The dolphin revisited
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Delphinus is one of very few constellations for which little imagination is needed to see how its name relates to the shape.
I took the following photo to try to locate the star HR Delphini (HR implies it is now designated as variable) which is the remnant of the bright nova seen in 1967. I have marked its position as the intersection of 2 white lines.
Canon EOS5DMk2 24-105mm lens @ 100mm 65 x 10s f/4 ISO3200
I had photographed the nova in October 1967, when its visual magnitude was 5.2 (not yet at its maximum of about 3.5). As a schoolboy I used very crude equipment. I had a Kodak Brownie box camera. I had removed the front plate of the camera, with its very small lens. Behind that was a metal plate with a large hole so I mounted in front of that a simple magnifying glass. That had a decent aperture. I focussed it by putting tracing paper in place of film in the film holder. The camera was really acting mainly as a film holder, for 120-size roll film. The camera was attached piggy back on a 6 inch (30cm) f/8 Newtonian telescope. The scope was on an unmotorised equatorial mount and I used it for guiding the camera by hand. Despite the appalling optical aberrations of the magnifying glass, I did get a record and here it is:
Kodak Brownie + magnifying glass, 20min exposure
guided manually, 1967:10:23 18:58-19:18UT
Notice that the new photo above was taken without any telescope, from a light-polluted suburban site. There was some thin wispy high cloud and the moon, just past full, was only about 90 degrees away. Processing a large number of snaps through GRIP enabled me to show so much detail. Anyone with a digital camera and something to fix it on can do this - you don't even necessarily need an SLR. I did use my motorised HEQ5 tripod but with 10s exposures that probably made little difference.
Having taken that photo I was able to work out where HR Del is (consulting books and other sites). Here is a cropped area from the new photo above, around HR Del. I have annotated it according to the finder chart given in Robert Burnham's "Celestial Handbook" (Dover revised edition, 1978).
The standard visual magnitudes of the identified stars (according to Burnham) are as follows.
- A = 7.8
- B = 7.9
- C = 8.3
- D = 9.0
- E = 9.1
- F = 10.44
- G = 10.95
- H = 12.26
- J = 13.08
I used my photo above as a map to enable me to find and photograph the same region through my 254mm telescope. This is the result (prime focus, configuration [A]):
Canon EOS5DMk2 254mm Newtonian @ 1200mm 65 x 10s f/4.8 ISO3200
Can you see which star in this telescopic view is HR Del? I think the difference between the two photos (camera lens versus telescope) is very interesting. No wonder I could not find the star at all by using published hand-drawn finder charts, nor with PC-based planetarium programs because the portrayal of stars on the PC screen is too different from the reality seen through a telescope eyepiece.
I added new facilities to GRIP (making version 9.10.14) and went on to estimate the magnitude of HR Del, as illustrated on a page about how to estimate magnitudes. The result is shown in this graph, fitting a least-squares straight line to the data:
The fit is better than I had any reason to expect, for at least two reasons. Firstly the faintest reference star, J, has an even fainter neighbour. Unfortunately GRIP detected both stars together, so the brightness measurement for J must be too large:
Perhaps some boundary editing facilities are needed in GRIP.
The second reason why the fit should not be so good is that the brighter reference stars are saturated: the brightest pixels are clipped, so their measured brightnesses must be too low. The profile option of GRIP's image measurement menu can be used for checking whether this has happened.
It is important to set the exposure time and ISO sensitivity for each photo so that none of the reference stars is overexposed like this.
Here is an extract from the telescopic image above, contrast enhanced and scaled to 100% (ie, 1 pixel on this page equals 1 pixel in the camera). HR Del is at the intersection of the 2 short white lines. Reference stars F, H and J can also be seen more clearly, towards the right of this extract. Because Delphinus is near the plane of the Milky Way, there are a very large number of faint background stars. It can be seen in this enhanced view that J has two fainter neighbours almost touching it.
I said above that I think hand-drawn finder charts are very difficult to interpret and so are planetarium programs running on a PC. The latter cannot portray stars realistically because they attempt to keep up with the changing sky in real time.
So I am attempting to work towards more useable finder charts. I believe finder charts need to look more like the view seen in a telescope and it ought to be technically feasible to generate them from existing data. I have added a facility to GRIP to enable any square portion of the sky to be plotted photo-realistically from publicly available star data sets. More about GRIP's star charts here. Here is a chart produced by GRIP for the region around HR Del. That star has been clicked so that it has a cross against it and a message box shows some details of it from Hipparcos:
I have compiled the following table to show how reference stars from the BAAVSS finder charts and Burnham's chart can be identified in the Hipparcos and Tycho data. I think that in future reference stars should have identifiers from those data sets to help people identify them unambiguously.
|BAA ref no||BAA magnitude||Burnham id||Burnham magnitude||Hipparcos id||Hipparcos magnitude||Tycho id||Tycho magnitude|
|1||7.97||1642 2024 1||8.00|
|2||8.34||C||8.3||1642 186 1||8.55|
|3||8.48||1642 448 1||8.58|
|4||8.81||102239||8.91||1642 416 1||8.86|
|5||9.38||1642 730 1||9.56|
|6||9.64||1642 296 1||9.81|
|7||9.75||1642 830 1||9.89|
|8||10.08||1642 962 1||10.14|
|9||10.22||F||10.44||1642 764 1||10.47|
|10||10.33||1642 1000 1||10.50|
|11||10.75||1642 4 1||10.85|
|13||11.16||1642 981 1||11.28|
|C||102040||6.43||1642 835 1||6.43|
|E||1642 833 1||7.99|
|F||A||7.8||1642 90 1||7.61|
|H||102295||7.85||1642 3003 1||9.06|
|B||7.9||1642 748 1||7.85|
|D||9.0||1642 526 1||9.01|
|E||9.1||1642 670 1||9.38|
|HR||102190||12.31||1642 998 1||12.31|
Notes about the table:
- The 4 BAA reference stars C, E, F, and H appear on the BAAVSS charts but are given no magnitudes there. You can see from the table that they are not the same as Burnham's reference stars having the same letters.
- Stars that have no Hipparcos or Tycho identifiers do not appear in those data sets. Generally this is because they are fainter than the stars those projects covered. That is why I should go on to use bigger data sets.
I have also annotated a GRIP-generated star chart (from Hipparcos and Tycho data) to show all of the BAA reference stars which appear:
Should all finder charts look like this in future? GRIP could produce them automatically, given lists of the required reference stars.