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ASA DDM85X-SL - Keeps getting better

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#26 BPO

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Posted 26 October 2012 - 08:11 PM

If you already have a good geared mount and guiding setup, there is probably little reason to ditch it in favour of one of these direct drive mounts, unless you really want to get rid of the guiding rig.

For a new setup, it makes a lot more sense. Once the user gains the necessary experience and has trained the mount, an external guiding rig becomes unnecessary; Indeed, counter-productive.

But it needs to be borne in mind that these mounts are a major step up in most ways. They are much closer to professional observatory technology than other kinds, and the demands on the user's ability and knowledge increase, at least at first, although the pay-off is significant.

#27 SL63 AMG

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Posted 16 November 2012 - 01:09 PM

Are there any differences or advantages between guided and unguided tracking (and vice versa) if mount is correctly polar aligned?
Why would I shoot unguided if I can guide (disregarding you don't have to worry about guiding star or seeing)?


I believe the advantages are significant.

With this mount, you have no backlash, no periodic error and without guiding, you eliminate a whole series of other potential issues, such as difficulty finding a guide star for long FL image. You eliminate issues such as how to guide, with an off-axis guider or a guide scope that may have flexure issues. Where do you place the guide chip, in front of or behind the filters, such as SBIG's cameras.

Certainly, guiding is not impossible, but I think it is a challenge in and of itself for beginners. So if you are going to spend hours learning how to setup and autoguide and solve all the nuances surrounding autoguiding, why not learn this mount and eliminate the need to autoguide altogether?

Then, once you have learned the mount and you decide, heck, I'm bored and want to learn something new, you can still autoguide using this mount!

In fact, I am going to learn to do it with my AT12RC so that I can take very long NB guided exposures at 2432mm focal length.

The joy is, however, that when using my ASA N10 or a small refractor, or even the AT12RC for exposures of 20m or less, I don't have to autoguide.

My polar alignment right now is >6 arc seconds error in RA and 6-8 arcseconds of error in DEC.

I can turn on the mount and start a 5 minute image and get perfect stars zoomed to 8x full resolution without even running MLPT.

Seriously, that's amazing.

#28 korborh

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Posted 16 November 2012 - 09:02 PM

Auto-guiding is actually quite simple to do. Also finding guide-star in OAG is easy with calibrated FOV and planetarium software.
What is perhaps much harder is to maintain delicate and large pointing models and tune a complicated open loop system and hope that un-guided works :fingerscrossed:

#29 SL63 AMG

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Posted 16 November 2012 - 09:51 PM

Auto-guiding is actually quite simple to do. Also finding guide-star in OAG is easy with calibrated FOV and planetarium software.
What is perhaps much harder is to maintain delicate and large pointing models and tune a complicated open loop system and hope that un-guided works :fingerscrossed:


There is no hoping about it. The proof is in the pudding.

A single 30 minute unguided luminance -35c binned 1x1 900mm FL F/3.6 unprocessed.

Image taken 39 minutes ago. No MLPT, just point and shoot.

Will your mount put this target dead center on every slew and stay there for 30 minutes without moving a pixel? :question:

Incidentally, my current polar alignment error is <=6 arcseconds in RA and <=12 arcseconds in DEC. it's unlikely you can even get your mount that closely polar aligned.

Let's not forget that Autoslew corrected for the polar alignment error, tube flex east and west, collimation error and synch offset and sent corrections to the mount in real time.

:shocked:

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#30 korborh

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Posted 17 November 2012 - 02:19 AM

I routinely take 30min subs with <2" FHWM stars, 100% yield at 2800mm FL with OAG and AP1200 mount. The mount is 30" Polar aligned RA/DEC and I did not tweak further as it is not necessary.
Plate-solve + sync + slew lands on each target dead on center of chip, only a few pixels off. I don't use modeling - just simple plate-solve + re-slew.
Un-guided may be useful when one wants to take many images per night e.g. supernova survey. Un-guided is also more forgiving with refractors where flexure is minimum. With long FL mirror based scopes, flexure is large and less accurately modeled.
I have not yet seen an un-guided raw image with <2" star FWHM in 30min subs.

#31 jjongmans

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Posted 17 November 2012 - 06:48 AM

It would be strange if you couldn't accomplish good autoguided results with an AP1200. Same counts for the DDM85.
The reason I don't want to autoguide is because I'm using automation software like CCD AutoPilot and I like to refocus during my subframes. It takes time to reacquire the guidestar and needs time to settle, I don't like that. If you guide with a low min. motion setting, you'll guide on the seeing with lots of unneeded corrections. If you use a high min. motion setting, you'll be too late to correct the error. So with autoguiding you constantly need to adjust the parameters to have optimum guiding, something I don't want to do in an automated setup.
So I think there won't be a big difference in results from an autoguided and unguided setup, it's just another way of working.

#32 korborh

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Posted 17 November 2012 - 10:57 AM

The only autoguide settings I have needed to change (only sometimes) is aggressiveness to match the seeing. One does not need to mess with other settings much at all if the system does not change.

To get the best long exposure data, it is desirable to use closed-loop guided systems and not open loop. Also. I am not sure un-guided using these mounts is any easier than autoguiding.

#33 orion69

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Posted 17 November 2012 - 06:30 PM

I don't really care if I guide or not, it's the result I'm interested. If guiding is not the problem (and so far it never was) I don't see why I wouldn't use guiding with this excellent mount. Problem is I don't have permanent setup and fast polar aligning is required.
Btw, if you look at my images you'll see they are all 30 min subs except one...






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