Astrophotography is an interesting challenge, but a challenge anyone with a telescope can take on. The pursuit has been made infinitely more rewarding since the move from film to digital. By letting a camera’s sensor “collect” photons over time, the telescope can reveal images not visible to the naked eye, no matter how dark the sky. And unlike the days of shooting with film, digital imaging results can be seen almost immediately.
However, taking images of dim points of light at night still has plenty of difficulties, one of which is getting the focus spot-on. Getting it right was a LOT of trial and error. Things were made less frustrating when Pavel Bahtinov gave the world the Bahtinov Mask. The mask made use of an interesting phenomenon known as diffraction. It is very clever and the use of a diffraction mask made focusing by eye easier, but still subjective. That is because even though such a visual-only technique can be pretty good for observing (subject to the expertise of the user), it is usually insufficiently precise or repeatable for astrophotography.
In order to to address those deficiencies, Dr. Winter of Gold Astro designed the GoldFocus system. For those interested in the details, there is an extensive discussionon focus and collimation on the Gold Astro site. The Gold Astro method, by using a combination of a proprietary mask and software, is an objective approach to achieving critical focus, unlike the subjectivity of the experience of the user. While the principle of the GoldFocus mask is the Bahtinov mask, the pattern is different. Gold Astro’s masks are designed for use with astro-imaging setups. The minimum requirements are a digital camera, preferably a specialized astro-imaging camera, but DSLRs will also work. Masks differ depending on whether they will be used with a Fastar-capable telescope.
The mask is constructed from black plexiglas. The slits are cleanly cut, with no rough edges. Since we plan on using the GoldFocus masks with Hyperstar lenses, both of ours have center cutouts. The larger mask also has a removable cover for the opening for when the mask is used with a rear mounted camera. Along with the masks, the GoldFocus package also includes focusing software. The software allows the user to capture a steam of images for focus analysis.
As we can remove the secondary mirrors from our scopes, we can mount a camera either in the rear or the front. Each option has its respective advantages and limitations. In mounting the camera in the rear, standard film and DSLR cameras may be attached with a simple T-adapter. By mounting the camera in the front, the focal length will be reduced and the exposure time significantly shorter, but it does require the addition of a relatively expensive Hyperstar lens.
If we were to mount the camera in the front, we would use our Starlight Express H9C instead of a DSLR. To accommodate, the GoldFocus mask has a hole slightly larger than the diameter of the body of the H9C. The mask slides down over the body and sits on the front of the telescope with the camera protruding from the center.
The mask will also work if a dew shield is in place. GoldAstro actually recommends this option when possible as it solves the problem of the camera cables interfering with the mask. This is often the only option when using regular rectangular astronomical cameras or DSLRs. Of course, when the camera is in the rear, the mask just goes on the front of the scope with no obstructions.
The mask for the Celestron was a custom design for the Hyperstar. Regardless of how the masks are mounted, they are held on by tension with the aid of three nylon screws. They slide along the slots and allow for easy centering.
So for users expecting to use the GoldFocus with a Hyperstar lens, plan on either using the camera through the mask’s hole (first choice) or inside a dew shield (second choice). For us, we will be using the first option with the Celestron C6 on the CG-5 ASGT mountand will try both options with the Meade 10″ LX200. Look for the Setup article later in the Spring and the FirstUse write-up in the Summer.