Decisions decisions

[MarkS]

Forget the rule of 600 - I see this rule repeated time and time again in forums.  It dates back to the days of film, trying to avoid obvious trailing on a photographs printed at some standard size.  But we tend to look at images at full size on big monitors where the trailing becomes obvious.

So, I prefer to think in terms of star trail length which is give by own rule of thumb formula:

Star trail length in pixels = focal_length * exposure_time / pixel_size / 15

where:
exposure_time is in seconds and pixel_size is in microns.

If I used the rule of 600 on my 600D at 20mm focal length it would suggest an exposure time of 30 seconds.  But the 600D pixel size is 4.3 microns so this would give a star trail length of :

20*30/4.3/15 = 9 pixels

Do I want star trails of 9 pixels?  Not in general.  That's why I don't use the rule of 600.

To find out the star trail length produced by the "rule of 600" for your own camera then use this:
star trail length = 40/pixel_size

Then you can make your own judgment on it.

Mark

[Kenny]

Thanks all. Very useful clarifications.

Interesting experiment this evening with the Canon 450D and Nikon lens set at 18mm. Steady camera mount, no tracking. ISO-1600. f/3.5. 30 seconds exposure was far too long (unsurprising) so didn't bother this evening. 15-20 seconds gave the best looking image, starting to pick up clues to some Messier objects, but still a surprising amount of star trailing (6-8 pixels on the full size image). 10 seconds wasn't enough, leaving a very grainy dark frame with much less detail than the longer exposure images.

The focus may have been marginally off which would obviously compound the problem. Unlike the other evening where I was able to use this Moon for sharp manual focus, I really struggled this evening to focus on stars (or Jupiter).

[MarkS]

The focus may have been marginally off which would obviously compound the problem. Unlike the other evening where I was able to use this Moon for sharp manual focus, I really struggled this evening to focus on stars (or Jupiter).

Are you using the kit lens (which has no focusing scale)?  Don't rely on the auto focus, even with a bright object like the moon, it does not perform reliably enough.  So set it to manual focus.  The best tip I can give you is to set the zoom (if it is a zoom lens) at one extreme or the other (e.g. 18mm) and then stick on a couple of small strips of masking tape - one on the focusing ring and the other adjacent to the focusing ring so that the act of focusing moves one past the other.  Draw a focusing scale on these in pencil. 



Find the precise focus point on the scale you have drawn by taking a series of test shots on a tripod.  You can do this in daylight on a distant object or on a trailing star field at night.  Once you have found focus then tape the ring precisely in that position so it can't be accidentally knocked.

As you perform your test focusing, always move the focusing ring in the same direction (either clockwise or anticlockwise) because the lens mechanism has a fair amount of backlash so if you mark an exact focus point and then approach the same marked point from the other direction then you won't be exactly in focus.  I actually draw an arrow on the masking tape to indicate the direction to turn the focusing ring.

This might all sound like a palaver but seriously, focusing is by far the biggest issue that folk have, precisely because they don't tackle it in a careful systematic manner.  The kit lens can produce quite extraordinary results when used carefully.  By the way, if you want to use the camera at mid zoom then tape the zoom ring into a fixed position before using the same procedure for setting focus.

I tape over the AF/MF switch also so it reminds me to remove any tape fixing the lens in position before using it in autofocus again

The other big tip is to use a dew band around the lens because generally speaking the lens will dew up very fast in typical UK imaging conditions.

Mark

[RobertM]

For focus I rely on the fact that nearly all lenses produce different colour fringing intra and extra focal.  I focus till that colour fringing is minimised.  Admittedly finding a suitable star might be a challenge in a wide field but live view zoom makes all the difference.  If I can't use a mask (Bhatinov) then that's the method I use.

HTH
Robert

Just to qualify, in using this method the star has to be close to the centre of frame.  If you use one near the edge then that will likely be a slightly different focus position ie less than 'perfect'.

[Kenny]

Thanks.

So far have avoided using the kit lens for night photography as a couple of reviews said it wasn't very good. I'm using the Sigma DC 18-200mm lens which is only manual focus. It has only a very basic focus scale, nothing you could accurately use so your idea of using tape and focusing in the daytime is definitely worth a try. I can't see much through the viewfinder or Liveview, certainly not enough to focus on anything but really bright objects (like Jupiter last night) but even that was a challenge. I was more or less going to infinity then left a bit. p.s. also taking a shot then checking the focus by zooming in with the playback mode.

What is a dew band?

I took a few frames of Orion and Canis Major last night. Having problems stacking them with Registax (another story) so tried manually stacking them as lighten layers in Paint.net. Interestingly, and annoyingly, the stars don't align across the whole frame. When I align the stars in the central region, the further out to the edge you look the greater the alignment gap (very noticably). Not sure if this is an error I've introduced. Going to have another go.

[MarkS]

What is a dew band?

I took a few frames of Orion and Canis Major last night. Having problems stacking them with Registax (another story) so tried manually stacking them as lighten layers in Paint.net. Interestingly, and annoyingly, the stars don't align across the whole frame. When I align the stars in the central region, the further out to the edge you look the greater the alignment gap (very noticably). Not sure if this is an error I've introduced. Going to have another go.

A dew band is one of these:
http://www.firstlightoptics.com/dew-prevention/astrozap-dew-heater-tapes.html
They need a 12v power supply

Lenses have distortions - that's why the alignment is failing as constellations pass through the field of view.  Most stacking programs will do their best to correct the distortion they find.

Mark

[Carole]

I took a few frames of Orion and Canis Major last night. Having problems stacking them with Registax (another story)

Kenny, have you tried stacking in DSS (Deep Sky stacker), Registax is really for stacking AVIs I didn't know you could stack DS images in it.  DSS is free:
http://deepskystacker.free.fr/english/download.htm

I thought there was a more recent version than this, but this is all I could find.

In this you can load your lights, darks, flats and bias.

Carole

[Kenny]

I took a few frames of Orion and Canis Major last night. Having problems stacking them with Registax (another story)

Kenny, have you tried stacking in DSS (Deep Sky stacker), Registax is really for stacking AVIs I didn't know you could stack DS images in it.  DSS is free:
http://deepskystacker.free.fr/english/download.htm

Oooh. Thanks. Will give that a go later today.

[Kenny]

What is a dew band?

A dew band is one of these:
http://www.firstlightoptics.com/dew-prevention/astrozap-dew-heater-tapes.html

What size would fit a dslr.

Do you attach it to tbr front or nearest the camera body?

[MarkS]

It wraps around the cylindrical lens body, as close to the front lens element as possible (since you are trying to heat the front lens element to prevent its temperature dropping below the dew point).  If your lens body is 3" in diameter buy the one for a 3" telescope.  If it is 4" in diameter buy the one for a 4" telescope etc.

You need a 12volt supply to drive it.  A dew controller is an optional extra so you can control the amount of heat it delivers but there's nothing wrong with running it full power.  You may well need full power in any case to push sufficient heat through the plastic body of most modern lenses.

Mark

[The Thing]

What is a dew band?

You can make them quite easily if you have a soldering iron. PM me if you want to have a go as I have instructions and a box of bits left over from my DewItYourself dew heater kit business (now closed) which you are welcome to have a rummage through. That way you can make one just the right size.