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Herzy

Terrible Tracking!

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Once I get enough money I'm going to be purchasing a AVX mount with a C6-N optical tube. I have about a month before I'll have enough money saved to get this setup. In the mean-time I want to do some imaging with my Nexstar 90SLT.

For some reason I haven't had any luck with this setup trying to image. The VAST majority of the photos I take with it have some serious star trailing. I don't understand what I could've done wrong, I think I have all the settings correct. I suspect the problem with Nexstar 90SLT is balancing... my DSLR is really heavy and it might be messing with the tracking pretty badly because I cant balance both sides out. However I haven't seen anyone else with this mount encounter this problem. :/ I know what field rotation is and thats not the culprit, I'm only taking 30s exposures.

Just in case I'm just entering my settings incorrectly I will type out my exact procedure.

1) I level the mount.
2) I enter my longitude and latitude.
3) I enter my time (not in military).
4) I pick daylight savings time and some other settings.
5) I do skyalign on three stars that are far away from each-other in the sky.

Thats all! The align seems to be pretty accurate when Im just using an eyepiece but the second I put in the camera it goes bonkers.

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if your finderscope is perfectly aligned to your main scope then try alignment with the camera attached,pick out the three brightest stars you can

and see if they match up through camera viewfinder,if you cant see anything in viewfinder take a snap @ high iso to see if star is in view

see how it goes then,or drift align on a star to see how far off you are then adjust mount

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"The align seems to be pretty accurate when Im just using an eyepiece but the second I put in the camera it goes bonkers."

This suggests to me that the balance is wrong. Try balancing it with the camera on first.

Peter

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Have your yet tried just using your dslr with a lens on the mount with no telescope.

This would help confirm whether there is a tracking issue or if telescope and camera is just too heavy.

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Let's see one of the images.

30 seconds is far too long for a long focal length scope on this alt-az mount. This isn't a mount for DSO imaging. It is great for solar system imaging, but not for DSOs. 

Your current set up:

- is alt-az so field rotation will always be an issue, especially at 30 seconds.

- the tripod is very unstable, and the merest breeze or ground vibration will be apparent in the image.

- the gearing inside the mount isn't designed to give perfect tracking unlike a $1000 mount.

- you are using a long focal length scope, 1250mm, this makes everything appear "worse" (vibration, tracking errors, alignment errors etc).

If you like tinkering, fair enough, but I would say if you've set your sights on the equatorial mount, I'd just wait for that.

Good luck.

James

 

 

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If you want to try and maximise the chances of getting the best possible tracking with this set up:

- use a chunky power supply, like a large battery or run from the mains.

- look at ways to stabalise the mount (people put sand in the legs, or hang something heavy from the midpoint).

- get the kit to be as balanced as well as possible with the camera attached; pointless balancing before the camera is attached.

- when imaging, are you using a remote shutter release or having to press the button on the camera; the latter will result in vibration.

- are you locking up the mirror to limit vibration from the camera?

- make sure you leave a good gap between focusing and taking an image; vibration can continue for a good 5 seconds or more after a re-focus.

- when undertaking your alignment, can you use a reticle eye piece? or if not use your highest power eye piece and a Barlow or Powermate so you really can get the star as close to the centre of the FoV as possible as it is this which is the one factor you can influence which determines the subsequent tracking accuracy.

- when undertaking the star alignment, are you always ending the movements using the up and right arrows only, to always mesh the gears in the same way and help to reduce in the impact that backlash may be playing in your system which again may hinder the accuracy of the sky map you are teaching the mount.

- do you have much noticeable play in either of the axes? with the power turned off to the mount, and no scope on the mount, how easy is it to turn either the altitude or azimuth axis by hand? If they do not move, that is good, if you are able to relatively easily move them you should aim to tighten these up. Report back and I'll give you instructions on this.

- have you tried different parts of the sky, as some will give you more noticeable field rotation than others: there is info out there for your given latitude what areas of the sky you can image for what duration, but again with a long focal length this will always be a limiting factor.

 

As I said before, I'd just wait the month :)

James

 

 

 

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turn off daylight saving, the clocks are not forward yet so we are on standard time :thumbsup:

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1 hour ago, happy-kat said:

Does America even do daylight saving?

Usually but as I found out when there, not in Arizona for some reason.

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Sorry for late reply. The mount doesn't turn in either axis without the motors turning them unless i push it hard (which I'm afraid to do). 

It's probably a good idea to do the alignment with the camera in, but that doesn't fix the balance issue. The problem is the camera is very heavy and it pulls down on the back end of the scope. I don't have a very long dovetail bar so i can just slide it to balance. I thought of just adding weight to the other end of the telescope to fix the balance but Im afraid to exceed the limit of the mounts carrying payload.

Everyone was mentioning how this scope isn't very good for DSO imaging, and I know that. This is just what I've got until I can upgrade.

No I haven't tried imaging with just the camera, you need adaptors to fit the camera onto it that I don't have. I wont buy these because it will only delay the time it takes to get an equatorial mount.

When I align I do use my highest power eyepiece and a barlow to ensure accuracy.

Finally I will work on getting the tripod more stable but the problem doesn't seem to be the tripod. When I get tracking errors (almost every time) its always in one direction and a clear trail, its never jumping around.

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Is it possible to see at least one of the inages?

It is reasonable to image DSOs with that scope, the limiting factor is the mount.

I'd give it a go at strapping something around the corrector plate end to counter the weight of the camera, like some ankle weights, or a bag of sugar sealed in something waterproof and attached very firmly. But as I said before, this mount was never intended to be used for "long exposure" DSO imaging.

Whack up the ISO and drop the exposure to 5-10 seconds, and just take 10 times as many subs.

James

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If the camera has a live view screen and you have a zoom function on that, and there was a cross hair or target / bullseye function too, you could just leave the camera in all the time and do your alignment with it and get each alignment star in exactly the same place on the live view screen at full magnification. 

Is your tracking rate set to sidereal?

james

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Sorry, I don't have a picture to post. I delete just about all photos because my mount only seems to have a 5% success rate. I will be adding weight to the top to balance it out. What is tracking rate mean? I probably sound stupid but I haven't messed with the tracking rate at all since I've gotten the scope.

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Page 20 of the manual:

http://www.celestron-nexstar.de/aktuell/bilder_2010/1265965139_nexstarmaksltma.pdf

 

The tracking options are:

- off, so the mount will GOTO your chosen target but the mount won't try to follow it as it moves across the sky, therefore the object will scoot out of the FoV very quickly.

- sidereal, you GOTO your target and the mount tracks that target at sidereal rate (most things in the sky).

- lunar, you GOTO a target and the mount tracks at lunar rate, which is pretty similar to sideral but not exactly so, and I'm not clever enough to work out if this would impact on you for a 30 second exposure.

- solar, as for lunar, but at solar tracking rate which is much closer to sideral than lunar.

 

Have a good look for images you took, it would be really useful to see these. You might have kept one somewhere.

James

 

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There is an earlier thread on this issue from this user with an image I'll post a link if i find it

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If you want to do imaging you should study up on the right telescopes towards that end. Because there are better telescopes made for doing imaging.

The AVX is very capable with a guide scope and PHD2 to help it. But not that great without, for visual good, but cameras catch what our eyes and minds can adjust for. Like the gearing's small quirks as it tries to mechanically stay on course. I made a time lapse video of my AVX following Orion's Nebula without the guidance of my Autoguider. It really shows how the mechanics of the mount bounce along.

And to anyone who has or is considering an AVX, I recommend battery power is the best way to avoid drive problems. (Based on two warranty issues with mine.)

If you would like to invest in a Newtonian type telescope, look for those that are specifically made for photography, like an Astrograph series, or other reflectors (SC, RC, etc.) specifically made to so Astrophotography works. The bonus is that a telescope made for Astrophotography can be used for visual astronomy. But often those made for visual astronomy are not that great for astrophotography, their mirrors and systems just aren't designed for it. Resulting in getting the sort of results you have become annoyed with.

There are plenty of good learning and informational video's on You Tube to explore. One I always liked is Forrest Tanaka.  And he has one specific to the AVX. (He is also very much EAA. Electronically Assisted Astronomy.)

So there are a few words of caution for you. Enjoy your astronomy! :icon_biggrin:

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I know all about the AVX. Im getting the C6-N with it because it's a great bargain for only $100 extra. It isn't compatible with 2" eyepeices so coma will be a problem. I'm going to convert my 90mm MAK into a guide scope and buy an autoguider soon after. 

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