CDK17 F4.45 reducer

[gregbradley]

I finally received the reducer for my CDK17 yesterday.

I installed it straight away (I had to grind the thread down for the adapter into the FLI filter wheel which has a shallow depth before the
adapters hit the filter screws and stop it turning - done it before with adapters - easy for the maker to get it wrong).

I installed a Vixen 95VMC guide scope with the ST402 (now repaired finally).

I was concerned I would get flexure. But 15 minute exposures show totally round stars - awesome.

This setup is going to show up any misalignment and there is a tad of that. Planewave has graciously said they are shipping a replacement focuser after the spacing shims fell out of mine after an accidental loosening of a plate they are under.

So mine is a tad out and there is some minor star distortions in 2 corners that I expect the new focuser to solve. I will check collimation while I am at it.

17 inches of F4.45 gets a bright image pretty quickly. A 15 minute Helix sub at 2x2 binning shows the real faint outer neb in one exposure. Cool!

I'll post some images taken with it later on.

Here is what it looks like:

http://www.pbase.com/gregbradley/image/136344979

Greg.

[CDKPhil]

That sounds like a great result.
I am considering the focal reducer for the 12.5 takes it form f8 to f5.3.
I take it you could not fit your OAG with the focal reducer in place?

Cheers
Phil

[UniPol]

That is one serious set up you have Greg! I'm sure Sam Spade would exclaim " the stuff that dreams are made of ". Hope you get to do some decent imaging considering the amount of time needed to gain the necessary technical knowledge not to mention procuring various adaptors et al.

[gregbradley]

That's right. Backfocus is severely limited so its just the reducer which fits into the visual back and the guidescope for guiding or a self guiding SBIG camera. Perhaps a QSI with the built in OAG may work but then the 8300 chip is not a good match for this scope (arc seconds per pixel are too small unless you have 1 to 1.5 arc second seeing at 2000 metres altitude??)






Thanks Steve.

Yes there is a certain amount of setup time and fiddling with things but I have to say the amount fo fiddling required for the CDK is minimal.

Greg.

[Bassnut]

Thats a worry, so whats the BF with the reducer?.
Do you think the full rotator/Ao gear etc is possible even without the reducer?. I think I saw what the BF was once, but I forget, is it more than say 8 inches?.

[Bassnut]

I just looked. The CDK is 8.8in from the back plate and 5.8in with the focuser racked in. Not sure what to make of that, doesnt look good for my gear. The BF on a 16" RCOS is 10in !! (has a secondary focuser). Geez, it seems money is the only solution for usefull proper specs, damb it!.

[gregbradley]

I think you'd be able to get your setup on one. Not 100% sure though. How much room do you need?

Planewave are coming out with their own rotator soon so perhaps it will be more compact.

Certainly the reducer means STL11 or ST series cameras only for self guiding. It gives 1959mm focal length with my scope and the Vixen guide scope works well luckily.

Greg.

[Gama]

Greg, you can get a longer back focus by replacing the focuser with a Clements type. http://www.clementfocuser.com/
I've seen this done to some Planewave scope and you should have ample space once its on.

Also, whats the viewing like using the reducer ?.
Some time back, I was tinkering on the border whether to buy one for my 20" CDK, but just lost track of time.. So im again thinking of one mainly for visual use.
Lastly, you shouldnt get any flexing with your guide scope. As i mentioned elsewhere, i have the Vixen 110mm CAT as my guide scope, and i also have a longer focal length than your scope too, and i get round stars were ever im pointing. The scopes and guide plates Plane Wave sell are really good.

Theo

[gregbradley]

Thanks for the link Theo. That is very helpful. So far it is working just fine so no need to change anything but it may interest Fred who is considering a CDK with his ST10XME and OAG setup.

I haven't looked through it visually as it is attached to the camera and plugs into the visual back.

I imagine it makes the view a bit sharper (because its wider field) and wider.

Yes the little Vixen makes a great little guide scope. So light and yet quite long focal length and the focuser is very smooth.

I have very little trouble in finding guide stars so far. I did try an Astrotech 66ED but I got flexure. But then I was using a
diagonal which could easily flex.

Greg.

[pluck]

Greg,

What's the FOV and resolution of your system before and after adding the FR?

Paul

[gregbradley]

2959mm focal length before and 1953mm afterwards. Its a .66X reducer.
Aperture is 17 inches diffraction limited. Not sure what that works out to in arc seconds theoretical resolution. But in reality it is seeing limited. The 1953mm is more manageable with a larger pixelled camera with my local seeing.

Greg.

[pluck]

Thanks Greg,

If you're using the KAF-16803 then at those focal lengths your image scales are roughly 0.6" per pixel and 0.9" per pixel without and with the focal reducer respectively. As such, it makes sense to use it under your skies (you're on the border of over sampling). The very nice field of view (now over 1 degree with the reducer) is the real clincher.

I'd love to seem some test shots - simple star fields would be fine - to see the quality at the edge of that 1 degree FOV. A flat field would be interesting to see too.

Nice.

Paul

[Bassnut]

Well, your lucky, look at that big guide cam waving in the breeze on a pokey draw tube :P ;). Bolt the cam to the rail and get 20mins. Looks like theres a bolt hole on the side just made for the job.

[gregbradley]

Next time I use it I'll be taking fresh flats as I just recollimated it.
I'll post something.



Good tip Fred. I'll check that out. I probably can go 30 minutes asis though. I haven't tried it yet.

Greg.