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Disclaimer: All the how-to directions below are for information only. They are not intended to be sold or used for unlawful purposes. The author takes no responsibility if they cause the reader bodily harm, loss of life, or cause the damage of any property, person, dog, cat, or any other animal. They are presented for information only and all liability is on the part of the reader.
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Imaging the Planet Venus and building a UV pass Barlow
Safety level: Low The risk is mainly for destroying your existing equipment but with the proper care no damage will occur.
Skill set and level: Need to be familiar with working and handling glass, comfortable with voiding warranty on optical part
Time to completion: 1 week (most of this time is waiting for the delivery)
Materials needed:
Used or new cheapy 2x barlow, Orion or Celestron type
Small precision screw driver
Lens tissue or standard tissue paper
Elmer's Glue
Tooth pick
Black Magic Marker
Sheet of paper
Scissors
Discussion Topic Venus Imaing: Venus is a one tough planet to image. It is extremely bright but the caveat is that that brightness is attributed to the reflection of the Sun off the planet and its proximity (closeness) to the Sun. To make matters worse it is only up during the waking hours of the day and the planet itself is shrouded in sulfuric acid cloud cover. At first glance through the telescope the planet looks like a moon because it typically appears like a crescent. Because of the cloud cover and solar reflection the planet shows up best at the UV part of the visible spectrum. Knowing this, if one uses a Violet #47 filter on a telescope you can easily see the planet. The violet filter has about 30% transmission but it only passes a deep purple color and all other wavelengths are blocked. Below is an image taken with a Televue 76 telescope (3 inch aperture), a couple of 2x barlows and a Violet #47 filter. No surface detail can be seen but the planet shape is very sharp.
To get a better shot with more detail requires a much larger aperture lens or mirror. While the 76 was crisp and sharp the required filtering is such that more light will have to be blocked. The current filters out there for doing this do not pass enough light for a camera to detect when using a 76mm aperture. In 2008 I had saved up for a 130mm aperture refractor so I decided to give Venus another try. I stayed up all night imaging Jupiter and wanted to get a shot of Venus in the early morning hours. I had already imaged Saturn and Jupiter and the results from those imaging sessions had been excellent with the new refractor. Detail was excellent but those planets are larger in the scope and do not come up at dawn.
I started with just the Violet #47 but found that the image of the planet was too washed out to get surface detail on. The image was also wavy looking and appeared to be boiling due to the atmospheric turbulence in the morning. I quickly realized that a good filter to cut some of this noise was a standard Baader UV/IR filter. I had believed it was the IR causing all the issues due to the rising Sun. The combination of filters worked and I took the shot below.
Cloud detail can clearly be seen in the image. The image is not as sharp as I would like but the details can be seen. For the price of the filtering it was a cool discovery. To get better results I decided to purchase an AstroDon UVenus filter. This filter is a narrow band filter that further blocks the light in such a way as to pass only a small portion of the UV light. It is reported to pass 90% of UV unlike the competing Venus filters from Baader Planetarium. It specifically passes the wavelengths from 325 to 381 nm on the light spectrum. At 325 the light is next to impossible to see with the naked eye. 381 is also next to near impossible. The filter also states that because of the superior filtering it does not require a separate UV/IR filter. It appears like a win win situation.
http://www.astrodon.com/products/filters/uvenus_filter/
After a week of waiting I finally got the filter. Upon trying it I found that it did not pass enough light for use with a 130mm aperture. Because of this problem I reverted back to my old standby of UV/IR filter and Violet #47 filter to get the shot below. Detail can still be seen but is still not optimum.
The fact that the new filter did not pass enough light was pretty shocking since the initial image with the other filters provided an extremely bright image of Venus. I started looking for other possible issues in the imaging train. I read in an old Sky and Telescope magazine that multicoated barlows typically block uv and ir to help with achromatic telescopes light separation. This further filtering is probably not the sole cause of the darker image with the new filter but it would not hurt to add as much light to the imaging train as possible. There is a store online called CVI Melles Griot that sells all things Photonic.
http://www.cvilaser.com/PublicPages/Pages/Default.aspx
I found that the 25mm size -100mm focal length was close to the size of the lens in the barlow and the singlet made from fused silica came in a non-coated version. The cost of the lens was 75$. It arrived after another week of waiting. The quality of the lens was great and true to the order it was non coated. Comparing it to the other lens proved to be pretty easy in regular light. The image below shows both lenses side to side held at an angle to show the lamp on the ceiling fan. The coated barlow lens shines really bright while the uncoated lens lets the light pass through.
To build the Venus barlow I had to take apart a barlow that I had purchased a long time ago but never used. I originally bought it because the gunmetal gray color looked cool. It is a Celestron OMNI barlow.
HOW TO: First unscrew the lower lens housing on the barlow. Place the head of a small screw driver in one of the slots of the lens retaining ring and rotate the housing. Work both slots even if the ring does not initially turn. The evenly placed pressure will eventually loosen the ring. Be extra careful not to scratch the lens. Once the ring is removed place a tissue paper over the opening and turn the lens housing over. The lens will drop down and you will have it in the tissue. Wrap it up for safe keeping.
Now that all the parts are dissembled take the uncoated lens and darken the sides of it with a black magic marker. This will help with the internal reflections. Place the lens in the housing and make sure it is level.
There is a little slop in the housing because the lens does not seat all the way down. This is ok. We will need to glue it in place with the Elmer's glue and will be using the shims of paper to help center the lens. Cut 4 shims approximately 1 inch long by 1/4 inch wide. Place the shims equidistance around the lens in the housing. Use a syringe or small tooth pick to place some glue in the areas around the lens not taken up by the shims. Be careful not to place any on the shims. Once the glue dries take out the shims and fill in the remaining areas with glue too.
After all glue dries put barlow head back on the body and give it a try. This barlow is specific in what it can be used for imaging. Due to the nature of the singlet lens used it will be best suited for narrow band imaging and should work will for monochrome type imaging of items like the Sun, Moon, and Venus.
To be continued after first light...........................
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