What FL for guide scope?

[troypiggo]

I'm using my 600mm ED80 for imaging with DSLR mounted on it. Was recommended this because it has a bigger aperture and I know it's a very popular and highly recommended scope for widefield imaging. So I have used my 900mm ED100 with SPC900NC webcam and PHD for guiding. I've only been able to attempt this setup a couple of outings due to weather.

I was having trouble keeping the guide star in the box on PHD initially. Did a much better alignment last time (a pseudo drift align using finder scope - didn't have a illuminated reticle with crosshairs, but do now ;) ), and actually did get some decent exposures, but in PHD the star was still constantly moving outside the guide "box" and the application was beeping and flashing while it tried to get it back on track. Yes, it had gone through all the calibration type procedure and did appear to be working despite all the warning bells and whistles.

I am sufficiently inexperienced to know if this is normal or not, but last night I was thinking that it could be that 900mm is too long a focal length and the stars are moving too much, even more than the imaging scope (600mm).

On one hand this could be a good thing. Means that PHD Guiding will be much more sensitive to movement and the resulting images in my DSLR should be better for it.

On the down side, not sure if it's due to the smaller aperture or the narrower field of view, but seems hard to actually find guide stars in the PHD window.

I was thinking that perhaps a wider guide scope with bigger aperture might show more potential guide stars. Something like this Orion ShortTube 80 Guide Scope I saw for AU$259 (https://www.bintelshop.com.au/Product.aspx?ID=7877).

What is your opinion? Should I get this guide scope, or keep using the ED100? If I get it, should I keep the ED100 or sell it to pay for the guide scope? I do intend to get something like a 9.25" reflector in the future.

[troypiggo]

Whoah, been reading some more about this:

http://www.wilmslowastro.com/tips/autoguiding.htm

Talking about ratios of focal length of imaging scope versus guide scope, pixels of imaging camera versus pixels of guide camera, sub-pixel capabilities of the guiding software, arcseconds (what?)... getting compli-ma-cated...

Summarising my equipment considering the above -

Imaging gear: ED80 600mm f/7.5 with Canon 30D 3504 x 2336 px image size.

Guiding gear: ED100 900mm f/9 with SPC900NC 640 x 460 pixels.

Proposed guide scope: 80mm f/5 400mm FL with above SPC900NC.

I have no idea how or what to calculate arcseconds.

[Nighthound]

Troy, there are two things that are working against you with your current guide scope. A Narrow FOV and a high f/ratio. It's no wonder you're having trouble locating a star bright enough to guide on many times. When imaging at 600mm, a 300-400mm guide scope would be plenty and offer a wide enough FOV to help locate your guide star. Mounting the guide scope on adjustable rings will allow even more flexibility by allowing you to point the scope independently in any direction to find a star. I would try for around f/5 to help resolve more stars for the guide cam as well. There are just some objects that have very few bright stars around them so having all the benefits of the things I described will be very helpful.

Also if you and I had very accurate, high quality mounts we could take longer exposures(3-5 sec.) through PHD which would also help with dimmer guide stars. But our mounts require more frequent corrections so we need to stay closer to 1-2 sec. and that's where a faster focal ratio scope helps. Remember that when seeing gets bad the guider can start over correcting or become confused by the erratic fluctuation of star brightness. The higher the focal length, the greater that fluctuation becomes. I tried using a barlow with my Sky 90 for guiding and had worse results. I'm sure if seeing was excellent I would have done better but how often do we have those conditions?

I guide my 800mm Newtonian with my 500mm Sky 90 with no problem. Once you get a feel for PHD and get a guide scope around 1/2 to 2/3 the FL of your imaging scope you'll be fine.

Are you planning a focal reducer/flattener for your imaging scope?

[troypiggo]

Thanks Steve. That's what I was thinking. So the 400mm f/5 guide scope I linked is in the right ballpark for my 600mm ED80.

Do you think I should sell the ED100 900mm to fund it? I know the answer is probably "got the money to keep it? you might want to use it...", but I'm going to have a hard time justifying this to my treasurer.

And maybe down the track, if I do go for 9.25" reflector, around 2350mm FL, the ED80 will be too short for guiding but the ED100 will be in that 1/3 range? Or should the ED80 be fine. I was hoping so.

Are you planning a focal reducer/flattener for your imaging scope?

I don't think I have enough experience to answer that, since I've only taken the one image :) Not sure why you ask. I didn't think there was any problems with the stars being elongated on the outer edges of my shot, and thought that's what they were for.

I bought my gear used, and the guy I bought it from threw in a Mogg focal reducer, but it was sort of damaged in transit. It's just a plastic tube with a lens, and the lens fell out. Both sides of it appear to have different curvatures and I'm not sure which way to glue it back in. So to date I haven't worried about it. If you know, I'll give it a go.

How does that affect the guide scope stuff?

[Nighthound]

I'd say you have to decide if the scope is one that's a useful tool for the type of shooting you do. It's not a good deep sky scope as far as image quality(false color) but could be pretty good for planetary work with a web cam.

I was asking about the reducer/flattener for your imaging scope. It would flatten the field AND reduce the focal ratio. The lower focal ratio produced would put your guide scope even closer to the focal length of your imaging scope. I would look into a decent quality FR/FF if you're going to do it. William Optics has one and Astro Technologies new .8 reducer is said to be awesome(Bobby here uses one).

All refractors will have some field curvature by the nature of their lenses. Just as reflectors have coma and require correction as well.

[troypiggo]

Ah, thanks for the clarification.

Spent a lot of money on this hobby recently and after the initial setup gear, am trying to be a little more thrifty. I'll think about it a bit longer. Thanks again.

[Adrena1in]

I use my cheapo Orion ST80 for guiding, and got okay results. When I tried a switch, and imaged through the ST80 but guided with my 1200mm f/9.5 it all went to pot. "Why?!", I asked myself. If I'm guiding at a relatively high FL, surely it will be super-accurate, and there'll be no problems imaging through the little 400mm. But whether it was atmospheric conditions, or what, my guiding camera had really problems keeping the guide star central, and it wobbled about all over the place, often losing it completely.

Guidescopes really don't have to be good quality. My ST80 was £80, but a £20 cheapo would probably do the job.

[troypiggo]

Just following up after some more thought and reallocating funds...

Went with a ST80 400mm f/5 guide scope.

I also met up with a local astrophotog from IIS (http://www.iceinspace.com.au/forum/index.php)and saw the difference between my cheap/beginners webcam and a decent CCD camera. The difference in resolution and ability to pick up stars is amazing, and much less noise. So I've also ordered a Meade DSI II Pro CCD camera (monochrome with LRGB filters) on special (https://www.bintelshop.com.au/Product.aspx?ID=7160). Reckon that should sort out my guide star woes :)

[troypiggo]

I've got maaaaaaaaiiiiiiiiiiiiilllll!!!! :)

Will post piccies soon, ebann ;)

[Adrena1in]

Nice, let us know how you get on with it.

[ebann]

I've read somewhere that for optimal guiding, the guidescope should be about 2x the FL of the imaging scope! In my case I would use my TV Ranger to guide my 400mm lens.

[chris.bailey]

I've read somewhere that for optimal guiding, the guidescope should be about 2x the FL of the imaging scope! In my case I would use my TV Ranger to guide my 400mm lens.

That very much used to be the case but with sub-pixel guiding software it has turned very much the other way around.

[dpastern]

Troy - if the mount is accurately polar aligned, there should be next to no movement from the guide star for at least several minutes. Since it's moving, I suspect that it's not accurately polar aligned, and phdguide is working super hard to try and keep it guided for you.

Dave

[troypiggo]

Yes, that was the original problem Dave. This was all before I realised how to correctly drift align. PHD was working way too hard. Once I figured out the drift aligning it all worked much better - provided I could find a guide star. I think the finding guide stars was caused by the decreased field of view of the 900mm scope and the reduced light gathering capacity because it was only f/9 so fainter stars weren't visible.

(Hopefully) got more options for guide stars now with 400mm field of view and f/5 focal ratio.

[chris.bailey]

Ideally the guide star should be of mid brightness (if you get what I mean) i.e. neither too hot or too cold :-)