Guidescope or Off-axis?

[Mariner]

Any advice on which of the above 2 options is better or easier or whatever advantage over the other? Going by the advice, I may be putting up some stuff on the market soon.

[cloud_cover]

Depends on the primary imaging scope. A guidescope is much easier to use in that stars are easier to find but because it is attached to the imaging scope, it may flex with respect to the main scope resulting in poor guiding.
An OAG, on the other hand, does not have this issue but has a small and dim FOV due to the size of the pick off prism. It thus benefits from a more sensitive camera (certainly unusable with the Nexguide) and finding guide stars may be a bit more of an issue.
One of the simplest setups to use, if you have a short focal length scope, is a 50mm finder/guider.

[Mariner]

Is a 50mm finderscope adequate as a guidescope? Current scope is an F/10 SCT.

Also, would like to know how an autoguider works. Does it work simply by just plugging it into the GEM's autoguider port? Or are there other stuff that need to be done?

[orly_andico]

you need software to control the autoguider, PHD (Push Here Dummy) is the most popular.

a 50mm guide scope may or may not be sufficient for an f/10 SCT.

i am using a 50mm guide scope, my image scale is 9.71" per pixel. your typical f/10 SCT would have an image scale of about 0.7" per pixel.

following the rule of thumb that the image scale of the guide scope must be 5X to 10X of the main scope, 9.71 / 10 = 0.97"

it would be better to go for 5X only so 9.71 / 5 = 1.9"

so... a 50mm guide scope is insufficient. I put a barlow in mine to extend the focal length.

if you use the 0.63 reducer in your SCT, that will reduce its focal length a lot. So the 50mm guide scope will then be sufficient.

OAG is really hard, I have a couple OAG's and even made my own on-axis guider (with an IR cold mirror). if you go OAG, make sure to have at least an SX Loadstar autoguider camera.

[MooEy]

Not very possible. Let's take a recent DSLR with a pixel size of 4.3 micron.

Assuming working at f/10
206.265 / 2032 * 4.3 = 0.44"/pixel

Assuming working at f/6.3
206.265 / 1260 * 4.3 = 0.7"/pixel

A 50mm finder has a focal length of roughly 180mm. A QHY5 have pixel size of 5.4 micron.
206.265 / 180 * 5.4 = 6.2"/pixel

Even with subpixel guiding at 1/5, you may not get round stars.

Off-axis guiding may not work well either, the scope needs to be well corrected and give round stars at the edge for the tiny prism to pick up.

Spacing is another issue. Most correctors/reducer/flatteners are designed for 55mm of spacing from the rear of the corrector to the sensor. Adding an off-axis guider may exceed that distance.

~MooEy~

[orly_andico]

Mooey makes some very good points. The problem with OAG is back focus and the abberrated stars at the edge of field.

For the first issue, a very low-profile OAG like the TSOAG9 (which is 9mm thick) would be one approach. This is a spendy OAG though.

For the second issue.... well MetaGuide claims that it can find the centroid even of highly abberrated stars. I have not tried it.

Probably a good way to side-step the issue is to use a self-guiding camera (like some of the SBIGs), a camera with a built-in guide port (like the QSI WSG series, or some Starlight Express models).

Personally I have not had much success with OAG - because the guide stars are VERY dim in the OAG, so you need a pretty good guide camera. I will probably revisit it. One approach I have tried is to put a 0.5X or 0.3X reducer on the guide camera (!!!) the SBIG STF cameras do this (built-in reducer in the guide port).

[mymoon]

Now I am lost.

According to this article only f ratio matters; focal length and aperture does not matter.

Where did I go wrong.

Cheers

[Astrosiao]

Now I am lost.

According to this article only f ratio matters; focal length and aperture does not matter.

Where did I go wrong.

Cheers

F ratio is a function of focal length and aperture.

[mymoon]

F ratio is a function of focal length and aperture.

That's the physics

I am referring to Table 1 and Table 2 of the above SBIG article.

The article gives the impression that Guiding accuracy is dependent on focal ratio only.

Cheers

[beginner]

Alan article seem to point out that guiding accuracy is solely dependent on F/num irrespective of focal length of the guide scope. Interesting...hor. Or have I misunderstood his article meaning wrongly?

If right, then can guide a C9.25 with a Orion 50 mm guiderscope, right?
180mm/50 =f3.6, wa perfect leh. Not far out from his 100mm

BTW, Mariner, care to share what you plan to image with your large scope?