questions about ins and outs of newtonians

[adman]

I am in the research stages of getting a new scope and would appreciate it if someone could fill in some of the gaps for me about newtonian reflectors:

1. what are the good brands to get and are there any to avoid?
2. are there different features to look for that make your life easier / better if you get a newtonian?
3. are there any extras that don't usually come with the scope that are considered essential?
4. What are the most common optical issues with newtonians

I already have an EQ5 mount and just need the OTA. I would spend up to about $1000. At the moment it would be mostly visual use for me - but with a view down the track to dabbling in imaging (I am slowly getting the impression though that you don't dabble in imaging so much as get sucked in hook line and sinker....)

Thanks in advance.

[mental4astro]

Hi Adman,

You ask some very technical Qs. They tell me you've mainly used refractors? I'll try to give some enlightenment.

1:Mass production OTA's that tend to be consistently good include GSO & Skywatcher. These are readily available in Oz. Look for primay mirrors advertized as parabolic.

2: A two inch low profile focuser. These tend to be less intrusive than focusers found on refractors meaning they will be less likely to intrude into the tube obstructing light. Easily accessible collimation screws for the secondary & primary mirror, including vertical adjustment for the secondary.

3: Fast (ie: f/5 or less) newtonians can & do 'suffer' from coma. The shallow light cone they form means that with most EPs & with astrophotography the outer edge of the field of view does not focus at the same time as the central area. This is not a defect but an artefact of the optical arrangement. 'Coma correctors' are lens arrangements which assist in eliminating this effect. Should you get one, be aware that there is not a one size fits all. There are a various out there & each better suits different focal ratios.

4: Newtonian optics are not held fixed in their cells like the lenses of refractors. Hence they do move, ever so slightly with transport, so they need to be recollimated frequently. Not hard to do, & there are many aids to help. It's a touchy feelie thing.

Just some points to which others can elaborate more on.

Mental

[adman]

Thanks Mental,

so looks like the basic accessories for a newt are:

1. Coma corrector ~$200
2. some kind of collimation device ~$100
3. Depending on whether the OTA is sold with tube rings and dovetail plate that might be another ~$100 for those two

On the Andrews website there is a Black Diamond 8" OTA for $449 - plus all of the above is a little less attractive....

Adam

[mental4astro]

Adman, if it's mainly visual you are after, a coma corrector is not something that is really necessary. Some ep's deal better with this than others, especially at f/5. My big dob is f/4.5 & I had the chance to compare the image between my GSO Superview 30mm & a TMB Paragon 40mm. The GSO is useable, but with small but obvious 'seagulls' in the outer third of the field of view. The TMB should have shown much more obvious 'seagulls' for its focal length, but they were tiny, tiny, tiny. A trully brilliant ep. I want one. No coma corrector used here.

Yes, a collimator is necessary, but the GSO laser collimator is $60. Looking at one myself too. Much easier to use with my monolith than the cheshire eyepiece.

The OTA's appear to come with mounting hardware from Andrews, plus finder. Call 'em to find out.

All up this is about half your 1k budget, assuming you have ep's.

[RobF]

I'd 2nd Alexander's comment re the coma corrector. Its not something you need to rush into at all for visual. You're better off putting the money into better eyepieces, but even then standard eyepieces are a great place to start these days. For a Baader MPCC for example you have to use special spacers on the end of each specific eyepiece.

I'm biased, but bang for buck its very hard to go pass a Newt, and my 8" has given me lots of flexible enjoyment (ease of transport, setup, visual/imaging).

For collimation, I'd make sure you buy a cheshire first, before playing with lasers. Cheaper too. You'll find lots of threads here where people have got into all sorts of trouble with a laser - their chief advantage is ease of use day/night for a quick tweak. There are some common collimating situations a cheshire shows up well but the laser can't or won't easily show.

Would be worth posting in the wanted area for a 2nd hand newt too I'd suggest once you know what you want. People often move on to more expensive gear later on, so its not unusual to be able to pick up an OTA for $300-400 2nd hand in great condition.

[adman]

Thanks Rob,

I am slowly coming around to the idea of a newt, and it is great to get some info off those who have already been there.

My other concern is how stable it will be on my mount - I have an Orion SkyView Pro - an EQ5. Quite solid with a low weight refractor on it, but can't help wondering what something larger and heavier will do to it.

I am thinking around 8" - maybe 10 depending on weight (for my mount) and portability (for my car). I think that will keep me going for a while.

Adam

[rmcpb]

Your eq5 would be suitable for visual, even though you will have to be very flexible sometimes to get to the eyepiece, but it would only be suitable for lunar photography and some bright planets.

Coma should not be a problem for visual, maybe you should be looking at an f6 8" which is much easier on eyepieces. I'm sure you would find some good deals in the for sale section.

Cheers

[adman]

Hi Rob

where do you think most of the problems would lie in imaging with this mount? Is it in the periodic error, or just plain old physical stabilty?

Adam

[RobF]

If you're thinking about deep sky imaging Adam, you'll almost certainly need a 2nd scope for guiding, 2nd camera, a bunch of wires, dew covers, DSLR or CCD camera. When you add up the weight of that lot to your planned 8" OTA you're getting into 12-13 kg area. HEQ5 can do it (just), but many say EQ6 gives you more room to grow in the future if you can afford it.

EQ6 and HEQ5 have better gears and better stepper motor sensitivity and guide port.

Astrophotography can be a bit like banging your head against a brick wall for fun sometimes. You really don't want to be working past 1/2 or 2/3 rated weight of mount or it all just gets too off-putting. :)

For visual or planetary (video) work EQ5 should be fine though.

[adman]

It should be good enough to get me going - and probably keep me occupied visually for a while. There's plenty to see. But it sounds like I will be coughing up for a new one at some point!