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Equipment Discussions >> Mounts

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zawijava
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Reged: 10/06/07

Loc: Wells, Maine 04090
Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5795477 - 04/12/13 10:05 PM

Glad I could help....due to the weather here in the Northeast for the past month or so I have yet to try the ASPA routine myself. I've had the AVX out a few times and the goto's seem really good, can't wait to ASPA and take a few photos -Tim

Quote:

Tim,

Thank you very much for clarifying this. It makes sense to me again. Hopefully it will stay that way.

I can also say that I have seen the behavior you describe. The further away I slewed from the synced star used for ASPA, the less accurate it was.




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Patrick
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Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5795678 - 04/13/13 12:05 AM

Quote:

I am making the assumption that a synced star is one of the two alignment stars. Is that correct or is a sync something different?

What about the calibration stars? Do I need to recalibrate after changing the alignment stars? Do I need to recalibrate after a ASPA?





I find it easier to turn the mount off after ASPA and redo the goto alignment, especially if the polar alignment was way off. You can unsync and replace alignment stars etc, but to me it just seems easier to move the mount to home position, turn it off and on, and start from scratch.

Quote:

Out of 9 frames that were 90 seconds each, 4 had well rounded stars and 5 had slightly oval stars. I did mave the mirror lockup set for the DSLR to prevent vibration from the camera shutter. Not sure if the slightly rounded stars were caused by periodic error or some other cause. It was not windy and the scope was well balanced. Are my results typical for unguided shots?




Regarding your round and oval stars, I believe your results are about average. One of the myths out there is that if you get your polar alignment dead on you will not have tracking issues. The truth is that at short focal lengths and with shorter exposures, moderate polar mis-alignment will only be seen from frame to frame as a gradual movement of the star field across the frame.

If you're seeing star trails and ovals in your subs it's most likely caused by periodic error in the mounts drive gears.

Patrick


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Jeff2011
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Re: Celestron VX Mount Tracking Test new [Re: Patrick]
      #5795740 - 04/13/13 01:05 AM

Quote:

One of the myths out there is that if you get your polar alignment dead on you will not have tracking issues.




Thanks Patrick. That is what I suspected. I had a similar result with my EQ platform even after a lengthy star drift alignment.


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freestar8n
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Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5795812 - 04/13/13 04:00 AM

I didn't mean to cause commotion and have everyone outdoors in an unsyncing party - but it is something simple to try *in case* GoTo is less accurate after ASPA far from the polar alignment star. Syncing shouldn't cause GoTo to be horrible far from the star - but it may make it a little worse.

Patrick's comment above is good because although you don't need to power off after ASPA - some people may prefer to do that because the alignment process steps you through adding calibration stars - and it avoids button presses and menu navigation.

I think the options after ASPA are the following:

1) Do nothing and leave it sync'd

2) Unsync only

3) Unsync and replace the 2 alignment stars (not much menu navigation involved here, so it isn't bad). Although the calibration stars are "gone" - the calibration should still be good after ASPA because only the orientation of the mount changed.

4) If you really want absolute best pointing you can go ahead and do a full realignment with fresh calibration stars - and if you do this then Patrick's approach of powering off may be most efficient because it prompts you through the process.

But in all this stuff - you may not need to do anything after ASPA, and what you do depends on how accurate you want the pointing and how much you mind adding stars again. For me, when imaging I do nothing after ASPA because I just need it polar aligned and then I need to find the target and start imaging - and improved accuracy isn't needed. Sometimes I sync on the imaging target itself in case I need to go away and come back to it, to focus for example.

Frank


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cn register 5
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Reged: 12/26/12

Re: Celestron VX Mount Tracking Test new [Re: freestar8n]
      #5795922 - 04/13/13 05:22 AM

I don't think this debate will get much further without some data in the form of measurements of pointing errors for a variety of sequences of aligning, ASPA and sync.

It's quite easy to measure alignment errors - do a slew to a bright star, centre it and use the identify function on the HC. The selected star should be the first object and you get it's distance from where the mount thinks it should be.

I did some tests like this when I was checking out the mount and found that after a 2+3 alignment the average pointing error was less than 2 arc minutes, I saw values between zero and 4.5 arc minutes, mostly zero and 1.

The polar align error was just over 1 deg in azimuth and 5' in alt.

I didn't do an ASPA and check this again but it would be easy for someone who has a clear sky to try this. Also check the effect of sync.

Chris.


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Jeff2011
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Re: Celestron VX Mount Tracking Test new [Re: cn register 5]
      #5796472 - 04/13/13 11:41 AM

Quote:

The polar align error was just over 1 deg in azimuth and 5' in alt.




Thanks Chris. I am now interested in the polar alignment accuracy itself. How did you determine that? I was planning on following up an ASPA with a CCD/DSLR star drift alignment test.


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cn register 5
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Reged: 12/26/12

Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5796637 - 04/13/13 01:29 PM

The HC shows the polar align error.

I think an ASPA should give a good place to do a drift alignment if you feel that one is required. You will have to redo the alignment afterwards because the mount will not be aware of the drift alignment corrections.

Chris


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Jeff2011
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Reged: 01/01/13

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Re: Celestron VX Mount Tracking Test new [Re: cn register 5]
      #5796676 - 04/13/13 01:55 PM

Ok you were checking the correction that was made during ASPA as reported by the hand controller. I was interested in comparing the resulting ASPA to a star drift. Yes I see the need to realign after making adjustments during a star drift. However based on the results of my test shots so far I can say that the ASPA is very close.

This has been a very helpful thread to me. I want to thank everyone for their advice.


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bilgebayModerator
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Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5796703 - 04/13/13 02:22 PM

Quote:

However based on the results of my test shots so far I can say that the ASPA is very close.




This is very good to know indeed. Thanks.


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FTLAUDSKY
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Reged: 01/21/13

Re: Celestron VX Mount Tracking Test new [Re: bilgebay]
      #5797539 - 04/13/13 09:50 PM

OK...I am newbie and have the VX mount now for about 2 months. Keep in mind this is my first mount and it took me about 2 hours to figure out how to get it working. From then on I always would do a 2+4 and by the 3rd or 4th star they would be right in the middle or off by just a little bit. I never ever did a ASPA until recently. I am fortunate and can see Polaris from my backyard clearly. I have align Polaris in the middle of my non reticle EP. This was wrong obviously. After reading about the ASPA I have now incorporated that after my 2+4. Took me a few times to get it right and turn the reticle EP so the star would drift and move along the lines. Afterwards I turn off the mount and do another 2+4. My unguided pictures would start to show star trails after about 60 seconds. With guiding I can do 2-3 minutes. Unfortunately for me after around 3-4 minutes my picture turn white and are oversaturated even with the IDAS V4. Sucks being in the city. I am also by the beach and wind is a constant problem. I am happy so far with the VX just wish it could handle more weight. I am about at the 1/2 load with my scope and gear.


PS - I am now learning how to properly configure the brain settings in PHD and watching the graph after each adjustment to see if it get better or worse. I can tell you I have both RA and DEC moving up and down not far from the middle line. The movements though seem choppy so need to work on the aggressiveness.


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Jeff2011
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Re: Celestron VX Mount Tracking Test new [Re: FTLAUDSKY]
      #5798100 - 04/14/13 11:06 AM

Quote:

My unguided pictures would start to show star trails after about 60 seconds. With guiding I can do 2-3 minutes. Unfortunately for me after around 3-4 minutes my picture turn white and are oversaturated even with the IDAS V4.




I have been able to get 2 minute shots unguided with round stars at 430mm focal length. When you use your reticle ep to align, do you Barlow it? Using a Barlow should help with the accuracy.

I have the same problem with light pollution. I have a 2" baader sky glow filter which helps, but I am probably going to be limited to 5 min subs once I start autoguiding.


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Jeff2011
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Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5816294 - 04/22/13 08:29 PM Attachment (8 downloads)

The clouds finally cleared and I was able to run the test I have dying to do. I wanted to do a star drift to double check the ASPA. I did the following procedure:
1. Rough aligment on Polaris
2. 2 star alignment
3. 4 calibration stars
4. Polar align mount (ASPA) on a star close to where the meridan crosses the celestial equator
5. Unsync
6. Realign the two alignment stars
7. Perform a DSLR star drift aligment as per http://www.cloudynights.com/item.php?item_id=2838

Here are the results. After the ASPA, the star drift showed that the polar alignment was off in Azimuth but not in Altitude. After correcting for Azimuth, I took some test shots. Star trailing was evident after 3 minutes. This was no different than when I took test shots after just doing the ASPA. For unguided 2 minute shots, ASPA seems to be good enough at 430mm focal length.


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orion69
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Reged: 05/09/10

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Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5816329 - 04/22/13 08:44 PM

Why are people so obsessed with unguided imaging with low end mounts? I does not make sense, you just don't do that. As for 2 min of unguided imaging, sometimes it will be ok, sometimes not. It's not repeatable (because it's a low end mount) and that is why it is not relevant.

As for ASPA, it's very usable for AP, nearly all my images were shot after ASPA.

Edited by orion69 (04/22/13 08:46 PM)


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Jeff2011
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Re: Celestron VX Mount Tracking Test new [Re: orion69]
      #5816392 - 04/22/13 09:19 PM

Quote:

Why are people so obsessed with unguided imaging with low end mounts?




Because us low enders are facinated by our equipment and want to see what we can get out of it after paying a pittance for it.

In all seriousness, that is good advice Knez. To a seasoned astrophotographer as yourself, I can see how my topic may be irrelevent. This is all still rather new to me, so I like to find out what the equipment is capable of. And yes, I have been finding out that unguided imaging is unrepeatable. Some of us just have to learn the hard way.


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