I have a takahashi fc100d on a cg4. I want to get a larger scope to use on nicer nights when I am going to be out for several hours. I care most about lunar and planetary. I was thinking about getting a 10 inch dob or a cat that I can mount on my cg4. What are my best options?
Always a lively discussion.
I see you live in Illinois so I assume that like around here, the seeing usually sucks...out loud sometimes. But sometimes it can be quite good. So, I recommend a Newt in the 8-10" range for planetary. That aperture seems to work really well for me for solar system viewing here in the midwest.
Actually I have a local friend who is looking to do exactly the same thing, a 10" F6 Newt for the planets and looking for some advice so here was my response"
"Hey Nick. I hope you are well.
I see you're getting ready for the planets this year. 10" is a great planetary aperture for around here.
My experience is that once you get below ~25% CO, the differences are really small. I personally like ~ 17-18% if there is a standard diagonal size that will get me there. In your case, that would be a 1.75" diagonal, which is a standard size. However anywhere between 15-20% is fine but at 15%, it can be a bit more challenging to collimate the scope because the fully illuminated field is very small.
By far the biggest factors to good planetary views with a Newt are:
1. Excellent optical quality of the primary and secondary.
2. The ability to get and maintain excellent collimation no matter where in the sky the scope is pointed. This includes the focuser and primary & secondary. At F6, the scope's sensitivity to,and keeping good, collimation is much easier than at F 5 or below. I assume you will be using the bino-viewers so you need to make sure the focuser will not sag under the weight of the viewer.
3. Great thermal cooling ability which includes a fan on the back of the primary mirror as well as a boundary layer fan scrubbing the primary's surface. It's best to have a thinner primary mirror to help with cooling.
4. Did I say excellent optical quality of the primary and secondary?
I also recommend that when the scope is pointed south, the focuser be put on the east side of the OTA. That's because most breezes will come from the south, north and west and will not blow your body heat across the front of the scope if the focuser is on the east side.
Some lessons hard won over the years.
I also suggested a solid tube.
So, I hope this helps and take what you like and leave the rest.