Best Mount for Imaging (£5k-£6k Budget)

[jjongmans]

Checked the values of my DDM60; RA 11.4" and DEC 4.8" accuracy.

[ribuck]

jjongmans, looking at other mounts they quote pointing accuracy in ArcSeconds, e.g. the ASA DDM 60 claims to get <15" ArcSecond RMS accuracy, and i think the GM1000HPs, Paramount MX etc is very similar in comparison. These seems to be a huge difference than the mesu which quotes 2.5 ArcMinutes.

So does the mesu 200, have it's own pointing model software to bring this in line with other mounts ?

[perfrej]

The Mesu uses the Argonavis controller, a third party add-on and not something that comes from Mesu himself. It does sport a TPAS system that models geometrical errors. I do not know how good that is in reality.

/p

[jjongmans]

With the Sitech controller it is possible to create a pointing/tracking model just like the ASA and 10Micron and I expect similar accuracy, but you need the extra encoders.

Pointing isn't as important as tracking accuracy. Pointing can be fixed with plate solving, this enables the worst mounts to point accurate.

[jjongmans]

@Per

The Mesu comes with a Sitech controller nowadays. The Stellarcat / Argo Navis combination couldn't be operated remotely, so the Sitech replaced them and is fully remote controllable.

[LDUNN1]

Rich, from what I've read in this post I doubt my comments will make a difference, & I nearly didn't bother posting, but I figured I might as well give you a little of my experience, because I see some parallels between us.

I have been using an nEQ6 (not too dis-similar to you) for just under a couple of years, but mid last year started 'thinking' & looking at better mounts, I wasn't really expecting to do anything about it, but just wnated to better educate myself on the options. The next obvious step up the ladder to me was the G11, but after reading a fair bit online about them & chatting with a few folk who I knew used them, I decided against the G11. There was too much talk about 'tinkering' with the mount to get it working well for my liking. I just wanted something that worked with minimal effort & fuss (which also seems to parallel some of your comments).

During this time I'd read a number of user reviews on AP various AP mounts, Mach1, 900 & 1200 mainly. One type of comment kept on surfacing - how these mounts just got on with the job & 'dissappeared', not getting in the way of the user, becoeming invisiable to the user with minimum fuss & effort. These sort of comments resonated with me.

By chance some months later, I stumbled upon the chance to pick up a secondhand AP900GTO on a portable pier (I mainly travel out to dark sites) with a bunch of other accessories, available at a reasonable price (which was still eye watering for me). Cut a long story short, I ended up going for it.

It may not have all the bells & whistles of the latest designs, & may not be at the cutting edge of technology, but it does seem to deliver on the other user promise of working with the owner not against them.

Things I like about it:-

I wasn't sure if I'd get on with the portable pier or not. Having tried it, I think its great, very quick & easy to level as the adjusters are at the top of the stay's & a few turns where you can keep your eye on the bubble level continuously & you are done.

The separation of the RA & DEC axis is so quick & easy, & makes the AP900 far easier to move as a transportable mount vs the nEQ6 - it is not just the reduced weight that you carry, but also the two parts are easier to hold than the nEQ6 was. I didn't really have a problem as such with the nEQ6, it is just that the AP900 seems so much easier.

The AP handset is great. I did not really get on that well with the nEQ6 handset & quickly moved to laptop control of my EQ6 - but that might have been me not giving the nEQ6 handset enough time, & also my relative inexperience at the time. Either way, I now use the AP handset, I can have multiple locations stored in the handset that I just call up depending upon where I am observing from. quick & easy, no need to enter date, time or location details after the first setup.

I 'park' the mount in the same position for tear down at the end of each session, this allows me to reassemble the mount at the start of the next session in the same position for set up & then just 'resume from park' from the handset. Quick & easy.

I rough align with the polar scope & then use the calibration tools on the handset to fine tune polar alignment. The process is that you point the scope (I usually manually push it) to a known star & fine tune centre it using the handset. Then enter the polar calibration routine & select the star that the scope is centred on. The scope then springs to life & motors back to where it thinks Polaris should be. You then centre polaris using the manual ALT-Az adjustment & select your calibration star from the handset, the mount motors back to the start & you use the cursor keys on the handset to finely centre the star, then 'ok' that & the mount motors back to Polaris - it is an iterative process where you get closer & closer to spot on between the two stars as you motor between the two & your manual ALT-AZ adjustment becomes more accurate, & after a few iterations you can exit the calibration routine. It seems to work pretty well for me & you can select different stars mid iteration if you desire. The manual ALT-AZ adjustment bolts are so free & smooth vs the nEQ6 - really chalk & cheese, this aspect of the AP mount is really lovely in comparison & just feels 'quality'.

I can't really comment on exact tracking accuracy I've been getting, I've always been guiding so can't claim any perfect 30' unguided subs, but I have had some of the flattest guiding graphs I've ever seen, & I think the AP's are reasonably well respected in their tracking abilities, & I have no reason to doubt that, & at the end of the day, thats what we really seek.

Another big difference is that I felt that I was on the limit of imaging with the nEQ6 with the payload I was putting on it (no where near the quoted 25kg!) - if I got everything perfectly set up, it was okay on the nEQ6, but sometimes my balance was out & the nEQ6 would struggle to cope & sometimes the clutches would slip if I swopped back end gear & the nose was too heavy - then you'd lose your 3 star alignement, which was a pain. The AP easily copes with the payload, & the clutches never feel like they are going to slip if you just remove kit from the back end.

If I had a permanent setup, then maybe the AP900 would not be such a great option, but it seems to be pretty good to me for how I currently use it as a transportable to dark site solution where I have to setup & tear down at the begining & end of each session.

If I were to buy again, I'd be looking at the AP, Para's & the other options suggested in this thread, but I'd still strongly favour ease of use when making the purchase decision.

All the best with your purchase, hope you buy the right solution for your needs & wants, its a significant investment, you want to get it right.

[ribuck]

Thanks for your feedback lawrence, it's very much appreciated. At the moment i have a few months to kill before i buy, so i'll keep looking into each mount and do more research, hence asking questions about the Mesu.

As for all the latest bell's and whistle, i would prefer to have a more modern / future proof mount. For me having the encoders, is probably going to be mandatory and not an Optional add on at a later date at great extra expense.

I guess the 2 reasons i want encoders are :-

1) if i move the mount or release the clutches, i dont need to go through the whole hassle of polar-alignment and creating pointing models all over again.

2) Sometime when guiding, you get small cloud rolling in that can throw off camera guided tracking, which then needs to be restarted / or need user intervention. If i can go unguided, no cloud can interfere and the worst that happens is that the cloud ruins that particular sub, but doesn't throw off the imaging process.

Rich.

[LDUNN1]

........Rich, I may live long enough to regret my purchase ;-)

[ribuck]

Just dont look at the price of the encoder upgrade kits......lol. One of the mounts i looked at i think it was the Mach1, i think someone said the encoder kit upgrade was several thousand dollars :-(

[LDUNN1]

.........I find the best practice with any astro kit is NOT to look at the price........

[ollypenrice]

Olly, i'm still reviewing mount options in the £5k-6k max price bracket, and in the thread in noticed that you hld the mesu 200 in high regard, so had a few questions for you.

Firstly, i noticed that the pointing accuracy is measured in ArcMinutes (2.5') i think, whilst other mounts in comparison seem to be measured in ArcSeconds. How do you find the pointing accuracy, and how is the pointing model controlled ?

I didn't see any mention of any encoders on the mount, so if you release the clutches, do you loose all your configuration and then have to do a full polar alignment / pointing model again ?

Just want to make sure i review all the options properly.

Sorry, I lost track of this thread.

The Mesu does have encoders, the latest ones with twice the resolution of Yves' (the one I use.) I haven't measured the pointing accuracy of the Argo Navis controller but it has never, so far as I recall, ever failed to put the object on the chip at 2.4 metres FL. However, it doesn't have clutches, being roller drive. My routine is to push it, powered off, to a single alignment star and centre that in the finder, then powerup, perfect the alignment in the camera, confirm alignment and that's it. Slew to your target. Once guiding I rarely even bother to check the PHD graph because it just gets on with it. If I'm unsure of whether to go for L or RGB I do check it because, of course, I only want L on nights of very good seeing.

Olly

[Ajohn]

Interesting to note that the AP900 spec's +/- 3.5 arc sec tracking accuracy before correction and no mention of the dreaded rms which will always make things look better. The AP900 comes out at 1.87 arc sec rms. I think piers are great too - much quicker to set up if levelling is needed.

One question I would ask concerning the various mounts that have been mentioned and the encoders. Do any of them allow the clutches to be disengaged, scope swung manually and then re engaged while maintaining go to accuracy? This needn't cause any problems at all as far as the drive rates are concerned but I'm surprised that a couple of arc second pointing accuracy can be maintained as that would need better than 650,000 pulses per rev out of the encoders. The usual way of getting that level of accuracy is by interpolating between pulses from the encoder by timing them and this is why drives slow down during the final stages of pointing at something. I would have thought a more achievable level of pulses per rev would be of the order of 4000 or maybe 2 or 3 times that but as far as hand slewing goes the 4000 for instance only gives a pointing accuracy of 5.4 min not seconds with some limited scope for interpolation.

John

[ribuck]

Hi John i believe on the asa & 10 micron, you can disenage the gear move it manually and then re-engage the gear and it will still keep it's accuracy. in terms of stepper resolution the 10 micron GM1000HPS apparently has more than 10 million increments (interpolated).

[perfrej]

Rich and John,

You can loosen the frictions and kick it around at your leasure. The encoders always know where they are.

As for encoder resolutions, I recall 1.2 million absolute positions per rev from some 10Micron documentation. This is then interpolated as well so resolution is rediculously good.

On a side note, I am feeling wild and crazy tonight and am testing long subs.. The moon is ruining everything anyway so I might as well test unguided performance. So far I have concluded four half-hour subs with no losses (apart from a slight field rotation from my 2' polar alignment error, clearly evident at 30 minutes). Currently one-hour subs are well underway and will tick for the rest of the night.

This is exciting! Here's a 100% sample of an unprocessed (mild-stretched) single 30-minute sub at focal length 1000mm:

Stay tuned for one-hour subs in a while

/per

[Ajohn]

I'm inclined to think stinking fish which is why I asked. Something doesn't make sense. The spec on the AP900 is pretty spectacular and I suspect would still needs hand work as AP used to do on the mount I own. They lapped the worm which basically means running the wheel and worm in a fine abrasive and carefully adjusting them as they wear into each other.. On that particular mount the worm is then accurate and driven by a servo motor who's speed is controlled by an encoder. The encoder ensures a more accurate drive speed for the worm than can be realistically achieved with a stepping motor. Any gears in the train can be lapped in the same way but it's a more complicated procedure. The encoder can be at the motor output or some way down any gearing that is involved but higher and higher pulse counts are needed as they are run at lower and lower speeds. I don't think an encoder is feasible on the actual scope drive shafts - might be wrong but would love to see a spec for a suitable encoder. Like worm wheels they have accuracy limits anyway. As far as I am aware 0.1% is the sort of precision that can be expected from normal high precision units. That can be improved in the same way as stepper drives are by subsequent gearing but this means that the encoder revolve several times for one rev of the actual scope axis. This is why systems loose the go to alignment when the clutches are released.

Anyway if you do want to hand slew and retain pointing accuracy I would try and make sure they really do this before buying. It's an area that has interested me because of the time a typical drive takes to slew through the worst possible angle - 180 degrees. I suspect steppers could offer a solution to this but the manufacturers seem to be locked into being able to run the drives with a maybe 8 rather small batteries - even only 6 in some cases, along with rather small cheap steppers.

Like many I suppose I dream of a remote observatory and slew rates are more important then. Even with a fast slew rate of say 7 degrees / sec as slew of 180 degrees still takes rather a long time.

John

[ribuck]

Holy Moses, you're going to try a 1 hour sub unguided ? You had me sold at 30 mins.

OK time to lay your cards on the table per, for a newbie imager using 500mm fl am I going to find the mount hassle free once I've mounted on a pier and done the initial alignments. I'm planning it the mount to be a permanent fixture in the OBS and no field trtips

[perfrej]

John,

This is a quote from Renishaw's encoder specs:

The RESA is a one-piece stainless steel ring with 30 µm pitch absolute scale marked directly onto the periphery.

The unique single-track format of the scale incorporates the absolute position data and phase information in one single code, which combined with RESOLUTE's advanced optical detection scheme provides far wider installation tolerances than traditional multi-track absolute encoders.

The RESA offers impressive accuracy with resolution to 0.00075 arc second, while RESOLUTE readheads feature low Sub-Divisional Error (SDE) and low jitter, making this encoder system suitable for the most demanding precision applications.

For high-speed applications, RESOLUTE is capable of speeds up to 36 000 rev/min, even with 27 bit resolution. This unique combination of speed and motion control capability make RESOLUTE particularly well suited to applications, such as high-performance lathes, which need high speed for turning operations, very smooth velocity and solid positional stability for precise C-axis contouring and orientation.

10Micron uses Renishaw encoders as far as I can remember (but I may be wrong). 10Micron is a long time precision mechanics firm in Italy and Baader Planetarium has helped them with the encoder addition to their high precision mounts in the QCI series. It appears to be a good marriage

And just for your dreaming pleasure, the GM2000HPS that I have on the balcony pier will do 20°/s max slew rate. That speed, however, is a prohibiting factor in the relationship with my neighbors (concrete balcony, metal pier, people below). Also, at -15°C the controller sometimes throws motor warnings when using high slew rates. Irritating but not a problem as the encoders keep track of everything. So 7°/s is my usual setting, and for all-nighters I use 5 or even 3. Doesn't matter if you're not building a model and need to slew around like crazy.

/per

[ribuck]

John' your post didn't make much sense to me maybe your spell checker went mad. so are you saying you don't believe the mounts capabilities can be so good ?

[perfrej]

Rich, there are no promises and no wonderland. I usually get 20-30 minutes with no problems and I never drop a sub due to tracking errors. The GM1000 is a smaller mount, but other people that I have been in contact with have managed 15 minutes at 1500mm with no hickups. My GM1000 is slated for the dirst days of February and I will know more then

But the short answer is: yep, should do it!

/per

[ribuck]

Per are you trying for the 1hr sub tonight, as I would really be interested to see the results.

[perfrej]

First one started tickin' a while ago... Stay tuned...

/per

[ribuck]

Really keen to see the results, but I think I'm pretty much sold on the idea of the gm1000hps already.

[ribuck]

Per hows that 1hr sub coming along ?

[Ajohn]

Anything is possible with a Linux spell checker but apart from an "as" that should be an "a" it all looks ok to me.

Yes I was suggesting that these drives may not do what they "appear" to do but it seems suitable encoders are available.

John

-

[ribuck]

Per has already posted can 30 main unguided and that is more than enough for me and the results speak for themselves. He is currently trying a 1hr unguided sub as we speak, so will be interesting to see results.