Hi was wondering if anyone knows if achro,s are at all usefull for imaging.
I know your best off with an Ed scope but I have decided to invest most of my budget on the mount and just see where that takes me.
I will eventually save for a decent scope but I would like to at least practice in the mean time.
Would something like the f5 skywatcher on the classifieds be suitable for instance.
Any advice much appreciated.
Brad
The main difference between Achros and APO scopes is the focus of the different wavelengths of light, in an APO the visual colors focus pretty much together for practical purposes, this is done by using glass with varying refractive properties which unfortunately tend to cost more. In an Achro for most uses its not so noticeable but the wavelengths don't converge at the focus point quite so well, this usually shows itself in the blue end of the spectrum, as a result it's not so focused, and just as you defocus a star it spreads wider, but in this case only the blue, the end result is blue halos around the brighter stars, it's there on all the stars to some extent, your big bright stars really show it up.
So is it usefull for astrophotography, yes, but with caveats..... If you are using a narrowband filter then converging of different wavelengths is not a concern, if you are using a RGB filter wheel focus for each color separately, in a OSC you will need to photoshop out the halos to some degree.
As in all things in life you tend to get what you pay for, a more expensive scope should apart from APO also have a better focuser, better baffling etc etc.
Commonly it is considered that the mount is where the dollars go first, as with a good mount your stars will not show poor tracking ( provided its aligned etc), if you have a poor mount it doesn't matter how good a scope is you have stars wil be eggy or worse.
If experience is anything to go by, most people buy and sell scopes several times, so regardless of what you buy now, you will at some stage upgrade it. There's no perfect answer, it comes down to your budget.
I will just add, that starting to learn imaging with a small scope might be more enjoyable, due to less demanding guiding for shorter focal lengths. One more thing I would consider is the F ratio; although F5 will require shorter exposures than say F6 or F7 (for the same aperture), but it will also be more demanding in terms of correct focus and any deviations from optical axis, for example camera tilt, will show up more on systems with smaller F numbers. Smaller scopes also weigh less, good news for most budget mounts. Having said all that, the scope you mentioned is relatively cheap, has a weight of only 4kg and its focal length of 600mm should not be too demanding for guiding.
I started imaging a few years ago with a 1200mm Newtonian on EQ3 mount, unguided...and I still had fun with it and learnt a lot, although my space photos were horrible
I have imaged with an achro and it can be done but never as good as a good ED refractor or a coma corected Newt. A Baader Semi-APO filter on the achro can stop some of the CA and a longer fiocal length helps. In terms of low cost entry to imaging it is hard to go past an imaging newt, where you can get a new f4 or f5 10" newt for less than a 80mm ED refractor. Remember shorter f# means shorted exposures for the same target, and as always aperture is king.
Hi Brad, I have a Saxon branded 5"f5 achro its a great rich field scope and a fantastic guide scope/finder scope for my 10" newt. Fast f ratio achro's have lot of CA to deal with. Like Glen said a Baader Semi-APO filter will tame the worst of the CA. I have had some success with a light polution filter. Anyway here are a few snaps, all single frames.
The first one 5"f5, second one with LPB filter, the third with LPB filter and some photoshop tweeking and the last one with a 4"f7 achro for focal length CA comparison.
Rick
With the achro, you will get blue bloating of the stars, even with an LRGB method and separate focusing of each of the colour channels. The achro just smears the blue light too far across the spectrum.
For astrophotography, f/ratio is Emperor. For instance, a massive 16" RC at f/8 will yield only 1/3 the number of photons per unit area of CCD sensor compared a small 3" apo at f/4.8. Or, put another way, a 12" newt at f/4 will give exactly the same number of photons per unit area of sensor as a 6" newt at f/4.
So why bother to go to larger apertures for AP? Two words: image scale. In the above example, the RC has a focal length almost 9 times longer than the little apo, and the 10" newt has one 1.66 times longer than its 6" counterpart. That means that the pixel resolution increases, and if your CCD pixel size matches the focal length, you get more detailed 'zoomed in' images of galaxies etc.
I get blue halos with my ED doublet - I think RGB imaging through a refractor is probably best done with a triplet.
From my limited experience, surely large aperture helps when imaging, but faint details can also be successfully captured with small telescopes, and small telescopes are generally more 'user friendly', especially when starting off astroimaging adventures.
I might be bragging, but this image taken with a 4" doublet from a balcony only about 2km from Brisbane's CBD compares surprisingly well with those taken with 17" telescopes.
Having shot with Triplets, EDs and Achros, there is definitely no arguing that the triplets and ED have better imaging quality. This being said, as a beginner I would not worry too much about this. There is a far bigger learning curve in imaging with things like guiding, exposure setting, post processing etc. These are the things that make a difference and are the hardest to master. Someone with a very nice Triplet APO and top of the line Mono CCD is going to produce images nowhere near the quality of an experienced user with a 'lower quality' setup
One could argue that by buying better it gives you room to grow, but with this hobby things become costly quickly and no harm in 'dipping your feet' with a simple setup. I dont have much time anymore to image so downgraded to a f/5 achro and sold my Mono CCD Only use my DSLR and I know the quality is not the same as when I used to shoot with a better set up but end of the day I am very happy with how things are panning out (and extra $$$ in the wallet ).
Below is my second attempt at my f/5 achro on a full moon's night using my DSLR (original link here). As you can see, there is quite a bit of CA around the bright stars but still very much a usable image and enough for me I have a Semi-APO filter on the way so will do a comparison once it's here.
I would say give it a shot, if you like it then there's plenty of room left to learn about imaging with, and if you dont then these scopes go pretty cheaply and quickly around here 2nd hand (or keep it for visual use).
Clear skies and remember, every learns here from mistakes, not by getting it right the first time
Thanks to everybody for there great advice and very encouraging photos.
I like what Meru is saying about mastering the hard yards and maybe upgrading once I've earnt the right to own an expensive scope.
I'm going for an EQ 6 for a mount and it's tempting to say OK you've got a half decent mount now why not a decent scope as it makes sense to do it right the first time so everyone at the shops keep telling me.
I just think maybe until I get used to guiding, processing ,polar alignment so on I should just go with a cheap frac with a bit of appature and a smallish F number and work my way up slowly.
the images everyone has posted gives me a lot inspiration and maybe my money is best spent on a guide scope, photo shop so on.
Once again thanks to everybody who took the time to help out a newbie I will bug you all again when I try to set up my new mount.
Cheers Brad
PS look forward to seeing the comparison shot with the filter Meru.
Last edited by Willow127mm; 11-04-2015 at 02:49 AM.
Sorry forgot to ask Rick what his plan of attack was to get those shots of M42.
They look really nice mate for a single frame but I see what you guys mean with the stars still I would be over the moon to get shots even close to what you guys are getting.pardon the pun.
PS is it just me or does Queensland have a monopoly on Astrophotographers?
Cheers Brad
Last edited by Willow127mm; 11-04-2015 at 02:46 AM.
Brad, If you're are going to start with a cheapish refractor and hopefully move up later I'd suggest the good old 80mm f5 Achro as put out under different brands. When you decide to move up and get serious it makes an excellent guidescope and\or 'grab & go'. Second hand gets them for under $200.
The problem with the "garden variety" 80mm F5 achro, is that it only has a 1.25" focuser. For imaging you really want a 2" focuser, and it is practically mandatory if you your are suing a DSLR.
Sorry forgot to ask Rick what his plan of attack was to get those shots of M42.
They look really nice mate for a single frame but I see what you guys mean with the stars still I would be over the moon to get shots even close to what you guys are getting.pardon the pun.
PS is it just me or does Queensland have a monopoly on Astrophotographers?
Cheers Brad
I only have a RA drive on my old mount, so a bit of drift aligning gets me around 30 sec exposures (but usually 20 sec to try and keep the stars round). I use a canon 1100d and try for 3200 iso, if the weather is cooler, but if too noisy drop it to 1600 iso which seems to be the best general setting on the 1100d.
That first one with the 120mm f5 was shot at 6400 iso and 15 sec.
Cheers Rick
I have a similar achro to Rick and Barry, a Sky Watcher 120mm F5. You can kind of get away with it better on some objects than others. A nebula filter will reduce a lot of CA, as with the eta pic here. But M42 (no filter) is horrible.
Does it have to be a refractor? Small reflectors are very cheap even brand new these days. They offer a big bang for your buck. On the down side they need collimation and a coma corrector for photography. But they've won me over. APO refractors are too expensive for me. I have an ED100 (which is great) but a bit slow compared to the Newtonians.
Thanks again for everyone's help it's much appreciated.
Looks like I might be getting my EQ6 tomorrow can't wait to start doing my head in.
Time to start thinking scopes I suppose so all the advice will come in very handy.
I will post my first image as soon as I can.
Cheers Brad
I was discussing the use of an Achro last week at the astro club and someone said they do it with a violet 30 filter There were some other letters in there but my skatty mind has reduced the info to V30 M - which may or may not be correct.
He did however say that his pics are well beyond beginners photos and not to be sneezed at and he is a very well respected member of the club.
So his comment may be translated as - close to but not alongside ED.
Personally I think it may be a great idea and I'll try a filter on my achro before I scoff too loud at the idea and extend my wallet to an ED or APO
Hi Jennifer thanks for your input I really like the idea of using filters only I don't know where to start some of the other members have mentioned filters and judging by there images are getting pretty good results.
Do you know what the exact name of that particular filter was called?
If anyone knows about this or any other good filters please let me know.
thanks again
Brad.
I've done quite a lot of testing with filters and achro refractors. There are a few examples in the thread. But to boil it down, the best bang for your buck is to get a 48mm yellow photo filter like the Hoya K2 and just screw it into the camera adapter T thread. Use manual white balance in the camera to get it neutral first or you will clip the blue channel. Heaps of them on ebay if you can't find one locally. But I still think a reflector would be better.
Only use my DSLR and I know the quality is not the same as when I used to shoot with a better set up but end of the day I am very happy with how things are panning out (and extra $$$ in the wallet 
).
I have a Semi-APO filter on the way so will do a comparison once it's here.
