What Mount should I go for?

[Rumbold2010]

Hi All,

Hope you’re keeping well! Today I took delivery of a impulse purchase ( one I’m very happy with ) - An Orion Optics CT8. My intention is to use this to delve into a little more Astrophotography, at least for the next couple of years, this will be using my DSLR. I’ve managed to confuse myself a bit with research into what mount would be best in terms of handling payload and remaining within suggested tolerances. The OTA comes in at 7kg so, allowing for a few other bits and bobs, I’d imagine I’ll be somewhere between 10-15kg all in. I very much like the look of the Skywatcher AZ-EQ6 GT but in truth, is this a bit of overkill for my intended use? 
 

thanks as always, clear skies!!

 

Andrew

[MarkAR]

Get the one with the biggest payload you can afford, it will be a lot more stable than something just on the limit.

I have the AZ EQ6 and it is a lump but it does the job well. You wont regret overkill on a mount.

[DaveS]

You can never be over mounted, go for the biggest payload you can afford.

[MarkAR]

Just had a thought, I think my payload is similar to yours. 

Also it's AZ for observing or EQ for AP, best of both worlds.

[ollypenrice]

Payload is not the priority. Accuracy is the priority - and accuracy can be inadequate because the mount is overloaded or because it is simply not able to deliver the tracking accuracy required by the resolution of the system. A mount which will work at low resolution will not necessarily work at high resolution with the same payload.

I assume your scope is this one https://www.astroshop.de/fr/telescopes/telescope-orion-optics-uk-n-200-900-ct8-carbon-ota/p,57566 so it has a 900mm focal length. As a rule DSLRs have small pixels (though not all do) but if you use this calculator http://www.12dstring.me.uk/fovcalc.php you'll be able to discover your imaging resolution in arcseconds per pixel. Whatever that resolution is, divide it by two to get an idea of the tracking accuracy you need.  If your imaging resolution is 2.0 arcseconds per pixel your mount must deliver an average guiding accuracy of half that, so 1.0 arcsecond. If you are imaging at 1.0 arcsecond per pixel the mount must deliver an accuracy of 0.5 arcseconds. A very good EQ6 with autoguiding, carefully set up, will usually deliver about 0.5 arcseconds. This will support an imaging scale of 1.0 arcseconds. Even then, your local seeing (the stability of the atmosphere, not the clarity of the sky) must play ball.

I don't want to bombard you with information overload but there are common errors on the forum about mounts. The main ones are, 1) if the payload is not exceeded the mount will work. No, it must deliver the tracking accuracy the system needs. 2) round stars prove good tracking. No. Equal errors in RA and Dec produce round stars despite bad tracking and lost resolution.

Olly

[MarkAR]

Good info there Olly, I'm imaging at 1.454"/pix and Ekos is giving me between 0.4 and 0.7 RMS in general over a whole nights run including dithering, guided. Thats a 115EDT with 0.8x reducer on AZ EQ6.

I think the AZ EQ6 will handle a CT8 ok.

 

[robbymain]

Hi Andrew

I’m very much looking forward to picking up a used CT8 from a fellow SGLer once lockdown rules allow and will be putting it on my classic green Vixen GP-E. It should be ok for visual use (I’ve had a precursor to OOUKs VX8 on a GP before and it didn’t complain) and but I’m hoping to find a used SX2 before too long that has a photographic capacity of 12kg - I do agree that it’s better to have plenty of loading capacity spare if you plan to do much imaging at all!

All the best with the CT8, perhaps we can swap notes once we have some use under our belts?!

 

[Rumbold2010]

That sounds like a great idea - let’s do so and good luck!! Clear skies...

[Rumbold2010]

On 05/05/2020 at 20:24, ollypenrice said:

Payload is not the priority. Accuracy is the priority - and accuracy can be inadequate because the mount is overloaded or because it is simply not able to deliver the tracking accuracy required by the resolution of the system. A mount which will work at low resolution will not necessarily work at high resolution with the same payload.

I assume your scope is this one https://www.astroshop.de/fr/telescopes/telescope-orion-optics-uk-n-200-900-ct8-carbon-ota/p,57566 so it has a 900mm focal length. As a rule DSLRs have small pixels (though not all do) but if you use this calculator http://www.12dstring.me.uk/fovcalc.php you'll be able to discover your imaging resolution in arcseconds per pixel. Whatever that resolution is, divide it by two to get an idea of the tracking accuracy you need.  If your imaging resolution is 2.0 arcseconds per pixel your mount must deliver an average guiding accuracy of half that, so 1.0 arcsecond. If you are imaging at 1.0 arcsecond per pixel the mount must deliver an accuracy of 0.5 arcseconds. A very good EQ6 with autoguiding, carefully set up, will usually deliver about 0.5 arcseconds. This will support an imaging scale of 1.0 arcseconds. Even then, your local seeing (the stability of the atmosphere, not the clarity of the sky) must play ball.

I don't want to bombard you with information overload but there are common errors on the forum about mounts. The main ones are, 1) if the payload is not exceeded the mount will work. No, it must deliver the tracking accuracy the system needs. 2) round stars prove good tracking. No. Equal errors in RA and Dec produce round stars despite bad tracking and lost resolution.

Olly

Thanks a million for the info Olly, not overload but also not info that’s familiar to me so lots of food for thought. As it  happens, I eventually settled on an EQ6R Pro Mount, hopefully here next week so the learning can begin in earnest. I’m expecting a very different experience to that my CPC925 has given me the past 5 yrs. 

Andrew