small refractor and other items for imaging. advice required

After much browsing of the web i am noticing that all DSO photography is done with apochromatic refractors, generally in the range of 80mm - 110mm.

I am planning my next year purchases now ( so i have time to save!) and am planning on a few big purchases:

1. A fast apochromatic refractor, 80mm min.
2. dovetail and rings to mount this to my 6" SCT.
3. Orion Starshoot autoguider.
4 Light pollution filters.(Il be using a canon 400D)


I have been doing my research regarding this but would like to hear some advice from expirienced users.

For the refractor, I was looking at a Vixen A80SS
telescopes.net/doc/2500/mftr/Vixen/item/VIXEN-2601

I know i would have to replace the focuser and visual back to a 2" one, but this scope is within my budget (500 euros MAX, preferably lower). would i be better getting one with a longer focal length ( such as a megrez or something 2nd hand)? what are the main things i should look for in an apochromatic scope? I know my budget is extreemly restrictive here and that most of the refractors for imaging are well over the 1000 euro mark, but I am happy to look for 2nd hand gear and Im not looking for the best items going, just a solid performer.

For the mounting rings and dovetail, I can see that I can get losmandy rings for mounting this, however a dovetail to fit onto my C6S SCT is a problem. Having never viewed a dovetail before, I cant speculate if i could modify( cut) one to fit my scope. can someone advise on this?

Regarding the autoguider, im going for this as it means i dont have to modify a webcam ( not an option as i dont have the required skills) and it seems to be a pretty complete autoguiding package, and can be gotten for just over 220 euro. would anyone advise me saving for an extra few months and going for something better or would this do the job fine?

Regarding light pollution filters, im quite in the dark here. I want to get filters that fit into the diagonal/scope so that if i ever get a dedicated astro imager, I wont have to replace a ton of filters as they are designed only to fit onto the DSLR.

I realise im taking a bit of a jump here, but from browsing online forums it seems that much of this is the minimum required for half decent DSO imaging.

Thanks for the advice all

After much browsing of the web i am noticing that all DSO photography is done with apochromatic refractors, generally in the range of 80mm - 110mm.
I am planning my next year purchases now ( so i have time to save!) and am planning on a few big purchases:
1. A fast apochromatic refractor, 80mm min.
2. dovetail and rings to mount this to my 6" SCT.
3. Orion Starshoot autoguider.
4 Light pollution filters.(Il be using a canon 400D)
I have been doing my research regarding this but would like to hear some advice from expirienced users.
For the refractor, I was looking at a Vixen A80SS
telescopes.net/doc/2500/mftr/Vixen/item/VIXEN-2601
Thanks for the advice all

The Vixen A80SS is an achromatic refractor not an apochromatic. The Vixen ED80S (f/9) and the Vixen ED80Sf (f/7.5) are the apo versions costing $900 and $799 respectively. Vixen ED scopes generally enjoy a good reputation for quality scopes at competitive prices.

Fintan - have you considered buying a CCD imager for the 6" SCT and forgetting an APO?

The SCT would give you a wider choice of DSO's than a 80-110mm refractor. You would also be able to go deeper or at least take shorter exposures - that will be kinder to your mount.

If you go the piggy-back route and choose to guide with the SCT and image with the refractor, prepare for a rough ride. Flexure will kill you unless you have a rock solid mating between the two scopes. It may also put a lot more strain on the mount - the extra weight may be too much for it - have you checked payload capacity?

And don't make the mistake that an autoguider will happily kiss your tracking woes goodbye - for autoguiding, you still need a pretty crisp mechanical system (mount, rings and scope-to-guidescope connection).

I hate to sound like I'm hexing you - but given your enthusiasm, I'd hate to see you suffer severe astrophotography woes...

If you can get your hands on Ron Wodaski's CCD book (can't rememebr the title - not his new one though) that gives great ideas for differing imaging setups.

By the way - second-hand is definately the way to go on a tight budget - don't forget, CCD's and refractors have barely any moving parts and brand-name second hand versions will perform as well as the out-of-the box stuff.

HTH

Dave

Unless it's a proper apo, I wouldn't skimp buying an achro or "semi-apo" for astrophotography as you'd be wasting your time.
Just my €0.02

Thanks for the replies all, much appriciated. to answer why i want to obtain the setup described above:

dmcdona: The SCT would give you a wider choice of DSO's than a 80-110mm refractor. You would also be able to go deeper or at least take shorter exposures - that will be kinder to your mount.

Yes, I can go deeper with the SCT,but It is also a much slower scope (being F11) and thus will require longer exposures, which will easily show any weakness in the mount. Also, yes, the SCT with the larger apature can go deeper, but it can cover less sky, only 54 arc minutes, compared to an Orion ED80 which covers 135 arc minutes, or another affordable APO, the Barska AE19070 which covers 147 arc minutes. As many nebula and DSO's are well ove 1 arc minute in size, a wider field of view is preferable to taking many exposures of a small region of sky and trying to stich them together in photoshop. (NOTE: Field of view measurements were calculated assuming that the telescope is using a 26MM plossl with an apparent FOV of 52", which I believe is pretty standard, as couldnt find a formula to work out FOV of a scope without an eyepiece).

Quote dmcdona: If you go the piggy-back route and choose to guide with the SCT and image with the refractor, prepare for a rough ride. Flexure will kill you unless you have a rock solid mating between the two scopes. It may also put a lot more strain on the mount - the extra weight may be too much for it - have you checked payload capacity?

I have a CG5 GoTo. It is rated to carry up to 10Kg's comfortably. I have read many reviews that state it will carry a bit more than this with no trouble( see Dave Grennan's previous setup). So, with my C6s OTA at 4.5 Kg's, and an 80mm refractor at about 2.5KG, and being generous with other equiptment ( Camera, mounting rings, telrad) at 2 kg's, im still below the manufacturers treshold. And from the reviews i would not be afraid to push the mount to 12 kgs.

Quote dmcdona: If you go the piggy-back route and choose to guide with the SCT and image with the refractor, prepare for a rough ride. Flexure will kill you unless you have a rock solid mating between the two scopes. It may also put a lot more strain on the mount - the extra weight may be too much for it - have you checked payload capacity?

This is why I am looking at losmandy dovetail and rings. again, google has provided reviews that state that as long as they are mounted correctly they are rock solid.

Quote dmcdona: And don't make the mistake that an autoguider will happily kiss your tracking woes goodbye - for autoguiding, you still need a pretty crisp mechanical system (mount, rings and scope-to-guidescope connection).

Im under no illusions that autoguiding is a magic solution to all my astro imaging problems. Sooner or later, if i want to move on in astrophotography, I will need to autoguide. The CG5 is capiable of about 30 seconds unguided. I dont want to have to take 60 exposures for a 30 min total exposure. exposures of 5-10 mins are possible when autoguiding this mount. so although im sure there will be many hurdles with autoguiding, it will reduce the number of exposure significantly, and allow me to aim slightly higher in the future.


I know your not trying to put me off Dave, and thanks for the advice. Its questions like these that are good at making me question why I want such a setup. Also makes me investigate further into the setup im looking for.


One thing i am having trouble with is working out how much quicker one scope will be over another. i.e, the refractor would be much faster ( 2 - 3 stops faster) but the SCT has more apature and can catch more photons. If anyone can help me work this out It would be great.

If I have failed to take some factor into consideration, or my statements above are incorrect in some way, please let me know so that I can make a more informed decision.

I see where you're coming from...

Any astrophotograhpy setup is a balance or a compromise. In this order, you need a good mount, a good imager and then a good telescope.

If you're mount is optimised, then the next step is to move onto the imager. The imager should be matched to the telescope and to the "seeing" - you're looking at getting a pixel size that will give you somewhere around 1 to 2 arcsec per pixel - if you can bin the imager, all the better for those nights of poor seeing. If you can get a a tec cooled camera, better still - especially for DSO's - you really want to get the noise down as much as possible.

No rings/dovetails are rock-solid - believe me. Whilst the weight of your proposed APO is small, flexure will still be an issue you will have to contend with. There are two ways out - buy a selfguiding imager with an in-built guide chip (e.g. SBIG) or get an off-axis guider with a pick off mirror. That'll negate a second scope and will eliminate flexure issues.

Also, beware of autoguiding issues with your mount - even if it can go 5-10 mins with an autoguider, you will still have the issue of periodic error. Even with guiding, you will not eliminate this. I don;t know what the PE of the CG5 is but I'd imagine it would be in the tens of arcseconds. That will certainly limiy your exposure timne - not the ability of your mount to be autoguided.

If you are dead set on using your current mount, telescope and adding a refractor plus a guiding camera, prepare for a very steep learning curve and average results.

I'd strongly recommend you:

1. Decide what you want to get out of astrophotgraphy
2. Read and research equipment and processes
3. Take the plunge and buy the stuff that will give you what you want out of 2 and 3 above.

One other thing on the focal ratio - shorter focal ratios will require less exposure but at the expense of a wider field of view. If you have a really fast scope with average pixel sizes, a DSO might look like a very small blob in the middle of the image.

Google Ron Wodaski's CCD Calculator - this will show you, with actual images, what your telescope/imager combination will give you. Its free software and will help you a lot.

HTH

Dave