Successfully Guiding an 8" Celestron SCT - Any ideas?

[PhotoGav]

I had a good session yesterday night - though a tad chilly! I was concentrating on trying to get guiding going with my Celestron 8" EdgeHD on an HEQ5-Pro, with an EOS60D attached, guiding with a QHY5-II through the 9x50 finder, using PHD2. I tried some 300s subs and 600s subs. In the dark the 600s subs seemed to be good so I set off a run of 11 subs. On processing today I see that the stars are not as tight as I would like... I presume that this is down to guiding.

So, any tips, hints, advice and suggestions on how to guide a monster focal length with a tiddly guide scope would be much appreciated please!

I calibrated with a calibration step of 4000ms, anything less wasn't moving things far enough - does that sound about right? The graph was pretty steady and quite flat, hunting around the centre line, no drastic spikes or nastiness. I didn't touch any of the other settings, so they were at the default values.

Thanks in advance.

[Leveye]

Please post an example of the stars you got that night. Initial balancing and alignment of the mount is pretty critical at that focal length. The mount needs to be at your "true" north or south position and this gets overlooked quite often.There are world charts online for that. Just pointing it with a compass it not good enough.My mount has GPS so that helps. Then your polar alignment is critical and then multi star alignment after that. It all adds up to perfection. Also there are settings for just how much leeway you want to give guiding in pixel movement. You can make that tighter. Focus of course needs to be spot on.

[PhotoGav]

Will do, just popped home for a cuppa - laptop is in office... Will post a rough pic in about half an hour!

[Thalestris24]

Hi there

Not sure as I'm only just getting the hang of guiding myself. However, the EdgeHD has a focal length of 2032mm? Whereas the finder is only 200mm. I would suggest that ratio is maybe too great and you could need a longer focal length guidescope. I think the main scope/ guidescope resolution ration shouldn't be greater than 4:1. Remember, the PHD only reflects what the guidescope can see. Also, a 4 sec calibration step sounds rather long.  I have a 9x50 and qhy5l-ii and use a calibration step of 2150ms to guide my 150pds.

I'm sure others here will be able to advise. There is a good phd guiding tutorial on here: http://stargazerslounge.com/topic/188777-phd-guiding-basic-use-and-troubleshooting/?hl=%2Bphd+%2Bguiding+%2Btutorial#entry2037281

That will take you through the calculations and settings you need to be concerned about.

Hth.

Louise

[swag72]

I would suggest that you are really pushing it Gav to be able to guide with your finder guider. I bit the bullet with the C9.25 and went for an OAG - With the reducer in the imaging train I'm only working at 1370mm - I'd not try a finder guider with that, let alone more focal length.

[red dwalf]

Do you use a focal reducer on 8" sct edge ? Would be better for exposure times if the focal ratio of the imaging scope was reduced

[PhotoGav]

I know, I think I have fallen off the edge... Ha ha! It looks as though more expense will be required to maximise the new scope - OAG & reducer are on my wish list.

In the interim, I am going to try putting a 2x Barlow on the finder camera - that brings the ratio down from 12:1 to 6:1. The focal reducer on the scope and barlowed guide camera would give a ratio of just over 4:1...

More small steps up the learning curve.

[PhotoGav]

Sara, which OAG did you go for?

[PhotoGav]

Here's a pic - hardly any processing done yet (as you can see!), but the star shape is evident:

Here's a 100% crop:

[swag72]

I got the Celestron Radial guider, but I bought it second hand with the scope. Whichever one you look for be mindful of possible connection to a reducer further down the line.

[Russe]

I think - perfection makes the difference. If we are imaging at 300s but can't at 600s b/o eggy stars, then smth small must be wrong.

Polaralignment I do with EQMOD - that should be pretty accurate.

I used to use alignmaster for perfecting polar alignment, but have given that up.

I rather just slew to three stars around the object I'd like to image and center these / sync with EQMOD. Then go to the object (maybe use astrotortilla at this point).

There are no movable bits on the finder guider anymore.

[willuk2010]

Interesting thread, I'm using an EdgeHD 8", with the .7 focal reducer.. For guiding I have the extra dovetail on top, with an Altair Astro 10x60 with guide focuser and an ASI120MM as guide cam.. I have no interesting pictures to show yet, as I'm still a newbie and learning to polar align properly! Hopefully soon

Sent from my iPhone using Tapatalk

[Luke]

It costs a small fortune, but I'd suggest taming the focal length a little bit with the Celestron 0.7 reducer for the Edge HD8 (at least it costs less that the reducers for the bigger Edge HD's!), bringing it down to about 1400mm. Even with an OAG I found a C11 with the 6.3 reducer challenging for 600s with a Canon 450D on an NEQ6, which I think worked out at about 1800mm focal length.

[AstroAdam]

I know that the maximum I've ever seen successfully guided with a finder guider is 1500mm.

Even my 1200mm Newt is a big ask when it comes to finder guiding, as I don't have a permanent setup.  That's why I'm more inclined to use a shorter refractor for imaging.

[Psychobilly]

I used to use a 500mm f8 t-mount lens with a DSI Iic and PHDGuiding on my CPC800 working at f10 and f6.3...

On the odd occasion when imaging at f30 I used an ETX105 at f15 as the guidescope...

Peter

Badly typed on my Galaxay S4 in Tapatalk4

[PhotoGav]

Thank you for your responses all. Looks like some further expense is inevitable, but we all know that is the case with AP whatever you have! I just wonder at what point it might slow down, let alone stop...

@willuk2010, what bracket are you using for the finder scope on the dovetail? Also, is the dovetail one of the ADM ones with the radial spacer mount thingies?

Not sure where to take this now without buying a reducer and an OAG. I think I will give the Barlow a go and see if things are improved and what the max sub length is that I can acheive. The trouble is, these galaxies are pretty small and faint, so at f10 long subs are a must. If I can only do short subs I'm pretty limited with my choice of targets.

Anyway, keep at folks, the number of new image posts has been way down recently. Hopefully there will be a flurry of pics following these recent clearer nights!

[PhotoGav]

Interesting Psychobilly. So, essentially you are saying that a longer focal length guide scope would improve the accuracy of guiding. Or indeed this could be achieved with an OAG?

[Psychobilly]

The CPC 800 working at f30 has a FL of 6096mm ... So I decided to use a longer FL guidescope as at the time I had a 300/4 lens as the "normal" guidescope...

It would probably have worked but I didnt want to risk it...

Peter. ..

Badly typed on my Galaxay S4 in Tapatalk4

[Psychobilly]

OAG's are great when used with a sensitive guidecam to givd decent chance of finding guidestar with the pick off prism...

Badly typed on my Galaxay S4 in Tapatalk4

[willuk2010]

Hey @PhotoGav

So i have the extra dovetail, like this on top > http://t0.gstatic.com/images?q=tbn:ANd9GcSgJdUv9drlcBY2sh6DTV5HGnPfScispN9mAKAW5OCVUCyGNeWlmtQZEbIi

And then, just the LEFT part of this > https://www.altairastro.com/images/D/SW-DUAL-VBAR-LBAR-265MMDSC_3324-800w.jpg

This is how it looks > http://www.astrobin.com/full/81661/0/  (ignore the mess on my table!)

Must admit, its quite heavy now...(i need an extra counter weight already) but, its a start, all i wanted was to get the guidescope ontop of the main scope, i didnt want it hanging off the side like it did with the standard Altair Astro bracket that came with the 10x60 finder.

Good luck, and keep me posted, we are in the same posistion, with some of the same kit!

Cheers,

[Christopher Davenport]

Just a penny worth but I always drift align my setup once I have focus. Your heq5 once well drift aligned should easily give you 300sec subs, without guiding, thus less work for guider, which then only needs to nudge if you up in 500sec zone.

I also put a 2x Barlow on the extension nose of my guide cam to increase focal length of guider.

See equipment finderguider in sig

[PhotoGav]

I just tried putting the 2x Barlow on the guidecam, but I couldn't achieve focus...

Sent from my iPhone using Tapatalk

[PhotoGav]

Hey @PhotoGav

So i have the extra dovetail, like this on top > http://t0.gstatic.com/images?q=tbn:ANd9GcSgJdUv9drlcBY2sh6DTV5HGnPfScispN9mAKAW5OCVUCyGNeWlmtQZEbIi

And then, just the LEFT part of this > https://www.altairastro.com/images/D/SW-DUAL-VBAR-LBAR-265MMDSC_3324-800w.jpg

This is how it looks > http://www.astrobin.com/full/81661/0/  (ignore the mess on my table!)

Must admit, its quite heavy now...(i need an extra counter weight already) but, its a start, all i wanted was to get the guidescope ontop of the main scope, i didnt want it hanging off the side like it did with the standard Altair Astro bracket that came with the 10x60 finder.

Good luck, and keep me posted, we are in the same posistion, with some of the same kit!

Cheers,

Looking good! Does it work?!

[PhotoGav]

Just a penny worth but I always drift align my setup once I have focus. Your heq5 once well drift aligned should easily give you 300sec subs, without guiding, thus less work for guider, which then only needs to nudge if you up in 500sec zone.

I also put a 2x Barlow on the extension nose of my guide cam to increase focal length of guider.

See equipment finderguider in sig

So, I failed to get the guide cam focused with the 2x Barlow on it. Could you post a pic of your guide cam setup please?! Also, which Barlow are you using?

Cheers.

[willuk2010]

Looking good! Does it work?!

Hey Gav,

Not had a chance to try it out yet, as i was saying its added a bit of weight.. so im in need of another counter weight, im waiting for that to come in at my local Astro shop..

Im still going to give it a go later hopefully, and see how it goes!