IR/UV cut filter in a Mak/Casse

[Stargazer33]

Hi,

I am in need of some serious help!

I have a Skymax 102mm Mak/Casse and a Baader Neodymium filter, but I am still getting really bad Red/Blue shift in my images.

I have read on SGL and other places that reflector type scopes don't really need IR/UV filters. The Baader Neodymium filter is supposed to have an IR filter built into it anyway, so why am I still getting images like this? This is of Venus by the way - it's not an incredibly bad image of the crescent Moon!

Do you need an IR/UV cut filter for imaging no matter what type of scope you have? Also, why isn't the Baader Neodymium filter cutting out the IR at least, even if it can't do the UV? Finally what is the difference between an IR/UV cut filter and a pass filter? Presumably the cut filter cuts IR/UV and the pass filter allows IR/UV to pass but why would you want to allow IR/UV through?

Taken with a Skymax 102mm Mak/Casse, through the mirror diagonal and a Revelation Astro 2.5x barlow & a Baader Neodymium filter, with a Philips SPC900NC.

Thanks in advance, Bryan

[riklaunim]

That's atmospheric dispersion. The lower the target is the stronger color shift you will see.

IR/UV cut filter cuts UV and IR and pass only visual wavelengths. Those filters are needed for color cameras to get proper color balance for objects that have colors (Venus doesn't). Without such filter or similar (like the neodynium) infrared would add more red color to the image (as red pixels respond to infrared).

IR Passing filters (like Baader IR pass, Astronomik Pro Planet or even red visual filters) pass nearly only infrared and are used with mono cameras for sharp luminance images.

[Cornelius Varley]

The colur shifting may not be due to the filter and IR filter wouldn't do this. You have a diagonal in your imaging path which will degrade the image and the colour shift could also be due to atmospheric conditions.

A UV/IR blocking filter cuts out ultraviolet and infrared light, cameras are sensitive to those wavelengths but are invisible to the humans eye. They also focus at a different point to the visible spectrum so, if not fitlered, will may the image slightly fuzzy. Webcams have a filter built into the lens, which is removed for astro imaging, so the filter needs to be replaced with one in the nosepiece. IR pass filters do the opposite of a cut filter and allows only IR light to reach the sensor, usefull for LRBG imaging where the luminance frame is shot using a pass filter or lunar imaging with a mono camera.

Peter

[Stargazer33]

Thanks guys,

That really clears things up.

So, if I take out the mirror diagonal and get a UV/IR cut filter then the image will be sharper and there should be a better balance of red/blue in the images of planets like Saturn, Jupiter and Mars.

Also if I wait to image the planets until they are closer to the meridian then again I will get better images due to less atmospherics.

Crickey and I thought my OU course was difficult!

Thanks again, Bryan

[Cornelius Varley]

If you take the diagonal out of the image path you might get a small improvement. I suspect the colour shift is mostly, if not all, due to atmospherics.

Peter

[riklaunim]

For Venus usually infrared filters are used. Dispersion is nearly zero for them, and seeing is also limited. For planets IR-RGB images can also be made (although color webcam can't use all pixels for infrared imaging).