature drops more than about 3 - 4 degrees F then I re-focus.

I try to use exposure times that don't saturate the field stars.  There will always be some bright stars in the field that will saturate no matter what you do but my exposure times are calibrated to reach a point where the camera's read noise is buried in the sky background and no more -> 10*readNoise^2.

I am also very careful during processing of color images not to do anything that will bloat the stars.  I don't ever adjust the histogram white point and I am careful with the mid-range stretch not to brighten up the image [and the stars] too much.  For these images I focused on saturating the colors rather than brightening up the image with the histogram tools.  That tends to keep the star profiles smaller.

Lastly, I use Photoshop's minimum filter on the stars.  You can select the stars - Select Color Range --> Highlights - and apply lightly - 0.3 to 0.5 pixel with the "preserve roundness" selected.  It works great for de-emphasizing the moderately bright stars.

That is about it.

Join main@ap-gto.groups.io to automatically receive all group messages.