Reccommended Focal Reducer

[Biglewey]

Set-up is Skywatcher Skymax 127 MAK and altair astro  Altair GPCAM2 IMX224 Colour. I've just had the first look with it in the scope and some questions I'm sure you can help me with.

1. The magnification - which I suspected from the calculations I've done on the scope and ccd size - is very high.  From comparison the view on my laptop looks about the same as with a 10mm eyepiece. Which I calculate to be 150 X (I know that you cant compare really but its just an indication). so quite high and indeed too high for a lot of stuff since at that mag focus and aiming is not easy. So, lower mag is required. I guess a focal reducer is need probably 0.5 to give 75x. Any recommendations for a reasonably priced but good quality one based on experience please.

2. I assume that since the scope is f11 as is that the focal reducer will halve the focal length from 1500mm to 750 mm - will this make it in effect f5.5 and therefore provide a brighter image?

3. What is the max focal reduction I can use to capture bigger (more difficult and for later) stuff like Andromeda.... and I assume its not good to "stack" multiple focal reducers??

Thanks for any guidance and information for a keen to learn newbie. 

[cjdawson]

For my Meade LX-90, I've got two focal reducers both from Meade.  The F6.3 turns my F10 scope into F6.3, and the other is an F3.3 (bet you can guess that that does)

The F6.3 works well.  The thing to note about it is to know that the distance from the FR to the imaging plane is important - get it wrong and you end up with artefacts in the image.

The F3.3 I've not actually tried yet.   This is sold for CCD use only.   Apparently it will produce a lot of Vignetting.

These devices are supposed to make the scope behave like either an F6.3 or F3.3 scope with the aperteur unchanged so the exposure times will be shorter.  The side effect of doing this is that the overall magnification of the image will be changed, so whilst you will get a brighter image, it will be smaller so less detailed.

[ollypenrice]

We really do need to forget the word 'magnification' in imaging. It is so much easier to think things through if you use terms that have a real meaning. The terms that matter are focal length, chip size, image scale, pixel scale and field of view.

You have a long focal length and a small chip so you have a very small field of view. That's the way to think about it. You are also working at F11 so your captures will need to be very long.

You can only do so much using focal reducers. For example, the old Meade F3.3 mentioned above is only workable on the tiny webcam chips with which CCD imaging began. It will not cover even modestly sized Sony CCD cameras and certainly not DSLRs. It isn't just a matter of vignetting, it will produce huge distortion off axis. It would be nice just to be able to buy a 0.5x focal reducer for any scope you like and have a fast instrument giving a wide field and, with the tiny chip of this camera, such a reducer might work. It wouldn't work with a bigger chip.

However, it would take forever to shoot Andromeda with this camera because the target is huge, the chip is tiny and the focal length is long. I modelled your setup in SkyMap Pro assuming a 0.5x reducer giving a focal length of 750mm. The little white square in the middle is what you'd get on the chip. This camera chip is too small for extended DS objects but it could do small bright ones (Ring, Blue Snowball etc) very nicely.

Olly

[Biglewey]

Thanks for the advice and correction...

However, I do understand that magnification really has no real meaning in astrophotography that's why I said " know that you cant compare really but its just an indication". Ive done the calculations and got roughly the same figures as you.  I guess it comes down to the long focal length of the scope but its my first scope and fits my needs in other ways so need to match the ccd camera up better.

I was just looking for experience of reasonably priced reducers to ease my early attempts since with the current set up positioning, guidance and focus are all very critical - the same as it is with my 10 mm eyepiece (no apology for the comparison).

I find the scope easier with the 25mm eyepiece in all respects so looking for a 0.5 x reducer would similarly ease things and give easier exposure.

So far ive seen Revelation 0.5x (£20 ish), Anteres (£40 ish) vs Celestron and Meade units at circa £95.

Anybody got the Revelation or Anteres units to comment pls.

[nedstark]

I've used a Revalation 0.5 reducer on my QHY5-II/Skywatcher 150mm f4 reflector and found it OK.

For FOV with various cameras/eyepieces I use the 12dstrings web site  - a good tool