OAG + dslr

[alacant]

Hi everyone

Quite a bit of interest in OAGs recently, so I thought I'd post my result.

Slim a few sizes from your stars, a few kilos from your mount and guide for as long as your sky will allow. Recommended. HTH and clear skies.

 

 

[jjosefsen]

It sure seems like you have gotten excellent results with it. So thumbs up!

I went the piggyback finder guider route instead, because it was cheaper (already had the finder scope).

An added bonus is that I can use sharp cap for PA, not sure your fov will allow that?

 

[alacant]

On 20/12/2017 at 14:13, jjosefsen said:

the piggyback finder guider rout

Hi. For short telescopes, I'm sure a separate guide telescope is adequate. I'm guessing the limit is around 1000mm.

On 20/12/2017 at 14:13, jjosefsen said:

sharp cap for PA

 Have a look at PHD2's drift align or DARV in APT.

Cheers and clear skies

[ollypenrice]

56 minutes ago, alacant said:

Hi. For short telescopes, I'm sure a separate guide telescope is adequate. I'm guessing the limit is around 1000mm.

 Have a look at PHD2's drift align or DARV in APT.

Cheers and clear skies

Of itself focal length doesn't matter. The key number is resolution in arcseconds per pixel. 

Olly

[jjosefsen]

1 hour ago, alacant said:

 Have a look at PHD2's drift align or DARV in APT.

Cheers and clear skies

Yeah I was looking at them, and honestly found it a little confusing compared to the Sharpcap PA routine, though this requires a rather wide FOV.

If/when (lets face it.. It's probably when.. :D) I get my hands on a scope with a longer FL, an OAG is definately going to be the way to go.

[jjosefsen]

41 minutes ago, ollypenrice said:

Of itself focal length doesn't matter. The key number is resolution in arcseconds per pixel. 

Olly

And is there a "rule of thumb" number for that, when an OAG is generally better than a seperate guide scope?

[alacant]

12 minutes ago, jjosefsen said:

when an OAG is generally better

Hi. IMHO, speaking from recent hands on but novice experience level, it's always better. It's easier to setup, lighter and it eliminates flex you didn't know you had. It improves -becomes more noticeable- as the focal length of the telescope increases. Varible: focal length. Constant: your camera. HTH.

[ollypenrice]

51 minutes ago, jjosefsen said:

And is there a "rule of thumb" number for that, when an OAG is generally better than a seperate guide scope?

The key rule of thumb is that an OAG is always better on a reflector where the mirror can move. Right now Tom O'Donoghue and I are capturing some very nice tight data at 0.9"PP using a simple guide scope and TEC140 refractor. My experience of an OAG was that it worked well but that in some parts of the sky guide stars were marginal. These tended to be the parts of the sky, outside the plane of the Milky Way, which are attractive to high res setups looking for small galaxies. Obviously in these regions there are fewer potential guide stars. Our disadvantage was that we were using a large chip which forced the prism away from the bright part of the light cone. Our advantage, though, was that, using a very accurate mount, we could happily run 4 second guide subs, long enough to record good signal on faint stars. Had we needed to up these to six or more seconds I'm sure the mount would have handled it. However, I also use less accurate mounts which like guide inputs ever second or less and these might challenge an OAG working in a star-poor part of the sky.

Olly

[alacant]

22 hours ago, ollypenrice said:

guide stars were marginal

Oh dear. What camera? 

[knobby]

I can feel a 'show me your OAG' thread ...

 

[alacant]

12 minutes ago, knobby said:

'show me your OAG'

No, please. The excitement. It would go viral. The SGL server wouldn't be able to cope!

[ollypenrice]

11 hours ago, alacant said:

Oh dear. What camera? 

Lodestar with 4 second subs. You're fnding the OAG dead simple and it often is, but in the circumstances I've outlined above (long FL giving small FOV, large sensor using most of the light cone) it is not necessarily so simple. The fact that we could use long subs made marginal stars workable. Had we been working at EQ6-type PE we'd have needed shorter subs and this would have been a problem. I'm not knocking OAGs, I'm just sounding a note of caution. Mind you, if you have a long FL reflector you don't have any choice. If you do, though, a mount with low PE will allow long enough subs for the OAG to work OK.

An idea that seems to have died the death was a built-in guidescope seen on a prototype GEM shown by Astrotrac some years ago. This would eliminate half the potential sources of flexure and could have been calibration-free, I'd have though, once set up.

Olly

[alacant]

48 minutes ago, ollypenrice said:

Lodestar with 4 second subs

Hi. Fine here with a zwo120 and 2s cycles. Maybe you left the filter over the sensor? It wasn't accurately in focus? If all else fails, try binning it. HTH.

[alacant]

1 hour ago, ollypenrice said:

EQ6-type PE

Have a look at PHD2's PPEC guiding for RA. HTH, cheers and clear skies.

[alacant]

On 24/12/2017 at 09:43, ollypenrice said:

You're fnding the OAG dead simple and it often is

OK. Now having tried various types, I think I see what you mean by OAGs being difficult. Here are two OAG setups I've tried. The first is with a zwo clone, the second with an actual zwo.

The first is simplicity itself. With the imaging telescope focused, push in the guide camera until you see stars. That's it.

The second is the nightmare scenario so often described. m4 bolts to the rescue, it is the shape of the camera which determines the ease of setup.

You have been warned!

Cheers and clear skies.

[ollypenrice]

6 hours ago, alacant said:

OK. Now having tried various types, I think I see what you mean by OAGs being difficult. Here are two OAG setups I've tried. The first is with a zwo clone, the second with an actual zwo.

The first is simplicity itself. With the imaging telescope focused, push in the guide camera until you see stars. That's it.

The second is the nightmare scenario so often described. m4 bolts to the rescue, it is the shape of the camera which determines the ease of setup.

You have been warned!

Cheers and clear skies.

One of the glories of doing this for a living is that you get to see all sorts of stuff, sometimes very expensive stuff, not working...

But isn't it nice when it does!!!

Olly

[steppenwolf]

On 22/12/2017 at 22:33, ollypenrice said:

My experience of an OAG was that it worked well but that in some parts of the sky guide stars were marginal. These tended to be the parts of the sky, outside the plane of the Milky Way, which are attractive to high res setups looking for small galaxies. Obviously in these regions there are fewer potential guide stars.

I have only ever had a problem once with an OAG and my 1070mm focal length Esprit 150 and that as when trying to image galaxies for the reasons stated by Olly. I was using a relatively large sensor (APS 'C') so the OAG prism was a long way out from the centre of the light path. To compound the issues, the OAG I was using (SW) had a small circular hole in the turret for the guide light to pass through rather then a rectangular or elliptical hole and the guide sensor (SXV guide head) was very small. Changing this set-up for my QSI camera with its built in larger diameter turret and a larger guide sensor (Lodestar) I have never had a problem finding a suitable guide star since.

Olly's words of caution are well founded! Using an OAG with a long focal length telescope requires, a wide throated OAG turret, a large and sensitive (mono) guide camera sensor and relatively long guide exposures (I use 3 second exposures).

[alacant]

12 hours ago, ollypenrice said:

doing this for a living

You don't need to do this for a living to try stuff, just a group of like minded people, cool beer and clear skies!

9 hours ago, steppenwolf said:

the OAG I was using (SW) had a small circular hole

OK, so with barren spring skies about to spring upon us (sorry, couldn't resist!), I'll not pass on my guide telescope just yet;)

[ollypenrice]

2 hours ago, alacant said:

You don't need to do this for a living to try stuff, just a group of like minded people, cool beer and clear skies!

 

That's exactly how I make my living! It's a 'ard life...

Olly

[RayD]

1 minute ago, ollypenrice said:

That's exactly how I make my living! It's a 'ard life...

Olly

Grrrrrr 

[ollypenrice]

6 minutes ago, RayD said:

Grrrrrr 

On the other hand unplugging and re-plugging USB leads 700 times a day does grow old...

lly

[RayD]

1 minute ago, ollypenrice said:

On the other hand unplugging and re-plugging USB leads 700 times a day does grow old...

lly

 True.  However, still a rather nice way to earn a living, and a jolly good job you do of it Sir.

[alacant]

2 hours ago, ollypenrice said:

It's a 'ard life...

LOL. Easy on the imaging eh?!