Camera Dipping

[MikeeJC]

Hi.

I've just started getting into taking photos of the night sky using a DSLR.  I'm currently having issues with the camera 'dipping' slightly once I've framed the star/planet, locked the head and then gently let go of the camera body.  It usually goes out of frame if I use a long focal length.  I used to use a cheap pan & tilt head but have just upgraded to a Sirui K-30X ball head.  The issue isn't quite as bad now but hasn't gone away completely.  Is this normal and is there a way to stop it from happening?

Any help appreciated.
Thanks, Mike

Edited August 11, 2020 by MikeeJC

[vlaiv]

Hi, welcome to SGL.

Planets and stars move across the sky. This is due to earth's rotation about its axis. Your camera is probably pretty solid - but things simply move out of the frame because - well, they move all the time.

Not everything is moving at the same rate though and the time it takes for it to leave the frame depends on focal length - as you've noticed.

In a nutshell - this is what happens. Sidereal rate is about 15 arc seconds per second. Arc second is very small measure for angles - 360 degrees in full circle, 60 arc minutes in one degree and 60 arc seconds in one arc minute.

Depending on the size of your sensor and focal length that you are using - you can calculate how fast things will move out of the frame.

http://www.wilmslowastro.com/software/formulae.htm#ARCSEC_PIXEL

For example - Canon 750d with 200mm lens will be at 3.82"/px. It has 6000px across the sensor, so that is 6000 x 3.82" = 22920"

If we center planet and then let it drift - it will drift for half a field of view or 11460 arc seconds. At speed of 15"/s it will take 764s for it to drift out of FOV.

This is strictly true only at celestial equator. Once you start moving in declination (towards Polaris) - this apparent speed drops with cos(declination) factor. At declination 60 degrees it will only move at half this speed - ~7.5"/s.

In any case, if you want this to go away - you need an astronomical mount that can track for you. In particular you want something called equatorial mount.

There are very nice compact - star tracker type mounts that will track the stars for you, like this one:

https://www.firstlightoptics.com/skywatcher-star-adventurer/skywatcher-star-adventurer-astronomy-bundle.html

What focal lengths do you use and what do you want to image?

[Tenor Viol]

Hi Mike

What focal length are you using? The Earth rotates 15 degrees an hour or 1 degree every 4 minutes - that's twice the width of the full Moon in 4 minutes. At around 900mm the Moon will fill a full frame completely.

If your lens is long and heavy then the leverage will be significant and will require a very robust mounting mechanism to stop it sagging