Mount Advice

[Shibby]

I've not had my scope long, I'm still a beginner, but having tried my hand at imaging, I'm already sick of the Alt-Az Synscan mount that came with the scope (a SW 130P Newtonian).

Now I've seen a second-hand Celestron AS-GT CG5 GOTO for sale for what looks like a decent price, so I'm hoping you chaps could advise me on whether this would be a good choice?

What I want from the mount:

- To be able to take unguided exposures of a few minutes (I can currently only manage 30s at best with the Alt-Az) using my current scope with Nikon D70 attached. How long can I reasonably expect to track unguided, bearing in mind the FOV is quite small at ~0.6deg?

- Support a heavier OTA at some point in the future, although not necessarily for *un*guided imaging.

- Be fairly easy/cheap to upgrade the setup for guiding in the future.

Many Thanks,

Lewis

[ollypenrice]

Unguided exposures of a few minutes are really only possible on very expensive mounts, ones costing over six thousand pounds. Some people cajole the EQ5 and 6 into giving a couple of minutes at shorter focal lengths but the process is not easy and these cheaper mounts (alas mounts are very expensive) are quite variable in accuracy.

I have only had one of these Celestron mounts down here and it was dead on arrival so I didn't learn much! I wouldn't write them off on that experience because it can happen to anyone. It is a pretty light mount and not very future proofed for you but I think it is supposed to be on a par with the EQ5. I'm sure it comes out of the same Chinese factory judging by the look of some of the bits.

A bigger mount would be the EQ6.

None of these will do long unguided imaging but autoguiding is not that difficult.

I have not found the electronics to be all that reliable on our our own Chinese mounts. Not that bad but certainly not bombproof, so buying second hand has some issues. The price of the bits as spares is outrageous by comparison with the whole lot new.

Olly

[Shibby]

Thanks for the reply Olly.

I suppose then, I should probably wait and try save up for a better mount or start considering autoguided imaging. It'd be fairly depressing to fork out on the new (to me) mount only for it to make little difference. Although I suppose this could at least spread the cost...

Would I be right in assuming off-axis guiding is the way to go to keep weight down and make a mount like this CG5 more viable?

[MartinB]

I have another suggestion. Forget future proofing by the way, just accept you will always want something better than you have - first law of astro imaging! Try to get a second hand vixen polaris mount. The Super Polaris was replaced by the Great Polaris (a few minor tweaks only) and then there is the GPDX which is heftier and often comes with Star 2000 goto (so more expensive). You should be able to get a motorised SP or GP for under £200. Don't worry about goto but make sure it has a polar alignment scope. This alignment scope is very good and will give you an excellent alignment good for a few minutes with short focal length scopes (yours included).

These mounts were designed to be premium quality. these days no one is making small mounts of this quality. With a bit of care and tweaking the tracking error of these scopes comes in around 10 arc seconds which is very good. They are happy with up to 7kg load and you are right, you can keep the weight down by off axis guiding.

Stick to 2 minute exposures (which will be fine) and with a bit of care and attention you should be able to use 90% of your sub exposures.

I have a high end Takahashi mount but it is a Super Polaris I will be taking to the SGL5 star party. This is the mount I use out in the garden when the Tak is running in my obsy. So it's future proof, you might just want another mount to go with it at some point!

[gary1968]

I had a CG5GT and persevered with it for imaging for a year or 2, without much joy. I couldn't get it to guide reliably, sometimes it was fine other times it was off in Declination. I think it was down to poor balance, lack of effort on my part probably. In its defence thought it was a fantastic mount for visual work.

In the end I upgraded to a HEQ5 Pro and cant really praise it highly enough, it is great for visual and imaging, using PHD it guides straight out of the box, but then I do take great care to ensure the balance is as good as I can get it.

So, IMO, save a bit more and get yourself a HEQ5, either Syntrek whith which you get full goto with the use of a EQDIR module and a laptop, or a Pro version with full goto in the hand set.

HTH,

Gary

[ollypenrice]

Martin makes a good point about the Polaris, though. They do appear at great prices.

Olly

[EA2007]

Unguided exposures of a few minutes are really only possible on very expensive mounts, ones costing over six thousand pounds.

Hmm, not entirely accurate, i have an NEQ-6 Pro and i have managed to have it tracking successfully for 30 minutes.

Though i do agree that an unguided mount is not going to give as long a tracking as a guided mount. the celestron CG-5 mount is good but it is noisy and can't handle as much weight as the EQ-5 Pro or EQ-6 (incl. N & Pro versions)

Good tracking is all down to a good firm alignment and set-up process, which occurs from:

a) good motors

plenty of power

c) level surface

d) weight balance

e) NCP alignment

f) 3 star alignment

I went observing a good few times at uni and we had two EQ-6's one evening, the lecturers set up one of them and the other was left to the students, it has to be said that the student mount didn't track longer than 90seconds, while the lecturers was okay for a good 5 minutes.

This is just my findings, many people may argue you can only get a few minutes, that's cool.

I was going to get the CG-5, it will probably do you well if your quite new to it all.

[davew]

[ i have an NEQ-6 Pro and i have managed to have it tracking successfully for 30 minutes ]

Hi,

What are you refering to when you say " Tracking ? " I think Olly means imaging. Is this what you mean too. Or do you mean a star stays in the field of view of an eyepiece for 30 Mins ?

Dave.

[russ]

[ i have an NEQ-6 Pro and i have managed to have it tracking successfully for 30 minutes ]

Hi,

What are you refering to when you say " Tracking ? " I think Olly means imaging. Is this what you mean too. Or do you mean a star stays in the field of view of an eyepiece for 30 Mins ?

Dave.

No, that will be tracking in the eyepiece for visual. Not even a £10,000 Paramount will give 30mins unguided imaging capability. I used to get 90-120secs out of the EQ6 unguided with a Canon 300D and ED100. And that was with a 50% failure rate too.

The ASGT is okay as a visual mount with the GOTO but its not an imagers mount. Martin has the best idea, track down a Vixen SP or GP(E). They are made to higher standard than the EQ5, CG5 or LXD75.

Russ

[EA2007]

tracking; visually and photographically.

though I have no means to prove this........

Yeah, Vixens will give better tracking. A little more costly though.

[ollypenrice]

Quote:

Originally Posted by ollypenrice

Unguided exposures of a few minutes are really only possible on very expensive mounts, ones costing over six thousand pounds.

Hmm, not entirely accurate, i have an NEQ-6 Pro and i have managed to have it tracking successfully for 30 minutes. (Quote)

Look, sorry but my point was not about what you call 'tracking' but about 'exposures' and is entirely accurate. Exposures are photographic and require a tracking precision measured in a couple of arcseconds, not the couple of degrees required to keep an object in the eyepiece of a medium field of view. My own EQ sixes will hold an object in the field of a metre focal length instrument with a 25mm EP for hours, not half an hour. All night quite probably, I have no idea. But they will absolutely not deliver the tracking precision required for imaging at a metre for more than a minute or so.

There is a difference between visual 'tracking' and imaging 'tracking' and the poster should not be misled on this. A good bit of cash could be at stake and it is important to remain within one's competence when giving advice, I think.

Olly

[EA2007]

fair enough.

each to their own, just going on what I have come across.

anyway, this ain't helping Shibby, the CG-5 is a good purchase in my mind, just buy some ear plugs with it

[ollypenrice]

Indeed, but don't expect it to deliver unguided imaging precision. That's all I would want to flag up for Shibby.

Olly

(Gawd, did I really use the term 'Flag up.' Shoot me, somebody.)

[MartinB]

(Gawd, did I really use the term 'Flag up.' Shoot me, somebody.)

Always good to think out of the box Olly

[Deneb]

What ! Pardon ! Excuse Me ! Come Again ! Six thousand pounds ....

Most of my images were done unguided with a EQ5 & a CG5 mount, but you are limited upto 2mins exposures, that's if your Polar Alignment is good enough. I keep on saying "Patience, Patience".

It's beautiful when you see an image gradually take form.

Cheers

Nadeem.

[ollypenrice]

You are the acknowledged master of unguided imaging, Nadeem! (And, yes, the cheapest AP mount is over £6K, a Paramount is now down to about £9k - I'll take two...) However, it is important to specify the focal length you were using and I can't remember what that was. Also worth saying to Shibby is that if you do want to try unguided then a fast f ratio scope is an absolute must. Short focal length and fast f ratio. Hyperstars at under f2 really can do unguided on a decent mount.

Two minute exposures do limit your choice of targets quite considerably, I'd say. The faint stuff is going to struggle to make itself heard above the noise.

Olly

[Shibby]

Thanks everybody for the advice. Yes, I'm looking to change mount solely for the purposes of imaging. The Alt-Az actually tracks very well for observing - I can keep something in my (pretty narrow) FOV for up to an hour! (so long as I work hard aligning). But for imaging, it's no good because the stars "jiggle" and I get multiple images of each star. Even at 30s, it's dumb luck if I get a "keeper" sub.

Well... although the scope is quite fast at F5, I unfortunately have to use a 2x barlow to focus (hence the fore-mentioned small FOV). Just another of the many challenges presented by astrophotography...

I like the sound of these Vixen mounts. I can see a couple for sale within budget but they don't have the motors, so I'll have to keep my eyes peeled. I'm willing to put the effort in when it comes to aligning. Am I right in assuming that leveling the mount is far more critical when not using goto and the multiple star alignment features that come with it?

I think for now I'd be happy with achieving a minute or two without having to throw away 80% of the subs, as I do now. I can always move on to guided imaging later.

Speaking of which, I've just had a concern - is off-axis guiding ok with the barlow or does it need to be prime focus?

[MartinB]

The Vixen SP and GP are very unlikely to have goto at all. If you are working to a budget I would sacrifice that and get used to target finding with a Telrad. If you do end up with goto bear in mind that a good goto alignment doesn't indicate a good polar alignment, they are seperate processes. Goto wont help your tracking.

Dump the barlow. I'm not sure why you need it for focusing but if you do, find a better focusing routine!

A level mount isn't necessary for accurate tracking, however if your mount isn't level the polar alignment scope wont be fully accurate. If drift aligning the drift will be a bit off the north south axes a little just makes things a bit more awkward but isn't the end of the world.

It is unlikely that an early Vixen will support autoguiding. My Super Polaris has an AWR hand set with autoguide port but these cost over £200 to buy new. Adding goto would be a similarly expensive option. You would be better going for an HEQ5 or EQ6 if these things are important to you. However, if you are wanting to learn the ropes - get accurate polar alignment, find targets manually, choose targets appropriate to your set up and get to grips with processing an old Vixen is a great way to go.

[Shibby]

Nope, I'm not too bothered about goto, I'm *reasonably* adept at finding my way around the sky with the help of Stellarium. I really missed a trick here: http://stargazerslounge.com/completed/98859-vixen-gpe.html

People keep saying this, but I seriously can't ditch the barlow - it's the only thing that has worked in all my efforts. It's a real flaw of the 130P imho (not enough inward focus travel). A webcam or something might reach prime focus I suppose...

Here's a Vixen GP for £190, but no motors: U.K. Astronomy Buy & Sell

Another thing I'm confused about is the weight of the OTAs. I can't find this information anywhere. How much does the 130P weigh? And what's the largest scope the Vixen GP can comfortably cope with?

[MartinB]

Oh I see, yes a lack of back focus can be a real problem with these scopes. Means you can't get close enough in with a DSLR. If you are close there may be some adaptations you can make. Try pming Steppenwolf, he is expert in this area and is Sky at Night magazine's "Scope Doctor".

The load capacity of the SP/GP is 7kg. You count everything carried by the saddle (so the counterweights don't count). 7kg is a conservative estimate but is probably a good limit for unguided imaging. Your scope and camera should be well within this.

Are you going to SGL5? I will have my Super Polaris there.

[Cornelius Varley]

You can find alot of information about Skywatcher telescopes here

Peter

[MartinB]

One day Skywatcher will publish the periodic error data for their mounts...one day

[Helen]

Rob is selling a scope and a GPe with an RA motor http://stargazerslounge.com/sale/100619-items-sale.html#post1406334

Don't know if that fits the bill....

Helen

[Shibby]

Thanks all, I've PM'd Rob. Only thing I'm not certain about, how vital is the polar scope for aligning? I do have an illuminated viewfinder on the camera (with crosshair) - is this good enough?

[MartinB]

Depending on your familiarity and desired precision with drift aligning the polar scope will save you between 15 and 60 minutes on every set up.