Focal What?

[Matt Steele]

Hi All

Its been a short while since I asked for your advice ("Is This it?" 06 Feb), and I wonder if you could advise me a little more?

First off, I'm certain that VA is for me - especially after your overwhelmingly positive response.  I'm tempted by the PD Colour Camera (and kit) as a good place to start.  What I would be grateful to know is what focal reducer would you recommend?  There's simply lots of them on the market (ranging from £20 and upwards of £200) although they appear to do the same thing.

My set-up is a CPC 9.25, using stock 1.25" eyepieces etc, in moderately light polluted suburban skies (I can just make out the Milky Way with the naked eye on a good night).

Again, any advice is gratefully received.

Thanks!

Matt S

[DoctorD]

Hi Matt

I'd start with a basic x0.5 or x0.6 1.25" reducer at the lower end of the cost range which should get you started and can be used with the F6.3 SCT reducer later.

Keep a look out for second hand F3.3 or F6.3 - they come up from time to time.

HTH

Paul

[f15p5]

Hi Matt,

I would get the AVS .18 focal reducer. I would not waste money again on lesser reducers. I did and regret it. Waste of money. By the time you buy a cheap reducer and spacers, then a factory .63 reducer you could have bought a much better reducer. They are not cheap but will be the last reducer you ever buy for a SCT which they were designed to work with.

http://www.astro-video.com/f18-sct-focal-reducers.html

 I think he going to be selling the first batch very soon if not already.

Richard

[Dom543]

I would second Paul's suggestion of staring out with a simple and inexpensive 1.25" focal reducer.

The AVS 0.18x reducer has been an intriguing promise. But there are two purely technical issues that need to be kept in mind.

1. Spacing between the reducer and the camera's sensor must be accurate to the microns for this aggressive focal reduction to work without major distortions. AVS promises to manufacture the reducer in such a way that this spacing will be correct, when screwed on one of the two AVS cameras. Finding the correct spacing experimentally for a third party camera will be far from trivial.

2. Focusing is getting very tricky as one gets to focal ratios faster than 2.8. Fast camera lenses have very fine helicoids built in to help to do this. But basic telescope focusers are not designed to work with the required degree of micron accuracy. This is also the reason why there is no focuser for the Hyperstar. One has to buy the correct spacer for the particular camera model directly from Starizona.

Clear Skies!

--Dom

[Dom543]

I would second Paul's suggestion of staring out with a simple and inexpensive 1.25" focal reducer.

The AVS 0.18x reducer has been an intriguing promise. But there are two purely technical issues that need to be kept in

[DoctorD]

Hi Richard

I have no experience of the AVS 0.18 reducer, can it be used at other reduction factors?

The Astro Video web site says these are for 1/3" CCD's only - which would suggest that devices such as the Lodestar and SCB4000 with 1/2" CCD's may have problems.

It will be interesting to see the results with these larger CCD's as F1.8 from my SCT would be great.

CS

Paul

[f15p5]

Hi Paul,

As far as I know, only one reduction, not adjustable. If you want adjustability I would look at his varioreducer, I use it from .24 to .63 with steps at .38,.46,.54 in between in my small Apo. Don't know it it will work in a SCT though. Many 1.25" reducers cut out light in larger scopes with baffles using larger sensors. There is a formula, I just can't remember it right now. If I remember right, 9.25" is right on the cusp of needing a 2" reducer for full illumination using a 1/3" chip, I would think a 2" would be needed for a 1/2"?

You might want to email Matt and discuss it, he has done a lot of work with various combo's and can point you in the right direction. As far as focusing, not a problem if you have a good focusing mask. But like mentioned above by others, the distance between reducer and camera is critical for a set reduction, that is the biggest challenge. I don't know if Matt at AVS has played with larger sensors with his reducers, might be a grey area?

Richard

[Matt Steele]

All

Many thanks for taking the time to comment on this topic.  I'm most grateful to learn from your experiences.

Matt