Help with auto-guiders

[Athena]

Hello all

I'm starting to get quite interested in DSO imaging, I have tried quite a few DSO's now but am plagued with what I think is period error. Generally if I expose for around 10 seconds I get quite a bit of image drift. I don't think this is field rotation per se.

So, unless my reasoning is flawed I see three options open to me:

1] Buy a EQ mount (expensive option)

2] Buy a EQ wedge (fine but I have read some bad reports)

3] Auto-Guiding

With reference to auto-guiding, for example the:

Opticstar AG-130M COOLAIR auto-guider.

Would I still need an EQ wedge or would this guider work fine with the telescope mounted alt-azimuth? According to the documentation this guider will work with my NexStar 8SE but would it "actually" work?

Thanks everyone

Nikki

[adamsp123]

Nikki, I believe you will have problems with field rotation using an altazimuth mount, really I do think for imaging you need an equatorial type mount. If you do go down this route save your pennies and get the best mount you can afford so it is future proof if you end up with heavier loads on the mount. A wise piece of advice I was given that for imaging build from the mount up.

For a guiding cam many use the cheaper QHY5, as I have started doing, and PHD or other similar guiding software. Talk to Bern at modern astronomy for more on this.

I am no expert and learning the hard way, I am sure more experienced folk will offer you some sound advice.

Pizza

[Whippy]

Without not know what exact mount/SCT you have, but I would imagine you could buy a wedge that would make your mount much more suitable for imaging. I've never used one but several people do with pretty good results.

For autoguiding, I think the main two cameras would probably be the QHY5 or Meade DSI. The QHY5 is simplicity itself (I had mine running in 10 minutes) but the people say the Meade is more sensitive (makes guide stars easier to find) but you'd have to buy an extra interface to make it work for guiding. I use PHD for the software and again, it's really simple (PHD stands for Push Here Dummy!) but other use Maxim, guidedog and several others...

I know a friend who tried and failed miserably to guide with that Opticstar camera you mentioned. It appeared that it's just not sensitive enough for guiding purposes.

Tony..

[RobH]

Hello Nikki,

With an alt/az mount, you will only be able to go for a maximum of 30 seconds without seeing field rotation creeping in.

This will manisfest as the stars seeming to trail around the centre of the image. Guiding won't help.

Periodic error is caused by slight defects in the worm gear, and tends to repeat after one turn of the worm cycle, generally around 8 minutes or so. The better the mount, the less the PE, but they all have it to some extent, and this is generally trained out by recording it and then correcting for it.

As Tony says, the general concensus on guidecams seems to be the QHY5 or the Meade DSI.

The QHY5 has a built in guide port, the DSI doesn't.

I use a DSI pro mk1, but there isn't a problem with it as you simply connect to the computer, connect the computer to the mount (the Shoestring astronomy GPUSB interface is the way to go for this) and away you go.

If you are imaging with the SCT, you will need a wedge, but maybe have a go in alt/az first....I got some great results with a 10 inch SCT using 21 second subs but getting lots of them....this helps to make up for the short exposures.

If you find that you're enjoying it enough, and have got to the limit of what you can achieve with short subs, then move on to bigger things!

Cheers

Rob

[Beyond_Vision]

I would also add that you cannot use autoguiding in alt/az mode.

Regards

Kevin

[Athena]

Thanks everyone

It's the standard Celestron NexStar 8SE mount. My intentions were to improve tracking for short duration exposures (no longer than 30 seconds)

I know that field rotation is unavoidable on a alt/az mount. I was hoping that auto-guiding would improve tracking and help to remove period error. Currently only 5 images in 10 are suitable for stacking, I would like to get a better success rate.

I have found that anything over 30 seconds (longer as I approach the horizon) a small amount of field rotation creeps in. This increases over time but I can use software to remove this rotation.

Will auto guiding eliminate tracking errors?

I would also like to hear from anybody that has used a wedge with my equipment, it's the one with a single fork. I currently have an opportunity to borrow a wedge if I so needed.

I don't really want to spend a fortune on an EQ mount if at all possible. I have looked at some prices and for the equivalent equatorial GOTO mount is quite expensive. Unfortunately it would have to be goto as I don’t yet have the experience to find objects without it.

Thanks

Nikki

[adamsp123]

seen wedge this on ebay

http://cgi.ebay.co.uk/CELESTRON-WEDGE_W0QQitemZ270420521010QQcmdZViewItemQQptZUK_Telescopes?hash=item3ef651b032&_trksid=p3286.c0.m14&_trkparms=65%3A1%7C66%3A4%7C39%3A1%7C240%3A1318%7C301%3A0%7C293%3A2%7C294%3A200

[roundycat]

Nikki, re your last post above;

you are right about field rotation and alt/az but it can be a problem if you are not aligned closely to the pole even though using an equatorial mount. Auto guiding will improve tracking and it could remove all periodic error but it depends very much on the mount.

Why are only 5/10 images suitable? If the whole set up is giving you a good result once why not every time? Is this due to something more local such as ground vibration (you stamping your feet in the cold) or an effect of the wind.

I would be cautious about using software to remove field rotation. It might be able to 'round-up' stars but what is it doing with the nebulous bits. It is a little bit like getting rid of one fault by introducing another. Get rid of the field rotation and forget the software solution.

Dennis

[jgs001]

Nikki, whilst there are limitations in AltAz for field rotation, the area of the sky in which you are aiming will have a marked effect on the sub length before rotation becomes a factor. North/South and in a line between them up to the Zenith is the shortest subs. Low East and West provides the longest subs.

This will give some idea. Everything I've read indicates that the focal length is not a factor in field rotatio, but it will potentially show up issues with drive errors far more than a short focal length tube. I have used 2 minute subs low in the east with no sign of field rotation using an SLT AltAz mount, drive errors meant I lost 50% of my subs though.

[Catanonia]

Don't go with the opticstar. I have the more sensitive BW (mono) version and I am having heaps of problems with it. So much so I am giving it them back for a refund and getting a QHY5 probably.

[Merlin66]

Kevin,

Sorry to correct you, but you can auto guide in Alt-Az.

I use the QHY with a beamsplitter on the 10" and 12" LX200 in Alt-Az mode to guide my spectroscope. I can get good guiding for up to 20mins on a 30 micron slit.

Field rotation obviously is still an issue ( BTW I have a brand new Meade De-rotator for sale....) but definately 100% you can auto-guide in Alt-Az mode.

[Bigwings]

I asked David Hinds for advice on imaging with my SE8. Here is the reply. I hope you find it of use

In order to do any longer exposure imaging, you require a wedge for the 8SE. Tripods & wedges Accessories Celestron wedge, nexstar 6se and 8se

In order to guide for CCD imaging, you will need a guidescope. Any small and lightweight optical tube assembly will be OK. The only issue is fitting guidescope rings onto the SE body. The SE telescopes are not designed primarily with long exposure astrophotography in mind. Although long exposure astrophotography is possible with the 8SE, it requires some modification of the tube assembly to accept a pair of guidescope rings. It is also advisable to place a counter balance on the under side of the tube, to counter the weight of a guidescope, its rings and a lightweight guiding camera - e.g. - Photo & visual accessories Accessories Lvi smartguider

With the SE, it is best to use an SLR digital camera at the eyepiece position, to take astrophotos, to discover what issues there are. If you mount guidescopes and heavy CCD cameras onto the 8SE, you may find issues with tracking and pointing accuracy if too much weight is added. Try a digital SLR and one of these guiders - Photo & visual accessories Accessories Celestron radial guider

You might also want to use a reducer for wider angle images - Photo & visual accessories Accessories Celestron reducer - corrector

Please do not hesitate to contact me again should you have any further questions.

Kind Regards,

Chris

DAVID HINDS LTD

[Athena]

Field rotation obviously is still an issue ( BTW I have a brand new Meade De-rotator for sale....) but definately 100% you can auto-guide in Alt-Az mode.

Hi

Would a field de-rotator solve my problem? The concept of rotating the camera in the opposite direction seems sound but what does that do to the DSO being imaged? I assume that the DSO is rotating at the same rate as the star field!

Thanks

Nikki

[RobH]

In theory de-rotators are fine, but ask yourself why virtually no-one uses them?

Very tricky to get right apparently. It'd be interesting to hear reports from anyone who has used them.

If you're alt/az mounted, forget guiding. Your exposures will be short enough not to benefit from it, and if you've got trailing under 30 seconds, something else needs looking at.

If you want longer exposures, get a wedge and make sure it's aligned as precisely as possible to the pole.

Once that's done, triain the periodic error out of the system and see how long you can go unguided and keep tweaking things until you can't get any further. You'll find plenty of help here.

You should be able to get a couple of minutes I would have thought.

After all that is done, then look at going down the autoguiding route.

Be warned....autoguiding can have its own set of frustrating problems too, so the better you can do without guiding, the easier it becomes when you start.

Cheers

Rob

[Psychobilly]

Use the SE mount for Visual and get an EQ6.... get it connected to a lappy using EQDIR & Ascom drive it from your fav astro ap and Bobs your mothers brother...

Wouldn't a de-rotator need to plug into the mount as it needs to know where the scope is pointing.. they are an option for Meade's but haven't seen one for a Celestron... especially the Baby ones...

Peter...

[Merlin66]

Rob,

At bit OT, but the Meade de-rotator is a "plug and play" no fiddle no setting-up, nothing, it gets it's signal from the LX200 so it knows where the telescope is pointing and the degree of de-rotation required. All automatic; the user is protected from all the hard stuff.

Unfortunately only the Meade LX200 has the interface required.

[RobH]

Did you find it worked OK? I know that they are used in all the big observatory scopes these days (keck etc) so they obviously can work fine.

Cheers

Rob

[Beyond_Vision]

Rob the Meade derotator costs nearly as much as a good wedge and introduces a third motor which will have inherent errors.

Regards

Kevin

[Merlin66]

Rob/ Kevin,

The de-rotator works as advertised. No issues what so ever.

The "another motor another problem" I think is wrong. No more than a filter wheel, or a focus motor etc. What is not appreciated is the the fork mounting in Alt-az is far more stable with much less vibration than when it's mounted on a wedge. ( I know because I have both the 10" and 12" Lx200 and HD wedge.) The balancing is only in one axis not two so once again much easier.

The reason I don't use it, is nothing to do with it's capability but that my need for the spectroscope doesn't need field rotation correction. I don't do any long exposure imaging anymore.

Sorry for highjacking the thread, but the discussion may help others understand the options available.

[Athena]

Yeah, thanks everyone.

You know, I'm attempting to run before I can walk I think I need to learn my current kit for at least 6 months and then look at getting a good EQ6 mount.

Anyway, I am arranging to borrow a celestron wedge. I'll give that a try and see how it goes.

I think Nikki needs to learn so patience

Nikki

[RobH]

It's all so exciting that we all want it straight away Nikki

Get to know your gear well, and you'll learn loads, and be surprised at what you can achieve. It's really good groundwork.

That said, if I won vast sums somehow, I wouldn't go in small, considered steps at all....I rather like the Hale telescope

I've dug out a couple of my earliest images to give you an idea. These are 21 second exposures using a Meade DSI Pro (now my guidecamera) and a 10 inch alt/az mounted Meade LX200 classic at F10. About 50 luminance subs each, and 20 or so of each colour channel.

I must have a bash at reprocessing them sometime

Cheers

Rob

[simon e]

Well guess what, I have just found this forum and also have a 8 SE with similar issues.

However I am sticking with the mount and gonna have a go at the auto guiding. Setup when the stuff arrives will be.

8SE, with Meade Lunar Planetary Imager (LPI) w/AutoStar Suite, EBAY!! sitting on the top part of a Meade Off-Axis Guider.EBAY!!

With a Imaging source DFK 31BU03.H EBAY!! fitted to the other part of the guider. EBAY is a wonderful place. All going through me laptop. Oh and I have to make up a wedge for it.

I have been taking 20 sec frames without tracking of the Ring Nebula, which actually look ok, so I am hoping that the motors on the mount are ok.

Si