Advice on next steps wanted

[philherbert]

Hi,

This is my first post to SGL for DSO, although I have been reading for a while. I use a SkyWatcher ED100 on an HEQ5Pro mount with SynScan, and an un-modded Canon 350D at prime focus.

I can get 30 second exposures with minimal trailing, but am struggling to do much better. I am just starting to learn Polar Alignment adjustments from the SynScan menus. I'd be interested in advice on what to do next. Should I keep trying with the PA setup until I can get say 120 second exposures without trails, or should I just go straight to an autoguider setup? I also wonder if I should get the 350D modded, or get some filters to cut down on light pollution?

As you can tell, I am considering many options, but would appreciate others' thoughts on where to start. For the record, by far my best attempt at Orion is below.

Thanks,

Phil

[Freddie]

Welcome to SGL. I would suggest you try the DARV method (google it to find the details or search on here) which uses your cam to check and increase the accuracy of your PA before looking at the next steps. I would have thought your mount/scope/cam should achieve more than 30 secs with good PA.

[philherbert]

Thanks Freddie. Is this DARV method suitable with just a camera? I've not tried computer control of the mount or camera yet.

[AndyUK]

Yes - A warm welcome to SGL Phil... and may I start by saying that for an unmodded, unguided ED100, that's a VERY good result indeed .

I actually started out imaging with a very similar set up (a CG5 goto, an ED100 and a 40D)... Admittedly I didn't give as much attention to PA as I should have done, but by trial and error I  could sometimes get up to 60s at 900mm FL, and by taking enough frames, you can get some very reasonable results.   Accurate PA will certainly enable you to get longer unguided exposures, but ultimately you'll almost certainly want to get even longer exposures (believe me!) and then you'll really need to look into guiding options.

My first step was to get my DSLR modded (not being able to spell DIY, I had Astronomiser do mine for me, but many are able to do it themselves - I personally had a filter replacement mod so that I could still bring it to focus and use it as a terrestrial DSLR by setting a custom white balance) but very shortly after I then bought an EOS CLS clip filter....

However, I then discovered that the ED100 (@ f9) wasn't really ideally suited to DSO imaging - It's quite slow - So I opted for a faster scope (f5.3) which almost trebled the photons I was able to get for the same exposure length ...  but then I realised I wanted (needed) to guide, and that's where things started becoming REALLY interesting...

I'm not familiar with the Synscan PA routine (I'm an EQMOD convert so don't use a handset any more ) but you can do drift aligning with an EP prior to attaching your camera (it's best if you have a reticle EP), although you can obviously also drift align with your camera using software such as APT or Backyard EOS... I have to say I dragged my heels attaching my mount/camera to a laptop (mainly because it was a work PC ), but it's pretty much inevitable at the end of the day that you'll need to use one - There are so many advantages!  I tried to avoid it for as long as possible by using a Synguider, and must admit that, for a year, I had great fun with that set up (easily getting 600s exposures), but then came the attraction of the sensitivity of mono / narrow band imaging... and so began the slide into bankruptcy the darkside...

In answer to your question "Should I keep trying with the PA setup until I can get say 120 second exposures without trails, or should I just go straight to an autoguider setup? I also wonder if I should get the 350D modded, or get some filters to cut down on light pollution?"  I think the short answer is ultimately yes to all... it just comes down to the order you choose to do them... but guiding is certainly a game changer, but with that then comes the need to cut out the effects of LP... and you'll then want to capture the Ha light band which DSLR's, by default cut out...

[MikeODay]

How do you polar align now?  Does your mount have a polar scope?  I've had my scope and mount since Easter and for most of that time I just used the polar scope to help me get a reasonable alignment; just lately I've been trying to improve the alignment using the drift method as I now have the scope mounted on a pillar.

I have found that an auto-guider and reasonable alignment gets me to 2 minutes without any issues.  On the pillar I intend to try for 4 minutes (at ISO800 and f4 this will put the peak of my histogram near the middle due to local light pollution).  There is a calculator that will tell you the  rate of drift in elevation you can expect for a particular value of polar alignment error.  [ link: http://celestialwonders.com/tools/driftRateCalc.html ] This might help you work out how good your alignment needs to be to achieve particular exposure durations.  (example below)

Bye the way, I also use a Baader UHC-S nebula filter to cut back the pollution, etc, and bring out the detail in the Nebula targets I have been chasing.   It has definitely improved my images.

My recommendation, for what it is worth, is to work on your technique so you can get a reasonable alignment without wasting too much time and then invest in an auto-guider.  A hint that might help, depending on your circumstances, is to always place your tripod in the same location; having levelled the scope and polar aligned the first time and marked the locations of the legs.  When you finish the session, park the scope in the home position and then carry the tripod inside with the mount still attached and without adjusting the legs.  When you use it next place the tripod in the same location and you will greatly reduce the time you need to get a reasonable alignment.

Cheers

Mike

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

Example of results from calculator and how I use them ...

Plugging in a 10 arcmin alignment error, 1 minute exposure and declination of 0 deg results in a drift in elevation of 2.6 arcsec / minute. 

With my camera (Nikon D5300, 6000x4000 pixels, 1.5x crop sensor) on my scope (1016mm focal length) I have a field of view of ~ 1.33 by 0.89 degs (or 1.33x3600 by 0.89x3600 arcseconds). 

So a drift of 2.6 arcsec/min is equivalent to 2.6 / (0.89*3600) *4000 pixels / min.  Or a drift of 3.25 pixels / min. 

Likewise, a 1 arcmin alignment error gives 0.3 arcsec / min or, for me, 0.38 pixels per minute. 

[MikeODay]

Ah, perhaps I should explain why I care about accurate alignment when I have an autoguider...

Yes the autoguider helps to correct drift in declination but it is not perfect, particularly when the seeing is poor and the autoguider starts to chase the guide target.  But also, for longer exposures, despite guiding, if your alignment is too far out you will begin to see movement in the outer parts of your image due to field rotation. 

So, I need alignment that is good enough to eliminate visible field rotation with the ultimate goal of getting it sufficiently accurate that I can either turn off guiding in declination or at least tune the parameters so that it is only responding to the very slow drift.

[philherbert]

Yes - A warm welcome to SGL Phil... and may I start by saying that for an unmodded, unguided ED100, that's a VERY good result indeed .

I actually started out imaging with a very similar set up (a CG5 goto, an ED100 and a 40D)... Admittedly I didn't give as much attention to PA as I should have done, but by trial and error I  could sometimes get up to 60s at 900mm FL, and by taking enough frames, you can get some very reasonable results.   Accurate PA will certainly enable you to get longer unguided exposures, but ultimately you'll almost certainly want to get even longer exposures (believe me!) and then you'll really need to look into guiding options.

My first step was to get my DSLR modded (not being able to spell DIY, I had Astronomiser do mine for me, but many are able to do it themselves - I personally had a filter replacement mod so that I could still bring it to focus and use it as a terrestrial DSLR by setting a custom white balance) but very shortly after I then bought an EOS CLS clip filter....

However, I then discovered that the ED100 (@ f9) wasn't really ideally suited to DSO imaging - It's quite slow - So I opted for a faster scope (f5.3) which almost trebled the photons I was able to get for the same exposure length ...  but then I realised I wanted (needed) to guide, and that's where things started becoming REALLY interesting...

I'm not familiar with the Synscan PA routine (I'm an EQMOD convert so don't use a handset any more ) but you can do drift aligning with an EP prior to attaching your camera (it's best if you have a reticle EP), although you can obviously also drift align with your camera using software such as APT or Backyard EOS... I have to say I dragged my heels attaching my mount/camera to a laptop (mainly because it was a work PC ), but it's pretty much inevitable at the end of the day that you'll need to use one - There are so many advantages!  I tried to avoid it for as long as possible by using a Synguider, and must admit that, for a year, I had great fun with that set up (easily getting 600s exposures), but then came the attraction of the sensitivity of mono / narrow band imaging... and so began the slide into bankruptcy the darkside...

In answer to your question "Should I keep trying with the PA setup until I can get say 120 second exposures without trails, or should I just go straight to an autoguider setup? I also wonder if I should get the 350D modded, or get some filters to cut down on light pollution?"  I think the short answer is ultimately yes to all... it just comes down to the order you choose to do them... but guiding is certainly a game changer, but with that then comes the need to cut out the effects of LP... and you'll then want to capture the Ha light band which DSLR's, by default cut out...

Thanks Andy, you've confirmed what I feared - that I am just starting a very interesting and expensive hobby! It also sounds as though you have been through this already. I think I'll practice the PA for now, and prepare myself for the seemingly inevitable slide towards guiding, filters and CCDs!

Phil

[philherbert]

Ah, perhaps I should explain why I care about accurate alignment when I have an autoguider...

Yes the autoguider helps to correct drift in declination but it is not perfect, particularly when the seeing is poor and the autoguider starts to chase the guide target.  But also, for longer exposures, despite guiding, if your alignment is too far out you will begin to see movement in the outer parts of your image due to field rotation. 

So, I need alignment that is good enough to eliminate visible field rotation with the ultimate goal of getting it sufficiently accurate that I can either turn off guiding in declination or at least tune the parameters so that it is only responding to the very slow drift.

Thanks Mike (for this and your other reply). I think I'll stick with the polar alignment (I have a polar scope in the mount) for now. I haven't yet established a fixed position for the tripod - that should probably be my next project!

Phil

[Owmuchonomy]

Hi Phil I have a very similar set up to you. The best I could get using the polar scope followed by the handset polar alignment routine was 90 seconds. I have now added guiding and use the Drift Alignment that is built into the free guiding software called PHD2. I can now get 20 minutes if required. You can see some of my efforts on my link. I also now have a modded DSLR.

[philherbert]

Thanks Chris; hearing that you only got 90 seconds without guiding makes me feel better at my 30 seconds efforts so far!