Chomping at the bit for a new aquisition

[Rocket Pooch]

But the glass elements do expand.  If they had the same coeff. of expansion as steel then the whole apparatus would expand uniformly and there would be no shift in the image plane.  But glass expands at probably half the rate of steel so it compensates for approx half the shift caused by the tube expansion.  So we can divide the 3.5microns in half giving 1.85 microns - let's call it 2 microns.

Hmm this is not really right, because you have one steel tube and multiple glass elements, by the sound of the thread though it does sound as if you have hit the nail on the head and the expansion is an issue, especially due to the focal length?

Fay, have you bought that 8" TMB yet?  but seriously what do you want to image?  you have the ed80 and a zs66, surely next would be something in the 1 meter range to balance the setup?

For the focusing I do check my focus between filters, my RGB filters are bang on par focal but my astronomic ones are not, I manually focus using the focus aid in AA, but I do miss my electronic focuser, mus buy a moonlight for the ED80, and also the 8"ARC, the new newt also focuses rather oddly compaired to the easy focus of the ED80, basically as the focal length gets longer and if the scope is fast focus gets really critical.

Anyway that my 4p.

[Fay]

I was just looking at your setup, Chris. Is that a Europa 150?

Well, Chris, I would like a larger aperture, faster optics with good quality lenses, I don't want anything that I have to collimate or do not want to have to be more sensitive with focusing, although I have ordered Robo focus. I have to be careful with weight & bear in mind price & I would like to think of it as my last purchase & need it to be ok for me & just what I wanted. Nothing worse than thinking you have bought the wrong thing.

Obviously my imaging preferences are galaxies & nebulae.

Comments, please, Chris?     

[Rocket Pooch]

Hi,

Yes its the 150mm one, it needs a tweak but I think it will be ok as a fast 750mm scope.  Its also very very light and will not loose collimation easily because the mirror it tiny.  I'm going to add a fan to it, a curved secondary holder and flock it, then it should be very good.

I think you are going around the same loop as Robert and I.

If you want to image galaxies then you will need 1300mm to 1700mm focal length with our cameras to get the right image scale for the messier or NGC objects, there are some big easy galaxies like M33 M31 M81 etc and the kit you have is fine for that and also disperse nebula.  You will also need something quite fast for the galaxies because they tend to be dim as well.

But its is going to get difficult now, I can image at 2 meters with the 8" on the EQ6 on the pier, but not in the field, but at F10 my ARC (or is it ACF) is too slow for anything other than bright objects, so I use the Astro Physics reducer to get the thing to F6.7 and a FL of about 1340mm, that gives me .95 arc/sec per pixel.

I think you might be better of considering a good newt or one of the new astrographs like this, Skywatcher Explorer 190 MN Pro, or even the revelation 8" F9 Ritchey Chretien Astrograph.  There both fit the bill, as does a VC200L as well.

The thing with these and my 8" ARC is they have enough of a light cone to cover your EOS as well as the 314L (but the Skywatcher will not take a focal reducer making it a little limited) and they all have their advantages and dis-advantages.

The ARC 8" I have, Dew Dew Dew, I have both a dew heater and a metal dew shield on it, the opics are very good and is has a large flat field so my focal reduce is very good with this scope.

A VC200L you can get for about £500 second hand.  I had the VC200L it was a bugger to collimate, but it kept collimation perfectly and never dewed up, but I did not like the diffraction spikes.

The Skywatcher seems ok from the images I have seen, but again dew is going to be the issue, the revelation I have not seen any images yet, but on paper it looks like its spot on, but you will get big diffraction spikes again.

Unless you really just fancy a litte more focal length I don't think there is going to be an easy way to move forward without buying something a little expensive.  The 150 from William optics looks great, but its up in the £4k bracket, outside of that its going to be something like the Meade triplet I guess.

If however the budget was about a grand the Meade seems good, Jim has one so it would not be difficult to test the optics, olly also has one as well and he seems to be getting nice images.

Personally, I'm going to get or build a good newt about 1.5meter FL and sell the Meade at some point.  But I have a permanent setup and its sheltered from the wind.  So I'm ok.

But, don't get anything which will take the scope, camera and guide cam and dual bar over 12kg, or you will be looking at a new mount as well.

That's another 4p.

Before you but Jim, Mike and I have three of the scopes mentioned above, so you could probably have a look at them 1st.

[Fay]

Chris, that was good of you to give such an extended opinion, thanks.

I did not realise Jim had the Meade. There are a lot of negative opions on the Mede.

I am just going to digest & investigate the options you suggest.

So, I should consider a longer focal length, faster scope to enable me to get the more feint galaxies etc?  Is focusing more of a problem? Would I have to do much longer exposures, or not, if the scope is fast?

[Fay]

Chris, I don't think I could handle an 8", as I have to have a portable setup, also with my heavy dual plate I think it all may too heavy for the mount 

[Mike]

Oooeeerr missus !!   

[Rocket Pooch]

the carbon RC is very light

[Fay]

I'm talking telescopes, Mike : 

[RobertM]

Fay,

All scopes have the inherent weakness that they don't do everything so you always need more than one.

The VC200l is a nice scope and I currently have Mikes to try out (if the weather ever clears).  It is light for an 8", has an inbuilt field flattener, and I've seen some fantastic images taken with it but as Chris mentions you may not like the diffraction spikes.  The GSO RC looks very good on paper but I have a few reservations (like the aluminium tube rail between front and rear cells/carbon fibre tube combination) - it's still vapourware last time I heard.  The Orion/Skywatcher Mak Newt looks very interesting but is relatively heavy (as is the Meade ACF), someone on UKAstroimaging has posted some pictures with one but they don't look anything special for that type of scope so maybe he's just getting used to it.  There are also a host of 8" F/4 Newtonians out there - Orion/GSO/Vixen which can also produce exceptional images.

Don't forget you can always go for wide field rather than narrow and the images can look equally spectacular plus you get the most out of the mount you have.  For closeup work you need a bigger scope (more aperture and focal length) and with that comes extra weight and tighter guiding.  I'm not saying your mount can't cope but it will be more demanding.  Why don't you measure the weight of you tandem bar and guide scope together then take away from the 12Kg Chris has recommended and see what you have left for another scope.

At the end of the day it comes down to deciding what you want to do then figuring out how you want to achieve it.  Most of us want it all so where possible have a range of scopes to suite different circumstances.  Whatever you do make sure you scour the web looking for useful information.

Sorry if I've repeated anything Chris has said but I thought I'd get my own 1p in...

Robert

[RobertM]

Correction - the GSO RC does seem exist (well it seems to be in stock at BCF) and the dovetail rail is not now fixed to the mirror cells.

[Fay]

Thanks Robert, there is a lot of truth & food for thought in what you have pointed out.

I will have to try & have look at what imagers in the club take thru these longer fl scopes so I can see first hand what attributes they have over what I have already got. No good repeating what I already have.

Fay