Tracking without guiding on EQ5

[namreg]

Hi guys

Quick question from a noob.

I have an EQ5 and have recently purchased dual axis drives and a polar scope. I intend to venture into astro photography using my DSLR. Assuming I can make a reasonably good job of polar alignment using the polar scope, what length of exposure do you think I'll get away with before tracking errors start to intervene? Or is that like asking the length of a piece of string?

All advice gratefully received

Thanks

Doug

[gfletcher]

Hi Doug

If you get the polar alignment as accurate as you can you will be able to get up to 90 second exposures of objects close to the zenith ( the closer to the pole star, the longer your exposures can be).

If you started imaging say M42 , you would probably get up to 45 second exposures.

Be ruthless though on examining your images and discard any which show any star trailing. This maybe up to 50% of the total that you take.

Take as many exposures as you can.

HTH

Graham

[Black Knight]

I don't know what scope you have on your EQ5, but I have recently mastered getting 60 second exposures using my Canon 500D on my 200p and EQ5, unguided. Needs really good polar alignment though.

[namreg]

Thanks guys. Andy, I have the same as you, a 200p. A really good polar alignment will come with practice I suppose. Any tips?

[dph1nm]

Or is that like asking the length of a piece of string?

Yes! Depends what scale you want to show your images at. So, for instance, my HEQ5 has periodic error of 40 arcsec in 10 mins or so (which I believe is quite typical). If I was happy binning up my pixels to 40 arcsec (!) then I could image for 10mins (and more). More realistically, if I want 2 arcsec pixels not to show trailing, then I am restricted to about 30secs (maybe 1min if I am willing to throw away the worse exposures). On the whole, I find the periodic error restricts the exposure more than polar alignment.

NigelM

[namreg]

Blimey Nigel, that sounds depressing. I might as well just go out there and take a picture of the sky with my Canon Snappy and make do with that! Sounds a little hit and miss this astrophoto stuff.

[Black Knight]

Thanks guys. Andy, I have the same as you, a 200p. A really good polar alignment will come with practice I suppose. Any tips?

Use Astrobaby's tips here Astro Babys HEQ5 Polar Alignment. It refers specifically to the HEQ5, but applies to the EQ5 as well. You could also use the free PolarFinder app (just google polarfinder), that will help as well.

I've also started drift aligning which helps enormously. I don't have an illuminated reticule eyepiece, so I've tried using WCS from Polar Alignment - WCS - Fast and accurate polaralignment for astronomical mounts, using a CCD / webcam and drift alignment, which helps if you have webcam or whatever to use with it.

In no expert by any means but in general:

- start by getting rough alignment

- nail down the alignment with using Astrobaby's tips or PolarFinder

- then drift align using a reticule EP or something like WCS.

Oh, and er, it takes lots of practice. In my case hours and hours and hours of frustation. Until I finally got it right and managed a decent image. You may well progress better than I did!

[ollypenrice]

Well... it's only hit and miss without a guider, which is why most people use them. I did three consecutive all-nighters last month without dropping a single sub. The guider just did its thing. They are no longer expensive in the grand and rather relative scheme of things!

Focal length is hugely important in determining tracking tolerance. The longer the less tolerant, of course. So for this reason some camera lens imaging might sit well with your setup, though still tracking.

Olly

[Black Knight]

A couple of other things:

You will need a T-ring for your DSLR if you are going to do prime focus. Since you have the same OTA as me you will probably find it impossible to achieve focus with your DSLR. Two easy solutions:

- use a barlow lens. The major disadvantage here is that you are getting much less light on the cameras sensor, meaning you will need many more exposures to get a result.

- if you have the 2-in-1 Crayford focuser, you can unscrew the barrel of the 1.25" adapter on the focuser and attach the camera with T-ring directly to this. I didn't know I could do this until recently - this is what SW means by direct camera connection, and is by far the better option.

You will probably experience some coma around the edges of your images, but I was so chuffed to get anything decent at all I didn't care! . That's something for me to work on for the future!

[namreg]

Andy: I've attached the camera as in your second option - a very helpful guy at First Light Optics pointed that out to me. I literally just have the t-ring which screws straight onto the scope. So I'm not expecting focussing issues.

Olly: I do intend eventually to use guiding. In fact, that was my intention now until someone pointed out (again - First Light Optics) that I need an ST-4 port (ignorance is bliss). However, on a previous post someone directed me to Shoestring Astronomy that do a handy little conversion kit which effectively puts an ST-4 port on the hand controller of the dual drives - all for $7. It was then I decided that maybe I was trying to run before learning to walk, so I abandoned all that and am determined to learn the ropes, so to speak, before venturing any further.

[Black Knight]

Best of luck then, and post up your images when you've got 'em

[namreg]

Depends how good they are!

[dph1nm]

Blimey Nigel, that sounds depressing. I might as well just go out there and take a picture of the sky with my Canon Snappy and make do with that!

Actually you can get surprising good pictures even with 30sec subs (and with a 'Canon Snappy', actually, but that is another story)!

It is my impression, from reading the web, that most of the 'popular' EQ mounts have PEC of several arcsec/min. I don't think my HEQ5 is particularly unusual in this respect. I suspect it varies from mount to mount as well, so you might be lucky and get a good one.

NigelM