Skywatcher Star Adventurer

[alcol620]

Hi folks, me back again.

Had the Adventurer out last night for about 3 hours before it clouded over. This time I had:

Checked the calibration of the Polarscope. A reasonably balanced set up after putting the longer dovetail on.

However, even with 30s subs I could see slight star trailing, with 60s worse and obviously longer subs than this even worse.  I have attached a couple of examples to illustrate subs of 60s and 300s (I know this length of sub is likely to trail but it does show the direction of trailing) All the trails appear to be in the same direction across the whole image. Not in a circle or pointing to the centre of the image. 

I noted that over the 3 hours of taking subs, Polaris followed the circle in the Polarscope, presumably confirming that the mount was Polar aligned OK?

Presumably I should expect better Than 30s subs without trailing. Would anyone like to suggest what might be causing thetrails? Is it perhaps mechanical?

 

 

 

 

[Davey-T]

Have you done a 24 hour sidereal test, it looks suspiciously like my original SA.

Have a read of this thread

Dave

 

[alcol620]

Thanks as always Dave.I did do this check, but not very accurately. I am setting up to do it again now only very accurately this time. I saw mention of updating the firmware, I haven't done this as I am always worried about this going wrong in the download and nothing working afterwards.

Thanks again

[JamesF]

The impression I had was that the firmware update was only necessary if you wanted the additional features it provides.  In the general case I'm not sure it's necessary.  I could be completely wrong though.

James

[Davey-T]

35 minutes ago, alcol620 said:

I saw mention of updating the firmware, I haven't done this as I am always worried about this going wrong in the download and nothing working afterwards.

Not needed as it just provides more timelapse options.

Dave

[alcol620]

Thanks Guys. Fully loaded mount ready to go on a lap of the universe, but waiting till this evening when I have a good chance of being here tomorrow to record the end of the lap!

What I did find when I checked the mount over in preparation was 4 screws holding the Dec clutch were not tight.  This certainly wouldn't be helpful, but the 24 hour test goes on.

[Davey-T]

Worth checking the big screw in the bottom of the wedge dovetail and the two countersunk screws it dovetail clamp.

Dave

[thomasv]

I have to say I wouldn't be surprised if you get some trailing at 260mm focal length based on the periodic error I've measured for my SA, see towards the end of this post:

Some people report good results for up to 300mm FL without guiding, I can't see how I could do that with my mount, I wonder if there is a fairly large spread in the extent of PE between individual SAs. 

 

[Davey-T]

20 minutes ago, thomasv said:

 

Some people report good results for up to 300mm FL without guiding, I can't see how I could do that with my mount, I wonder if there is a fairly large spread in the extent of PE between individual SAs. 

 

Mine can do 90sec's reliably with 300mm lens, others have done longer, using 24mm it can do pretty much as long as you like only limited by light pollution.

Dave

[thomasv]

Just wonder if by 'reliably' you mean you don't have to discard any subs at 300mm 90sec? With the shape of PE being roughly sinusoidal, there will be some time periods with not much change for 90sec and some with a rapid change of error. I've only seen a couple of other measured reports of PE for the SA and if I remember correctly they were in the range of 30-50" peak to peak, so I may be unlucky with my mount.

Another slightly amusing thing I've just found recently is that, since I use the polemaster to align, I used to leave the plastic polar scope cover on, but the setting circles on the polar scope can actually touch the plastic cover.  With the RA clutch loose, rotating the axis by hand gives scratching noises as it rubs against the plastic cap. It might be in the manual which I've never read fully, that the cover is supposed to be left off for the whole session, not just during polar alignment.

  

[Davey-T]

43 minutes ago, thomasv said:

Just wonder if by 'reliably' you mean you don't have to discard any subs at 300mm 90sec?

Yes as long as I'm aimed at a sensible Dec.

I always leave the cover off so I can keep an eye on the PA.

Never measured the PE as I've never guided it.

Dave

Edited August 29, 2018 by Davey-T

[alcol620]

Right then guys, the 24 hour circuit of the Earth has been completed and I require some clever feedback. I had the lazer pointer tracking and at the end of 23 hours 56 minutes and 4sec (approximately) I was 4cm short of a complete circuit. The distance from the center of the RA axis to the marks on the wall was 141.5 cms so I make the angle of rotation to be short by 1.625 degrees.

Now guys tell me what this means in terms of running a scope with a focal length of 260mm and a camera with 3.8 um pixels? Is this the reason for my trailing stars with exposures above 30s?

Thanks

[JamesF]

18 minutes ago, alcol620 said:

Right then guys, the 24 hour circuit of the Earth has been completed and I require some clever feedback. I had the lazer pointer tracking and at the end of 23 hours 56 minutes and 4sec (approximately) I was 4cm short of a complete circuit. The distance from the center of the RA axis to the marks on the wall was 141.5 cms so I make the angle of rotation to be short by 1.625 degrees.

Now guys tell me what this means in terms of running a scope with a focal length of 260mm and a camera with 3.8 um pixels? Is this the reason for my trailing stars with exposures above 30s?

Thanks

I get it to be a shade under 1.625 degrees, but it depends what value you're using for pi I guess.  Anyhow, 1.625 degrees is 5850 arcseconds.  I reckon that makes the error two arcseconds every 30 seconds.

A focal length of 260mm gives you 793 arcseconds per mm or 3 arcseconds per pixel.

Assuming I have my maths right, I'm therefore going to suggest that no, I don't think that amount of inaccuracy will cause any significant star trailing over a single 30 second exposure.  Over a 60s exposure I don't think you'd see as much as is apparent in your 60s frame either, so I'm not convinced it's the main cause of your problem.

I suspect alignment may be more of a problem.

James

[alcol620]

12 minutes ago, JamesF said:

I get it to be a shade under 1.625 degrees, but it depends what value you're using for pi I guess.  Anyhow, 1.625 degrees is 5850 arcseconds.  I reckon that makes the error two arcseconds every 30 seconds.

A focal length of 260mm gives you 793 arcseconds per mm or 3 arcseconds per pixel.

Assuming I have my maths right, I'm therefore going to suggest that no, I don't think that amount of inaccuracy will cause any significant star trailing over a single 30 second exposure.  Over a 60s exposure I don't think you'd see as much as is apparent in your 60s frame either, so I'm not convinced it's the main cause of your problem.

I suspect alignment may be more of a problem.

James

Thanks James for you input, it may have been the loose screws, I will give it another run on the next clear light to check it now the screws have been tightened. I had the SA all loaded up so your answer is reassuring.

[alcol620]

Hi James

When you mentioned alignment possibly being more of a problem, were you referring to polar alignment or possibly alignment of the sensor with the scope tube?

[JamesF]

My first guess would be polar alignment.

James

[alcol620]

Thanks James, I will take great care when I next get outside, may be tonight?

[GuLinux]

Sorry if I might ask the obvious, but do you align by putting Polaris on the center of the polarscope reticule, or do you use a polar finder app to get the actual position of Polaris?

There's almost a whole degree difference between Polaris and the actual pole, depending on the target declination that might account to a lot!

[Davey-T]

7 minutes ago, GuLinux said:

Sorry if I might ask the obvious, but do you align by putting Polaris on the center of the polarscope reticule, or do you use a polar finder app to get the actual position of Polaris?

There's almost a whole degree difference between Polaris and the actual pole, depending on the target declination that might account to a lot!

Put it where shown by the app.

Dave

[GuLinux]

1 minute ago, Davey-T said:

Put it where shown by the app.

Dave

I wasn't clear, sorry I was just asking @alcol620 what he does, as that might explain his tracking issues, I wasn't asking for suggestions ?

[alcol620]

Hi guys, I put it on the circle at the point where it should be at that time. I think that is correct. I use the Jason Day positioning app. 

[RayD]

Hmmmmm I'm not totally convinced about PA with that much trailing.  I would be checking and eliminating anything mechanical such as clutch slip or a gear key-way grub screw come loose or something similar.

[alcol620]

Hi Ray

As my earlier posting said, I did find some screws loose, I haven't had the opportunity since to run with stars in the frame - only the 24 hour test performed. So next time out I'm hoping for the best. I know that the TSAPO60 and the ASI1600, does struggle to get round stars in the corners with the flattener/reducer even when great care is taken to get the spacing spot on.

[RayD]

3 minutes ago, alcol620 said:

Hi Ray

As my earlier posting said, I did find some screws loose, I haven't had the opportunity since to run with stars in the frame - only the 24 hour test performed. So next time out I'm hoping for the best. I know that the TSAPO60 and the ASI1600, does struggle to get round stars in the corners with the flattener/reducer even when great care is taken to get the spacing spot on.

Ah Ok I hadn't realised you hadn't tested since.  If it was spacing it would be different patterns, but this is definitely trailing being across the whole field, either as a result of really bad PA (but I would think it would need to be really bad to show up this much in 30s) or some other mechanical issue. 

[alcol620]

2 minutes ago, RayD said:

Ah Ok I hadn't realised you hadn't tested since.  If it was spacing it would be different patterns, but this is definitely trailing being across the whole field, either as a result of really bad PA (but I would think it would need to be really bad to show up this much in 30s) or some other mechanical issue. 

thanks Ray for reassurance re trailing rather than spacing, although there looks to be a bit of it too.