C9.25 Guidescope Showdown

[Stratis]

Ok then,

So I finally have my 9.25" SCT, my gorgeous 2" optical train all sorted out, and an HEQ-5 purring like a kitten even under all that load. I have cameras ready to attach, but at the focal length of 2350mm the Celestron is way outside the safe zone of most small-aperture subpixel guidescopes and finder/guiders.

Now I can invest in something premium like the 60mm Altair Astro guidescope, but as I started to look higher and higher in price I realised I was essentially buying a whole new scope to bolt to the top of my main scope, a setup I have seen before but never understood; now I do.

Good news is however, like many big-SCT owners my house is absolutely littered with small scopes; there's barely anywhere you can go around here without tripping over some tube or another.... so, I ask which of my baby scopes would you recommend to guide such a long-FL SCT?

Contender the First: SkyWatcher Heritage 90mm Maksutov

Aperture: 90mm Focal Length: 1250mm (f/13.9) Weight: ~1.2kg

This was one of my first scope purchases and I was lucky enough to get it around half price; with the Mini-Dobsonian mount (now converted to GOTO via SynScan HC) it is a spectacular tabletop telescope with enough aperture to pick out Messier objects and nice constrasty double-star performance. Punches well above its (minimal) weight, I've just never heard of anyone using a Maksutov as a guidescope before.

Contender the Second: Aquila ST80 Refractor

Aperture: 80mm Focal Length: 400mm (f/5) Weight: ~2.2kg

These things are sold everywhere under a a variety of brands and with many different fittings; this one seems to be the model I have. 2" Crayford, brass fittings, retractable dewshield and heavy tube rings in my case. Shows quite a lot of CA thanks to the short f/ratio but that's not an issue for a guidescope. This is much heavier than the Mak, but shows superior contrast and is easier to focus, and has tube rings adequate for a Stinger missile.

Contender the Third: TSL50D 50mm Guidescope

Aperture: 50mm Focal Length: 180mm (f/3.9) Weight: ~0.5kg

One of these little numbers is also available to me, which is pretty much the opposite end of the scale; very small, very light, incredibly short focal length and can be mounted directly into a finder shoe rather than requiring some heavy engineering. Fact is I do not know how well this will cope with a 235mm f/10 SCT under long guidance, but included for the sake of completeness. If I hadn't read so many warnings about guiding a long scope with a short, I'd probably be struggling with this right now.

So there you have it, my dilemma. I'd love to repurpose the 90mm Mak because I have a lot of affection for it, but at such high magnification will it find guide stars? Similarly the ST80, I enjoy using it as a travel scope but does it have the accuracy and is the weight too high? All good questions!

[Dave Smith]

I have successfully guided my C9.25 with an ST80 if that helps.

Dave

[Freddie]

Other problem with the Mak as well as small FOV, it is very slow @13.9 so you may have problems picking up guide stars

[ollypenrice]

Other problem with the Mak as well as small FOV, it is very slow @13.9 so you may have problems picking up guide stars

Indeed, it would be a total nightmare. The worst choice imaginable. You should go for the 400mm FL F5 refractor in my view. The little scope might do it but there is quite a difference in focal lengths between SCT and finder guider. It might work, it might not. Personally I'd always guide a large reflector with an OAG to be on the safe side.

I would never guide with a reflector because of the possibility of mirror shift. This is why OAGs are the safest bet for guiding reflectors.

Olly

[Stratis]

This is why OAGs are the safest bet for guiding reflectors.

But if I understand correctly, an OAG depends on the operator to manually guide? Is there a reliable way to autoguide through an OAG? I understand the issues with field of view and brightness in the Maksutov, I have a Baader Witty One which would allow me to locate guide stars significantly off the main optical axis but the challenge would persist. I was just hoping I'd be able to get away without having to mount a whole piggyback bar for the ST80

I am fortunate in that my 9.25 displays barely any mirror shift, in fact I haven't been able to detect any so far simply by focusing uphill and planning my exposures.

It's interesting that long-FL scopes are thought to be so troublesome to guide.... as we increase FL we decrease FoV, which is also vital. It makes me wonder what we could consider to be the 'ideal' guidescope for a large SCT if we're always stuck between two fires of low FoV and short FL. Perhaps a longer-tube refractor of around 80/1000? These can be had for very low prices and demonstrate a compromise at 1000mm length and 80mm aperture for reasonable FoV.

[Freddie]

With the OAG, just pop your guide cam on it and you are good to go.

[E621Keith]

But if I understand correctly, an OAG depends on the operator to manually guide? Is there a reliable way to autoguide through an OAG? I understand the issues with field of view and brightness in the Maksutov, I have a Baader Witty One which would allow me to locate guide stars significantly off the main optical axis but the challenge would persist. I was just hoping I'd be able to get away without having to mount a whole piggyback bar for the ST80

I am fortunate in that my 9.25 displays barely any mirror shift, in fact I haven't been able to detect any so far simply by focusing uphill and planning my exposures.

It's interesting that long-FL scopes are thought to be so troublesome to guide.... as we increase FL we decrease FoV, which is also vital. It makes me wonder what we could consider to be the 'ideal' guidescope for a large SCT if we're always stuck between two fires of low FoV and short FL. Perhaps a longer-tube refractor of around 80/1000? These can be had for very low prices and demonstrate a compromise at 1000mm length and 80mm aperture for reasonable FoV.

You may be confusing OAG with flip mirror.

OAG uses a prism to look inside the scope's optical path, but outside the main camera's viewing area (hence off-axis). You put a guide camera on the OAG to autoguide. I think OAG can correct for mirror flop, but guide scope definitely cannot

[ollypenrice]

But if I understand correctly, an OAG depends on the operator to manually guide? Is there a reliable way to autoguide through an OAG? I understand the issues with field of view and brightness in the Maksutov, I have a Baader Witty One which would allow me to locate guide stars significantly off the main optical axis but the challenge would persist. I was just hoping I'd be able to get away without having to mount a whole piggyback bar for the ST80

I am fortunate in that my 9.25 displays barely any mirror shift, in fact I haven't been able to detect any so far simply by focusing uphill and planning my exposures.

It's interesting that long-FL scopes are thought to be so troublesome to guide.... as we increase FL we decrease FoV, which is also vital. It makes me wonder what we could consider to be the 'ideal' guidescope for a large SCT if we're always stuck between two fires of low FoV and short FL. Perhaps a longer-tube refractor of around 80/1000? These can be had for very low prices and demonstrate a compromise at 1000mm length and 80mm aperture for reasonable FoV.

An OAG uses a guide camera (or can do) just as a guidescope does. What are the differences?

- The OAG guides at the same FL as the imaging scope images. WIth autoguiders this is no longer terribly important since they guide sub pixel.

- The OAG guides from the same light cone as that of the imaging camera. If the mirror moves, it moves for both. (How do you know you have little mirror shift and how do you quantify 'liitle?' At this kind of FL 'little' translates as 'a lot.' I image regularly with a 2.4 metre FL catadioptric. A little is a lot!!)

Long FL telescopes are not 'thought to be difficult to guide.' They ARE difficult to guide. OK, they wouldn't be if you put a gigantically large pixel camera in them or binned the camera you have down to 2x2 or 4x4 but if you want the resolution of which your rig is capable then, yes, long FLs are difficult to guide. Don't kid yourself otherwise.

Olly

[Stratis]

An OAG uses a guide camera (or can do) just as a guidescope does.

That's all I needed to know. I had read elsewhere that guiding with small pick-off prisms tended to reduce image brightness to a degree below the threshold of most autoguiders. If that isn't the case I couldn't be happier, it's less weight on my mount and less differential flexure to deal with.

(How do you know you have little mirror shift and how do you quantify 'liitle?' At this kind of FL 'little' translates as 'a lot.' I image regularly with a 2.4 metre FL catadioptric. A little is a lot!!)

I know I have little mirror shift because I've imaged unguided with this scope up towards the zenith with no problems as long as I don't attempt to cross the meridian, which my mount doesn't allow me to do without a flip anyway. I also know, as I've loaned the OTA to a local friend with an Advanced VX, who has imaged over the meridian unguided and had little issue.

Since I clearly do not regularly image with a 2.4m primary, it is surely understandable that my mirror shift issues will be vastly diminished vs an optical system with ten times the aperture and many more times the mirror mass. On this scope and mount, I haven't noticed mirror flop in a single image or test as long as focusing direction is carefully controlled. The only time I found mirror shift an issue it was during planetary observing with stacked barlows.

Long FL telescopes are not 'thought to be difficult to guide.' They ARE difficult to guide. OK, they wouldn't be if you put a gigantically large pixel camera in them or binned the camera you have down to 2x2 or 4x4 but if you want the resolution of which your rig is capable then, yes, long FLs are difficult to guide. Don't kid yourself otherwise.

Noted.

The main question though was what sort of FL guidescope would be an appropriate balance between the slow/narrow FoV of a Maksutov and the fast/wide FoV of an ST80. Do you have any insights into the ideal optical geometry to guide one of these SCTs?