Guide Camera

[Graeme1858]

Hello all

Could anyone advise what the sensible selection criteria is for a guide scope to go on a Celestron 9.25 F10 2350mm SCT please?

Rother Valley Optics are selling a second hand Celestron 80mm complete with rings and brackets for £90 (https://www.rothervalleyoptics.co.uk/se ... ckage.html)

Or I could get a 50mm guide scope from Ebay for £45

Is there a rule to select guide scope focal length as a function of telescope focal length?

I'm fully aware of the old adage "you get what you pay for" but if the £45 unit will do the job I can get that and CLS filter as well for Christmas!

Regards

Graeme

[yobbo89]

can you fit an off axis guider ? a calculator gives me 204 mm fl for the absolute minimal recommended guide scope. i think 600mm will be fine. i don't know how the rings will go, you will probs get some sort of flexure,short exposure at a few mins will be less effected to small amounts of flexure ,but long exposures (10-30mins) might call for an off axis guider or a sturdier guide scope mounting connection

[UlteriorModem]

I would also recommend OAG as it eliminates a lot of pitfalls, mainly being flexure. But it can be a real pain to set up and get working correctly.

In the meantime here is an article. Scroll down to part 3 regarding focal length.

https://agenaastro.com/articles/selecti ... l#aperture

[Graeme1858]

can you fit an off axis guider ? a calculator gives me 204 mm fl for the absolute minimal recommended guide scope. i think 600mm will be fine. i don't know how the rings will go, you will probs get some sort of flexure,short exposure at a few mins will be less effected to small amounts of flexure ,but long exposures (10-30mins) might call for an off axis guider or a sturdier guide scope mounting connection

Thanks Yobbo

With 204 fl as a minimum the 50mm Ebay guide scope is too small at 190mm. A 60mm version has 240mm fl but for another £30 I can have a pukka Celestron 80mm/600mm fl guide scope. Then again the 60mm would reduce flexure. My guide camera is a QHY 5-11, half decent but not top of the range so a bigger guide scope would improve guiding greatly.

An off axis guider opens another subject for research! Presumably the same get what you pay for rule applies.

Decisions decisions!

Regards

Graeme

[Graeme1858]

I would also recommend OAG as it eliminates a lot of pitfalls, mainly being flexure. But it can be a real pain to set up and get working correctly.

In the meantime here is an article. Scroll down to part 3 regarding focal length.

https://agenaastro.com/articles/selecti ... l#aperture

Thanks, that's a very informative article.

The OAG is a neater option but "a real pain to set up and get working correctly" is rather off putting!

My CGX mount should cope with the SCT and the 80mm guide scope easily enough. The OAG is a step up in expenditure but the 80mm has no mounting rail. Although as an electrical engineer I could probably find a 100mm aluminium busbar I can take to my router and find a solution!

Regards

Graeme

[UlteriorModem]

I've got one of these around here somewhere I could sell for a good price. Just don't use it anymore.

Send me a pm if interested.

[Graeme1858]

I've got one of these around here somewhere I could sell for a good price. Just don't use it anymore.

Send me a pm if interested.

An OAG or a guide scope?

Regards

Graeme

[JayTee]

Just to backtrack a little: the ratio of imaging scope FL to guide scope FL is roughly 4 or 5 to 1. So the 80mm falls right in line with this rule of thumb.

Cheers,
JT

[Graeme1858]

Just to backtrack a little: the ratio of imaging scope FL to guide scope FL is roughly 4 or 5 to 1. So the 80mm falls right in line with this rule of thumb.

Cheers,
JT

Thanks JT

That would make the 60mm a tad small then.

Regards

Graeme