WTB- Newtonian (need help)

[kimo]

Hi,

How good is TS optics 8" f/4 Newtonian with carbon tube? any other recommendation from other Brands? want to use for imaging. I need focal length of 750mm to 800mm F ratio 4 or 5.

Thanks
Kimo

[orly_andico]

The carbon tube is really for weight. Not strength. You will likely need to reinforce the tube under the focuser like I did. Frankly I'd go with the steel tube if it needs reinforcement anyway. Because carbon doesn't dent, it breaks.

You will want a Paracorr or the Baader Rowe corrector. Stay away from the MPCC, it adds spherical aberration. The Paracorr adds 15% focal length so an f4 becomes f4.6

The Rowe has no Barlow effect but has a lot of back focus.

At some point you may want to upgrade the focuser.

I have done all of these, and the result is decent. Large aperture, fast focal ratio, fairly inexpensive. But it's a big heavy wind grabber. My setup is over 15 kg.

[kimo]

thanks. should I require coma corrector for small CCD like DSI 2?
many user comments bad and some good about 8" Newtonian with F/4. may be, good to go with 6" F/5 750mm with dual speed so its easy to collimate. its light weight and shouldn't have tilting problem right?

[orly_andico]

I think the small sensor will not require a Paracorr. On my 8300 chip, the middle 20% of the sensor has round stars.

At f/4 collimation is rather critical. I am using a CatsEye XLKP and a TeleCat. And an Astrosystems barlowed laser.

I suspect a good laser with a barlow would suffice, the overkill collimation tools I have left over from my bad experiences at f/3 with the now-sold Keller reducer.