Altazimuth goto mounts and imaging?

[sploo]

Having done a bit of simple astrophotography using a DSLR+camera lens+Skywatcher Star Adventurer I've picked up a nice simple noob starter scope... a 300P flextube Dobsonian (yea, I know).

Despite my complete lack of scope experience, I'm so far enjoying the views through the scope (and probably should be asking lots of questions about that); but my mind is turning to the photography question.

In terms of tracking, I assume the altazimuth GOTO mounts would be able to find (and maybe keep) a particular star or galaxy centred in the field of view, but that they would not be able to correct equatorial rotation - such that you'd continue to look at the right object, but it would slowly rotate in the view?

My 300P doesn't have the GOTO system but I'm mulling over ideas for adding motors - either altazimuth, or maybe a Poncet Platform for the whole scope+mount to sit on.

[PeterCPC]

Have you looked here

Peter

[sploo]

4 minutes ago, PeterCPC said:

Have you looked here

Peter

Interesting, thanks. So... my take home is that short exposures (10s-30s) may be feasible with only tracking on the altitude and azimuth axes. I assume that multi-minute exposures would still be out of the question though?

[PeterCPC]

2 minutes ago, sploo said:

Interesting, thanks. So... my take home is that short exposures (10s-30s) may be feasible with only tracking on the altitude and azimuth axes. I assume that multi-minute exposures would still be out of the question though?

I think that you need to experiment but, if you want long exposures, then an EQ mount would be required IMO. Planetary imaging is ok with an AltAz.

Peter

[sploo]

Grand. Many thanks.

Of course, I could try putting the 300P tube on the Star Adventurer 😆

Edited February 13, 2020 by sploo

[rob_r]

I have the SkyWatcher AZ GTI mount for my mak127. I have also tried taking photos with my kit with a couple of decent results, nothing much worthy of posting here. From my short experience for just over a year now, I find that anything over about 12 to 15 seconds and field rotation starts to become apparent as the stars start to deform. 30 seconds and the stars definitely trail. I got some decent photos of M42 on just stacking about two dozen 2 second exposures. Planetary is all good, managed a few single shots of Jupiter and Saturn last year. I got EQ5 mount recently but still not had chance to unpack it as yet.

Edited February 13, 2020 by rob_r

[sploo]

Just the info I was after - thanks Rob.

It does occur to me that with a camera mount that itself could rotate, a combination of alt, azimuth, and camera rotation, would probably allow long tracking. Obviously it's much more complex (physically and logically) than having a single equatorial axis, but for larger scopes I do wonder if it would be more cost effective than the amount of bulk required to create an EQ mount with a similar load capacity.

That said, probably a Poncet platform would make more sense - although they can only usually track for some tens of minutes before needing to be reset.

[happy-kat]

The longer the focal length the quicker flaws will show such as stars elongating.

There is a member with a 250mm on that thread, light bucket but uses short exposures.

[sploo]

31 minutes ago, happy-kat said:

The longer the focal length the quicker flaws will show such as stars elongating.

There is a member with a 250mm on that thread, light bucket but uses short exposures.

Yep. My background is photography rather than astro (so I had to get my head around what "magnification" meant in telescope terms). I've had reasonable results with 30s exposures at 400mm on a full frame DSLR with the Star Adventurer; but that was a camera lens, so relatively small and light in telescope terms.

The 300P is 1500mm if used as a prime lens, so that would be much more of a problem. Maybe OK for a quick burst of images of the moon, as the "500 rule" means that I might get away with 500/1500 = 1/3s exposures. No good for DSOs though.

[happy-kat]

The Moon is likely ISO 200 and around 1/100-200 second exposures or take video and process using say registax.

DSO likely ISO 800-1600 1-30s take lots and stack say using DSS

Fun either way

Edit, sorry assuming you have motorised your mount in altaz

Edited February 13, 2020 by happy-kat

[sploo]

A rather crazy combo of a 400mm f/5.6 lens, with stacked 1.4x and 2x teleconverters (1120mm f/16) works pretty well for me with a Canon 5D4 in 4K video mode. The 5D4 has a 1.74x crop in 4K video mode (for an effective 1950mm focal length), so fills the sensor quite nicely with the moon. 1/50s exposures (at 25fps video) seems to work pretty well; especially when on the tracking mount.

For DSOs I've only really tried M42.

I haven't motorised the mount, but I was pretty impressed with the design of the Dob mount on the 300P; simple, and does the job. I'd considered trying to make a lighter version, possibly with castors. However, looking more into tracking mounts I see there's an equatorial mount called an "Open fork". It's actually just an altazimuth, but where the azimuth axis is tipped to point at the pole. I think that could be an interesting possibility for a tilted Dob mount, with the az axis motorised.

[2STAR]

On 13/02/2020 at 12:12, sploo said:

Having done a bit of simple astrophotography using a DSLR+camera lens+Skywatcher Star Adventurer I've picked up a nice simple noob starter scope... a 300P flextube Dobsonian (yea, I know).

Despite my complete lack of scope experience, I'm so far enjoying the views through the scope (and probably should be asking lots of questions about that); but my mind is turning to the photography question.

In terms of tracking, I assume the altazimuth GOTO mounts would be able to find (and maybe keep) a particular star or galaxy centred in the field of view, but that they would not be able to correct equatorial rotation - such that you'd continue to look at the right object, but it would slowly rotate in the view?

My 300P doesn't have the GOTO system but I'm mulling over ideas for adding motors - either altazimuth, or maybe a Poncet Platform for the whole scope+mount to sit on.

You need a goto, alt/az is ok if using sharpacap and a CMOS OSC, SC makes slight adjustement to keep object on track and minimise Field rotation, the latest CMOS cameras from ZWO are also  good on a alt/az mount.

For really good images it has to be EQ though.

eric