Nexstar 8 and Atoguiding

[Zakalwe]

Noobie question time again......

I am starting to dabble in astro-imaging. My NexStar has an autoguiding port on the mount. Given that it is an alt/azimuth mount, is there any advantage to buying/building an autoguider? I would have thought that field rotation will limit the exposure length, which would mean that autoguiding would be superfluous?

Any thoughts or comments?

[SteveL]

Am pretty sure that the Celestron AltAz mounts will refuse to accept ascom pulse guide commands if its in Alt/Az mode (as my Nexstar 8 GPS used to bitch about). Unsure if it would accept commands via the ST4 port in AltAz.

[Zakalwe]

So is the autoguider port only really useful if the scope is mounted on a wedge?

[SteveL]

I cant see how autoguiding an AltAz is useful in the first place If you have the NS8GPS and want to do long expsoure imaging, you will need a wedge... or move to an EQ based mount.

(And I am speaking from experience of owning a NS8GPS)

[Zakalwe]

I cant see how autoguiding an AltAz is useful in the first place If you have the NS8GPS and want to do long expsoure imaging, you will need a wedge... or move to an EQ based mount.

(And I am speaking from experience of owning a NS8GPS)

Yeah..that's what I was thinking...why autoguide on an alt-az, when field rotation would scupper any long exposures.

[MartinB]

I autoguided a wedge mounted NS8. The port accepts standard ST4 commands. It actually guided very well, the periodic error was very good.

The pic below shows my old set up minus the ZS66 which was held in the rings. The rod below the scope took a counterweight to balance the system. The motors slewed ok but I did slow the slew rate down to reduce any strain on the gears.

The crescent image was taken using this set up. I used a focal reducer and the focal length at the spacing used was 1800mm. 12x10 minute exposures. It was early imaging days for me and it is slightly out of focus but the stars are round and shows the quality of tracking the scope and mount can deliver

A much better arrangement is to use an off axis guider which gets around the clumsiness of piggy backing and also problems with mirror flop (which my NS8 never actually suffered from). The M27 image below was taken using an SX Active optics system which uses an off axis guider and a tilting mirror to rapidly correct guide errors. This is a brilliant system but is complicated to use and it's probably best to get to grips with ordinary off axis guiding first. I think the pic shows that the optics are well up to the task.

The point is that the scope tracked very well indeed. I measured the PE at +/- 10arc secs which is amazing, better I suspect than most EQ6s.

If you are starting from scratch then an equatorial mount is a more flexible choice but if you have an NS8 don't let people try to pursuade you that you can't use if for long exposure astroimaging. You have the added benefit that the NS8 is a much more pleasant scope to use visually than any equatorially mounted set up, even when it's on a wedge. The down side is that decent wedges aren't cheap:(

I eventually spent a lot of money on a Tak EM200 since I wanted a wanted to use a wider range of scopes and after this it didn't make sense to keep the NS8 for imaging.

[Zakalwe]

Martin, Steve,

Thanks for the replies, much appreciated.

I think that I will have to decide what I want to do. I could get a wedge for the NS8, which comes with some drawbacks.

Or I guess I could de-fork it, get a dovetail from somewhere and mount it on an eq mount (I have a focal reducer on it's way to me to speed up the 'scope).

Or I could go down the dedicated imaging scope.....the Orion Optics 10" is giving me strange stirrings in my loins.......

[MartinB]

Another alternative is to keep the NS8 and use for visual and planetary and sort your self out with a low cost DSO set up using small refractors. Don't be in a hurry to ditch the NS8. You don't need to spend a fortune to get a decent set up especially if you are prepared to buy second hand. Old Vixen polaris mounts are great performers.

[Zakalwe]

Another alternative is to keep the NS8 and use for visual and planetary and sort your self out with a low cost DSO set up using small refractors. Don't be in a hurry to ditch the NS8. You don't need to spend a fortune to get a decent set up especially if you are prepared to buy second hand. Old Vixen polaris mounts are great performers.

Martin,

Thats sorta along the lines of what I was thinking....I can either retain the NS8 for Lunar and planetary stuff, and get a DSO setup. The low cost of the Orion is attracting me...the 8" got a very good write-up in November's Astronomy magazine.

The alternative is to either wedge the NS8 (expensive for what it is, awkward to use, and I lose the "grab and go" accessibility of the fork mounted NS8), or to de-fork it and stick it on a guided eq mount.

Decisions, decisions,

[MartinB]

The problem with a 10" newt is that it is quite demanding of the mount, not just the weight but also the size (windage and leverage). The focal length is quite long which also puts heavy demands on mount quality.

The wedge set up is far from awkward to use compared with an equatorially mounted 10" newt. It has the big plus of not needing a meridian flip and a disadvantage of not being able to image close to the NCP because camera gear catches at the bottom of the forks.

The Orion is a fine scope as are the Skywatchers but that choice of scope will be significantly more challenging and expensive (mount wise) than a small refractor. A small refractor is one of the best imaging tools known to man. Don't be deceived by small aperture, these scopes can be fast and deliver a large field of view. My most used scope is a 106mm refractor (a very expensive one but that's another matter!)

[Zakalwe]

Arghh....too many choices!

And your 106 Tak.......way way out of my budget. A very drool-worthy piece of kit though:icon_salut:

[SteveL]

...and I lose the "grab and go" accessibility of the fork mounted NS8...

Indeed you do, it turns into some kind of Borg creation with a wedge...

[MartinB]

But it doesn't need to be a Tak! An Skywatcher ED80 is a great scope and now there is a dedicated field flattener it performs superbly across a large chip. They also keep cropping up second hand. Astroimagers are gear incontinent, there is a steady stream of perfectly good stuff being discarded for this months latest upgrade. The beauty of this is that you shouldn't try to go for a once and for all purchasing solution. Cut your teeth on relatively inexpensive but high quality kit and then you will be able to make your own choices in future depending on which way you want to go. When you come to sell on you will hardly be out of pocket.

[Zakalwe]

Indeed you do, it turns into some kind of Borg creation with a wedge...

I am sure I can see a couple of square-inches where you haven't stuck something on to that poor NexStar. Tsk, tsk.....amateurs:p:D

[dazza1639]

Sorry to resurrect an old thread but it seemed appropriate. I'm looking at the new atik gp guide camera and it doesn't have an st4 output, so my question is can an equatorial wedge mounted Nexstar GPS be pulse guided? I use nexremote on the computer connected through the handset via USB serial converter.

Thanks for your help

Darren

Darren

@SalAstroSoc & @astronut1639

www.salfordastro.org.uk