70 mm mak, good guidescope???

[Starcrazzy]

Hi,
I have been after a guidescope for a some time..i have been useing a cheap ebay reflector, with limited(actually no) success..I have found a 70 mm mak with a Fl of 860 mm..from andrews for 160 bucks, i know its bit cheap, but im not after image quality, just something to guide my eq5 which drifts in ra..Do you think that if i barlowed the mak it could guide my 8 " newt (f6)??cheers for any tips
Ps..it will need to be manually guided as there's no auto guide capabilities..

[allan gould]

I had thought of the same scope for autoguiding but was told [by And***]that the image quality of this scope was far inferior to the 70x500mm refractor for $150 from Andr***s Comm. I purchased the 70x500mm and with a barlow find the weight etc acceptable for autoguiding. Remember that a mak has a floating mirror that can give a different image shift to your main scope. You may want to use a cheap3x barlow to get the FL up to ~1500mm. Thats why I went the refractor route.

[[1ponders] (Paul)]

I agree Allan. While the idea of the 76 mak sounds good with it's longer focal length, unless there is some way to prevent mirror shift I'd suggest the refractor option.

[JohnG (John)]

Got to agree with everyone here, not good to use a moving mirror scope to guide with. A 500mm f/l refractor fitted with a 2 x Barlow would be fine, gives 1000mm guiding a 1200mm f/l main scope, you should have no problems guiding with an Illuminated Reticle eyepiece.

JohnG

[Starcrazzy]

ok..thanx...but you've lost me with this image shift??what does this mean??woul this also be a problem for a 80 mm reflector?

[allan gould]

The mirror in a mak or schmit cassegrain are shifted along a retaining bolt by the focus mechanism of the scope. Thus the mirror is movable. Inherently when the scope tracks across the sky [following the object being photographed] the mirror may shift due to gravity or temperature effects thus creating a pointing discrepancy between your image thru the main scope and the image as seen through your guiding scope, which can lead to trailing or double images. Thus to avoid this you use a refractor which has only one moving part [hopefully] which is the focuser mechanism. Thus when guiding, the object and guide star stay relatively fixed to each other. Your mounting for the guide scope MUST be absolutely rigid or you will get flexture which will result in bad guiding. This is why some use an off axis guider which sees the same field as your main mirror and you pick off a guide star from there and manually guide on that star. If the mirror moves then your guide star will move correspondingly and you will hopefully guide out the error.