Tracking issue with Celestron Advanced VX mount

[rodgerK]

I recently purchased a Celestron Advanced VX mount from Astronomica but I see they have ceased trading now    I have used it a few times and it seemed OK but I always use a finder-guider on my telescope as well using PHD Guiding to ensure good tracking over a longer period.  However, lately even with the guiding running, the PHD software was struggling to keep on track.  So I did a test last night with and without PHD running.  Both exposures of M51 were ~120 seconds long.  See the attached results.

To me this implies that without the PHD guiding, the mount is hardly tracking at sidereal rate if at all.  I expect a mount of this quality to be able to track OK for at least 2 minutes with only an 80mm APO, finder and camera  attached (~ 4kgs).

Now this maybe a setup issue  but I check in the menu and its setup for EQ-North and Sidereal.

Any one got any ideas ?

[PeterCPC]

When I have my AVX Polar Aligned using ASPA I can get about 4 minutes unguided with my ED80 and camera attached. Have you done the ASPA polar align - I usually need to do it twice to get it well Polar Aligned. With guiding using PHD and a 50mm guide scope, I can get at least 10 minutes without trails.

If you mean Astronomia, FLO has taken over warranty issues for them.

Peter

[rodgerK]

Yes - I have done the ASPA twice now.  I am not over confident its working particulary well though as the second time I had to move the mount manually quite a bit again. 

Even if its a poor polar align -  these results are suprisingly poor for a 2 min exposure and no better it seems than a static mount.

Is the better solution to go and buy the optional polar scope ?

Yes I did mean Astronomia.

[PeterCPC]

Well I don't have a Polar Scope and see no need for one if the ASPA is working correctly. When you have completed your second PA what sort of error are you getting? The star trails indicate a poor PA. Are you sure the clutch knobs are not being impeded by the motor housing? This has happened to me twice.

Maybe you should communicate with FLO to get their take on this as they will now deal with warranty issues and it's a recent purchase.

Peter

[rodgerK]

Whilst performing the second PA, to get the star back centred I had to move both the Lattitude and Azimuth adjustments knobs more than I expected.  From memory more than I did on the first PA.

Yes I have seen the clutch knobs get in the way -  I use a motorised filter wheel as well as the CCD camera and I have to be careful the cable from it does catch the knobs or the motor housing.  But for this exposure the scope was practically vertical and well away from any impediment.

After seeing your first reply I have now contacted FLO for their advice.

Thanks.

[Scorpius]

I recently purchased a Celestron Advanced VX mount from Astronomica but I see they have ceased trading now    I have used it a few times and it seemed OK but I always use a finder-guider on my telescope as well using PHD Guiding to ensure good tracking over a longer period.  However, lately even with the guiding running, the PHD software was struggling to keep on track.  So I did a test last night with and without PHD running.  Both exposures of M51 were ~120 seconds long.  See the attached results.

To me this implies that without the PHD guiding, the mount is hardly tracking at sidereal rate if at all.  I expect a mount of this quality to be able to track OK for at least 2 minutes with only an 80mm APO, finder and camera  attached (~ 4kgs).

Now this maybe a setup issue  but I check in the menu and its setup for EQ-North and Sidereal.

Any one got any ideas ?

I have no idea really but I have the AVX and it's performed flawlessly so far so I'm wondering if it might be something with your set-up (probably PA). I know others may disagree, but I'm a proponent of the optional PA scope and never use ASPA . The big disadvantage of the polar scope is having to adjust it initially so the polar axis of the AVX and the crosshairs in the PA scope are pointing in the same direction. This is a tedious but necessary procedure but only needs to be done once provided the reticle doesn’t get knocked out of alignment at a later date (been there done that). Another minor annoyance is the fact Celestron doesn’t include an illuminated reticle in their PA scopes but this can be resolved by setting up a small, red, LED flashlight at the right distance so you can see the reticle in the dark. Lastly the reticle's not been updated for Epoch 2015 but if you get to the point that's an issue, it's probably time to consider drift alignment which I haven't needed to get 10-15 min subs when guiding.

So that's the disadvantages of a PA scope but the one big advantage is consistent and repeatable results which can be achieved relatively quickly once you get the hang of it. Then it's a 2-star alignment with 4 calibration stars and you're off to the races. There’s no need to star align twice with the PA scope and the mount doesn’t even need to be on because you're physically aligning the mount/scope to the NCP. In my mind this takes the mount’s built-in software completely out of the equation so I know the polar axis is pointing in the right direction with no concern that ASPA is applying some type of "correction factor" to compensate for the mount not being positioned correctly. Keep in mind that ASPA doesn't use the pole star to align so in the background it's "guessing" where the NCP is located and the accuracy of that guess is based on the quality of your star alignment which is done before rather than after when using ASPA. This means ASPA and Goto star alignment are "joined at the hip" which is why I prefer two independent procedures. That's not to say ASPA isn’t an accurate method but I do think it's more time consuming and has more room for error.

So having said that and for what it's worth - if you're considering investing in the PA scope from Celestron (about $45 US) - to determine if there’s  a problem with your mount - here's how I PA the AVX.

Face the peg on the tripod north with the tripod’s feet resting on a solid surface. I use vibration isolation pads on concrete pavers recessed in the ground and have made index marks on the pavers which help me get rough alignment close enough that I don't run out of adjustment on the azimuth knobs once PA begins. Make sure the adjustment knobs on the legs are tight and the top surface of the mount is level in two perpendicular directions (others may disagree with this as well). Make sure the spreader bar is fully seated between the legs and tighten everything up real snug. When you put the palm of your hand on the tripod’s top plate - and apply slight pressure in different directions - there should be absolutely no "rocking back and forth" At this point you'll want to carefully place the mount on the tripod and firmly tighten the knob that holds it in place. Next install the counterweight bar and the weight (an index mark on the bar is a big time saver once you’ve determined the balance point ). Then carefully place your scope on the mount and after confirming it’s fully seated in the dovetail – tighten the knobs. Next is to install all your accessories (cameras, filter wheels, etc) and then balance the scope. First balance in RA by adjusting the counterweight and then balance in DEC by moving the scope back & forth in the dovetail. The manual outlines the procedure and indicates balancing is very important and I believe that’s true based on past experience. Now the next part is VERY IMPORTANT! - the white index marks on the mount’s housing must be precisely aligned before attempting PA. Then loosen the RA clutch and rotate the scope until the constellations on the reticle (Big Dipper and Cassiopeia if you’re in the Northern Hemisphere) match their actual orientation in the sky. This may seem somewhat subjective at first - since you don’t actually see them in the PA scope - but it’s very important because it determines where you’ll place Polaris in the reticle therefore you want to take your time and get this as close to perfect as possible. You do not have to rotate the polar scope in its housing to complete this step since you’ve already adjusted the reticle to match the mount’s polar axis. If you rotate the scope instead of the RA axis, the it will get loose in its threaded housing and that makes it impossible to get accurate PA. Lastly, use the manual Alt/AZ adjustment knobs to place Polaris exactly in the center of the little circle indicated and the optical axis of your OTA will be pointing very close to the NCP. Now you can complete a two-star alignment with 4 calibration stars and enjoy good tracking the rest of the night especially when guiding...

Here’s an LRGB image of M51 recently captured using the AVX which consists of multiple 600 sec luminance and 450 sec RGB subs (2x2 binned). This was with guiding and the Celestron 8” SCT at 1280 mm focal length. I’ll wager your AVX can get similar results if you take the time to get accurately polar aligned no matter which method you choose...  

[rodgerK]

Thanks for this detailed story. My scope is house in my own observatory so once I get this right if will stay right. I may consider buying the polar scope. I also have a HEQ5 mount with polar scope and this issue never appeared with that mount.

[knobby]

Just an off the ball thought ... Is the mount set to sidereal rate , I'm sure there are a few other rates available

[dph1nm]

Those star trails are exactly along lines of constant Dec, so I don't see how it could be PA. It could be exceptionally bad periodic error I suppose.

NigelM

[lonebod]

Stupid question I'm sure, but is you scope correctly balanced?

[rodgerK]

Hi

Yes good spot they are essentially not going at the right sidereal rate.  I have done some rough calucalations and if there was no tracking, the light trails from a 2 minute exposure would span ~ 30 mins of arc which is about half the picture size.  Those star trails appear to be ~ 3 mins of arc long so its tracking but at ~ 90% of sidereal rate.  Is it possible its stuck on lunar rate ?.  I have double checked the menu system and its set to EQ-North and sidereal rate.

FLO have come back and we have agreed I will try a complete setup again including doing a reset to factory settings.

Hopefully this is me and not the scope at fault

[Scorpius]

Those star trails are exactly along lines of constant Dec, so I don't see how it could be PA. It could be exceptionally bad periodic error I suppose.

NigelM

This is why I mentioned I really have no idea so hope I didn't waste the OP's time with my lengthy dissertation on polar aligning the AVX. As the old saying goes "a little knowledge is a dangerous thing" but I did learn something about recognizing bad PA so thanks for that info.

Only other thing that comes to mind since it's a DEC tracking issue is the possibility the DEC clutch is slipping. I have to crank it down pretty tight to keep it in place so maybe it's worth checking that just to eliminate as many potential problems as possible.

Cheers and clear skies to all...

[rodgerK]

Clouds and poor weather are stopping me checking this out. I will post when I have retried the setup again.

[Thalestris24]

Hi

I find my AVX quite tricky to balance because of the stiction (I currently have a 130pds + 1100d + finder guider on it). I do it by judging resistance to hand pressure. I've found it quite important to balance properly in RA, DEC - and vertically.

Louise

p.s. I've never used the ASPA but Astrotortilla for gotos plus the PHD2 drift align tool are very good