Testing New Camera

Interference Fringes

May 13th was a lovely day with 72 degree temperature, scattered clouds, light breeze, and, frequently, good seeing. It seemed like a good day to try different Barlow lenses with my new ZWO ASI174MM camera.

The first picture below, a 5-image mosaic, was made with a 3X Barlow lens. A large magnificent prominence adorns the northeastern limb and a string of sunspots are arranged on a diagonal line running down to the right from the prominence. (Click on the image for a larger view.)

The 3X Barlow seems to give better results than a 2X Barlow. I'm quite satisfied by the resolution of details in this image. Unfortunately, the picture is ruined by dark vertical bands. These are interference fringes caused by multiple reflections of monochromatic hydrogen alpha light within the optics of my system. I've encountered interference fringes before with my higher power Barlow lenses, but those fringes, often called Newton's Rings, were circular, indicating multiple reflections between a curved surface and a flat surface. The vertical bands above are almost perfectly straight, more like interference fringes caused by two nonparallel flat pieces of glass. These fringes were not apparent on preview images in the FireCapture camera operating software, so I wasn't aware of their presence until I later processed the images.

Sometimes interference fringes can be removed by slightly tilting the camera. This has worked for me in the past, but now the effect will not be immediately visible on my laptop. I'll have to process future images to see if tilting actually removes the fringes.

I calculated an image scale of 0.53 arc seconds per pixel for the 3X Barlow lens with my Lunt 100 mm telescope. The 3X angular field of view is 17.2 arc minutes by 10.8 arc minutes. The image above was reduced from full size to two thirds size because this seemed to give the most pleasing resolution.

Next, I tried a 5X Barlow lens. The following picture is a 15-image mosaic of the Sun's equatorial region reduced to half size from the original.

Once again, annoying vertical interference fringes ruin the picture, but the details and resolution are very nice. (Click on the image for a larger view.) 

I calculated an image scale of 0.32 arc seconds per pixel for the 5X Barlow lens. The 5X angular field of view is 10.4 arc minutes by 6.52 arc minutes.

Sunspot groups 2345 on the left and 2339 on the right show in good detail in the next 4-image mosaic made with the 5X Barlow. The image has been reduced to two thirds size. Click on it for best detail.

Finally, check out this detailed 5X Barlow single image of sunspot group 2339 which has been reduced to two thirds size. Interference fringes seem almost completely absent. White flares are present near center. (Click on the image for the best view.)

The previous image is among the best I've ever achieved for detail. I might use the 3X Barlow for my workhorse lens in the future if I can remove the interference fringes. 

Video file sizes from the ZWO camera are much bigger than files I get from my DMK41 camera. A 400-frame AVI ZWO video produces a 0.92GB file! A 1,000-frame AVI video produces a 2.3GB file! My image processing software, Registax6, would not handle the large 2.3GB file, so I will be limited to fewer video frames if I continue with Registax6. This really isn't much of a problem because 400-frame or 500-frame videos produce very nice images.