Skyliner 250PX Flex Tube Auto Tracking Dobsonian

[DrRobin]

Hi All,

Testing and results

----------------------

I wanted a method of measuring the performance and accuracy of the motors on both axis without any tracking and be able to separate out the different variables, here is what I did:

In daylight I used the Synscan I centered the scope on a distant TV aerial on the roof of a house. I then power cycled the mount and cleared all PAE data.

Using the position information the scope read 0d 0'0" in Alt and 0d0'0" in AZ.

I then slewed the scope 10 degrees to the left on rate 9 and then slewed it back on rate 9 dropping to rate 1 as the TV aerial was centred. I was careful to always approach the TV aerial from the same direction. I then looked at the position information, it should be the same, but it was over 1' (minute) out in AZ.

I then realised that the two bolts that hold the scope to the drive plate were slack and also I have two batteries, so tried the second with a 10 degree slew on rate 9.

This time the mount came back to within 6" (yes seconds) of it's original position in AZ.

I then tried Alt, doing the same and raising the scope up by 10 degrees on rate 9. When it was re-centred it was 8' (minutes) out. Making small movements on rate 1, with no backlash, gears engaged, I noticed that the motor and position moved but the scope didn't. There may have been up to 2 minutes of movement before the scope moved.

I therefore realised that the clutch nut was either too tight or not tight enough. It turned out to be too tight.

Once it was slackened by about 3/4 of a turn, I repeated the test and this time it was perfect in Alt, say about 5 seconds, repeatable.

I then tried some 60 degree AZ and 30 degree Alt slews on rate 9 using the Park facility to re-centre the scope on my TV aerial, the results it was within 5 seconds in both axis.

I therefore think I have bottomed the problem, the battery change was responsible for a small amount of the problem, the loose nuts a bit more, but the over tight clutch nut making probably the biggest issue.

I had a fairly long session to balance the scope and found that because the weight of the scope isn't balanced around the pivot (the two truss tubes at the bottom cause a problem) it's almost impossible to get perfect balance over all Alt angles. I opted for a weight that gives a slight up force for most angles as I do most of my observing from South through West to North and therefore using mainly down arrow. A slight up force means the gears are engaged all of the time.

Finally I decided to measure and set the backlash, but couldn't find any instructions how to do it, so I devised my own method as follows:

Centre my TV aerial using a home made cross hair eyepiece out of the 25mm which came with the scope, and then move the scope in the opposite direction on rate 1 until the scope moves. Note how much the position data has changed. I then programmed this in to the backlash setting and tested again making slit adjustments until the scope moves in both directions straight away. I ended up with 43'00" backlash in AZ and 40'00" in AZ.

I would be really interested if someone else who has good tracking repeats my experiment and compares the results for accuracy and backlash.

Of course, I still don't know how well it will track, but my measurements indicated a lag in scope position compared with what the encoders said and that was what I say in practice last Thursday.

Regards

Robin

[Space Cowboy]

Very impressive test Robin! The most thorough examination of this scope setup I've seen.

I meant to ask did you check the voltage read out on the synscan handset when using different batteries?

Not sure what you are referring to by "2 loose nuts"?

I must confess to being ignorant of the backlash settings its something I've never used.

I will try and do a similar test with my setup. I guess this is exclusive to those using the goto handset.

[DrRobin]

Hi Space Cowboy,

The two nuts hold the OTA to the drive plate and dovetail. The two on the driven side were fairly loose, to the point where the OTA moved in the circular black plastic and dovetail fixing, the other two on the other side just needed nipping up a bit. I have attached a photo to help you find them.

I didn't measure the battery voltage, actually didn't know the handset did that and I thought about using a multimeter but then if the volt drop was coming in the cable I might not have seen it. The first battery I used was slightly smaller in capacity, but still sealed lead acid, but when I swapped battery I also swapped lead (I make these up myself) so maybe there is a bad joint in one of the leads.

I had a 127 Mak Goto before this and never played with the backlash, it's quite interesting when you get I right. Say you are using the right button and then overshoot, if you press the left button, the motor spins up quickly to take out the backlash and then if your using a low slew rate slows down. The result is that you don't have to wait ages to recenter a target.

Anyway looking forward to seeing your results as soon as we have a few we will all know the sweetspot for clutch nut setting. I reckon a few arc seconds is going to be just great, but it does show how sensitive these scopes are to clutch nut tightness, too slack and the scope nose dives (unless exactly balanced) too tight and the drive can't overcome the friction.

Something else I have noticed the drive gear connects to the OTA using the same friction nut, so too slack and the motor will spin without moving the scope.

Sorry picture will have to wait, my IPad won't let me attach one, had to use the PC. I also included a picture of the fairly big counter weight added to the scope.

Regards

Robin

[DrRobin]

Hi,

Just been out to measure battery voltage, good tip that. The bigger of the two batteries measured 13.1V dropping to 13.0V when AZ motor was moving.

The slightly smaller battery measured 12.1V, dropping to 12.0V. I suspect with the over tight clutch nut the slightly smaller battery was struggling to cope. Both batteries are fairly new, although the smaller of the two might not have been fully charged, whereas the bigger one was.

I have only used the scope from my back garden and have a summerhouse with mains power put in, so might start float charging the batteries during obs sessions in future. That or I need another larger battery. I am using around 5 and 7Ahr, good capacity for the size and weight and I sit them on the base plate so as to avoid wires tangling.

If my camera battery goes flat it's annoying, but if the mount looses power it's a real pain.

Do you know if these scopes are sensitive to battery voltage?

[DaveGarland]

Ok, so the 200 version is an F6. But the 250 version is an F5. So witch one would be better for planetary imaging with a webcam? Am I right in thinking that although the 250 has a bigger aperture, the 200 would give a larger image? How does all this work? I'm a little confused.

Dave.

[Crunchard]

Nice going Robin.

I use fridge magnets for my counter-balance

[Space Cowboy]

Ok, so the 200 version is an F6. But the 250 version is an F5. So witch one would be better for planetary imaging with a webcam? Am I right in thinking that although the 250 has a bigger aperture, the 200 would give a larger image? How does all this work? I'm a little confused.

Dave.

Both scopes have the same focal length so image size is the same but brighter with the 250 meaning extra magnification can be used. Aperture is king with planetary imaging.

Thanks for the pics Robin just twigged those nuts are inside the OTA. For sure they must have effected the scopes slewing accuracy.

I too started with the skymax 127 GOTO and remember tracking going off the boil when voltage dropped below 12v (using AA batteries).

[DrRobin]

Hi Everyone,

I am no great expert on telescopes and what's best, but I am not sure the dobs are the best for planetary imaging. Like Space Cowboy, I had a 127 Mak Goto and I would say for the money that was a fantastic scope for Jupiter and Saturn. I just sold mine for Just over 300 quid and the reason for getting the 250 dob was to see a few more DSOs.

I had both for a few months and Saturn was brighter in the dob but the focal length of the 127 Mak is longer so Saturn was bigger.

I think for planetary I would go for a 180 Mak or bigger if the money will allow. You will probably get a decent size Celestron SCT for the same money as a 250 Flex tube Goto and it will be smaller and easier to use.

Regarding battery voltage, I never used the AA cells except as a back up on my 127 Mak and then they were NiMH cells. I am guessing that low battery voltage makes the motors perform a little worse as would the inability to deliver highish peak currents, but I haven't measured either yet. We have a nice meter and current probe at work, perhaps I will being it home and make some measurements.

Regarding counter weights and magnets, mine is a 1Kg weight lifters weight from Tesco (about £2) and a strong magnet from eBay (see another post on here). The magnets are about £3 and I bought two magnets and two weights. The bolts on the magnets are a little short, so I used the large cup washer (again nicked from work) which centres the weight and allows the nut to go on the thread.

If anyone wants a cup washer and can't find one in the shops send me a message and I will put on in the post if we have a spare.

[Space Cowboy]

I reckon for £650 the auto dob 250 takes some beating for planetary imaging on a budget. It will overpower a Mak 180 especially with fainter planets like Saturn. You would have to spend well over a grand to get a scope with similar capabilities. Here are a couple of images I've taken this year. :

Stargazers Lounge - Space Cowboy's Album: Jupiter - Picture

Stargazers Lounge - Space Cowboy's Album: Saturn - Picture

[Moonshane]

the tracking on my 16" dob is great. I can keep things in view for an indefinite period. my system is called 'my hand'

[DaveGarland]

That's nice, those pictures have just made my mind up good n proper. I'm gonna get me one of them. It won't be until the new year unfortunately due to the difficult economic climate (the wife wants a laminate floor!). What barlow, camera etc did you use on those pics?

Dave.

[Moonshane]

those planetary images are excellent given the kit involved.

[Space Cowboy]

That's nice, those pictures have just made my mind up good n proper. I'm gonna get me one of them. It won't be until the new year unfortunately due to the difficult economic climate (the wife wants a laminate floor!). What barlow, camera etc did you use on those pics?

Dave.

Jupiter was taken with my new DFK camera and 5x powermate (f30) and Saturn with spc 900 webcam and 3x tal + extension (f34). Hoping to improve on the Jupiter image if I can get some good seeing in the next few weeks.

Shame you can't get the scope while Jupiter is at its peak

P.s here is an example of the tracking (when its working properly) 1 minute clip during imaging session (f30) roughly 1500x magnification:

[DrRobin]

Hi,

Nice video of Jupiter and yes it is tracking nicely. I am hoping that mine now tracks as well as that.

I got a few 80 sec pictures of M101 with the Supernova without much movement on the stars recently, but Deep Sky Stacker wouldn't stack them, still the individual frames show it was tracking, but a few minutes later and there was great long star trails. I would say I got one good picture out of 5.

When I increased the exposure to 180 seconds, I didn't get one decent frame. Still that was before all my balancing and tension nut setting work, now we just need a clear sky.

I have attached the best photo, it's a bit grainy, would be better if it was 10 frames at 180 secs stacked.

[DrRobin]

Hi,

I am still having problems, so decided to do a little research in to Goto alignment and accuracy. Not wishing to cloud this thread, I decided to start a new thread at

http://stargazerslounge.com/discussions-scopes-whole-setups/159938-skywatcher-az-goto-alignment-accuracy.html#post1989516

Any contributions to try and get the definitive description/procedure would be most welcome, I am fed up with loosing half my observing session to aligning or correcting scope position.

[Space Cowboy]

I must say Robin if you are getting 80 sec exposures as good as that then your tracking is pretty good for an Auto Dob. I can't get anywhere near that.....the best i could achieve back in the spring with M42 was 15 secs and even then the stars were trailing.

[DrRobin]

Hi,

Unfortunately 80 sec exposures of that quality wasn't repeatable. I only got one other like it in about 10 pictures. I know dobs aren't reckoned to be that good for astro photography but my dob doesn't even keep objects centred in the eyepiece for more than a few seconds and looking at the specs that just isn't right.

For example earlier in the year to keep Saturn centred in an SPC900 and 2x barlow, I had to make constant manual corrections when using the auto-track. I have seen your 1 minute video and I was no where near that.

I want to get the mount tracking and aligned as well as I can before I bite the bullet and start guiding. I have everything I need to have a go at guiding but this is just more to set up and I am not sure it is right to use guiding to cover up poor alignment errors in the first place.

E.g. the GOTO accuracy is supposed to be 2 - 5 arc minutes, I am lucky if it is even 30 - 60 minutes, sometimes objects are outside the FOV of the 25mm eyepiece and sometimes even outside the FOV of the standard finder, so we are looking at massive errors and might as well not bother with the GOTO.