How to image Ison

[johnb]

So never imaged a comet, if I point the scope in the right area and get some 300 sec exposures am I going to see a trail or will I need to be more scientific than that ?

John B

[Freddie]

300 may be a touch too long but somewhere between 2 and 5 mins is the right ballpark. You can then use the comet tab in DSS to get different stacking options.

[johnb]

Thanks, so will it be obvious if I have caught a comet ? John B

[Freddie]

Yes, it will not be a small point of light but a "fuzz". It will be easy to see it moving against the background stars between images in say a one hour imaging session.

[badgerchap]

Isn't 300 sec 5 mins? I think it'll just improve and improve as the months develop, so just keep trying and see what works best, there's a lot of time yet before perihelion!

[laser_jock99]

ISON is going to become so bright that any point-and-shoot camera will get great pictures............well, we can only hope!

[earth titan]

ISON is going to become so bright that any point-and-shoot camera will get great pictures............well, we can only hope!

Fingers crossed.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[IanL]

You could try guiding on the comet rather than the stars. Metaguide has a shift-guiding mode that allows this:

http://www.astrogeeks.com/Bliss/MetaGuide/MetaGuide5xx.html#__RefHeading__58_1601665524

Alternatively, if you are/can use EQMOD with your mount, it has a custom RA/Dec tracking rate setting.

In both cases you need to know the current RA/Dec rates for the target. You can use the MPC Ephemeris calculator to determine the correct RA and Dec rates for a given object, time and location:

http://www.minorplanetcenter.net/iau/MPEph/MPEph.html

(It appears to be broken at the moment, expect it will get fixed later today. I will see if I can get it working later and give you more detailed instructions on using it).

You should then be able to get a longer exposure without needing to stack, since you are tracking the object not the stars.

[johnb]

Thanks for that. John B

[IanL]

Okay the MPC site is back up now, see http://www.minorplanetcenter.net/iau/MPEph/MPEph.html

Fill in the form as shown below:

You need to substitute your latitude and longitude (in decimal degrees EAST from Greenwich, if you live in the western hemisphere add 180 to your normal longitude!) and altitude in metres. Then click the Get ephemerides button at the bottom of the screenshot and it will return a list of dates and times with various figures. You want the two "sky motion" columns for EQMOD, the first is RA and the second is Dec in decimal seconds.

As you will see ISON is hardly moving at the moment relative to Earth so long exposures with normal tracking should be fine, but as it gets closer I imagine it will need a different strategy as I have set out above until it gets really bright.

[laser_jock99]

You could try guiding on the comet rather than the stars.

I had wondered about that. If the comet nucleus was small and bright enough could you pick it out with the guide camera and just guide off that?

[chrisshillito]

You need to substitute your latitude and longitude (in decimal degrees EAST from Greenwich, if you live in the western hemisphere add 180 to your normal longitude!) and altitude in metres. Then click the Get ephemerides button at the bottom of the screenshot and it will return a list of dates and times with various figures. You want the two "sky motion" columns for EQMOD, the first is RA and the second is Dec in decimal seconds.

You can also the MPC site to create a file of data that EQMOD can use for custom tracking (use V1.27b from the EQMOD group). In this way you can create a tracking file covering a month, and have different files prepared for the various comets you want to follow. Note that it isn't necessary to enter your site location on the MPC site. EQMOD doesn't use Alt/AZ data and site doesn't affect RA/DEC rates.

http://www.youtube.com/watch?v=MK3YSyVgcLA&feature=share&list=UUVrU4T3b2d25cMaqbGNBxQA

Chris

[IanL]

I had wondered about that. If the comet nucleus was small and bright enough could you pick it out with the guide camera and just guide off that?

I guess it all depends on how small the nucleus is, f-ratio and focal length of your scope. it if looked star-like enough it might work, but would it be bright enough to be picked up by the guide cam? Suspect not in most cases, and once it does get bright enough it's likely to be too big and/or not star-like and so not much use for guiding. The metaguide approach seems to be that you still guide by tracking on a star, but it adds the known movement rate of the comet to the 'lock' position of the star so that it follows the comet even though it is actually guiding on a star.

[russellhq]

What happens to the stars if you guide off the comet? Would they end up trailed?

[IanL]

Yes they will end up trailed if the exposure is long enough, but that is what makes these pictures interesting, since it shows in an obvious way that the comet is moving differently to the background stars. How long "long enough" is depends on the rate of movement of the comet, which in turn depends:

- Distance from the Sun - when it is far out from the Sun it will be moving slower and then speed up as it swings in, around the Sun and then slow down on the way out again.

- Distance from the Earth - objects further away from us appear to move more slowly against the background sky than objects near to us.

- Angle of view of the orbit. If the object is coming towards us, or going away, then its apparent motion will be slower than if we are seeing it pass by. (Think of a car passing you on the road, when it is a long way away it appears to move slowly relative to the landscape, but as it passes right by you it whizzes past).

Generally speaking, when the comet is far away and very faint, it is moving slowly, which allows the longer exposures with less or no trailing. When it is nearer and brighter, it will appear to move faster, and if you need to to a long exposure then one of these comet tracking/guiding processes becomes much more necessary.

[badgerchap]

I suppose you could use a star mask from a shortish exposure if you wanted to remove the trailing, but what merit this would have I don't know.