At what point do I need to guide?

[DaveS]

OK. As I get my old 8" Newt up and running again I want to re-purpose my Megrez for imaging (Assuming it's OK for the job) with my DSLR. I went to the FLO site and started putting together a package:

WO reducer / flattener,

T-Canon adaptor,

Skywatcher ST80 guide scope,

Skywatcher guide camera package,

ADM dual Vixen bar.

At which point my credit card had palpitations, even before uttering the dread word "Astronomik".

Now, I've seen in other threads poeple saying that if the subs are short (<1min) with a relatively short FL scope then you dont *need* a guider.

Is this do-able?

I know in an ideal world I should have the full package, but if I can get away with just the Reducer and T-ring, then I could put some money into a UHC filter, and budget for a (Better) guide package at a later date.

[IanL]

The bad news is there is no simple answer to this one. What are you hoping to image first off? Some of the brighter objects can produce satisfying results with short exposures and very little kit, but clearly the more challenging targets are a different story.

I assume the scope is around f/6.9? Not particularly fast; you'd obviously do better with a reducer, and assuming it is one of their 0.8x reducers you're then looking at maybe f/5.5 which is better but still not super fast. Your limiting factor without guiding is going to be a combination of the PE of the mount and how well you have polar aligned it.

The good news is that it would be pretty cheap to find out what your unguided limit is at f/6.9 since all you'd need is the appropriate T Ring and adaptor; hardly going to break the bank so why not buy them first and try it out for a few nights? If you think you're getting somewhere then you might decide to buy the reducer, or realise that you are going to want to invest in a guiding set-up and plan accordingly.

By way of encouragement, take a look at this one:

The kit is listed in the gallery entry, but basically it is a set of unguided exposures through an f/10 SCT, 10 second, 30 second and one 60 second. The polar alignment was shoddy (basically squinted up the fork arms at Polaris a bit), the tracking was poor as the RA drive is just a simple DC motor with a modification to add a potentiometer so the drive rate could be dialled in by eye (changes with the temperature!), etc.

Your existing gear is already an order of magnitude more suited to the task, so I'd expect you could dabble for a bit and decide whether you want to invest a lot more cash later.

[steppenwolf]

Ian's advice about 'sucking it and seeing' makes good sense. If you find the limit is too low for your purposes then with a native focal length of 558 mm and effectively 446mm with a reducer/flattener then you could consider using a finder/guider for autoguiding as this is lightweight and would cost less than a side by side arrangement.

Let us know how you get on.

[DaveS]

Thanks guys.

As I'll be using a DSLR I want the stars found to the edges so I'll get a WO Refucer iV, which should also pull the f-ratio down to 5.4 or so, and add a 2" Baader UHC-S to be getting on with.

To start with I'll only be going after the easy objects, M42, M45, Double Cluster, the usual suspects , keeping the subs below 1 min I think.

Beyond that, or if the tracking proves to be a bit "iffy" I'll look into a guideing package, better than the skywatcher one, I think.

I've already replaced the Skywatcher dovetail with the ADM dual Vixen / Losmandy one, ready for the 8" when it's finished.

[dph1nm]

At 660mm FL, with a Canon 1000D, I can get around 1-2min exposures on my HEQ5 before the periodic error causes a problem (assuming you are accurately polar aligned).

NigelM

[DaveS]

Good to know, thanks. So at 490 (610*0.8) mm, 2 mins should be do-able.

[perkil8r]

Let's get this into perspective. I'm a newbie, I use an EQ5 unguided with both st80 and 200p (not at the same time. I can get to 100 secs with 100% success (no bad subs) with either.

Example of my poor attempts:

orionsword101112a by Perkil8r, on Flickr

[kirkster501]

Steve Richards in his book suggests that you should move onto guiding as soon as you can to save so many exposures - and time - being binned. And Steve is the expert so that's what I am about to do as well....

[Gina]

Yes, I think you need guiding for any serious progres into the world of DSO. Bright objects like M42 Orion nebula with nice short subs of a minute or less are fine with something likre an ED80 with focal reducer and focal length of half a metre. But you'll soon run out of those.

[DaveS]

@perkil8r

That's rather good . If I can get anything like that I'll be well chuffed.

@kirkster501 and Gina

I don't intend to *stay* unguided, just to begin with.

My cunning plan us to "get my feet wet" with the easy stuff, tracked but unguided, while taking my time working out the most cost-effective guide package, given that I may want to take this further than just a DSLR.

It also means I won't overload my credit card with one huge purchase.

[Gina]

@perkil8r

That's rather good . If I can get anything like that I'll be well chuffed.

@kirkster501 and Gina

I don't intend to *stay* unguided, just to begin with.

My cunning plan us to "get my feet wet" with the easy stuff, tracked but unguided, while taking my time working out the most cost-effective guide package, given that I may want to take this further than just a DSLR.

It also means I won't overload my credit card with one huge purchase.

That's the way I did it - started unguided.

[DaveS]

Well, been an' gorne an' dunnit now

Just pushed the button on a WO reducer IV, T-ring and Baader UHC-S filter from FLO.

Appologies in advance for all the clouds

[perkil8r]

With a HEQ5 you should be able to push over 2 mins quite easily as long as you get your PA good enough. If I can do it on an eq5 you should have no problem.

[DaveS]

Well, my Reducer, T-ring and UHC filter have arived, now waiting for the clouds.

[perkil8r]

I've just come in and packed away after a very successful testing session. I wasn't aiming on capturing anything in particular so focused on a random star in pegasus. Fired off some subs wanting to see how long an exposure I could take with the st80 on the eq5 unguided. My previous cautious limit had been 100 secs with 100% success. Tonight I managed 5 mins unguided with 5 out of 5 completely free of star trails. at 6 mins I got only 2 out of 5 successful. So my new limit is 5 mins.

[ollypenrice]

Start guiding immediately!

M42 is an easy target??? No it ain't, it's one of the hardest targets in the sky with a staggering dynamic range. It's the only target for which I've found I needed to combine different sub exposure lengths. M45 is also difficult.

Seriously, guiding is imaging and imaging is guiding, or so it seems to me. Bite the bullet.

Olly