SW 200P on a HEQ5 - How much trouble do I ask for?

[hjw]

After a couple of years imaging with the SW 80ED (and still very happy with it) I feel an itch. I've been imaging galaxies lately and am considering a faster scope with more focal length. So, I am toying with the idea of a 8" f/5 reflector. Problem will be the limitation of the mount. I presume, there are people in this forum who use(d) this combination and my question is basically how bad will it be. Do I ask for a world of frustration or is it doable. I guess I will have to consider off axis guiding as my ST80 will definitely push the mount over its limit. 

Thanks!

HJ

[andyboy1970]

I imaged with my 200p & St80 on my HEQ5, close to the limit (Imaging) at 13Kgs but it worked fine.

The belt mod made a huge difference to the guiding performance.

I used three 1Kg Dumbell weights to balance the setup.

Don't use that setup these days as it was quite restrictive now I image with a small CCD (ATIK16Hr)

[ronin]

The HEQ5 should hold it well, but willask what is it that you want from the setup?

If the ED80 is f/5 then the image will be no brighter, the additional focal length of the 200P will give you a bigger image however. In effect your exposure times will remain about the same but the image on the chip is a bit bigger Size is approx:

Dia in mm = tan( Ø ) *1000; (1000 being the focal length of the scope and Ø is the subtended angle of the object in degrees).

Should be a 2* and a 1/2* in there but lets stick to close approximations.

Just a bit concerned that you are thinking 5x the diameter means a much shorter exposure time, it will not.

[hjw]

6 minutes ago, ronin said:

The HEQ5 should hold it well, but willask what is it that you want from the setup?

If the ED80 is f/5 then the image will be no brighter, the additional focal length of the 200P will give you a bigger image however. In effect your exposure times will remain about the same but the image on the chip is a bit bigger Size is approx:

Dia in mm = tan( Ø ) *1000; (1000 being the focal length of the scope and Ø is the subtended angle of the object in degrees).

Should be a 2* and a 1/2* in there but lets stick to close approximations.

Just a bit concerned that you are thinking 5x the diameter means a much shorter exposure time, it will not.

Thanks Ronin,

The ED80 (600mm x 80mm) has a f/7.5 the 8" is f/5. Theoretically this is a 2.25x difference - take away the obstruction of the secondary mirror I should be able to half my exposure time - or get twice as much photons on my sensor. Combined with the smaller FOV I am hoping for a clearer image of some smaller galaxies. At the moment I am taking in excess of 3hours and am still struggling with noise.

It is encouraging however that 2 out of 2 responses so far indicate thet the HEQ5 can handle it

Clear skies

HJ

[happy-kat]

I am only and avid reader, but you don't mention a focal reducer with your Ed80, would that not help you get faster/wider?

[almcl]

Not quite what you are asking, but for two years I have been imaging with a SW 200 P on an EQ5.  I was told (more than once) that this was way over the mount's capacity.  The scope + ST80 guidescope  + camera weighed 13kg.  And to be sure there were some nights when it definitely didn't work, but there were quite a few when it did.  Almost all those in my album were with this combination have a look (and a laugh).

I have now switched to an Alt Az EQ6, mostly to get rid of the DEC backlash, but I think your HEQ5 will do just fine.

[hjw]

2 hours ago, happy-kat said:

I am only and avid reader, but you don't mention a focal reducer with your Ed80, would that not help you get faster/wider?

The idea is to get faster with a smaller FOV not wider.

2 hours ago, almcl said:

Not quite what you are asking, but for two years I have been imaging with a SW 200 P on an EQ5.  I was told (more than once) that this was way over the mount's capacity.  The scope + ST80 guidescope  + camera weighed 13kg.  And to be sure there were some nights when it definitely didn't work, but there were quite a few when it did.  Almost all those in my album were with this combination have a look (and a laugh).

I have now switched to an Alt Az EQ6, mostly to get rid of the DEC backlash, but I think your HEQ5 will do just fine.

Thanks - the whole setup on an EQ5, now that's adventurous. I had a look at your pictures. Nothing there to laugh about - very nice! Yes, I think I will be alright! And one day I might get a bigger mount too.

Cheers

HJ

[RikM]

An 8" f/5 Newt on an HEQ5 is vey doable but most folks step up to the NEQ6 for a tube of that size. If you can shield it from the wind in an obsy or close to a fence etc. you will have an easier time of it but is perfectly possible.

[ollypenrice]

When people say that x telescope works fine on y mount with z camera I always want to ask them how do they know? I spent 2 years imaging with Yves Van den Broek using a 14 inch ODK on a Mesu with an SXVH36 at 0.66 arcsecs per pixel. We got pleasing results - at least to us. Here's an example.

But were we really gaining anything at 0.66 arcsecs per pixel over, say, what we might have achieved at 1.0 arcsecs per pixel? I simply have no idea. Round stars tell you nothng much. They tell you that your tracking may be excellent or that it may be lousy but equally lousy on both axes and so producing a random error.

I think that a good starting point is to say that your average guiding error in arcseconds needs to be no worse than half your imaging scale in arcseconds per pixel.

In changing your setup to have more focal length you are looking for more resolution. It is not dead easy for anyone to tell you that you will get it using x mount at y arcsecs per pixel, so I'm not going to try to tell you whether you will or you won't. How big are your pixels?

Olly

 

 

[hjw]

22 hours ago, ollypenrice said:

When people say that x telescope works fine on y mount with z camera I always want to ask them how do they know? I spent 2 years imaging with Yves Van den Broek using a 14 inch ODK on a Mesu with an SXVH36 at 0.66 arcsecs per pixel. We got pleasing results - at least to us. Here's an example.

But were we really gaining anything at 0.66 arcsecs per pixel over, say, what we might have achieved at 1.0 arcsecs per pixel? I simply have no idea. Round stars tell you nothng much. They tell you that your tracking may be excellent or that it may be lousy but equally lousy on both axes and so producing a random error.

I think that a good starting point is to say that your average guiding error in arcseconds needs to be no worse than half your imaging scale in arcseconds per pixel.

In changing your setup to have more focal length you are looking for more resolution. It is not dead easy for anyone to tell you that you will get it using x mount at y arcsecs per pixel, so I'm not going to try to tell you whether you will or you won't. How big are your pixels?

Olly

 

 

Thanks Olly,

very nice image - something I will aspire to in the next decade or so :). To answer your question, my camera has a pixel size of 4.8µm. The resolution of my current setup is 1.65"/pixel while the 200P would give me 0.99"/pixel. The Dawes' limit on the 200P is quoted as 0.58", so theoretically it should make a difference. My biggest concern was the weight of the setup on the HEQ5. The max weight for the mount is quoted as 13kg. For AP I heard/read that you half this. The 200P + camera + guiding will be around 10kg or so.

Clear skies

HJ

[ollypenrice]

10 hours ago, hjw said:

Thanks Olly,

very nice image - something I will aspire to in the next decade or so :). To answer your question, my camera has a pixel size of 4.8µm. The resolution of my current setup is 1.65"/pixel while the 200P would give me 0.99"/pixel. The Dawes' limit on the 200P is quoted as 0.58", so theoretically it should make a difference. My biggest concern was the weight of the setup on the HEQ5. The max weight for the mount is quoted as 13kg. For AP I heard/read that you half this. The 200P + camera + guiding will be around 10kg or so.

Clear skies

HJ

I believe that the Dawes limit is rarely the limiting factor in imaging. 1 arcsec P/P is, in my view, at the sharp end of things for the Skywatcher mounts and if you are pushing at the payload limit then it may be hard to realize. It isn't an extravagent demand though. You would certainly need to be entirely sheltered from the wind. What is your current guide trace telling you abut the mount's accuracy?

Olly

[RichLD]

The HEQ5 will handle your setup as others have said, just don't try getting the results I suspect you desire when there's anything above a very light breeze outside - it's a big scope that doubles as a lovely sail 

[hjw]

Wind is probably the least of my problems. There are a few big trees on my property, and when I say big I mean BIG (https://en.wikipedia.org/wiki/Eucalyptus_regnans ) and my setup is close to the house. I think the 200P would make a nice addition anyway and might give it a go. As far as guiding is concerned I might have to improve my efforts a bit. I tend to be within 1 arcsec but not 0.5.

Thanks for all your help!

HJ