Guiding a DSLR+Lens with a main scope

[cuivenion]

Hi, would it be possible to guide my DSLR (500D) + 135mm lens with a Skywatcher 130p (650mm focal length) + GPCAM (3.75um pixels)? I don't have the setup at the moment to guide the DSLR + Lens with a finderscope. So I'm hoping to piggy back the DSLR on the 130p and guide with it (130p). I can use a 0.33 focal reducer with the 130p to bring the focal length down if that will help.

[wimvb]

It's possible, as in theoretically possible, but it's also overkill. If you have a good polar alignment and balance, you may not need guiding with a short focal length. Generally, the guider scope has a shorter fl than the imaging scope, and doing the opposite just isn't practical. As a last resort (as you don't have a guide scope) just test it, while you save up for a proper guiding setup. But beware that it may introduce more problems than it solves.

Personally, I have never bothered with guiding my 135 mm lens. Just balanced, polar aligned, and dumped bad subs. Getting guiding right takes from imaging time. And imaging time is precious.

[Tommohawk]

I agree 100%. Unless you need very long exposures for an exceptionally faint target, or are planning to use narrosband filter, you could probably get away with unguided. Needs good polar alignment of course. 

[cuivenion]

I'll see about mounting the dslr on the mount alone then. I imagine I'll only get 1-2 minutes unguided though.

[wimvb]

short fl is much more forgiving than a long fl scope. On my eq3 mount I sometimes only goft < 1min subs, at best I managed a few 5 minutes subs (once). With a 135 mm lens I got up to 6 or 7 minutes. That was with my dslr attached directly to a dovetail bar. Just experiment. If you dither between exposures (about 15 pixels in DEC and RA), stacked image quality will be greatly improved. (But it doesn't help improve tracking of course.)

 

Good luck

[cuivenion]

44 minutes ago, wimvb said:

short fl is much more forgiving than a long fl scope. On my eq3 mount I sometimes only goft < 1min subs, at best I managed a few 5 minutes subs (once). With a 135 mm lens I got up to 6 or 7 minutes. That was with my dslr attached directly to a dovetail bar. Just experiment. If you dither between exposures (about 15 pixels in DEC and RA), stacked image quality will be greatly improved. (But it doesn't help improve tracking of course.)

 

Good luck

Thanks, as an additional question what do you use as a counterweight for use with a DSLR + lens? My 5kg weight is much too heavy. 6 to 7 minutes sounds pretty impressive unguided even at 135mm. What mount do you use?

[wimvb]

I use a Skywatcher EQ3 PRO (EQ3-2 with goto) for my wide field imaging. I only have the lighter counterweight all the way up the cw shaft. A slight imbalance can actually improve tracking, as the motors are always pushing the same.

Here's an example with 7 minutes subs unguided.

 

[cuivenion]

I used to use a 135mm with a gpcam on a vixen gp. Never managed more than two minutes and the stars were'nt great at that exposure length. I must be doing something wrong. Probably the balance was well off as the set up was thrown together.

[wimvb]

That's what mediocre imaging nights are for: experimentation. So that it all comes together on those rare, clear and dark nights. 

[bobro]

My counterweight is full of sand.

[cuivenion]

32 minutes ago, bobro said:

My counterweight is full of sand.

Cheap as chips, I like it!

[Nova2000]

2 hours ago, bobro said:

My counterweight is full of sand.

Nice idea. But doesn't the sand move? Is it totally filled? 

[Astrosurf]

I've not done it yet, but as with Bobro's set-up, you could try mounting the DSLR and guiding set-up on a side-by-side bar? I don't know much about FLs with guiding though.

Bobro, how did you make that bar? It looks like wood.

Alexxx

[cuivenion]

Yeah, even if I use my gpcam + finder to guide, it's still going to be higher resolution than the camera + lens. So it would be good to know if I could guide like this, looks like I might have to try it and see.

[Atreta]

On 6/15/2017 at 09:48, Astrosurf said:

I've not done it yet, but as with Bobro's set-up, you could try mounting the DSLR and guiding set-up on a side-by-side bar? I don't know much about FLs with guiding though.

Bobro, how did you make that bar? It looks like wood.

Alexxx

Hi Alex, 

I did something similar with my guidescope,  i had it drilled under its holders and secure it on a regular dovetail with a screw and a butterfly knob(don't know the english name for it). It holds pretty strong.  I didn't have the time to test it yet due to lack of clear skies.

Added some pictures of it.

[Astrosurf]

Thanks Frank. That's a good, cheaper option.