Fork-mount SCT for video astronomy

[Ben Ritchie]

I've mostly gone over to the dark side of video astronomy with a Watec 120N+, and for various reasons i'm wondering about moving it to a fork-mount SCT system - I rather fondly remember the simplicity and relative compactness of the 8" LX200 GPS I used to own. I need something that takes up less room than a 5" f/8 refractor on a GEM.

My thoughts are to go for a CPC 925 or 1100 with f/3.3 reducer, which should be fine with the small chip of the Watec and would give a similar image scale to the one I currently have with the refractor. But wondering if i've missed anythign I should have thought of - e.g. with an Alt-Az do I get bitten by field rotation? I can't remember how long you can integrate with on a SCT, it was a long time ago.

Undecided about 925 vs. 1100, usual aperture vs. focal-length/cooldown thoughts. Will also get used for some visual. No objection to Meade, but they seem very expensive these days unless I can find a used one.

[Dunkster]

Not trying to be funny, but have you seen the size of the CPC

[Ben Ritchie]

Yes! I did say 'relative compactness' I know they're big, but on a pier with OTA pointed down they take up a fair bit less space than a big 5" refractor plus GEM with counterweight bar, and much quicker to move in and out when necessary - my LX200 was a case of stick it outside, turn it on, wait a couple of minutes while it got a GPS fix and LNT did its stuff, then a two-star align and go. The GEM is 20 minutes setup even with a crude 'good enough for visual' polar alignment.

[Dunkster]

I've recently had a play with the new VX mount and - coming from an EQ6 owner - it's substantially smaller and lighter. I didn't go for the CPC originally because I can't lift the weight (and the ridiculous prices of the Edge HD CPC...) and the EQ6 is more manageable in that respect. Still effort, but breaking down into pieces it's doable. The VX on the other hand, I could lift easily - I tried this without the ota but with counterweight, and with the tripod retracted it's not that big. The tripod of a CPC is pretty big in itself, but like you say, the footprint remains the same regardless.

Not sure about the field rotation though... I guess that would depend on the integration time period.

For the ota size... I don't know the resolution of your camera, but given that it's using integration anyway, aren't you just as well off with a C8 than go through all the extra effort with a bigger ota? Of course, greater light grasp and resolution and all, but isn't the frame integration somewhat compensating for the light grasp? (forgive me if I misunderstand )

[seiko]

Not trying to be funny, but have you seen the size of the CPC

hey, are you trying to scare me! I've ordered mine already!

[Dunkster]

hey, are you trying to scare me! I've ordered mine already!

LOL boy are you in for a surprise :D

If you're under about 6ft tall, it's taller than you and likely heavier

[Ben Ritchie]

For the ota size... I don't know the resolution of your camera, but given that it's using integration anyway, aren't you just as well off with a C8 than go through all the extra effort with a bigger ota? Of course, greater light grasp and resolution and all, but isn't the frame integration somewhat compensating for the light grasp? (forgive me if I misunderstand )

You're quite right - the Watec integrates for up to 10s so you see far deeper than you would with Mk1 eyeball, the other benefits are that you can use broadband filters quite effectively (local LP is a bit of a pain) it that it's nice for groups too - I have a 17" LCD TV bolted to the wall of the observatory and it's much easier to point at that than wonder what the heck they might be able to see through the eyepiece. If it was only video then it would be a toss-up between the CPC 800 and 925, i'm thinking larger partly because it's nice for occasional visual too even if cool-down is becoming a pain at that size. The CPC 925 may be the sweet spot in my analysis-paralysis, although it might ultimately be settled by what turns up used.

But i've not owned a fork-mount SCT since 2006 and never did video back then anyway, so wondering if there's any reason that i've not thought about that makes it a bad idea - obviously field rotation is a killer with larger sensors and longer integration times, but i'm guessing that 10 seconds with a half-inch CCD at an effective reduced focal length roughly in the 600-1000mm range should be ok.

[Dunkster]

Someone cleverer than me would be able to calculate the angular rotation relative to the size of the pixels and therefore the maximum exposure you'd be able to get away with. Failing that, I think you're going to be OK for 10s, maybe more. I'll be interested to hear how you get on, as I'm in a poor LP area and interested in video too.

Regarding cooling... if it's permanently mounted in your obsy, that should minimise/eliminate cooling time unless we have a hot summer with cool nights... what are the odds?

FWIW, the 1100 is only 3kg heavier than the 925 (29kg vs 26kg for the fork+ota), but gives you 40% or more light grasp than the 925. Worth considering if you're interested in non-video visual too.

[Peter Drew]

I had great results with a 12" SCT and a Watec 120n operating at F3.3, no apparent rotation problems at 10 seconds integration.

[Stu_2011]

LOL boy are you in for a surprise :D

If you're under about 6ft tall, it's taller than you and likely heavier

Don't try and scare people now.....seriously unless you have any major back problems then the CPC is quite manageable. I'm only 5'4" and I move my CPC925 around without too much problem. I move the tripod and then the OTA and its quite easy really.

Mine is all set-up and ready to go tonight, looking forward to it.

Sent from my iPad using Tapatalk

[DoctorD]

Hi

I use my C8 at F3.3 and F5 on an iOptron Minitower and a SDC 435 video camera.

No problems with field rotation so far.

I agree that the biggest advantage for me is the ability to view any DSO from my light polluted garden.

HTH

Paul

[seiko]

I had great results with a 12" SCT and a Watec 120n operating at F3.3, no apparent rotation problems at 10 seconds integration.

Ha ha, well mine arrives tomorrow so I'll get to see it myself. Either way though, I'll be coping with its size one say or another

[djetman]

Hi,

I use an 8" SCT with f 3.3 focal reducer and have no problem with field rotation. I'm using a Samsung SBC 4000. It seems to work very well.

Dennis