Autoguiding with an SCT

[blinky]

I am having a major problem with my autoguiding at the moment. I used to use a DSI but I have recently changed to using my DMK as MaximDL seems to like it better. The problem is that I am imaging with my ED80 which means the SCT is relegated to guiding duties with the aforementioned DMK, but my guiding is terrible, even after drift alignment and it makes no diff if I use PEC or not. I am using a n F6.3 reducer and am wondering if, since the DMK is a small chip CCD I could extend the distance to the reducer to increase the reduction factor? Or would I be better looking for a 2nd hand F3.3 reducer?

The other worry I have is that since the guide graph is all over the place when guiding at F6.3 will this mean I cannot use the SCT for imaging when I guide through the ED80? I am hoping not and the guiding is all over the place due to the very small pixel size of the DMK and when using my QHY with the bigger pixels it would be OK - does that make sense?

[Merlin66]

Which mounting are you using?

Have you tried with PHD?

I use the ST80/ QHY5 guide combo with a C9.25 on a NEQ6pro (and the 12" Lx200) with no issues. I also use a Vixen modified beamsplitter (similar to OAG) on the C9.25 with a DMK and DSI II (with the x0.63 reducer) again with no problems.

I'd keep the reducer back focus close to the design distance of 110mm.

[Mike_F]

Hi Blinky,

Conventional (received) wisdom is that SCTs do not good guide scopes make. They're generally of a very long FL and, crucially, the primary mirrors are prone to moving - especially if you're imaging around the meridian. (Hence OAG tends to be a popular route with them, as it inherently compensates for mirror-shifts.)

I note you're trying to resolve the FL issue with reducers - but that obviously won't help mirror flop/shift. You don't mention what brand your SCT is, but some Meade OTAs can be fitted with a mirror lock to resolve that.

Out of interest - do you get much image-shift when you're focusing? If so, it might indicate there's enough free-play in the primary to be causing the erratic guiding.

Mike.

[blinky]

I am using a Meade SCT But I dont think it can be fitted with mirror lock. Maybe I should look at getting an OAG - I have tried them before but never had much luck!

[Merlin66]

Certainly the Meade can be fiited with the mirror lock. IMHO based on ten years of using them (10" and 12") I never had any issues with mirror flop during normal usage - just remember to run the mirror up and down the baffle occasionally to spread the glue - I mean the grease!

I can keep a target star on a 25 micron spectroscope slit for as long as you want.

Which mounting are you using?

PHD -is it any better for you?

[blinky]

I have the same sort of issue when guiding with PHD or MaximDL, I am using an HEQ5

[blinky]

What I am really wondering is, will the DMK show up errors more readily due to it being smaller pixels than the DSI? The DSI, although flaky in Maxim, produced a nice flat guide graph - I have redone the mount polar alignment and drift align at the same time as changing to the DMK and my guiding is terrible, I am not sure if it's the alignment or the DMK showing up smaller errors that would not affect the DSI.

[Psychobilly]

I used to guide using the CPC800 SCT with a Celestron 0.63x FR when imaging through the Megrez72 using a DSI IIc and PHD. I added a 500mm f8 T mount telephoto and moved the DSI IIc to that to free up the SCT for imaging...

I have added a TS EOS fit OAG with a DSI IC to the SCT for long focal length imaging to help overcome any mirror flop issues..

I tend where possible to use dimmer guide stars and exposure of 2-4s to avoid chasing the seeing...

Billy...

[Merlin66]

Blinky,

I think we need to see the guide graph...

It's difficult to make any other suggestions without seeing more data.

The DMK has been successfully used for guiding and if the parameters are set up correctly then there should be no issues.

BTW I don't recommend the x0.33 for anything!

The only time I've used mine was with a Star Analyser grating to get smaller star images and spectra. IMHO certainly NBG for imaging.

[FraserClarke]

How well do you need to guide? If you're guiding with a telescope with a 2m focal length and 5 micron pixels, your guide graph could look as rough as a badgers botty, and you may still not see a jot of movement on your 600mm 7.4micron pixel imaging camera... Your over sampling the guiding by a factor of nearly 5, so you can probably tolerate a lot of jitter.

As Ken says, would be really useful to see a guide graph.

[BlueAstra]

I guide with a Lodestar via an Orion OAG after the 0.63 FR on the C8 SCT with PHD. I've never had a problem guiding it ( or the Meade which is piggy backed on the C8). The Lodestar has almost the same size pixels as the DMK, but it may be more sensitive. I generally guide on a dim star with an exposure of 2-4s.

[blinky]

This morning I tightened everything up and I also added 2 bits of wood between the top and bottom plate of my pier adapter - I read somewhere that wood absorbs vibrations so am hoping this does some good.

Tonight is supposed to be clear (still 100% cloud here ATM) so I will do a drift polar align and also redo PEC then see what my guide graphs are like.

[ollypenrice]

I was going to say the same as Teadwarf but with less imagination!!!

When piggybacking as you did and guiding through the SCT I got a hideous guide trace but round stars. 1.6 metres guiding about 330mm. Not a problem.

On my old Meade classic there is no provision for a rock solid lock but Telescope House did a bodge gadget that works well. A threaded bar (studding) goes in through the transit bolt hole and then into the mirror holder (not too far!!) and then a spring goes over this and is pushed into light tension against the back of the scope by a nut. That pulls the mirror back permanently and is pretty good, given how lousy the basic system is to start with. You could make one in a tic.

On the wider issue, Ken is one of those geniuses who gets an LX200 to guide accurately. I am one of that large collection of numbskulls who never did, so I would say don't throw any money at it...

Olly

[blinky]

Well it's good to know I should not be expecting a perfect flat trace then. The worry I had was that I would not be able to image through the SCT due to the dodgy guide trace but I guess when using the QHY with it's bigger pixels it's less of an issue. Hopefully tonight will be one of those nights when everything works.....

[martin_h]

Ive imaged through an 8" SCT/QHY8 using a ST80/QHY5 as a guider and had no problems.

[blinky]

And that was with your HEQ5 pro? Thats good news if it was.

[martin_h]

Yes it was, the ST80 and QHY5 add hardly any extra weight, the balance point is only about 2" off center from the puck, I have to take out the thumb screw and put a bolt in on that side because the head fouls the saddle....its all good fun!