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Chomping at the bit for a new aquisition

Started by Fay, Dec 09, 2008, 11:56:36

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Fay

I really fancy a triplet telescope. Has anyone any opinions? Bearing in mind I have a SX M7, Atik, Canon, camera's &  ED80 & WO ZS66, scopes.

I cannot afford a fortune but do not want to waste my money, obviously.

I was looking at WO 110, FL770mm F7, but a bit expensive.
Meade 5000 127, more my price, FL952 F7.5
Any other makes anyone can suggest?

Thanks

 
It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

Mike

Go for the WO over the Meade. Anthing by Orion or Skywatcher is all teh same company (and in most cases identical scopes). If you feel really flush there is always a Takahashi Sky 90 which is really nice and reasonably priced.
We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology. Carl Sagan

RobertM

I think the Sky90 is a great scope but make sure you get the Mk II which has a better collimation cell (same optics though).  Also the reducer/flattener is rather expensive but you could save yourself a bundle by getting the whole lot together.  Tak fixtures and fittings can soon add up so the more you get as a bundle the better.

Scour the Cloudy Nights reviews and Excelsis ratings sites for information.


Rocket Pooch

How about a TMB 8" Fay, I can then borrow it.

If not a nice 12" F5 Newtonian, I can then borrow that as well.

or

A nice Vixen 260, that would also be nice to borrow.


Tony G

Fay,

Hide any of your black dresses and stilettos, or he'll be borrowing them as well.  :-?

Tony G
"I'm normally not a praying man, but if you're up there, please save me Superman." - Homer Simpson

Rocket Pooch

Bloody hell I did not know Jim had told you what we got upto in France this year  :o

Fay

I have eliminated the Meade.  Now it's:

Tak TSA 102 Triplet FL816      F8

WO 110 Triplet        FL770      F7

Tak Sky 90 Doublet   FL500     F5.5

Interested in your expert opinions please.

All very expensive :cry: :cry: :cry:


Fay
It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

Mike

They are expensive, but if looked after they will give you a lifetime of fantastic images. If it was my cash i'd go for the Tak triplet without a shadow of a doubt.
We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology. Carl Sagan

Fay

It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

Fay

Mike, I want a really good quality telescope, that would be it, no more, it has to be right for me & would have to last.
What about my mount, would spending a lot on a scope be a waste if the mount does not match the scope quality?
It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

Ian

Is there any question about the quality of your mount at the moment?

What about it do you think is wrong?

RobertM

Fay, look at the images people are taking with those scopes with your cameras.  If you are really interested in the Sky90 then you are more than welcome to either borrow it for an evening or use my setup to try it out (visually or for imaging).  I have both losmandy and vixen adaptor plates so you shouldn't have any trouble mounting it.  My personal view is that it has a fantastic field of view and excellent optics when collimated.

On your choices:

TSA102 - Best optics but it's F8 so excellent if you have a larger camera or F6 reducer.
FLT110 - Big scope for the money but I would still get the TSA.
Sky90 - Yes it's a doublet but becomes a quadruplet with the flattener or extender and has the widest field of view.

I would see what comes up but check on the field of view and what you want to do with it !

Fay

Ian, there is nothing wrong with my mount, but I am aware that it is not in the luxury league, eg. Robert's Takahashi. If I could not get a lot better results, then it would not be  worth spending a lot on a telescope.

Robert, you have given me a lot to investigate & think about. Your offer is very kind. As you have said, it is best to try & get some sort of an inclusive deal, as the extra's that are needed add up to a lot more money.

Tony, nice to hear from you, I was going to put out 'anyone seen Tony fliers'. I think a nice red polka dot dress would suit Chris :lol: :lol: :lol: :lol:
It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

Ian

Fay, my point being, if you don't already know where your current mount is deficient, how do you know that the new one will be better? You're not substantially changing focal length, and your guiding is working well at that sort of focal length. You're not substantially changing the load on the mount either so that's not a concern either.

I think you'll see the difference with a better scope due to field flatness (FR depending...), correction, internal reflections and so on. Just review the field of view and make sure your favourite targets will still fit and then crack on. When you work out at what point your mount is letting you down, then think about a replacement.

Mike

Your current mount is fine for what you want to do. The HEQ-5 and the EQ6 are the best bang for the buck mounts on the market. Learn how to use it to its full potential with a quality scope on top and you will produce fantastic images (not that you aren't doing that already with what you already have).

We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology. Carl Sagan

RobertM

Fay,  The Sky90@f/4.5 has a f/l of approx 405mm, the ED80 that I guide with is 600mm so as you can see guiding is less critical.

Just looking at your M33 from Les G. and you have pin sharp stars there.  Maybe the problem is focusing or seeing - I've noticed that's difficult to achieve and keep with the supplied focuser.

MarkS


Robert,

Do you set your focus once for a whole imaging session?

I am becoming more and more aware that focus (on my C11 SCT) drifts and I need to refocus.  It seems to be consistent with falling temperature though I haven't quantified yet - this will be the next stage.

I reckon I need to refocus every 30 minutes or so under present (Wintry) conditions.  I also give the scope 90 minutes to adjust to ambient temperature even before I being imaging.

Mark

Mike

Focus on all scopes will meander through the night, especially if the temperatures changes a lot. Even more so if you bring your scopes from a warm room to outdoors where they can take hours to get down to ambient temperature. better to sstore them in a cold garage or shed.

Do you suffer from the same fate in summer also Mark? Is mirror shift a cause?
We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology. Carl Sagan

RobertM

Mark, I actually set it then only refocus if on change of target or filter.  My imaging setup is always at ambient temperature when I start (it's out all the time under wraps) and there isn't much change at the moment between  +4 and -2 degrees day to night.  I certainly don't notice the difference though MaximDL often identifies one of the images near the beginning of the session as the one with the best FWHM.  If you work out the coefficient of expansion of the aluminium over 5 degrees C you could probably have a stab at guessing how much the position changes.

If I was automating the imaging session then I would refocus after every 30-60 mins.  It's so quick as long as there's a bright enough star, which isn't always the case.

Mike

If anyone is thinking of buying a motorised focusser, a lot of the modern designs have automatic temperature compensation built into them.
We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology. Carl Sagan

RobertM


Fay

So, Robert, you have to buy a Robofocus motor & Easy Focus Stepper interface? Is that everything? Do you just connect the interface to the motor? Do you then have to make something to replace one of thescope focusing wheels?
I am confused as to how all this works & goes together. 
It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

Fay

It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

RobertM

The Stepper motor is connected to the EasyFocus via a ribbon cable which is the same as Robofocus.  The EasyFocus unit needs 12v power and the output is rs232 which is plugged into my 4-way Serial to USB unit so it just looks like another serial device.

There are various ways of connecting the stepper motor shaft to the focuser shaft but I use a aluminium shaft coupling so it's a direct drive.  The motor is held in place by a bracket made out of some spare Dural plate.  The couplings can be obtained in almost any size and material and vary in price from £5 to £150, the one I got was just over a tenner.  Will take a picture tonight so you can see how it all holds together.

posted anyway as I'd written it ...

MarkS

I don't think it is mirror shift.. The mirror is very stable - I can even do a meridian flip with no need to refocus.   However, I do take the scope outside from a warm house - maybe that is part of the problem - maybe it takes longer to reach ambient temperature than I am allowing for.  But I don't want to store it in a damp garage.

So just how big an effect is thermal expansion/contraction?

Warning: This next bit contains scary maths

My scope is steel which has a coefficient of expansion of 13x10e-6/deg C
It is 550mm long, therefore for every 1 deg C it expands by 7microns (13*550/1000)
Therefore, naively, the image plane also shifts by 7 microns
But, you might say, the light path is folded, therefore the image plane might shift by 2 or 3 times that.

So I did some rough optics calculations using approximate focal lengths of the mirrors.  It turns out that the image plane shifts by about 5x the expansion in the tube length i.e. it is behaving as if it were a simple telescope 2750mm long (which is suspiciously similar to the focal length of the C11).  I suppose that makes sense really.

So if the glass elements do not expand (i.e. they retain their original focal length) then the image plane shifts by 5x7 = 35microns.  At F10 this means that the image of a star widens by 35/10 = 3.5microns i.e. approximately one pixel for every deg C.

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.

In sum, for my C11, the image of a star bloats (defocuses)  by half a pixel in diameter for every deg C change in temperature.  So a temperature drop of 5C (which is quite typical) can lead to stars bloating by 2.5 pixels. This is pretty consistent with the defocusing I see over the course of an evening.

Mark

Fay

Mark, that is exactly the answer I came up with after doing my mathematical calculations    :o :o :o
It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

RobertM

I didn't bother to work it all out like Fay and yourself but those figures sound very plausible.

Mike

Well that pretty much explains it then  :o

Is your attic badly insulated? if so, keep it up there or do you have a larder? My tiny larder has a big air brick at the back meaning it is bloody freezing in there all the time and must be pretty close to the outside temperature. Would be agreat place to store my scopes if it wasn't full of a washing machine, microwave and all kinds of other crap.
We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology. Carl Sagan

Ian


Fay

Mine are all up the shed, nice & dry & cold.
It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

Rocket Pooch

Quote from: MarkS on Dec 11, 2008, 12:50:11
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?     
It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

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?
It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

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 
It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

Mike

We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology. Carl Sagan

Rocket Pooch


Fay

It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!

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
It is healthier to be mutton dressed as lamb, than mutton dressed as mutton!