GSO 6" RC with APCCD67 reducer

[woland]

Hello everyone,

I'm thinking about buying a new scope for astrophotography. So far I've been using a TS APO 65Q on HEQ5 with not bad results, however what I would like to get now is a scope that would be a bit faster than f/6.5. Now, it is important to remember that for imaging I'm using an Atik 314L+ which uses a small CCD sensor so a large flat field is not necessarily a must. I've been considering several different scopes that would possibly fit in what i need:

- WO Megrez 88 (f/5.6), focal lenght ~500 mm, doublet, chromatic aberration (irrelevant in narrowband imaging)

- TS 100 ED (f/6 perhaps could be reduced to f/4.8 with TV reducer but I'm not sure if a 1k euro doublet would produce good images at such a fast speed), 600 or 420 mm focal lenght

- TS 90/600 triplet with a reducer (f/5.3) - 600 or 420 mm focal length - most expensive solution, very good optics

- SkyWatcher 130 pds newtonian (f/5 but I'm not sure about the quality, I used to have an achro 80/400 and umm... never again), focal lenght 650, I already have a coma corrector

- GSO 6 RC (it's f/9 but could be reduced to useful speeds), 1370 or perhaps ~800 mm focal length

Now since I don't have an endless supply of money, I need to optimize the cost (that's why all those nice Borgs and Takahashis aren't included in my list, unfortunately). The refractors sound nice, however their focal length with reducers would be very similar to what I get with my TS65Q (420 mm). They are also the most expensive solution. The quality of the newtonian might not be up to par and I haven't seen many great images taken with it (perhaps I haven't searched well enough).

On the other hand I've seen some very impresive images takien with GSO 6 RC and Atik 314L+ on HEQ5.

Now what I would like to know is whether it is possible to reduce the focal length of this scope using APCC67 reducer a bit more than what is suggested by the manufacturer as an optimal reduction factor (x0.67), for example to 0.6 or 0.55? Theoretically and according to the telecompression graph (http://www.astro-physics.com/products/accessories/photo_acc/CCDT67%20&%2027TVPH.pdf) it should be possible. Obviously the useful field would change as well but perhaps it could still work with a small chip like the Atik's? Having this scope at f/5 and ~800 mm focal length would be really great.

I would appreciate your thoughts, inputs and suggestions. Maybe I'm not considering every option that I have.

Tomek

[johnrt]

Hi Tomek,

I use this exact set-up with the APCCDT67 reducer and I can confirm you can push it beyond the x.67 reduction by varying the spacing. The small chip on the 314l+ is very good for this, exactly how far you can push I cannot answer.

If you would like an idea of what is possible with this combo please take a look at my flickr page http://www.flickr.com/photos/skywatcher150/ or visit my website in my sig.

If you have any questions don't hesitate to ask!

John

[jnc71106]

johns images are mighty impressive with this setup. You can also get a 0.75x reducer for the 6" RC from telescope house http://www.telescopehouse.com/acatalog/Revelation-0.75x-Focal-Reducer-for-Revelation-RC-OTA.html

[woland]

Hi guys and thanks for your input.

John, your excellent images are what made me interested in buying this scope . As for pushing the reducer beyond 0.67x, yesterday I stumbled upon a photo on Astrobin (http://www.astrobin.com/46955/) taken with the RC and the reducer, and the guy who took it says that he pushed the reducer to 0.56 for APC-S chip (!), so I guess it should be possible with Atik 314L+, too.

So I think I'll be buying this one in the next couple of weeks (seeing how we've had several days of good weather, I wouldn't want to spoil it with a new scope! )

[swag72]

John posted an excellent review of his scope http://stargazerslounge.com/topic/171866-altair-astro-6”-ritchey-chretien-review/page-3?hl which I assume that you've seen.

[woland]

Indeed I have and I understand that this scope will require some adjustment out of the box, but I'm fine with that. I do have some experience with fiddling about an 8" f/5 newtonian as well as upgrading my HEQ5 with belt mod and new bearings etc., and I have to say I find some satisfaction in this part of our hobby

[johnrt]

So long as you know it will need at little adjustment out of the box then you will love the 6" RC, I have just ditched the focuser for a feathertouch, almost doubles the price of the OTA but it's a beautiful piece of engineering.

I am very impressed by that DSLR image and the 0.56 reduction, I didn't think it could be pushed that far on such a big chip. Who knows how far you can push the small 285 chip?

[GuillermoBarrancos]

What is also an interesting option I just stumbled upon, which gets you on about 750mm focal length, is a Celestron C6 SCT XLT OTA and the following reducer:

https://www.teleskop-express.de/shop/product_info.php/info/p2179_Optec-NextGen-Wide-Field-0-5x-Reducer-Corrector-for-Schmidt-Cassegrains.html

Never noticed that reducer before. But it brings the SCT down to same focal speeds as the average refractor.

Ofcourse this setup is quite a bit more expensive than the 6inch RC. But it's quite fast (F/5) and above all! gives you no difraction spikes.

You can ofcourse put the standard (and much cheaper) 0,63 FR on it, which brings it down to F/6.2 and a focal length of 945mm.

I thought to add this option to the discusion. For if People (like me) that don't like difraction spikes.