Can I avoid a focuser upgrade?

[Shibby]

My new optics take the shape of an MN190, but I'm having issues with my pre-existing OAG setup.

The focuser itself is not really at fault - there's not anything wrong with it per se. In my opinion, the issue lies with the SX mini filter wheel and its integrated OAG. In order to reach focus with the guide camera, I need to add an extension between the FW and the imaging camera. Unfortunately there's then not enough back focus to reach focus with the imaging camera. 

Unless I'm missing something, my only option is a low profile focuser. I suppose an independent OAG placed before the FW could work, although I don't know for sure if I have enough back-focus for that either... It seems a poor design, that attaching your camera directly to this FW means you can't focus the guide camera; it also means increased vignetting. Is a new focuser the only solution here?

For now, I've resurrected my mini guidescope but ideally I'd like to carry on with the preferred off-axis guiding. Any thoughts?

[sloz1664]

I have exactly the same issues with my Tal200K. I have the same SX miniwheel & OAG and I cannot achieve focus either. I have had to resort to using a separate guide scope. I have searched for a low profile focuser and haven't found an ideal replacement yet. Also the cost of an adaptor plate and focuser may outweigh the advantages of OAG.

Steve

[Thalestris24]

Hi

This is just a wild suggestion... Looking at a diagram of the fw, is it possible to switch the front and back of it - to maybe bring the guide cam closer to the focal plane of your imaging cam?

Louise

[Shibby]

19 minutes ago, Thalestris24 said:

This is just a wild suggestion... Looking at a diagram of the fw, is it possible to switch the front and back of it - to maybe bring the guide cam closer to the focal plane of your imaging cam?

I like your thinking Louise!! But this would mean that the prism would be covered by the filter carousel. I could potentially get a little of the prism protruding into the light path, but even then I'd be trying to guide on filtered light. A nice idea though!

32 minutes ago, sloz1664 said:

I have had to resort to using a separate guide scope. I have searched for a low profile focuser and haven't found an ideal replacement yet. Also the cost of an adaptor plate and focuser may outweigh the advantages of OAG.

The Moonlite focuser is a common upgrade for my 'scope but, like you, I'm not sure the cost seems worth it.

[Shibby]

So, the latest plan is to replace my filter-wheel and its integrated OAG with a separate OAG + FW, allowing me to mount the prism further forward in the train. One bonus is that I can upgrade to 7 filter positions.

My guess would be I'll need a low profile OAG, such as this on FLO, which I believe has a profile of 13mm. Thing is, I still can't be sure that focus of both cameras would be reached. Does anyone else have an OAG setup with their MN190?

[ChrisEll]

On 1/15/2018 at 16:52, Shibby said:

My guess would be I'll need a low profile OAG, such as this on FLO, which I believe has a profile of 13mm.

I bought something very much like that off ebay... it suffered from prism wobble and was returned. Even some of the more expensive branded OAGs suffer from this.

[Kbramley]

Hi Shibby, I have a MN190 and that exact OAG you linked to from FLO. I use mine with a Brightstar manual 4x2" filter wheel, Atik 383L+ and a QHY5L-II guide cam, I still have about 5-7mm spare back focus with this setup with the stock single speed Crayford (its an old mn190). Works a charm, however I do get a bit of sag with the stock focuser due to weight which causes focus shift across the image and not so perfect Flat correction,so im upgrading to a moonlite CR2. With this OAG you do need to make sure everything is nice and tight otherwise things will have too much play under load.

HTH

Keith