Thoughts on a finderguider for widefield

[Demonperformer]

I am looking at producing a "finderguider" for long (NB) widefield images using my DSLR.

The image resolution works out at ~23.5"/px using the 50mm lens and ~5.9"/px using the 200mm lens. I have a celestron 6x30 erect image finder [#93778] with its holder and a QHY5L-ii camera, neither of which are getting much (any!) use at the moment, so they would seem to be ideal to use.

My first thought was to unscrew the back end of the finder and produce a tube that would screw into the front end of the finder and also into the CS thread on the front of the camera. However, that would need to be about 6" long to achieve focus, which would make the whole thing back-heavy and lead to chronic instability.

So my next thought is to simply attach the camera at the back of the full finder, where focus is obtained with virtually no distance at all. And what I have read suggests that, if a guide star is slightly out of focus that can actually be an advantage, as a larger centroid makes finding the sub-pixel centre easier. Yes, the cross-hairs would cause a bit of an obstruction, but finding some sort of star on which to guide should not be impossible. I know it is common to use a bigger finder, but (i) accuracy does not have to as great as when guiding a telescope, & (ii) I would need to purchase one [more expense!].

I am thinking a 50mm long cylinder of 38mm diameter plastic, with a 22mm diameter hole half-way through the centre on one end and a 31.7mm diameter hole through the centre on the other end, with a thumbscrew to hold each of the components in place would do the job (see diagram). Or would it be better to make it longer to increase stability - 40mm long on the finder-end and 50mm long on the camera-end would be the maximum.

But, not being very experienced in the DIY business, it seems reasonable to ask those more experienced: Does this sound like a reasonable plan? Any thoughts on getting it right would be appreciated.

Thanks.

[Gina]

That looks alright to me (unless I've missed something)   I'm thinking I might need guiding for my new widefield rig for the dimmer DSOs.  Unfortunately there isn't room for an OAG so a separate guidescope is the only option.  Adding a guidescope to a lens based setup never seems easy.

[michaelmorris]

If I read your posting correctly, you are planning to use a finder scope for imaging scope using a DSLR camera - is this correct?

If the answer is yes, I would think your biggest issue is going to be image quality.  The optics in finder scopes are typically pretty poor and certainly are unlikely to be well corrected for chromatic aberration.  I would have thought that any half decent camera lens is likely to provide a better corrected image.

[Demonperformer]

Michael,

Obviously did not explain myself clearly. I am imaging with a DSLR + lens(es). But if I start doing widefield narrowband imaging, I am still going to have to up the sub-length, for which some form of guiding would be necessary, and a finderguider would seem to be the best option. [But I'm always willing to consider an alternative, if you have a better idea .]

Having considered the cost of even a small plastic rod and the excessive amount of that rod that I would have to drill-out [plus the fact that I don't have bits that size and so would have to buy them], I am now wondering if it might not be easier (and would produce a better end-product) to make some kind of resin-based product around a wooden-dowel mould. Not sure what is available to work with nowadays. Going to have to enter research-mode!

Thanks.

 

[Gina]

You need a 3D printer    Almost anything is possible

A camera lens is clearly going to be better than a finder-scope though people have used them.