AP1600 Absolute encoders or not

[cwjohn (Chris)]

Hi

Please excuse the long post but some context is necessary.

My first passion (other than my family) is sailing and I had set aside a considerable sum of money to buy a boat and sail around the Pacific. This would be my second trip in this regard. Unfortunately, I have been diagnosed with Stage 3 unresectable malenoma. Those of you who understand Malemoma will know that unless you are incredibly lucky you do not ascribe remission to the disease but rather describe it in terms of survivability statistics. Eventually, I am sure it will kill me but how long that will take lies in the lap of the gods.

In any case it will require me to undergo immunotherapy involving frequent injections in the short term of pbs listed compounds and probably later concoctions of whatever comes next at trials.

My long suffering and wonderful wife has a favourite saying.

The pessimist complains about the wind;
The optimist expects it to change;
The realist adjusts the sails.

William Arthur Ward

My second passion is astrophotography although I have not indulged it for many years and did not intend to indulge it for many years to come. However, clearly I am unable to indulge my first passion, other than for very short periods of charter, so "adjusting the sails" as it were, I am now changing focus.

We are building at Maleny on the Sunshine Coast on 25 acres and whilst I would not call it a dark sky site it is not bad. My first thought was to put together a simple system, so that if and when I pass, my wife would not have any problems disposing of the equipment. However, when I discussed this issue with my wife she encouraged me to go all out in building whatever gives me the most short term enjoyment. Further, she is quite adamant that if and when I pass, she will not sell any equipment but open it up to any local enthusiasts in the area.

So my plan is as follows. I already have a large shed being built on site to handle gardening equipment. I will convert part of that to a control room. I already have an F6 fluorite triplet, and a Tak Mewlon, with a Temma mount, both of which are superb for observing, and I will probably buy a 16" RC mounted on an AP1600 for astrophotography. These will be mounted in two domes.

I will buy cameras equipped with guide chips in front of the filter wheels so guiding should be no problem.

So all of this preamble leads me to my question. Whether to equip the AP1600 with Absolute Encoders or not?

In this regard the community seems to be split as to whether unguided imaging is possible. Those with 10micron mounts swear that guiding is not necessary. Others say this is a load of rubbish. The other issue I have is that previously I had the large Losmandy mount and between family and friends constantly moving the scope for visual observing I found that homing never really worked properly and I spent a lot of time rehoming the mount for photography use. However, I am assured that homing with the Paramount and AP work well. The obvious answer to this is to put a lock on the photography dome and to direct everyone to the visual dome.

In regard to AE on the 1600, it is not really a matter of affordability, but I do not necessarily want to spend 10 grand on something that I do not really need.

I would seek opinions on those far more experienced than I in this regard.

Thanks in advance
Chris

[JA]

Hi Chris,

I'm sorry to hear of your situation and wish you well in treatment and management.

On the issue of the AP mount with or without absolute encoders, you mentioned it not being a money issue, but that you might not want to spend it if you didn't really need it. Well, only you can judge "if you really need it", but you could also opt for a "lesser" Skywatcher EQ8 which has a 50kg capacity and absolute encoders for $6K new or around $4K on the used market.

https://www.bintel.com.au/product/sk...v=6cc98ba2045f

Keep well.

Best
JA

[cwjohn (Chris)]

Hi JA

As you say I can only judge if I really need AE, but I would like my judgement to be an informed one.

I am quite aware of the cheaper chinese mounts. Perhaps we could leave discussion of their relative merits for another thread as they are not relevant to my needs.

Thanks
Chris

[lazjen (Chris)]

From what I understand (and read since I do not have any mounts with encoders), the absolute encoders for an AP1600 for your situation are probably a luxury. Given that it's a great mount already and you're planning to buy a suitable camera setup for guiding, I doubt they are really necessary.

However, also from what I've read, it seems the encoders make life easier overall and make guiding "work better".

Personally, if I was in your situation and I had the money for it - I'd get the absolute encoders and enjoy it all.

Finally, I'm just "up the road" from you in Flaxton, working on getting my setup built, etc. No matter what you do I'd love to see the setup when you're done.

[DJT (David)]

Hi Chris

Might be also worth reaching out to the AP Yahoo group. There’s likely people there using encoders and can give you feedback.
Good luck with everything

[RickS (Rick)]

I'm a member of a small team sharing a scope at SRO in California. We have an AP1600 with absolute encoders. The scope is currently a Planewave CDK14 and prior to that it was a Ceravolo 300. We have done unguided tests while debugging guiding issues with both scopes and I wouldn't consider the results acceptable. That wasn't a surprise. IMO, unguided imaging is only likely to work with refractors.

Cheers,
Rick.

[Slawomir (Suavi)]

Unfortunately, not many will have direct experience in using AP mounts with and without encoders, so Rick's advice is gold IMO.

It makes sense that even a quality mirror system with a long focal length and long exposures will most likely be affected by some form of flexing. I would not even consider unguided imaging with long focal lengths, unless I was doing short exposure planetary imaging only.

Whether data quality is consistently better with encoder version when guiding - that would depend on a number of factors, such as seeing, imaging scale, exposure to wind etc. But perhaps I am stating the obvious.

Please keep us posted Chris

[Logieberra (Logan)]

I’ve got a 1600. Love it. No encodes. You can add them later/home install. Get just the big guy now; and accessorise later.

[Logieberra (Logan)]

Are you talking about a GSO 16” RC???

[ericwbenson (Eric)]

Hi Chris,

To encode or not...

I currently use a Planewave Instruments CDK20 telescope. It started its life on a Paramount ME, then later I upgraded the mount to a PWI Ascension 200HR (the PME was basically at or beyond it's weight limit and I felt performance was compromised). The A200HR comes standard with optical encoders (0.07" per tick). The only encoders worth talking about for improved tracking etc are the high resolution variety (14M ticks) , the 8K tick encoders are only good for visual.

For a short time PWI sold an A200 with no encoders, but I suspect they found machining the gears to the accuracy required too expensive/difficult and just not worth it considering the encoders basically do a better job at keeping the tracking smooth and have other benefits. Since the mount is very large the extra cost of the encoders (factory installed only) is reasonable (IIRC the diff was ~3K at the time, the A200HR lists for ~23K).

The PME with a very dense Tpoint model was supposed to allow for unguided imaging. The A200HR with a quite sparse (35 stars) is supposed to allow for unguided imaging. I can verify that both these statements are true, but not under all conditions. When does it work? Well that is an impossible question to answer in a general sense. Imaging scale, exposure length and OTA rigidity all contribute. So if you knew exactly what camera/OTA would be on the mount, what type of targets you wish to acquire, and what your expectation level of the data quality was, then maybe we could answer the question with some confidence!

In my case I am very picky about my data quality. I have generally very good seeing (FWHM~1.6" for 10-20 hr stacks) and a large imaging scale (0.5"/pix) with a f/6.8 telescope and I like to go after faint galaxies, hence long subs are standard (30min). Unguided for me would yield less than acceptable results with either mount.

But if I binned my images to 1"/pix and chased more nebulas, hence undersampled and 3-5 min subs, I think the A200HR would do OK. I regularly did collimation+ tiptilt adjustment for the OTA with 1 min unguided exposures, no problem. The PME in the same conditions would still struggle, a lighter OTA and coarser scale might do the trick, but without encoders you are relying on the PEC always working perfectly. The PEC can be good but still trails the real time (20Hz) adjustment an optical encoder brings.

If I was getting my dream mount I'd get the encoders ... wait I already did that!

Of course after my obs was finished PWI go and release the CDK700, 700mm aperture, encoders and direct drive in AltAz config, luckily I can't afford it and so I'm happy with what I've got.

I know I might have muddied the waters more, but it is a complex question!

Best,
EB

[cwjohn (Chris)]

Hi Eric

That is some rig you have and I looked up the 0.7m PWI which I see costs U$210K - awesome but expensive.

Unfortunately, my location's seeing does not justify a scope much bigger than 16".

I am convinced that the AP1600 without AE has the capacity to guide at 1" assuming Pempro applied and autoguiding used, this being easily beyond the seeing limitations of the site. The real question is whether AE actually improves performance beyond that given these seeing limitations.

I am also painfully aware that the location is on the top of a mountain and windy conditions can be prevalent. Many times in the past I found data was unusable even on a low flexure system due to wind gusts. Supposedly the fast control loop associated with AE will ameliorate these issues to a great extent. I have no way of knowing whether this is true, or the extent of protection the AE control loop provides.

n regard to unguided imaging I think there is sufficient evidence that at focal lengths of 3m and above unguided imaging over long exposure times is not feasible.

Thanking you
Chris