Last Monday I took this picture (single frame with DSLR and Tak FC-100DL) of beautiful Rho Herculis (m = 4.5 / 5.4; sep. = 4.0"):
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Posted 17 October 2019 - 08:58 AM
Excellent image, Ulrich.
A is blue - white alright on www.stelledoppie.it and in your image.
B seems a darker blue (?).
Kind regards from Aubrey.
Posted 17 October 2019 - 11:30 AM
"The two stars are hot blue types and have appeared white or greenish white to various observers. F. G. W. Struve thought them greenish-white and greenish, whilst Admiral Smyth recorded bluish-white and pale emerald. RWA found them both white with an 8.3-inch (21-cm) reflector at 96x."
(AN ANTHOLOGY OF VISUAL DOUBLE STARS, page 351)
Best regards to Ireland
Posted 17 October 2019 - 05:53 PM
It has been 3 or 4 years since I split Rho Herculis at 112X.
As I am splitting doubles in Cassiopeia, I might have to wait until 2020 to return to Hercules.
I will be giving a full report on Cassiopeia tomorrow Friday 18th October.
There are some goodies coming up.
Plus over here in Ireland we are having clear skies at last over the coming weekend.
Posted 19 October 2019 - 08:41 AM
Ulrich, I have a question. 4" separation and the doubles seems so separated in your image? What is your pixel/arcsec ratio ?
In my telescope/camera, that pictured separation is at least 15"
Well done, anyway.
Posted 19 October 2019 - 02:13 PM
Thank you for sharing your image.
Rho Herculis was just added to my observer’s list. Astonished that it wasn’t already on it!
Posted 20 October 2019 - 02:31 AM
I don't know my pixel/arcsec ratio. I used a Canon 650D (which has a sensor with 4.3 micrometer pixels), my Tak FC-100/900 mm and a Baader Hyperion 2.25x Barlow threaded into the T2-Canon adapter ring.
In skyandtelescope.com I found an interesting rule of thumb:
"This is where critical sampling then comes into play, meaning the image scale of your camera’s pixels must be smaller than the size of the detail that you hope to capture. A simple rule of thumb to achieve critical sampling is to increase your telescope’s focal length (usually with a Barlow) so that the focal ratio equals 6 times the pixel size in microns, or FR = P × 6. For example, suppose you have an f/10 SCT and a camera with 5-micron pixels. You would multiply 6 × 5 to get a focal ratio of f/30. So you would need a 3× Barlow."
Greetings to Milano
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