Focal reducers for Alpy!

[Steve Cuthbert]

Hi guys

If I remember Francois correctly at the Alpy workshop he said the Alpy was optimised for an f4 to f5 setup?. My scope is a 120mm f7 apo Espirit refractor with focal length 840mm so I would need to get the focal length down to between 480mm (f4) and 600mm (f5) to satisfy the Alpy criteria. I`ve seen a few and one I came across is a 0.67x from TS optics giving me an effective focal length of about 562mm (f4.6 ish) does that sound about right please?.

cheers

Steve

[David Salmon]

Hi Steve,

I have an astro-physics focal reducer (telecompressor) which is a nominal x0.67 with some latitude for adjustment by varying the spacing. I’ve been working through the dimensions in my set-up according to the instructions here:
http://www.astro-physics.com/tech_support/accessories/photo/Telecompresssor-techdata.pdf
and you might find that interesting.

I also noted that François mentioned a range from F/4 to F/5 for the Alpy, but the manual says F/5. I wonder what it actually is ?

David

[Steve Cuthbert]

Cheers for that David, it seems with focal reducers its not an exact science especially if you start adding extension rings and such like plus I don`t use the calibration module!!. Any luck with that Nikon Capture NXI software yet?

Steve 

[Robin Leadbeater]

Hi Steve,

With a 120 mm aperture scope, depending on your seeing, it might not be worth reducing all the way to f 4/5. In fact it might even be counter productive as your star image might be so much smaller than the slit width that it “rattles around” in the slit making it difficult to guide on the slit as there will be little or no overspill. It could also give potential wavelength calibration issues as the star location rather than the slit itself will start to define the spectrum position.  I suggest measuring some star FWHM under typical seeing conditions first and comparing this with the ALPY slit width (23um). The optimum is to have the slit ~equal to the FWHM

Re the ALPY focal ratio, I think the aperture of the ALPY optics will accept an f4 beam but like most optics, they work better stopped down a bit. I suspect you will get a sharper spectrum and potentially squeeze a bit more resolution out at f5 compared with f4, particularly at the ends of the spectrum. This is certainly true of the LHIRES. 

Cheers

Robin

[Robin Leadbeater]

Since we are near Halloween, have a look at these pages concerning focal reducers on Christian Buil’s site.

http://www.astrosurf.com/buil/isis/lisa_ri/index_en.htm

http://www.astrosurf.com/buil/dispersion/atmo.htm

The first one describes precautions to be taken with focusing using a focal reducer with a LISA but the second one, looking at the effects of the  interraction between chromatic aberrations and atmospheric dispersion is particularly frightening !

Robin

[Robin Leadbeater]

The key thing is that if your star image is already comparable/smaller than the fixed slit width, adding a focal reducer to reduce the focal length will not give you any more throughput or resolution even though the spectrograph might be able to accept a faster beam. You just end up adding more unwanted glass and risk chromatic aberrations.

As an example, taking Steve’s 120mm f7 scope, the 23um ALPY slit covers 5.6 arcsec of sky so in typical seeing the star is already smaller than the slit and reducing the focal length further will give no benefit. (If it was possible to reduce the slit width, you could take advantage of the extra potential resolution but the ALPY slit only has a single fixed slit width)

If we consider my setup though (An f10 C11 with 280mm aperture) The slit is 1.7 arc sec and my seeing is typically 3 arc sec so unless I use a reducer I lose a lot of light at the slit. At f5 the slit is 3.4 arcsec and a much better match to my seeing

Cheers

Robin

[Andy Wilson]

I think I suffered from the chromatic aberrations for a time. It looked as though I had a problem with the slit drifting across the star image, so sampling different parts of the image that had been colour dispersed by the Earth’s atmosphere.

Here is a plot that shows the result as the continuum varies, just looking at 2 points in a series taken 20 minutes apart. I’m glad to say I’ve not suffered from this problem recently.

Andy

[David Boyd]

Hi Steve,

My inclination would be to run your setup without a focal reducer to avoid adding more chromatic aberration and see how that works in practice. That also avoids the problem of having to get the spacing just right for the focal reducer. As a general rule, I like to keep things as simple as possible consistent with getting good results.

David

[Tony Rodda]

Gents,

Slightly confused… Given the discussion at the workshop i was left with the distinct impression that I need a focal reducer for my f10 SCT to get the best Aply match.  I have both Meade and Celestron reducers.  I’ve copied the following from an old community discussion and there appears to be a big difference in the respective focal lengths.

Which one is the best match for mating the f10 SCT to the Alpy?  Am I right in assuming the longer FL will produce less chromatic aberration? but if so what does that do to the SCT/Alpy focal length match?

“I recently measured the focal length of a Meade 0.63x and a Celestron 0.63x focal reducer – the Meade had a focal length around 145mm and the Celestron around 235mm. So the “optimal” distance between lens and focal plane is greater for the Celestron; about 50-55mm for the Meade and around 85mm for the Celestron. The focal length measurements were pretty crude (+/- 5mm?) but clearly a difference.”

Regards

Tony

[David Boyd]

Tony,

My comment to Steve was in relation to his f/7 apo refractor. For an f/10 SCT, I think you will definitely need a focal reducer with the Alpy.

The issue of focal length and positioning of various makes of focal reducers is a bit of a minefield. I have seen several figures quoted for the focal length of the Meade 0.63x focal reducer and even that there are two versions on the market, one made in Japan and one in China with different focal lengths. I have no idea if that is true. In my experience, the best approach is to believe nothing and measure it yourself. To get a reasonably accurate value, put the focal reducer at the back of any scope, point it at a distant object on a bright day and measure the distance from the position of the lens in the focal reducer to the point where the image is sharp. Call this F2. Do the same without the focal reducer and measure the distance from where the lens was in the focal reducer was to the new focus. Call this F1. It will be larger than F2. The focal length of the focal reducer is then FL=(F1*F2)/(F1-F2). If you don’t get a sharp image without the focal reducer you may need to rack in the focuser a bit and try again.

The focal reduction factor RF you have just measured is F2/F1. It will vary as you rack the focal reducer in and out. Every focal reducer is designed to operate optimally at the position where the reduction factor RF is the quoted value, 0.63 in the case of Meade. If the measured focal length is FL, the optimum distance from the lens in the focal reducer to the image plane, F2, in this case the slit of the Alpy, is given by F2=FL*(1-RF) = 0.37*FL. You may need to insert spacers to get as close as possible to this optimum spacing.

As a general comment, you want to keep the Alpy as close to the scope as possibly to reduce the possibility of flexure so go with the focal reducer with the shortest focal length provided there is enough space between the focal reducer lens and the Alpy slit to achieve the optimum spacing. Needless to say you should also use a high quality achromatic focal reducer in a situation like this.

Hope this makes sense. It is based on my experience and has worked for me.

David

[Robin Leadbeater]

Hi Tony,

See  https://britastro.org/comment/2420#comment-2420 for a comparison of two different setups

Dont worry too much about matching and focal reducers though. There is no need in general to match the focal ratio of the scope to the ALPY.  You can use any focal ratio scope with the ALPY down to f4. It will not change the spectrum shape or resolution. The only reason for using a focal reducer is to better match the size of your star image to the size of the slit so you can collect more of the light and go fainter. This depends only on the telescope focal length (not focal ratio) and your seeing. If the star is smaller or comparable to the slit, then you are better off without one. Even if the star is bigger than the slit, if the star is a bright one, you may still decide not to use a reducer to avoid the risk of chromatic aberrations. 

Cheers

Robin

[Tony Rodda]

Thanks Gents.  Really appreciate the guidance.

Regards

Tony

[Kevin Gurney]

If the ‘seeing’ (typically 3 arc sec as you say) is thought of as comprising  a uniform disc, and this covers the slit in a symmetric way, then I think the geometry gives (after a little work…) an upper bound on light loss.

I reckon that  my f10, 200mm SCT (Celestron 8SE) with angular slit extent of 2.37 arc sec. incurs  only 11%  loss (I think Robin’s setup is a bit more at 32%).  Having tried to get my 0.63 focal reducer to fit with a nominal recommended 110mm distance from reducer lens to sensor and failed (calibration module prevents this) I am therefore  inclined to try without.

cheers

Kevin

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