How to choose coordinates

General Questions
1

I am unable to image at my home so I am trying Roboscopes. I am new to AP and don’t really understand the coordinate systems. So far, I have successfully imaged two objects using Telescopius to set the RA and DEC coordinates. In both cases, the simulator showed an image of the nebula in the center of the simulator FOV so I knew I was in the right spot. Now I am interested in Sh 2-136, the Ghost Nebula.

When I go to that site using the search bar, the center of the simulator goes to 21 15 00 + 68 02 00. There I don’t see an image of the nebula in the center of the screen. There is a small bright object in the upper left of the simulator FOV. When I expand to full screen that appears to be the nebula but I am not sure. When I center that object in the simulator the coordinates are about 21 16 26 +68 15 36 which match the ICS coordinates I find in Sinbad for something related to Sh2-136 but I am not sure if Sinbad is referring to some related object or the Ghost Nebula. The Sinbad data page for Sh 2-136 starts with “Basic Data: GN: 21.5.8 – Reflection Nebula.” The 21.5.8 there matches the RA on the Telescopius data sheet.

So can anyone help me figure out what coordinates I should enter for this object? And, is the darn thing so faint that a newbie won’t be able to successfully image it with a few hours of data on Robosopes’ 16" 2740mm Dall-Kirkam anyway?

2

I have just checked Sh2-136 details and it seems to be quite a faint nebula. BUT I’ve also seen pictures taken with 6 inch telescopes so you should definitely be able to get something with a 16 inch one. But you’ll probably need to gather at least several lights to improve the SNR given how faint this nebula is.

Regarding the coordinates, the current database has the coordinates of the original catalog, which unfortunately lacks the seconds information for the declination. SIMBAD has updated coordinates based on recent observations. In this case the problem is even worse, with a difference of many arcminutes.

The good news is that the new targets database, which I’m working on right now, is based on SIMBAD data so that will hopefully cause less trouble in the future. In the meantime, you’d be probably better off with SIMBAD’s coordinates for the Sharpless catalog.

I hope this helps!

3

Thanks. It helps a lot that someone who understands this stuff confirms that my confusion is merited. :slight_smile: But I am still a bit confused by what the Sinbad coordinates are. As I noted above, the page head in Sinbad says: “Basic Data: GN: 21.5.8 – Reflection Nebula.” which is very close to the RA of the “empty” spot Telescopius goes to. But further down the page it has coordinates for the dim object I saw on in the Telescopius frame (21 16 26 +68 15 36). What I am not sure of if the Sinbad site is telling me that object is the Ghost or some part of it, or whether it identifies it as a different associated object. Just now I found a link to the World Wide Telescope here in Telescopius and for SH 2-136 it take me to the object at (21 16 26 +68 15 36) and displays an image of the Ghost nebula. I am going to give that a shot when the moon winds down unless I learn something new. This is a copy and paste from Sinbad. Is the line with ICRS coordinates referring to SH2-136 or to some other object?

GN 21.15.8 – Reflection Nebula
Other object types: HII (SH), RNe ([RK68]), ISM (GN)
ICRS coord. (ep=J2000) : 21 16 26.0 +68 15 36 [ ] E 2003A&A…399…141M
FK4 coord. (ep=B1950 eq=1950) : 21 15 41.5 +68 03 00 [ ]
Gal coord. (ep=J2000) : 105.0633 +13.2166 [ ]

Edit: After all this, I looked it up in Stellarium and that very clearly shows it at the 21 16 26 coordinate. So I will stay with that.

4

Next Monday I’ll release a big upgrade, in which I’ll replace the whole DSO database with a much larger and improved version, and data corrected using SIMBAD. All coordinates will be much better starting Monday noon UTC :slight_smile: