Total Newbie Query - is this drift or something else?

[Dedanann]

hello, I'm wondering if anyone can let me know if I am merely getting these trails and "double" stars in some of my long exposures because of drift or is it something else?

Full disclosure: New to AP, have only just purchased a SW PDS Newt and using Full Frame DSLR on IV set to 90-120sec exposures (f/2.8 on cam, ISO 100-800 [varied] and WB = K), I am not using any guiding equipment yet. The Mount is an EQ5 and it is most definitely polar aligned as best possible without any electronic assistance, merely using a polar finder and illuminator for now, but it's within reasonable alignment and some images come out amazing. The Tracking is a Sky Watcher RA and Dec motor that seems to work very well, or at least, I think it does, but using live view on the camera shows the focus star drifting, usually to the east, and this makes me wonder if the motor control unit could be at fault, or is a worm/gear issue or is it merely drift? I was tracking Arcturus tonight as it is a very easy star to track, but similar to when tracking Vega last night, going over 30seconds in exposure and I got these trails and arcs... so, maybe it is merely drift and I need to invest there, yes, yes, I understand the pros and benefits   but I just want to be sure it's not a mount and worm issue. The mount is level, on grass and has been polar aligned as best possible.

Anything anyone can tell me would be great. Again, total beginner. Thanks. 

[Dedanann]

for reference, same camera, same focus and nothing changed on crayford, same ra position (unsure about dec) and nothing touched, except exposure time on IV - which is just under 6 seconds on this to see if the camera was indeed focused - no filters on the camera obviously. but this image came out reasonably sharp for a dslr I thought, and next to no trailing on it. Actually, this was one of the first ten images I shot tonight at fast shutter speed and they all look pretty much the same. 

[Elp]

On IV? What is that?

Are you imaging through the scope? Increased focal length makes tracking more demanding.

Without autoguiding your best method to make sure your polar alignment is bang on is drift alignment (DARV method).

[Dedanann]

13 minutes ago, Elp said:

On IV? What is that? = Intervelometer 

Yes, imaging "through the scope" using a 2inch connector on t-ring. Focuses just fine, but the trails and double star made me think it must be drift, even though it has a tracking motor on it, obviously computer aided guiding is best, but will get to that much later on, in no hurry for that, but just wanted to be sure it isn't my mount and worm gear.

DARV Method, thank you for that, I will investigate and take my time to see if I can get something better with this until I get autoguiding later on, maybe the big fat fella in the red suit might be nice to me this year, we'll see

Edited June 21, 2023 by Dedanann

[Elp]

Double stars usually mean vibration of some sort (vibration will also show a non straight line). I'd also try shorter exposures, on a windy night for example I usually use 30s even at short focal lengths and this is with autoguiding. Shorter exposures overall in consideration of total imaging time will have more noise in the total stack compared to longer and less images in a total stack, but from my experience they'll both have noise regardless and a noise reduction post process will need to be added anyway to smooth the end result, I'd rather have more subs with proper stars.

Edited June 21, 2023 by Elp

[Steve Ward]

2 hours ago, Dedanann said:

DARV Method, thank you for that, I will investigate and take my time to see if I can get something better with this until I get autoguiding later on, maybe the big fat fella in the red suit might be nice to me this year, we'll see

Here you go ... 

https://www.cloudynights.com/articles/cat/articles/darv-drift-alignment-by-robert-vice-r2760

[michael8554]

Here are two sources of star trailing in an unguided mount:

1)  Polar Alignment, causes drift in Dec.

That would be "up and down" if your camera was oriented with the long side of the sensor parallel to RA.

Unguided the PA has to be much better than when guided.

Such that your 120 sec exposure shows no drift.

2) Almost all mounts have Periodic Error in RA.

A repetitive wave-like drift of RA towards east and then towards west ( "left to right" ) with a period of several minutes, the duration of the "worm period " of the mount, maybe 4 minutes in your case.

When unguided all you can do is find the slowest exposure that doesn't show drift.

You've already found out that 6 seconds doesn't, but you should be able to expose for longer.

Michael

Edited June 21, 2023 by michael8554

[AstroMuni]

8 hours ago, Dedanann said:

The Tracking is a Sky Watcher RA and Dec motor that seems to work very well, or at least, I think it does, but using live view on the camera shows the focus star drifting, usually to the east, and this makes me wonder if the motor control unit could be at fault, or is a worm/gear issue or is it merely drift?

In my experience I get trailing if I increase exposure beyond 30s without guiding on my HEQ5 pro. On some days I can get upto 60s but thats rare. So I am guessing that with decent polar alignment you should be able to get around 30s too. Try using a Y mask or a Bahtinov mask to improve focus and at a later point you could invest in a Coma corrector to improve the elongated stars in the corners. Good luck!

[Dedanann]

really appreciate the advice. Thanks folks. Lots to work with now.

[Dedanann]

2 hours ago, michael8554 said:

Here are two sources of star trailing in an unguided mount:

1)  Polar Alignment, causes drift in Dec.

That would be "up and down" if your camera was oriented with the long side of the sensor parallel to RA.

I'll do an experiment tonight to see exactly what is what, and I'll leave Dec motor detatched to see if there is the same result with just RA, and then the vibration is something I'll change later too.

not so bothered about the actual imaging results at the moment per sé, just getting to grips with Astro process. I've been photographing and filming for over 30 years, and done basic slr and lens AP in the past using the 600 rule without a tracker and got some okay pics, but never used a telescope until now, and defo not a newt and slr combo.  So, getting balance and everything right first is my main objective, later on I can worry about more equipment and so on and better images, it's about the enjoyment of the process for me, so this is great to learn. 

really appreciate the advice

[Dedanann]

9 hours ago, Elp said:

On IV? What is that?

Are you imaging through the scope? Increased focal length makes tracking more demanding.

Without autoguiding your best method to make sure your polar alignment is bang on is drift alignment (DARV method).

the scope is only a 130/650 (and most likely not really 650 as the tube is slightly shorter, so thus the focal length will be slightly shorter too, but that's good for AP no?) 

[AstroMuni]

7 minutes ago, Dedanann said:

the scope is only a 130/650 (and most likely not really 650 as the tube is slightly shorter, so thus the focal length will be slightly shorter too, but that's good for AP no?) 

if its the 130pds then the FL is very close to 650. The shorter tube is just to allow cameras enough room to reach focus.

[Elp]

650mm is fine, but still considered to be around medium FL, a short FL will be around 300-400mm, for dslr lenses around 50mm (24mm or less wide, 100mm+ telephoto). Good quality planetary imaging is around 1500-2000mm plus (double if using a 2x Barlow, I've also done it with a 370mm refractor with 2x barlow) to give you some context, but for planetary you just need to keep the planet fairly central in view, for DSO tracking needs to be more accurate.

The larger the scope the more critical your balancing needs to be in both RA and Declination. Newtonians are also more prone to movement from slight breezes due to their size.

Edited June 21, 2023 by Elp

[Dedanann]

I finally managed to get one decent photo without any guiding and a pretty good Polar Alignment on the recent night of the Sep New Moon. This is a stack of 6 x 3min exposures on M81 and M82 (with what I think is NGC3077, but don't quote me). Rather happy with that on a full spectrum unmod'ed DSLR and unguided. So with lots of work I finally got the right spot in my garden that is just about level enough to get good PA, but I am gonna build a solid 8inch pier to put the mount on and have permanently Polar Aligned, and then build a wee small retractable roof shed for it all. So nice to finally get something good through the 130/650pds, visually the optics are lovely and now that I know I can get something good with the DSLR, I am modifying an old D5300 with an IR Cut Filter and getting a choma corrector to start off with. Such  a tantalisingly rewarding hobby this. 

 I got a nice snap of the 7 sisters in the Pleiades, but no filter or anything so it simple caught sharp stars and not much else. Still nice though.