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Equipment Discussions >> Mounts

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WadeH237
Pooh-Bah


Reged: 02/24/07

Loc: Snohomish, WA
Re: Celestron VX mount [Re: Stew57]
      #5652443 - 01/30/13 02:21 PM

Thanks for the input Rod and Mark.

My suggested test is a response to Frank's assertion that the dec offset is both set by calibration and important to goto.

His statements do not match what I've been told and observed, at least for the CGE and CG5. I am curious to see if it's possible to demonstrate that the calibration affects more than just cone error.

I am planning on trying it myself to satisfy my curiosity. But like I said, it probably won't be any time soon. If anyone else has an interest, it could be something to try.

Understanding how these mounts actually work seems like a good thing.


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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Celestron VX mount [Re: Stew57]
      #5653588 - 01/31/13 03:58 AM

Quote:

The dec ofset is to get the initial stars closer. Once the model is mazde it has no relevance. The calibration (which is actually just one as the 4 are averaged together) will make a big difference as the mount WILL have cone error.




I actually disagree with this assessment. The dec. offset is very important because if it is wrong, the error in dec. couples into the measurement of RA - and both are wrong. On the other hand, an error in the RA index does not couple into dec., so it is less critical. The dec. error causes the measured separation of two stars to be way off - which would make the two-star alignment very unhappy. And the calibration stars don't act as a single star - they sample the error around the sky just like a more elaborate mount model.

On the topic of how to get best ASPA with this mount - if it doesn't have index switches I would align it to the marks carefully, and for best results I would use a fresh 2+3 or 2+4. If you don't want to take much time - it may be better to use a 2-star alignment that relies on a prior 2+4, rather than a fresh 2+1, which may be inaccurate because it didn't sample enough across the sky.

Frank


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: Celestron VX mount [Re: freestar8n]
      #5653742 - 01/31/13 07:37 AM

The marks give a starting position. BUT...it doesn't even matter what that position is _as long as you use it every time_ you can make your marks wherever you want them.

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DaveJ
Carpal Tunnel
*****

Reged: 01/07/05

Loc: NE Ohio
Re: Celestron VX mount [Re: rmollise]
      #5653788 - 01/31/13 08:27 AM

Quote:

The marks give a starting position. BUT...it doesn't even matter what that position is _as long as you use it every time_ you can make your marks wherever you want them.




+1 to that, Unc! I've proven that with my CGEM DX - both in RA and DEC.


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Stew57
Carpal Tunnel
*****

Reged: 05/03/09

Loc: Silsbee Texas
Re: Celestron VX mount [Re: DaveJ]
      #5654240 - 01/31/13 12:30 PM

According to Celestron engineer the calibration are averaged together to give one value.

Quote;
That said, the last 4 points in a 2+4 alignment each measure cone error against the "backdrop" of the two star alignment. In that sense, this is really just a 3 point alignment with the last point being an average of 4 measurements. Why would the 4 measurements of the cone error vary? One cause is mount flexure as the OP eluded to. In principle you could use some of the extra data points in the 2+4 to back out the mount flexure terms. The legacy handcontrol did not have the computational prowess, the code space, or even sufficient RAM to do that kind of computation. The NS+ has enough of all three (I think) to do that. Look for something like this to be available in the future.
End quote

I was the original poster and I was questioning the ability of calibration stars to cancel out mount flexure.


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freestar8n
Post Laureate
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Reged: 10/12/07

Re: Celestron VX mount [Re: Stew57]
      #5654567 - 01/31/13 03:42 PM

I think I saw that posting somewhere else on CN but I don't remember if I commented. Well I have a different take on the question, but feel free to ignore it.

Although the 2+4 doesn't have an explicit model for flexure and atmospheric refraction, it does accomodate more parameters than just cone, and a key one is the declination offset. The resulting pointing allows some "mush" and distortion across the sky to help reduce the error with the 4 stars. For my c11 on cge, and including stars down low and refracted, there would have to be some "mush" in the pointing to result in the 3-5' all sky accuracy that can be achieved. I have never seen a good write up of this stuff, but I studied it in detail some time ago.

So - I think there is already some accomodation for non-rigidity and refraction in the model, and it does very well with just 6 stars. I expect that when the new stuff comes out, with more horsepower, more stars, and an explicit model - it will do even better.

Frank


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186vett
member


Reged: 01/29/13

Re: Celestron VX mount [Re: freestar8n]
      #5655356 - 01/31/13 11:55 PM

Just looking for someone with real experience re: the new Celestron VX.
Thanks Everyone-Jerry


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dr.who
scholastic sledgehammer
*****

Reged: 01/05/12

Re: Celestron VX mount [Re: 186vett]
      #5655369 - 02/01/13 12:05 AM

What do you want to know Jerry? Several of us here, including myself, own and use one now...

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WonderMellon
super member
*****

Reged: 11/26/11

Loc: Maryland
Re: Celestron VX mount [Re: dr.who]
      #5655867 - 02/01/13 09:15 AM

My current setup is a Nexster 8GPS fork mount with HD wedge. I generally take 30 second unguided or 2 min guided exposures at F10. The system works well, but is bulky and unwieldy to transport and setup.

I have been looking at the EdgeHD 800 and AVX combination as I am hearing good things about both. It looks like the OTA, EQ Head and Tripod break down into more manageable peices for transport and I am assuming the AVX should be able to handle the 8 inch at F10 for reasonable exposure lenghts. (30 sec unguided and 2 min guided)

I would like to know if my assumptions are accurate. Would the Edge800 and AVX be a decent replacement for a fork mounted Nexstar 8 with a wedge?


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: Celestron VX mount [Re: WonderMellon]
      #5655890 - 02/01/13 09:22 AM

To some extent, yes. It will be easier to balance and easier to transport. My good, old CG5 never had any trouble with 30-second unguided to 10-minute guided (as long as I've ever want to go) exposures. Nothing beats the GPS in alt-az mode for visual use, though.

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Pak
super member
*****

Reged: 09/15/12

Loc: The Great Arc
Re: Celestron VX mount [Re: rmollise]
      #5656653 - 02/01/13 04:07 PM

Please tell me if this is correct.

Assemble on site. Line up marks. Point counterweight shaft over leg pointed north.

Level

Power on and do 2+4calib
Do an ASPA if you are going to hook a camera up.

Re-do 2+4 from the alignment menu (not powering off).

Go-to your target.

Find nearby bright star.

Set speed to 6 or 7 and slew over to that star.

Set speed to 3 to get it right in the center

Sync on that star.
Go-to your original target.

Good to go for the night? Total elapsed time, 10 minutes or less.

Anyone here do it differently?


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: Celestron VX mount [Re: Pak]
      #5656695 - 02/01/13 04:30 PM

Don't sync on some star. If you do, you will ruin your go-tos across the sky. Redo your 2+4 and begin to observe. Again, not necessary and will screw up your alignment. You can set the slew speed low if you want to, but there is no reason not to leave it on 9 unless you are worried about noise.

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cn register 5
scholastic sledgehammer


Reged: 12/26/12

Re: Celestron VX mount [Re: rmollise]
      #5656870 - 02/01/13 05:59 PM

More or less, except:
Level is put scope down where it was last time.
No need to do a second 2+4 alignment after ASPA.
No need to sync on bright star, use Precise align if the object isn't in the FOV.

It feels like 10 minutes but probably takes a bit longer, still it isn't a race.

Chris


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dr.who
scholastic sledgehammer
*****

Reged: 01/05/12

Re: Celestron VX mount [Re: cn register 5]
      #5656973 - 02/01/13 07:03 PM

Uncle R is spot on.

As to setup time... You can transport it with the mount attached to the tripod as it is light enough to do so. That's how I do it. Rides nicely on the shoulder that way. Just take off the weight bar.

In the field from taking it out of the trunk, putting it together, balancing the scope, aligning to viewing is under 15 minutes for visual and about 3 trips. Trips being from trunk to viewing site with mount/tripod and counter weight being one trip, OTA the second trip, and battery plus EP's and folding table the third. This would be the same for AP setup.

As I am just now learning ASPA since I am getting into longer guided AP I can't say how long that would take for someone who's been there and done that.

The 8" Edge rides real smooth on it with the mount not even straining and that is with a Stellarvue 10x60 and SBIG ST-ic guider setup, Canon 450D DSLR, and a Stellarvue 8x50 RACI finder.

I am 100% sure I can side by side mount my 102mm ES APO and the 8" for visual with zero problem, 85% sure I can do the same for AP, 80% sure I could put up the 127mm APO and the 8" for SBS visual, and 50/50 that I could put up my 127mm APO and the 8" for AP. Since the CG5 will take a C11 for visual no problem. My only worry is that the 127mm is a sail in the wind and may not balance well with the 8".


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Pak
super member
*****

Reged: 09/15/12

Loc: The Great Arc
Re: Celestron VX mount [Re: dr.who]
      #5657236 - 02/01/13 10:03 PM

When you crank on the knobs to adjust for the ASPA the mount can't possibly know how many turns of which axis you made so how could the alignment still be valid? That is why I mentioned doing another 2+4 or even just a 2 star so it can re-model. Isn't that right?

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jrcrillyAdministrator
Refractor wienie no more
*****

Reged: 04/30/03

Loc: NE Ohio
Re: Celestron VX mount [Re: Pak]
      #5657245 - 02/01/13 10:10 PM

Quote:

When you crank on the knobs to adjust for the ASPA the mount can't possibly know how many turns of which axis you made so how could the alignment still be valid?




It doesn't know how many turns you made - but it does know how far and in what direction you actually moved the mount (of course, if you didn't center the ASPA star when instructed to do so, it can't know what you did, but let's presume that we are doing it correctly). That's because it told you how far to move the mount and in what direction by offsetting the ASPA star. By centering the star, you are adjusting the mount as instructed, and as anticipated by the controller. Vixen's SS2K did the same thing years ago - worked the same way and no subsequent realignment was required.


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186vett
member


Reged: 01/29/13

Re: Celestron VX mount [Re: dr.who]
      #5657272 - 02/01/13 10:27 PM

Sir;
Just a quick summary would be appreciated.Your overall thoughts, disapointments, accolades, or any other opinions you might have would be welcome. Thanks-Jerry


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dr.who
scholastic sledgehammer
*****

Reged: 01/05/12

Re: Celestron VX mount [Re: 186vett]
      #5657324 - 02/01/13 10:53 PM

Cheers Jerry. My summary would be that it is a mount that takes many of the best features of the CGEM and that of the CG5 and puts them together making it a very good mount for someone who doesn't want to deal with the weight of a CGEM but wanted something with more power and performance than the CG5.

Other than that please see my more detailed comments in this thread. It really is performing to spec to date which is nice.


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186vett
member


Reged: 01/29/13

Re: Celestron VX mount [Re: dr.who]
      #5657426 - 02/02/13 12:00 AM

Dr.Who;
Thank-you Sir. Ordered the mount, with the EdgeHD8 from High Point Scientific today. All your input, & from others very helpful.
Thanks Much-Jerry


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dr.who
scholastic sledgehammer
*****

Reged: 01/05/12

Re: Celestron VX mount [Re: 186vett]
      #5657590 - 02/02/13 05:08 AM

You're welcome Jerry but please either call me Carson or The Doctor.

You are really going to like that scope for it's ease of use, light weight, and quality of views.

Be patient with the mount. An EQ mount can be tricky to learn but once you get the hang of it it becomes easy. Balance is hard at first but again with practice it get's easy. I like to mark the spot on the weight bar with a sharpie so I know where I need to put the weight and the same with the scope's rail. Just make sure to mark both after you find balance and have done the balance with everything on the mount including the EP you will use for alignment. The finderscope may make the OTA tilt a bit but that is ok. Once you turn it fully sideways and you have good balance this won't happen.

Another trick is to move the mount peg on the tripod from where it comes to you placed to the hole that puts it over a tripod leg. This way you use a compass not your mobile phone's compass to find North (don't forget to check for your location's magnetic declination), line the leg up with the compass point, and make sure your declination is set for your latitude. This should get you roughly polar aligned. After that do a 2 star alignment plus 3 or 4 calibration stars and the accuracy of the mount will pleasantly surprise you.

If you are thinking AP then wait for the focal reducer coming out for it or get a nice 80mm APO refractor. It is well worth the money spent if you are just starting out.


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