Object: eta Persei

Constellation
RA / DEC
Magnitude A / B
Separation
Position angle
Spectral class
Colour A / B
: Perseus
: 02:50:42 / +55.53
: 3.8 / 8.5
: 28.5"
: 301°
: M3I-II
: yellow-orange / white
Details sketch
Date / Time
Observing Location
Seeing / Transparency
Telescope
Eye-piece
Magnification / Field of View '
: 14/09/08 / 20:15
: Landgraaf
: 3 / 4
: Orion Optics UK 300 mm
: 12 mm Nagler type 4
: 133 / 37
Stacks Image 697
Observing report
My first impression is that this double lies within a very nice asterism, seven stars forming a kind of chair, and the eight star sitting on top of it. The 3.8 magnitude A-component shines brightly yellow-orange, in contrast to the white B-component. There are no other significant stars visible. This pair was already split at lowest magnification (46x). I personally find 133x the optimum magnification, because it shows eta persei within the asterism very well. Besides the A and B components, I also observed the C and F components, which also are part of the chair-asterism. I could not detect the D and E components.

About the colours there are very different observations, Webb sees them as “very yellow and very blue”, Smyth sees them as “orange and smalt blue", the Webb society reports them as “strong orange and moderately blue” while Sissy Haas sees them as “apricot orange and cobalt blue”. I myself did not detect the blue in the B-component, but maybe the 300mm aperture kills the delicate colour. Maybe I should have stopped the telescope down a bit. The colour of bright stars is more easily detected in smaller telescopes.
Notes
The F component was something of a mystery to me. When verifying my sketch with all the astronomy software I have available (Voyager, Astroplanner and Megastar) I could not find this star on any of the maps. This is very strange, because I estimated the star to be of magnitude 11 or 12. I could however find it on the Digitized Sky Survey (Aladin), and on sketches of other deepsky observers. I also found it on an image from an Austrian double star observer. He identifies one of the stars as the F-component of eta Persei. From there I moved on to the Washington Double Star catalogue (WDS), and there I also found it, but again with no other aliases or catalogue numbers. I couldn’t find eta Persei in the Tycho, the Hipparcos and the Guide Star Catalogue when searching the databases by coordinates I extracted from the WDS database.

However, when I checked the data from the WDS again, I could only conclude that the Austrian astro-photographer Peter Wienerroither must be right. In the WDS database, I find a position angle of 40 degrees and a separation of 60”. This matches exactly the position of the F-component as can be seen on the image of the Austrian photographer and the map I generated with Aladin. The image below, made by Peter Wienerroither, can be found in an observing report under this link:

http://homepage.univie.ac.at/Peter.Wienerroither/pwafods/02507+5554.htm

Stacks Image 708
Stacks Image 715
To check everything I generated a map from Aladin. As you can see on the image to the left, the position angle of 40 degrees and the separation of 60” or 1’of the F-component match exactly. On some websites I found GSC 0370400488 identified as the F-component, but as you can see, this star lies at a position angle of almost 300 degrees and a distance of 120” or 2’, so it could never be the F-component.

The stars marked with a yellow squares are the stars measured for the Hubble GSC. As you can see, not only the F-component, but also the C and D components (if you compare them with the image from Wienerroither) are not included in the GSC Catalogue.