Trails where there should be no trails

[SteveBz]

Hi Guys,

So last night was a lovely night in West Sussex.  I'm hoping for the same tonight.

In the absence of a proper SDLR, I had my phone strapped to the eyepiece which works for the Sun and Moon, but not for much else!  Anyway here is a 60 second exposure of Albireo.  My Sky Scan 2001 EQ3-2 mount has dual axis drives, so why any Star Trails (see below).

Regards,

Steve.

[Cornelius Varley]

I wouldn't rule out star trailing simply because the mount is motor driven, this can still happen if you are not properly polar aligned or the motors are not tracking correctly. It could also be a little vibration in the mount.

[MattJenko]

That trailing may well be because of a bounce, when the whole setup has been nudged or jerked, probably when you pressed the button. Do you have the ability to take a photo without touching the setup at all? 

[SteveBz]

14 minutes ago, Cornelius Varley said:

I wouldn't rule out star trailing simply because the mount is motor driven, this can still happen if you are not properly polar aligned or the motors are not tracking correctly. It could also be a little vibration in the mount.

Could be both. I can easily check alignment.

How can I check tracking? Maybe with a 4mm lens on an easy star, Vega maybe?

[SteveBz]

3 minutes ago, MattJenko said:

That trailing is because of a bounce, when the whole setup has been nudged or jerked, probably when you pressed the button. Do you have the ability to take a photo without touching the setup at all?

I can do that with a pair of head phones, but it doesn't happen with short exposures. It could be I bumped the tripod though.  I'll try again.

[SilverAstro]

To test for ( guard against) bounce hold a card screen in front of the scope an inch or three away. Press buttons etc, wait a moment or two for vibes to die down, move screen away ( without bashing the scope lol !  ) Replace screen in front of scope before the end of exposure.

A third hand is useful, or the kids would love to help

Thinking aloud : Can drive error be determined by turning off the drive for say 5or10sec, then on for 40sec, then off again for 5or10, (assuming 60sec is your max exp. ? experiment with times) that should make two 'ears' on the wiggle, if they all point the same way then drive rate error would be probable ?

[Steve Ward]

Try using the self-timer on the phone's camera , set to 3 or 4 seconds and you can press the shutter button gently by hand safe in the knowledge that any vibrations will have died down by the time it actuates. If your phone does not have one built in you can download an App thingy for it.

But remember that even if the tracking rate is spot on ( they usually are ) you will still get trailing after 60 seconds if you are not very accurately Polar-Aligned .

[ollypenrice]

First of all no mounts can provide photographically accurate long exposure tracking without being guided. The very expensive (£6000 to £20,000) 'unguided' mounts do, in fact, guide themselves from their own in-built absolute enconders. Most of us use an autoguider which takes test shots of the starfield every second or so in order that the drift can be measured to sub pixel level and a correction sent to the mount.

SilverAstro's trick with the card is usually known as the 'hat trick' because it can be done with your hat. The niftiest way, though, is to use an umbrella which you can hold in front of the objective while still holding the handle and remaining at the camera end yourself!

Looking at the nature of your trail we can come to some pretty secure conclusions. This is not much connected with PA. The arcs of the trails have too tight a radius for that. We see that, at each end of the arc, the star image is brighter and larger. The camera has therefore spent longer at each end of the arc than it spent moving between them. This may well be due to backlash in the gears since this is a well known visual result from a mount which oscillates across its backlash between the end positions at each side of backlash. Or it could just be a bump. You can feel for backlash simply by wiggling the scope very gently in RA and Dec.

In diagnosing these faults it is an enormous help to have the camera orientated, landscape or portrait, along RA and Dec. (Just make sure that one side of the chip is lined up with the counterweight bar.) That way you know which axis is causing the problem, RA or Dec.

Olly

[SilverAstro]

Hats are best kept for eating later when suggestions dont pan out !

RA orientation is why I suggested turning off the drive for a moment   I did inspect the dim stars ( so as to get a better idea of what was going on under the over-exposure) and thought at first we had a kinky gear train prob because of the jiggle in the middle of the lower one of the two, but it is not showing in the upper one, so do we have bad seeing as well to compound things ?

[SteveBz]

Hi Olly,

Thanks for your very detailed response. I can at least orient the camera (phone).  There is some play in the dec gear especially which probably needs adjusting.

Forecasr is good for tonight so I'll investigate further then.

 

Steve.

[SteveBz]

Hello Guys,

Well I spent at least an hour adjusting the polar finder scope with next door's television aerial (as per astronomyshed's wonderful youtube chanel), followed by compass alignment and then polar alignment when polaris was visible.  The result was no trails! But clearly eyepiece projection with my cellphone is not the right way forward. I probably need to invest in an dslr sometime.

Thanks for everything guys.

Steve.