Flats for a newtonian

Shooting Flats for a Newtonian

Ah yes, the bane of the digital imager's existence: taking the flat frame. I have read all of the posts over the years and was waiting until the time came when I could no longer ignore the dreaded flats. Well, the time finally has come, as I am now able to do long exposure ccd. I looked at the options available: sky flats, dome flats, t-shirt flats, light box flats. I was attracted to the idea of the light box flat as it seemed to be the most convenient and the most reproducible (intuitively). A known diffuse light source readily available to be used at any time you need to. Sky flats seemed a little less reliable intuitively. You need to be out there at the precise time of twilight and conditions are changing all of the time. Dome flats could be done, but the dome would have to be closed.

Many light box designs are out there. Some I found were immensely complex. I also needed a design that would be suitable for the Cave. Very light weight was a must as the box would have to rest on the focuser mounting. I finally came across a design that satisfied all of the requirements and was quite simple to build. Details can be found here Russ Croman is one of the better known astro-imagers and certainly by looking at his work, it would appear that the light box is quite effective! All of the materials and steps used to produce the light box below are essentially identical to those presented on Russ's site, so I won't repeat them here.

This is the 'telescope view' showing the last diffuser. This is what rests on the OTA. The inner depth is 7". The box width is 13.75". The OTA is 13" in outer diameter which includes the aluminum cap rings.

This is the top of the box showing the hole where the light source sits. You can see the first diffuser through the hole.

Here is the assembled light source which is a tap light as shown on Russ Croman's site. These you can buy for 3 dollars apiece from Target or Home Depot. Same with the small bungees. The whole project will cost around $25. The lights are a good diffuse source of light.

Here the box is in operation. It does rest on the focuser mounting but is extremely light weight, probably less than 1 pound. The materials are foam core and draft paper as detailed, once again, on Russ Croman's site (link above)

This is what the raw flat looks like. Pretty uniform. The exposure will depend on the camera and if you are using a filter or not. For my set-up it is recommended to expose to at least 50% of saturation of the ccd. The ADU counts can be read directly off of the screen if you are using CCDSoft. There is a slight degree of brightening in the center, maybe a few hundred ADU, but the real test of the flat effectiveness is in the application

Now this is what it's all about! The flat is supposed to recreate the optical quirks present in your system. In this case as you can see in the raw image on the left of M81, we have some vignetting happening. Very common with newtonians!! On the right is the flat-subtracted image. Quite a difference! This was my very first application of the light box. I could have increased my flat exposure time a little further, but I would say that it appears to do the job, wouldn't you?

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