Focal reducer sums

[Kaptain Klevtsov]

I'm struggling to get my head araound the calculations regarding focal reducers so I thought that asking here would save me from losing hair that I can ill afford.

The addition of a focal reducer means that the focus point has to be moved closer to the lens / mirror of the 'scope according to what I've discovered so far, both by research on t'internet and by practical experimentation. I tried tonight to use my Antares 0.5X focal reducer and failed because I don't have enough inward travel on the focusser. I have had this with the Newt. so I tried it on the frac with the same result. This time though I pointed at an upstairs window of my house where the boy had thoughtfully left a light on. At a distance of (guessed) 50 metres I could only just wind the focusser in enough to get the image in focus on the SC3 camera.

As I understand it, the FR works in a similar manner to a Barlow in that the distance from the FR or Barlow to the chip is what determines the focus point. If that is the case, do I need to move the FR towards or away from the chip to be able to achieve focus with this setup?

The thing that I want to do is reduce the inward travel of the focusser required to get the camera focussed when using the FR. I had considered moving the Newt. mirror up a bit using the collimation screws but with the frac this is not an option, as this would involve sawing the tube to shorten it. As both 'scopes show the same problem, I'm looking towards a way of making this thing work which doesn't involve 'scope mods as I think that I'm missing something obvious.

I'm at the point of thinking (ALFA car analogy) that having checked the fuses and the wiring and the gauges, maybe I am out of fuel and the gauge is working kind of position.

Modding two 'scopes is possibly the wrong way forward and, unusually for me, being prone to reach for the tool box first and ask questions later, (I may be getting more prudent in my old age) I thought that by asking, I might learn something the cheap way.

The plan B is to shorten the nose of the SC3, but this option is a little scary, as taking a hacksaw to the new toy is the best way that I can think of to make it not work anymore.

And if Arthur is watching, how does the nose bit come off? I tried to unscrew it but bottled out as I thought that I was using enough force to unscrew it if it wanted to play but it didn't. I have shied away from the grubscrews as plan C is trying to reassemble a pile of bits and I don't want to go there if it can be helped.

Captain Chaos

[Ant]

Hi CC,

You need to move the focal reducer clower to the chip. This will lower the effect of the FR but also (more importantly) reduce the amount oof in focus you need.

When I used the Toucam and wanted to use the FR I had to chop some of the nose peiceoff. It did work but sometimes it was a bloosy mission getting the filter on the thread!

One suggestion would be to buy a second nose peice and cut down one of them and leave the FR screwed in all the time.

Ant

[Ambermile]

Well, as Ant said, the closer the FR is to the ccd the less in travel you need but also the less FR you get. As for the nosy bit, yes it's screwed in. If it has gone tight on you just release the grubs, take the assembly out of the chamber and tap it gently on the table (honest). this should loosen it enough to allow you to unscrew it from the adapter.

Arthur

[daz]

Sorry to ask a question on CC's thread, but....

Am I right in saying that without moving the mirror on the newt, you CAN'T achieve focus with the FR anyway???

[Ant]

Not with a DSLR - but you can with the Toucam. But a little creative work is needed to house the FR closer to the chip than standard.

Ant

[Kaptain Klevtsov]

Sorry to ask a question on CC's thread, but....

Am I right in saying that without moving the mirror on the newt, you CAN'T achieve focus with the FR anyway???

Yup.

Neither can I with the frac. so that's why I thought that I had a different problem (other than mirror position).

Captain Chaos

[Kaptain Klevtsov]

Update, I cut the end off the nose bit, about 8mm, so hopefully that will help. The FR still screws in so the thre still works, plus the new shiny bit where it was cut gets covered by the filter/FR so I don't need to blacken it.

Hopefully I can test it this evening and report back. I'm going to have a bash with the frac first, maybe pointed at M101 and if I get time I'll stick it on the Newt. and see if it will work.

Bad news is Question Time needs to be watched tonight, should be fireworks, and I have work tomorrow so after QT is a nono.

Will let you know, clouds willing.

Captain Chaos