Off-Axis Guiding - Vibration ?

[malc-c]

I currently have an observatory set up with a SW Explorer 200P / HEQ5 setup.  Imaging is done via a modded Canon D400, with guiding via an QHY5 attached to an ST80 refractor piggybacked on the 200P 

I'm having a few issue with flex / alignment of the guide scope with PHD2 repeatedly reporting calibration errors, so I was considering the option of using an off-axis guider.  However, my concern is that the movement of the mirror in the DSLR camera will cause vibration that will affect the guiding ?

Comments please

[steppenwolf]

30 minutes ago, malc-c said:

However, my concern is that the movement of the mirror in the DSLR camera will cause vibration that will affect the guiding ?

I don't think this will be a problem as it is a transient vibration anyway and the next guide command (less than 3 seconds away) will soon correct anything caused by the DSLR mirror, however, even this transient is a small one in the grand scheme of things unless you intending to take very short exposures, which seems unlikely.

[alacant]

Hi.  I think the 400d has a function lo lock up the mirror a few seconds before it opens the shutter but the main advantage an OAG would bring to your setup is the 2 kilo load you'd remove from the mount. HTH.

[Tim]

Have never had a problem with my DSLRs when used this way.

[malc-c]

Thanks guys.  Just didn't want a PHD graph that resembled a cardiac trace

[malc-c]

One further question, are there any real advantages using an off axis guider over a guidescope other than detecting the drift through the same optics ?

[alacant]

15 minutes ago, malc-c said:

advantages using an off axis guider over a guidescope

See here.

[steppenwolf]

36 minutes ago, malc-c said:

are there any real advantages using an off axis guider over a guidescope other than detecting the drift through the same optics ?

Closer coupling between the imaging and guide cameras leads to negligible risk of differential flexure.

[malc-c]

Looks like I need to buy myself a Christmas pressie then

[michaelmorris]

I would check that you're going to have enough back focus to accommodate and OAG on your Newtonian.

[malc-c]

59 minutes ago, michaelmorris said:

I would check that you're going to have enough back focus to accommodate and OAG on your Newtonian.

Michael,  I was looking at this one from FLO https://www.firstlightoptics.com/off-axis-guiders-oag/off-axis-guider.html  so I'm hoping that it wouldn't have too much of an impact.  

It would be nice to hear from anyone using a similar OAG with a DSLR and 200P with a similar guide camera.  From the links on FLO's site I can't work out how to use the coma corrector I have, presumably that just screws on the front much the same way as it does to the D400 body in my current arrangement?.  How can I find out if the two focal distances for the DSLR and QHY5 are the same, or can you focus the guide camera independently to the main camera ?

 

[alacant]

Hi. I have one on loan. Good solid piece of engineering. It adds 16mm to the optical path so it will work with the GSO and Rowe coma correctors, but not the more common ones. It works for me as I don't need a cc for my 150mm f8. What cc-sensor distance do you need?

[malc-c]

I'll have to take some measurements as I can't remember off hand.  It was fitted to the scope 4 years ago and been left like that ever since !  

I'm sure if I ordered one from FLO and I couldn't get it to work they would take it back ???

[ollypenrice]

It might be cheaper and easier to fix the ST80 guidescope. These guide all our setups including the TEC when working at 0.9 arcsecs per pixel. Sources of flexure are:

Attachment or rear casting of OTA to the main painted tube. Check the three screws.

Droop in the focuser. Tighten the lockscrew down pitilessly.

Flex in the extension tubes. Araldite!

Flex in the attachments. Fix as required.

Cable tug. Cut some slack...

But the OAG is always a good solution if you're not changing kit around too much.

Olly

[kirkster501]

Moving to OAG is the single best thing I have done.  I approached it with great trepidation but it was actually very straightforward.  I can now get 20 minute subs whereas before, with a guidescope, my subs went "eggy" much beyond 10 minutes.  For the life of me I could not find flexure but it must have been present somewhere.

[alacant]

10 hours ago, kirkster501 said:

it was actually very straightforward

+1 Glad I'm not the only one to think so!

Flexure: tighten and stiffen as you may, it remains a mystery. You only know it's there when you change to an OAG.

[ollypenrice]

9 hours ago, alacant said:

+1 Glad I'm not the only one to think so!

Flexure: tighten and stiffen as you may, it remains a mystery. You only know it's there when you change to an OAG.

I knew it was there with the first OAG I used. It rocked on its turret so I had to build a steel strap to hold it still. This design fault was addressed by Starlight Xpress in a revised version. The forces of darkness are everywhere in this game! On balance I agree that the OAG is the solution of choice but I won't make the change myself till I find a need. It's perfectly true that flexure can be a devil to track down.

BTW, as Sara points out, you can't use an OAG on an autofocusing dual rig.

Olly

[malc-c]

Thanks for the input people.

I'm still in two minds, especially as money is tight at the moment following termination of my employment due to braking my leg  a few months back (yeah so much for understanding employers!).  My interest in imaging also comes and goes, so I'm now more of a casual observer than fanatic at every opportunity.  Maybe I could try and adjust the ST80 so that it's more in tune with the main scope, and hopefully dial out the errors I'm getting when calibrating PHD2

[Allinthehead]

3 hours ago, malc-c said:

termination of my employment due to braking my leg

Is this legal in the UK?

[malc-c]

If I had been there more than 2 years I would have had a case under UK tribunal law, but anyone employed less than 2 years would have to sue them, which has a lot of risk.  If I had been in a union then that would have been different.  I had been there 19 months.

It was a crap company to work for and run by a power crazed bitch... and I'm glad to be out of there to be honest.

[ollypenrice]

16 hours ago, malc-c said:

Thanks for the input people.

I'm still in two minds, especially as money is tight at the moment following termination of my employment due to braking my leg  a few months back (yeah so much for understanding employers!).  My interest in imaging also comes and goes, so I'm now more of a casual observer than fanatic at every opportunity.  Maybe I could try and adjust the ST80 so that it's more in tune with the main scope, and hopefully dial out the errors I'm getting when calibrating PHD2

I honestly believe that your problem can be solved. I've been guiding two rigs over literally thousands of hours using ST80s as guidescopes.

Olly

[malc-c]

Olly, thanks for the suggestion.  The ST80 does indeed give a nice bright star field to chose a suitable guide star .  I'll have a look at my guidescope setup and see if I can dial out the issues

[alacant]

20 hours ago, malc-c said:

still in two minds, especially as money is tight at the moment

Hi. Not sure whether this is the same elsewhere but astro clubs here -I use the word club loosly- are a great source for trying out stuff without committing to a purchase. I too was convinced that my setup was flex free until I tried an oag. No more fighting with those awful star reduction algorithms!

[malc-c]

I posed the question to FLO and received a reply this morning from Martin, which I would like 

As far as I'm aware you will not be able to reach focus with the DSLR camera if you place a OAG in front of the DSLR camera, it will push the camera to far away from the telescope to be able to reach focus. Reflecting telescopes have very little focus travel and using a OAG and a DSLR is not normally doable with the explorer telescopes.

So it would seem that getting on OAG would have been a waste of time.  I'll just have to dial out the issues with the ST80 and hopefully my guiding will be a little more precise 

[alacant]

31 minutes ago, malc-c said:

if you place a OAG in front of the DSLR camera, it will push the camera to far away from the telescope to be able to reach focus

Eh? Just remove the canon t adaptor and insert an oag instead. This will work perfectly with your telescope! HTH.