Focal reducer for Mak telescope

[dark star]

Hi,I am thinking of getting a focal reducer for my Intes MK66 Mak telescope which is 150mm f/12.

The Widescreen Centre in London advertise a STF 2 inch 0.6x apochromatic focal reducer for Mak telescopes. It is rather expensive (maybe this means it is good!) and I wondered if anyone has used one?

Also it states that the telescope needs a focal plane extension of not less than 180mm. Can anyone tell me what this is and how I measure it? The advert also says the focal reducer has a back focus of 70mm, how do I measure this?

I want the focal reducer as I am getting in to amateur spectropscopy and have been advised that a focal reducer will improve spectrum sharpness by reducing the size of the star image.

I won't be getting one for a while as I need to save up!

[Merlin66]

Don't know if its a good or bad reducer...

The dimensions quoted are - the distance inside focus where the reducer needs to be placed - 180mm ( if you can imagine focusing your scope so the image of the moon is 180mm behind the rear cell - then the reducer would be positioned at the face of the rear cell)

The other size tells you that the new focus will be 70mm behind the reducer.... so you'll need spacers etc to get the camera CCD at this distance.

The lower f ratio will only help if you intend using a grating spectroscope. You don't mention which camera you will be using?

With a DSLR you can position the grating up to 80mm infront of the CCD to maximise the resolution.

[Whippy]

Not sure that the STF reducer would do the job but Telescope Service do the actual Intes reducers, see here: Teleskop-Express: Astro-Shop + Fotografie + Naturbeobachtung .

HTH

Tony..

[dark star]

Hi, I am rather confused. Is the rear cell where the star diagonal, eyepeice etc goes at the back of the Mak telescope? If so do I need to calculate the distance inside focus going 180mm further back behind the rear cell, and if so how do I do this?

[dark star]

Hi, I forgot to say I have a Rainbow optics grating and Canon 20D DSLR.

I am also confused, is resolution the same as sharpness? If I decrease the distance between the grating and camera chip will this increase resolution/ sharpness?

[Ags]

I think you are just reducing the image scale - so the resolution stays the same even though the dots are smaller.

[Merlin66]

Yes the rear cell is where you would screw in the reducer. The distances are academic. The key for the reduction factor is to have spacers etc AFTER the reducer to make a distance of 70mm.

If your camera has a T thread nosepiece then you already have 55mm - this only allows for a 15mm adaptor behind the reducer to hold it in place - doesn't sound enough....

The longer the distance between the grating and the CCD the larger the dispersion in Angstrom/ pixel - so the image will appear larger on the frame.

The sharpness is the focus. Resolution, in a spectrum is the ability to separate fine detail -close absorption lines.

For the RO200 grating a distance of 50mm would be good.

[dark star]

I forgot to mention I use a 2 inch star diagonal when photographing spectra, does this make any difference?

Also I have a William Morris 2 inch SCT/ZS66 adaptor connected to the rear cell in front of the star diagonal, so the star diagonal and eyepieces will connect to the telescope. It is around 50mm long. Is there such a thing as a low profile version of this which might move the reducer further in and allow me to use a focal reducer?

[dark star]

Sorry, I'm a bit sleepy, that should be William Optics, not William Morris. If I remember correctly he was a socialist/ designer a long time ago!

[Ags]

I would love a "William Morris 2 inch SCT/ZS66 adaptor" - didn't know the Victorians made them! I can imagine the Arts&Crafts detailing!

[Merlin66]

Why include the diagonal in the optical train?

What output thread would be on the reducer? SCT?

Then a visual back - SCT to 1.25" would allow you mount the 1.25" nosepiece (with the grating) directly onto the rear of the reducer...

[dark star]

I use the diagonal because otherwise looking through the camera focuser is very uncomfortable. Also, I usually find the star visually using an eyepiece first, then put on the camera, if I don't use the diagonal I get a crick in the neck and have to sit on the ground when looking at stars higher up! The higher up the better to reduce turbulence form the atmosphere.

I suppose I could get a right angle viewfinder for the Canon 20D DSLR, then find the star visually and remove the star diagonal?

The focal reducer from the Widescreen centre has 'the following threads-input (inside thread) T2, output 2 inch with compressor ring'.

[Merlin66]

Hmmm,

I don't think you'll be able to get the correct backfocus of 70mm with a DSLR body and a diagonal.

For that reducer you'd need a Mak to T thread> reducer> 2" to 1.25" adaptor > 1.25"- T thread nosepiece with grating > T2 -EOS camera adaptor. Even with this minimum set-up the distance may be greater than 70mm - a diagonal will add at least another 60-80mm.....

I have a 90 degree finder viewer on my DSLR - helps sometimes....

[dark star]

I guess I could take off the diagonal when I have found the star, and use a 90 degree view finder with the DSLR.

The SCT adaptor (I think this is a visual back?) plus 1 inch to 2 inch adaptor plus T2-EOS adaptor adds up to approximately 80 mm. This appears to rule out using a focal reducer that needs 70mm of back focus unless I can find a low profile visual back to reduce the distance.

When I have the cash I will go to the Widescreen Centre and ask, as they are quite knowledgeable and one of the people working there seems to know a lot about imaging.

I don't use a T thread nosepiece with the grating, I attach the grating to a 1.25 inch eyepiece barrel and put this inside the 1.25 inch to 2 inch adaptor, just like when using an eyepiece.

[dark star]

By the way, the Intes focal reducers from Germany also appear to need 70mm of back focus.

[Merlin66]

Sounds like they are not really designed for use with DSLR cameras.

If you use the reducer at say 80mm distance the reduction factor will be greater and the imaging quality may suffer. If you are ONLY using this for spectrscopy then it may be acceptable. It's when you want to image across the full frame that aberrations will be noticable.

(BTW The standard SCT x0.63 reducer ie Meade/ Celestron needs 110mm back focus...)

[AndyH]

Just to confuse matters, Intes made a 0.75x focal reducer, designed specifically for their F12 MK66, MK67 and MK69.

APM & Telescope Service both had these on their pages, years back. Have put some pics on here,

Stargazers Lounge - AndyH's Album: Retro Astro adverts + Tal Manuals

Andy

[Macavity]

There used to be a nice focal reducer calculator at:

http://timosastro.1g.fi/

My Finnish is not so hot <G>, but no more, I sense?

[dark star]

I don't think a SCT x 0.63 focal reduceer would work as it is designed to correct aberrations (field curvature?) in the SCT, (to flatten the field) which don't exist in the Mak.

When I have the cash I will check with the Widescreen centre about using their focal reducer with a DSLR, but looking on the bright side I may have avoided blowing nearly 200 pounds on a focal reducer, as it doesn't seem likely it will work!

In the very long term, if I am feeling richer, I may get a CCD camera, but given the cost of good ones, this won't be for a long time!