GSO f4 Imaging Newts

[barx1963 (Malcolm)]

I have noticed that GSO/Bintel have these new f4 Newts that they are selling as imaging scopes. I am thinking about an imaging setup and have a thought that these may be an option, so I am trying to put together a set of pros and cons to help with planning. So far I have

PROS
1. Fast (or fastish!)
2. Similar to existing newts in dobs, so already know how to collimate
3. Could easily have a dob mount made for tem so they can do double duty, in dob mode is going to be a shorter OTA so easier to use, lighter.
4. Includes 3" focuser
5. Price 12" is $1000
6. Lots of aperture for the $$$

Cons
1. Long tube compared to a GSO RC, could be hard to handle on a mount, as I understand shorter tubes enable you to get closer to the theoretical weight capacity of a mount. Long tube may also cause issues if mounted on a tripod.
2. OTA is heavy at 18.5 kgs without tube rings, which is nearly the carrying capacity of an NEQ6 and two thirds of the capacity of a G11
3. Would need a Parracor or MPCC
4. Contrast would be not as good as with a good refractor, but may be better than an RC as secondary is smaller?
5. Not a lot of people using newts for imaging.

Any thoughts?

Malcolm

[Shiraz (Ray)]

Hi Malcolm. Good list.

I have an older f4 GSO 8 inch and from experience with that, would add the following:

1. Focus is touchy at f4, since the diffraction limited focal depth is only about 40 microns. Seeing effects will give you a bit more latitude, but you will need a good stepper motor focuser with automatic focusing software or you will be forever refocusing to try to track variations in temperature etc.
2. Mechanical alignment is critical at f4 - everything must be as near as possible to perfect or you will get distorted stars somewhere in the FofV. Expect to spend some time tracking down minor flexures or misalignments in the tube/focuser/MPCC/mirror supports/camera etc.
3. A 12 inch Newt OTA will act like a sail in even the slightest wind, so you will need a very strong mount - you will probably need something much heavier than an EQ6.
4. You will not be able to use adaptive optics at this focal ratio.
5. Optically mine is very good - able to resolve much better than seeing will allow, but an MPCC or similar is essential, since coma is a killer at f4. No problems with contrast at all.
6. At f4 you can use much shorter subs, so can possibly get by with slightly more differential flexure than you would with a higher FR and consequent longer subs.
7. Balancing can sometimes be a problem with a big lump of camera hanging way off the tube axis.

Scopes like these are not black box refractors where light goes in one end and a near perfect image comes out the other. You will probably have to spend a lot of time fine tuning and understanding the scope's idiosyncrasies, but when you do get it right, the results can be very satisfying and take only a short time - and the scopes you are looking at are a lot of capability for the money. I am certainly going to keep my f4, even though it can be frustrating at times.
Regards Ray

[Moon (James)]

I have one of the new GSO 200mm f/4 newts (and also the 8" GSO RC) and I have to say that I agree with most of the comments above.

4. Contrast would be not as good as with a good refractor, but may be better than an RC as secondary is smaller?

Theoretically true, but in practice you probably won't notice it for DSO imaging.

The only other point I would add is there is more vignetting on the Newt when compared to the RC.

Scopes like these are not black box refractors where light goes in one end and a near perfect image comes out the other. You will probably have to spend a lot of time fine tuning and understanding the scope's idiosyncrasies, but when you do get it right, the results can be very satisfying and take only a short time - and the scopes you are looking at are a lot of capability for the money. I am certainly going to keep my f4, even though it can be frustrating at times.

This is very true. They are both (especially the newt) "projects" rather than finished products. Think: focuser motor, corrector spacing, center spot, reinforcements etc)
If you want a no hassle plug and play experience you will need to pay a lot more for an RCOS / Planewave CDK / Tak etc.

James