Starting in imaging with SCT on a CGEM . OAG, Guidescope, or nothing?

[Mike_F]

So, I've had some fun visually with my old Skywatcher Explorer 200p and my recently acquired C11 on a CGEM. Given my light-polluted urban environment, I think it's time to start some imaging with the newer rig.

I went for the C11/CGEM combo because:

1/ The C11 is a nice light-bucket, altho a bit big possibly for the mount - but I'll get to that. Should be good for both planetary and DSO imaging.

2/ The CGEM is massively more stable / robust than my EQ5-GOTO. The loss of EQMOD capability doesn't really bother me ATM - we'll see how that goes.

I have a Canon EOS350D that I plan to use as my first imaging camera, but no guide scope or OAG as yet. I see ADM do some nice dovetail bars/rings for a guidescope, and I also see some nice OAG solutions - but I have some fundamental questions first.

• Should I spring for a guiding solution at all with this OTA/mount combo to begin with? Or is it worth playing with (P)PEC and drift-alignment to see what I can achieve without guiding? Or - is the CGEM really close to its limits with this OTA?
  
• If guiding really is essential with this mount/OTA - is there a general best-advice path for OAG vs guidescope approach?
  
• W/Regard to the above - the OTA is quite hard to balance already with two 15lb counterweights. The thought of bunging another 20lb of guidescope/rings/cam makes me worry about balnacing it even more. As such, I'm kind of leaning toward OAG already - but the number of people achieving fantastic results with (relatively) flex-free guidescope setups gives lie to how much of a problem flexure is these days with nice short SCT OTAs and good quality kit like that from ADM.
  

So - with this mount/ota/dslr combo - OAG/Guidescope or give it a whirl with nothing and see how it goes?

Hope you can help.

Mike_F

[Mike_F]

Hmm. No responses - which kind of indicates "DYOR" - Do Your Own Research. Right then..

So lots more reading/research has revealed:

• The C11 with its 2800mm focal length is going to be a nightmare to guide accurately. Even with a decent mount.
  
• No probs with that, I'm up for it - just need to understand how to go about it.
  
• I had been thinking of using an ST80 as a guidescope, mounted via an ADM mini-dovetail. I'm worried that even tho the ADM is a nice flex-free setup the ST80 is just too much of an FL mismatch to make accurate guiding of such a long FL main scope easy - despite the additional strength/stability of the CGEM when compared to my old EQ-5.
  
• What focal length would be ideal for guiding the C11 - 1400mm? Is a good barlow in a cheap guidescope like the ST80 a good enough solution to that? Or are the optics poor enough to make guiding difficult even with a decent barlow. Is it worth investing in a better quality guide 'scope?
  
• Would I be better off considering a longer FL / better quality 'frac for guiding anyway? (So that I can use it for wide-field as well as guiding the SCT?)
  
• Not being able to use EQMOD with the CGEM, I've bought a 2nd hand LVI auotguider, that I intend to use for guiding. (Mentioned in case it imposes any limitations on the guide scope that can be used.)
  

I'm thinking of an ATIK 383L+ for the imaging part of the train - having realised that my 350D won't support long bulb mode image-taking easily. My primary drivers behind that are that it's high-res cam, and a fear of low-res imaging devices - based on long experience in DSLR land-based photography. Does the astro-world mean that low-res isn't an issue? (i.e. are mosaics common practice or is everyone happy with "thumbnails" because of the sheer cost/difficulty/long processing workflows associated with larger pics?)

My experience with astro-stuff so far has been that you can get a reasonable result with inexpensive entry level kit. But.. if your intentions are to go further and produce better results - you have to accept that it's going to be a bit expensive mainly in terms of the time/effort you'll need to invest - but also in £££.

No probs with that, but I'm not what you'd call loaded - so making a decent choice first time-around rather than an expensive mistake is a primary concern.

Appreciate any insights you may be able to offer.

Mike.

[riklaunim]

I use Celestron Radial Guider with C8 for guiding. OAG seems to be the best thing, and at high focal lengths precise guiding is essential. As for camera for imaging. Smaller pixels (like those in KAF8300 of Atik 383) increase the scale of the image (and tracking errors) compared to images taken with bigger pixels (you could use bin 2 with the Atik). And there is a limited set of targets - planetary nebulas and small galaxies Both don't need a big FOV of the camera.

[SteveP]

Hi Mike

My thoughts for what they're worth

The C11 is a heavy scope. I'd check around and see whether the CGEM is capable of carrying that load once you've also added a camera, filter wheel(?), guidescope or OAG.

It also has a very long focal length - not ideal if you are just starting out imaging. I would seriously consider a focal reducer to get the FL down a bit

If you opt for the guidescope, assuming your mount can take it, then you don't need to worry too much about the focal length of the guidescope. I'm guessing that the FL of an ED80 size scope is around 400mm. That will be fine.

Atik cameras seem to have a good reputation but I've never used one so can't comment

I'm not sure I understand what you mean by low-res images. You might only see jpegs on sites such as this but that has more to do with limitations on uploading images to the site than it does with image size and processing.

HTH

Steve

[Helen]

You could use a focal reducer to make it easier for imaging.

I've got a C11, but haven't used it for imaging other than some planetary. I'm just testing an OAG option on my smaller refractor, and early results look promising. So I may give that a go with the C11 at some point - although I think putting a focal reducer in there might caused a problem with the distance to the chip using an OAG, but its got to be worth a go! I wouldn't fancy putting a guide scope on the C11 on my EQ6, as I'm already on 3 counterweights...,

Helen

[Mike_F]

My thanks to SteveP & Helen.

I had realised that a focal reducer would be required for wider-field stuff - but the penny hadn't dropped that it might also help with guiding.

As for the CGEMs capacity for accurate guiding of this heavy a load - the jury seems to be out. I can't find a definitive answer anywhere. Some claim good results, others complain. I think it's a case of "your mileage may vary" - and it's still a bit new for a general opinion to have been formed. We'll see how that goes.

I've reached an understanding that this is going to be a case of finding a start point, testing, assessing what problems I have, resolving, testing, assessing what problems I have.... ad nauseum.

However, what's been said so far is leading me down the OAG route to begin with. Because:

They do seem to be an elegant solution to flexure errors - and are lighter than guidescope options. (And the true capacity of my mount is a bit of an unknown..)

As well as weight, another thing to consider is cost. Even a cheapie refractor on some ADM rings and a dovetail is going to cost ~£240 delivered. An Orion OAG would see me up and guiding at £109. (Plus whatever it's going to cost for an adapter.. I note the Orion unit is 2" - my C11 has the original 1.25" visual back fitted)

Any comments or experience anyone has with that OAG? (Particularly in combination with the LVI guider?)

Thanks again, your insights & experience are most helpful!

Mike.

[Mike_F]

I use Celestron Radial Guider with C8 for guiding. OAG seems to be the best thing, and at high focal lengths precise guiding is essential. As for camera for imaging. Smaller pixels (like those in KAF8300 of Atik 383) increase the scale of the image (and tracking errors) compared to images taken with bigger pixels (you could use bin 2 with the Atik). And there is a limited set of targets - planetary nebulas and small galaxies Both don't need a big FOV of the camera.

Cheers riklaunim. That nicely describes my primary fields of interest; planetary, planetary nebulae and the smaller galaxies.

Hadn't understood the tracking implications of smaller pixels. Makes sense. Thanks.

[daz]

My honest opinion would be to start with something smaller/easier to cut your teeth and understand the basics.

Why not start by accurately polar aligning the CGEM and sticking a lens on the camera and image unguided?

There is enough to learn and figure out with just that!

I am not trying to put you off, but rushing down this path is the surest way to a) spend a lot of money unnecessarily and become quickly fed up when you can't get it to work

Just my opinion....

[riklaunim]

In my case it looks more or less like this:

And can give quite good results like so:

Or M77 without a reducer RGB/IrRGB:

The image scale is very high and it won't give poster-like big fields of view with small and point stars... But also I've seen some incredible images of globular clusters made with SCTs and advanced DS cameras.

Or you can put DMK on top of the SCT, add a CS lens and get view like this:

[RobH]

Hello Mike

I started imaging using a 10 inch F10 SCT unguided and alt/az mounted, and got some acceptable results, especially on planetary nebulae, which don't generally need very lon exposures unless you're trying to catch the super faint outer shells.....

No-one told me it was supposed to be hard, so it wasn't

Go down the OAG route....with a C11 you will have some issues with mirror flop whan crossing the meridian, and the OAG will deal with this.

If you go down the guidescope route, a guidescope of 400mm FL will be too short for accurate guiding of your long FL scope. With a guidescope, don't go for less than 1/3 of the focal length of the main scope. However, stick a x2 barlow on and it'll be fine....you don't need super quality in a guidescope.

As regards mount capacity, a general rule is don't go to more than 1/2 your mount's max payload when imaging.

If you carefully balance everything, you can go to 2/3, but I wouldn't go further than this.

A 383L, or similar 8300 chipped camera will be great on this scope, but you will need to bin 2x2 to get decent results.

Seeing in this country is not good enough to get the resolution that 5.4 micron pixels are capable of at long focal lengths, but if you bin it, 10.8 micron pixels will be perfect, and very sensitive too.

I'm going exactly down this route when my big SCT arrives, using a Starlight Xpress H18.

Definitely get a focal reducer. The Celestron 0.63 FR's are fine for this, and also flatten the field nicely Aand you'll find that the FOV is then perfect for objects such as M81/82/51 etc.

You will have a right pain of a time until you get your guidecam parfocal with your main imaging cam if using an OAG, so do this in the daytime and use a distant object to focus on.

Re. OAG's.

The Orion/Brightstar is junk...totally flimsy. Don't waste your time or money on this.

The Celestron OAG isn't a lot better but does work although I'm having wierd issues with long stars on mine

HTH.

Cheers

Rob

[Mike_F]

My honest opinion would be to start with something smaller/easier to cut your teeth and understand the basics.

Why not start by accurately polar aligning the CGEM and sticking a lens on the camera and image unguided?

There is enough to learn and figure out with just that!

I am not trying to put you off, but rushing down this path is the surest way to a) spend a lot of money unnecessarily and become quickly fed up when you can't get it to work

Just my opinion....

.. and very welcome it is too Daz.

That was kind of the original focus of my question. I also have a nice 400mm optically stabilised lens for the EOS that would make that a realistic option too.

But, since finding out that I can't remotely control longer than 30s exposures without making up custom hardware, installing custom firmware, or selling the body and replacing it with a 450D (or better) - I've kind of scrapped that idea. I always thought of the EOS as an interim solution anyway whilst saving the pennies for a dedicated CCD - so the thought of investing (yet) more in SLR kit didn't feel right.

Also, since reading up and finding out that guiding a 2800mm FL scope is a bit of a challenge in itself, it's clear that the idea of unguided subs is a non-starter.

My thoughts at the moment are to start with a cheap CCD (something like a fleabay 2nd-hand Meade DSI or similar) - and work on the guiding with that - before deciding whether to invest in the Atik. One problem at a time..

Thanks for your thoughts.

Mike.

[MartinB]

The C11 is actually lighter than the Meade 10" LX200 ACF. I use one of these on my EM200, the mount which Synta copied to make the EQ6. I have also recently bought a CGEM which has a similar capacity although I haven't actually used it for deep sky imaging.

I would only use OAG and, unless you are using an active optics unit I would always us a focal reducer such as the Celestron 0.63 with as much spacing as you can get away with (whilst avoiding flexion of the optical path and vignetting).

You also seriously need to consider a change of focuser. The Celestron SCT focusers are better than the Meade but the image shift is still a right pain at this focal length.

With long focal lengths (which I consider to be anything above 1500mm) everything becomes harder and your preparation needs to be exact - polar alignment, balance, focusing and tracking. Use periodic error correction - use your camera, guiding software and the free Celestron PEC programme to do this.

Everything is hard work but once you have things sorted the results can be well worth it. Be prepared for traumas.

[Mike_F]

Hello Mike

I started imaging using a 10 inch F10 SCT unguided and alt/az mounted, and got some acceptable results, especially on planetary nebulae, which don't generally need very lon exposures unless you're trying to catch the super faint outer shells.....

No-one told me it was supposed to be hard, so it wasn't

Superb.. You broke just about every rule in the book there.

As regards mount capacity, a general rule is don't go to more than 1/2 your mount's max payload when imaging.

Mm. This is where things get murky with the CGEM/C11. I'm anticipating issues here. But, as Celestron quote the same max payload as the NEQ6 I'm not sure what the alternative is without going for the super-expensive mounts. I'm playing the "suck it and see" card with this one.

A 383L, or similar 8300 chipped camera will be great on this scope, but you will need to bin 2x2 to get decent results.

Seeing in this country is not good enough to get the resolution that 5.4 micron pixels are capable of at long focal lengths, but if you bin it, 10.8 micron pixels will be perfect, and very sensitive too.

Great advice, thanks.

You will have a right pain of a time until you get your guidecam parfocal with your main imaging cam if using an OAG, so do this in the daytime and use a distant object to focus on.

Yep, have read about that particular PITA. I hoped the Orion solution for that in their deluxe OAG would help - but then you went on to say..

Re. OAG's.

The Orion/Brightstar is junk...totally flimsy. Don't waste your time or money on this.

[removed word]. That's one plan squashed already then.

What would you recommend then Rob? You mention the Celestron radial one as being ok, but I've also seen you call it "cheaply made junk" in another thread..

HTH.

Indeed it does sir, many thanks!

Mike.

[Psychobilly]

A Timer remote for the 350D is around £15-20 on Ebay.. or a serial shutter lead can be made for a few quid if you know which end of a soldering iron gets hot...... and DSLRShutter to run it is free (donations gratefully recieved)

Cable Release and Serial Port Cable for the EOS Digital Rebel

DSLR Shutter

I started out with an alt/az 8" SCT and DSI 1 Color...

Peter...

[Mike_F]

In my case it looks more or less like this:

Cheers riklaunim - nice looking setup there. You're obviously making that Orion OAG unit work well.

[RobH]

Hello Mike.

You're right, I did slag off the Celestron too, and it is cheaply made, but I think I was being very harsh that day as I'd nearly slung the darned thing out of the observatory (after a night of lots of ruined subs due to the pickup prism being loose and moving around ).

The problem with both the Celestron and the Brightstar is that they are adjusted and then clamped down using the tiniest grub screws and flop around a lot. The unit is soft aluminium, and so very easily loosens, and the threads are dead easy to strip, but if you want no flex, things need to be very tight.

They generally aren't made with very tight tolerances, but then you get what you pay for.

I'm still trying to find out why I have very elongated stars in the Celestron....it makes for rather wierd guiding as the software is trying to find the centre, so that may end up sitting in a drawer expensively doing nothing!

I've just had the Starlight Xpress unit arrive, and this is much more solid, but designed specifically for SX gear.

There are quite a few on the forum with lots of OAG experience who should be able to help out with a choice so it's worth asking on the forum, but it seems to me that what you absolutely do need is rigidity above all else.

Cheers

Rob

[riklaunim]

Cheers riklaunim - nice looking setup there. You're obviously making that Orion OAG unit work well.

It's Celestron Radial Guider

[Mike_F]

Hello Mike.

You're right, I did slag off the Celestron too, and it is cheaply made, but I think I was being very harsh that day as I'd nearly slung the darned thing out of the observatory (after a night of lots of ruined subs due to the pickup prism being loose and moving around ).

Aha. That explains it.

The problem with both the Celestron and the Brightstar is that they are adjusted and then clamped down using the tiniest grub screws and flop around a lot. The unit is soft aluminium, and so very easily loosens, and the threads are dead easy to strip, but if you want no flex, things need to be very tight.

I see the problem now.

I've just had the Starlight Xpress unit arrive, and this is much more solid, but designed specifically for SX gear.

<Mike clicks a link or two on the FLO site, and creates a "fantasy basket">

Mm. Nice. But, by the time you've factored in the lodestar, the guide cam and filter wheel - it's almost a £1K guiding solution. A tad more than the resale I could get for my incoming LVI unit.

There are quite a few on the forum with lots of OAG experience who should be able to help out with a choice so it's worth asking on the forum, but it seems to me that what you absolutely do need is rigidity above all else.

Indeed, that's clearly the next step. A decision on which OAG.

Thanks a heap Rob - much appreciated.

[Mike_F]

It's Celestron Radial Guider

Oh dear.

In my defence, they do look very similar from astro-shopping website photos - which is about as close as I've got to one thus far..

[MartinB]

Mm. This is where things get murky with the CGEM/C11. I'm anticipating issues here. But, as Celestron quote the same max payload as the NEQ6 I'm not sure what the alternative is without going for the super-expensive mounts. I'm playing the "suck it and see" card with this one.

The CGEM should carry the C11 without any great problem if you are using OAG. As far as counter weights go, you are better off with heavier weights inboard than lighter ones at the end of the shaft

[Mike_F]

The CGEM should carry the C11 without any great problem if you are using OAG. As far as counter weights go, you are better off with heavier weights inboard than lighter ones at the end of the shaft

Thanks Martin, that's great advice (as was your earlier post). I'd already anticipated that I'd need to buy another weight, as balancing the bare scope (and 2 finders) in RA already requires both of the supplied 8KG weights pretty much at the end of the shaft. Finding a genuine Celestron one for < 65quid plus extortionate delivery seems to be the hard part there..

Mike