Altazimuth tracking drift

[Astroguy99]

Hi all, been looking around here for a few weeks now trying to get my head around how to get started.

I have a Celestron 60LCM refractor and an Altair 130 mono camera which I'm loving using and seeing things I'd never see with my own eyes in suburbia.

My question....

I'm not sure why, but the mount really doesn't track well at all. I've got it connected to Sharpcap and Stellarium for slewing both through the POTH hub driver, but for whatever reason, it really has a hard time.

In fact, I can see the RA and Dec drifting in Stellarium when it does this. I've tried one, two star and three star alignments but nothing seems to keep it on track.

I can't use plate solving very well either because I can't get enough exposure for enough stars without the trails (I'm aware there's field rotation, that's not the issue here).

The mount I definitely tracking as I can hear it, and the trails are a lot less than when it's not. 

I had it working perfectly one time but for the life of me I don't know why slewing afterwards without changing anything else it just went again.

 

I'd appreciate any help anyone can offer to a real noob, but I'm seriously hooked and want to learn :)

[PoI-LordBlackwood]

I haven't had much experience with alt-az electronic mounts, but here's my view of them:

They're great for visual astronomy and help with adjustment of eyes, but not that good for tracking.

In my local astronomy club, I had the opportunity to try out an electronic alt-az mount (pretty sure a Nexstar), but was informed previously not to take photos using it due to the low quality. Being me, I went ahead with the T-mounting process... Not great. After about five shots, I gave up as it was too unstable and I was getting a lot of blur in my photo due to unnoticed vibrations. 

So I have stuck to an equatorial mount ever since. Not an electronic mount though, but stable enough. It's still a chore to manually turn the DEC wheel, but with some practice, I've gotten some good shots. If you have the budget, I recommend an iOptron SmartEQ Pro+ due to its relative affordability and excellence as a travel mount. But if not, then maybe an Orion Skyview GOTO if you're looking at sturdiness. 

Clear Skies 🙂

[PoI-LordBlackwood]

By the way, what are you looking to image? Planets, Nebulae, Galaxies or just stars

[Astroguy99]

Thanks for the info, appreciate it.

I've really only just started, I don't have the budget for more gear after only a week. I'll certainly keep the suggestion in mind for a few months down the line.

I was really only trying to find out if maybe there's a hardware or driver problem since it won't even reliably track for more than a few seconds with a DSLR

[Astroguy99]

Bit of everything really. Mainly stars and clusters at the moment since they're the only things that will plate solve and then track well enough.

Maybe that's the issue, alignment error?

[PoI-LordBlackwood]

Hmm I think that's the case...

Have you taken a wide-field photo of the Milky Way yet? I would say that's a good start to astrophotography

My first photos were from a phone and could only manage a 30-sec exposure, but with a DSLR, you probably can get quite a good photo.

I would suggest checking out the 500 rule for these shots if you're doing stationary shots, but I think your mount should probably do the trick for wide field. Now you just have to mod the camera for attaching to a mount... 

In any case, all the best for your imaging projects!

(Oh you have an Altair 130

Ah well let me find out how to properly align a alt-az mount)

[PoI-LordBlackwood]

 https://www.celestron.com/blogs/knowledgebase/how-do-you-polar-align-using-an-alt-az-mount-with-a-wedge

 

Ah there we go 

Do you have a wedge for your mount or is the mounthead fixed onto the tripod? If so then 😅 I'll have to do further research

[happy-kat]

There are plenty of us having a go imaging with an altaz tracking mount, it's a case of learning to work within the mounts capabilities. There's a thread dedicated to it I'll add it later.

Altaz mounts move in tiny left right up down movements

Getting the balance of what you put on it can be key, take time understanding what works best for your mount

Even though not an equatorial mount, I suspect you still have a set up placement routine to follow, ie level base and in which direction the mount should face and be level before turning on. On my altaz mount I had to program in the latitude. Read the manual for your mount make sure you are setting it up correctly.

Tripod, tripod stability is about the legs more than what flavour of mount head you are using. If you can do not extend the tripod legs fully. If there is a tripod tray put something on it like a brick or bag of sugar, a little weight there adds stability.

Because the mount is not equatorial field rotation will show in images and how far it will show is influenced by the telescope focal length and where in the sky you are pointing, East and West below say 65° will show it least quickly. 

Mono camera is a nice choice.

[happy-kat]

You do not need a wedge to image you do need to work within the capabilities of the mount.

Make sure your setup for your location is correct, and dates are in mm dd yyyy format.

So check your camera cable isn't snagging the mounts ability to track or any other cable.

It's a long thread so you may decide to work backwards.

Link to thread

[The Admiral]

There is an often cited myth that you can't do astrophotography with an alt-az mount. Not true at all as you will see if you look at Happy-Kat's link. For sure there are some limitations, like field rotation and exposure duration, but work within those limits and you'll be OK. If you already have the mount, don't rush out and buy an equatorial (or a wedge), spend some time with it and hone your skills and see if astrophotography really appeals.

I agree with all the points mentioned by happy-kat. My mount used to keep an object more or less centred throughout the session, so it should be possible (I just used the handset to run the mount). Keep your exposures below 30 sec to start with. Also worth checking that your mount is set to sidereal tracking and not planetary or lunar*. Leveling and inputting the exact time and location is also critical, including whether daylight saving time is being used or not.

Good luck.

Ian

* unless, of course, your target is a planet or the Moon ;<)

Edited June 26, 2020 by The Admiral

[The Admiral]

I should perhaps add a couple of other things. As happy-kat said, the tiny zig-zag movements that the scope makes to keep track can lead to a number of frames showing movement, and as a result sometimes a good proportion can't be used in the stack. Another thing is that field rotation is minimal in the Easterly and Westerly directions, maximum in the Northerly and Southerly directions, and increases with the altitude.

However, I do appreciate that you are concerned with the overall drift of the target within the frame with time, which the above won't affect. It may be perfectly adequate for visual observation however, and perhaps that is what the mount is primarily designed for. The scope itself is an f/12 design which will require long exposures on feinter deep sky objects, more than is really usable for alt-az photography, and will be the cause of your difficulty with plate solving. I am sure it would be more than adequate for the Moon, planets, and globular clusters, and possibly some select deep sky objects. Most of us Alt-Az imagers tend to use rather faster optics, say f/6 or faster, often in the form of camera lenses.

Looking on the Celestron site I see the mount does three tracking modes, Alt-Az, EQ-North, and EQ-South. What are the latter 2 modes for? Is it for use on a wedge, not that I would recommend getting one?

Ian

Edited June 26, 2020 by The Admiral

[SteveNickolls]

To add to what happy-kat and The Admiral have mentioned you will get a lot of valuable experience out of trialing your Alt-Az mount and if you catch the imaging bug the experience gained will hold you in very good stead when using other mounts and cameras. There is a book (it's perhaps getting a little old now since many portable mounts like the Star Adventurer type products have come on the market these past few years) that will be a useful bible and it's available from our sponsors-https://www.firstlightoptics.com/books/astrophotography-on-the-go-book.html You would find it very helpful in understanding the effects of field rotation and how to plan around the location and movement of targets over a session. It also goes into how many exposures are enough since your individual sub exposure times with an Alt-Az mount will by necessity be short. Luckily the advent of higher QE sensors with lower read noise have become the friends of Alt-Az imaging allowing many short images to be stacked without recourse to buying up front expensive and large EQ mounts to see if you like astro-imaging.

One more tip, one overlooked aspect of imaging is coming to grips with processing software that suits your style and approach, you have to find which product best works for you. However much progress can be made getting familiar with your software of choice and it can turn a poor result into something much more pleasing. Also as software matures more capabilities become available to create better results. One benefit of imaging is you can always go back and reprocess old images as you gain more experience or the software improves.

I wish you every success in your hobby.

Cheers,
Steve

 

[Oldfort]

2 hours ago, The Admiral said:

Also worth checking that your mount is set to sidereal tracking and not planetary or lunar*. Leveling and inputting the exact time and location is also critical, including whether daylight saving time is being used or not.

I will second this.

The other thing to try is to do your alignment using a reticle eyepiece, which  makes it much easier to get the centreing right.

[Astroguy99]

All fantastic replies guys, thank you.

The EQ N/S is for a wedge, yeah.

I've actually put a spreadsheet together to calculate the field rotation rate for any given latitude, alt and azimuth, which is really helpful for seeing the maximum exposure.

As someone said, the f-stop is a bit of an issue, but with a highly sensitive imager I assumed that'd help reduce the time needed. Light pollution is a bit of an issue here, but I have a screw in filter coming today which I hope helps to some degree.

I've been a bit iffy about using the eyepiece to centre for alignment and used the finder scope instead. I guess that's probably a no-no thinking about it but changing focus to put the camera back in every night probably won't work for me.

Also imaging from indoors without a garden seeing only 30° in alt and 50° in az probably isn't helping getting a decent alignment *anyway*.

I'll do as suggested and learn to hone these skills, following some of those suggestions about how to get better - the handset is set perfectly other than that.

Thanks again everyone, really appreciate it

[Astroguy99]

Just an update.

Slewing is within 2 arcminutes now and tracking is pretty decent, for a cheap scope anyway.

Turns out it was either backlash or poor alignment...or both. Spent a few hours tonight slewing to various stars and adding them to the pointing model in CPWI and I'm really happy with the result. I've set the backlash to 90 in every direction. No idea how it works but it's working.

There's still a fair amount of drift but objects are still in frame after an hour or so, which I can't complain about for the price I paid.

Just thought I'd update as you guys were decent enough to reply to a relative noob to astronomy.

Thanks again!! ☺️

[happy-kat]

That's great to read your update.

[The Admiral]

Glad that you've managed to tame your mount. I think the backlash setting just makes the correction movements take longer so that the mount can take account of the fact that for a short time the drive will be turning yet not actually moving the scope, if you see what I mean.

Ian

[Astroguy99]

9 hours ago, The Admiral said:

Glad that you've managed to tame your mount. I think the backlash setting just makes the correction movements take longer so that the mount can take account of the fact that for a short time the drive will be turning yet not actually moving the scope, if you see what I mean.

Ian

Yeah I think I understand, adds compensation on top. Which is then counteracted by the pointing model in CPWI (I assume), so it all works out.

Would be nice if Celestron actually explained this stuff, especially knowing that people with little to no knowledge will be buying these cheap scopes. Was tempted to return as well! Glad I didn't!

[Dr Strange]

Unfortunately you are limited to short subexposures with an alt/az mount because it does not track in a way that is compatible with how the earth rotates in relation to the sky. On the order of about 20 seconds or less. And there will be drift overall because of the way the mount moves right to left and up/down as opposed to along  right ascension and if applicable declination like an equatorial mount does.  A wedge will rectify this but honestly for many a wedge is a major pain to work with. Many times it is easier, less frustrating, and in the long run more enjoyable to simply get a equatorial (EQ) mount than to fiddle with either alt/az or a wedge. A good entry level model is the Celestron Advanced VX or the Skywatcher HEQ5. Both brands are owned by Synta but for some reason the Skywatcher line tends to perform better. My suggestion is to look at what you would spend for a wedge then apply those funds to a EQ mount. More expensive overall but much less frustration and fiddling with (including getting any backlash out and other things) it to make it work once you have the hang of it.

My philosophy regarding AP is that my hobby is to take "pretty pictures". Anything that isn't taking those pictures is not time well spent and takes away from the hobby. Ergo I do the best I can to minimize those other factors within the budget I have. Unless you are a gear head who enjoys fiddling or you are not willing/able to spend more for a mount designed to image with I would suggest you apply the same philosophy.