Friday, August 07, 2009

Stronger reducer for Solarmax 40?

With the sweatspot, we can either push up the power so that we only use the central part of the image, and then stitch the power up frames into a big uniform mosaic.

Or we can use a stronger reducer so that the whole solar disc lies in the sweatspot, that would again form a uniform image.

Make sense?

Problem is, I cannot push too much on the reduction factor since it would require excessive in-travel. But again I guess this idea could be employed to generate good and uniform solar image in H-alpha. Time to think more.


Willis Chan said...

I don’t think this is possible. A reducer can boost up the focal ratio. One function is to shorten the exposure time. This means off-principal axis light from the objective is being used. From photographic experience we know that the vignetting is caused by using large aperture, the off-principal axis part of the lens. However, if smaller aperture is used, the effect of vignetting will be reduced. Try to read Nikon 80mm F1.4 AF as an example ( The more the vignetting, the smaller the sweet spot is. In photography we can increase the size of the sweet spot by using smaller aperture. In telescope, we can limit the objective by a card board with a smaller hole at the centre. The drawbacks are reduced in resolution and increase in exposure time. Try to see if you can get a larger sweet spot. This is my thought.

Oldfield said...

These are two different issues.

Sweatspot in H-alpha system is one, and the one you mentioned is about vignetting. Vignetting is far easier to handle, for example, by using a flatfield.

Willis Chan said...

Just want to make sure that are "Sweatspot" & "Sweet spot" different?

Oldfield said...

Sorry for the spelling errors.

But keep in mind that the non-uniformity of bandpass and illumination for narrow band system is not the same thing like regular vignetting issue.