Altair Astro 152...imaging?

[Jay T]

Hi all, first post on here after having signed up about a week ago. I've been browsing around and seen some great tips on certain things. But I digress. I've just sold my 200P SW Explorer as I'm getting into refractors and astrophotography (in the near future) I've kept my EQ-5 mount and will probably upgrade it soon. I've been looking at the Altair Astro 110ED, it seems like a very good scope and I've read good reviews on it, but alas, I have been struck by 'apature fever' and can't bring myself to get anything less than a 6" even though achromats aren't that great for imaging due to CA. But if I used a semi-apo filter, would that help with photography?

[DaveS]

Unless you want to upgrade the mount to something in £5k class I'd keep my aperture small, there's a lot you can do with an 80mm ED, not even going as far as a triplet.

If you want a 150mm imaging apo, then think £4.5k minimum for a Skywatcher Esprit Pro triplet. And be prepared to add a flattener, and possibly a reducer as well.

Don't even think of DSO imaging with a basic achro.

Oh, yes, and get a copy of Making Every Photon Count, here:

http://www.firstlightoptics.com/books/making-every-photon-count-steve-richards.html

[Jay T]

Hi Dave, thanks for the quick reply. I have a budget of around 1k for the OTA, hence why I was initially thinking the 110ED with a decent finder/guider scope. Deep sky is ideally what I'd like to get into. So would you recommend I stick to the 110ed then?

[ollypenrice]

I may be one of the reviewers whose reviews you read.

Aperture, in astrophotography, has to be considered in context. The context is complicated and aperture out of context is not a virtue.

In my opinion, in terms of deep sky imaging, you should follow a 'selection workflow.'

1) How good is your mount and can it handle the weight and focal length of the scope you choose? I italicize focal length because this is a bigger spoiler than weight. It would be more accurate to ask about the pixel scale at which you aim to image. Do you understand this? If not, I'd do a bit of research. It's important. The accuracy of tracking needed depends on the arcseconds per pixel of your system. 2 arcses P/P is considered sweet. It's not easy to guide but it's do-able. Below that, if you can't guide it and don't have the seeing, then you are wasting your time and would be better with a shorter focal length giving more arcsecs P/P and faster results with a bigger field of view and no loss of resolution.

DSLRs have small pixels. I see no point in using them with long focal lengths of whatever aperture. Using them with short focal lengths and lots of aperture (fast F ratios) makes sense, however.

2) What do you want to image? Lots of targets are big. They need short focal lengths to fit on the chip. Personally I think that well below a metre and beyond 1.5 metres are the really useful focal lengths. In between you can enter a kind of no man's land too short for galaxies and too long for nebulae. I do use a 1metre FL but I use it with a huge chip which opens up the view.

3) Focal ratio speeds up imaging at the square of itself. F5 is 4x faster than F10. There are caveats (yet again!) but enough for one post...

4) Beware of 'cheap and fast F ratio.' It will need a lot of tuning.

5) Achromats are useless for DS imaging. You would be so much better with a small apo.

85mm apo;

140mm apo (more detail, smaller field of view);

Focal length matters. Focal ratio matters. Colour correction and flatness of field matter. Out of these contexts, aperture does not matter in the least.

Olly

[ronin]

Aperure tends to be a visual consideration, it is not so much for astrophotography.

A 152 for imaging would need you to up the budget for the mount to the extent of not buying a scope.

A good imaging scope is in the 80mm size.

It tends to be easy to mix the ideas of visual and imaging up, however the 2 aspects are to a fair extent opposite to each other. Almost the time they "meet" is when you decide that you are happiest with a good quality small scope for visual and that aperture is not king.

[Jay T]

Thanks for the replies guys. Ollypenrice, some great information there. I'll take it in to consideration when finally deciding my refractor. I suppose it's just where I have been used to a 200mm for so many years, I still want a large scope. But from the research I've done and from what you guys have said, it's not all about size.

[DaveS]

Sorry for not getting back sooner,

For a £1k you could do worse that this:

http://www.iankingimaging.com/show_product.php?id=1605

Will give you budget for a finder-guider and a T-ring for a DSLR, assuming and hoping you do at least have a DSLR to hand. It's flat-field as it comes, and is respectably fast.

A couple of other questions. Do you have goto synscan on your EQ5? If you don't already, budget another £295 for a kit from FLO

http://www.firstlightoptics.com/skywatcher-mounts/synscan-pro-goto-version-3-upgrade-kit-for-eq5.html

And do you have a laptop available that you can hook up to the mount and cameras? You'll need one for guiding and mount control, it will also help with camera control.

But in the meantime, please read MEPC at least twice before spending any more money.

[Jay T]

Hi Dave, thanks for the links. I don't have much kit at the moment having only just moved away from visual astronomy. I'm getting a mount upgrade in September at the Herstmonceux Astro festival as it'll probably be a bit cheaper from the trade stalls and maybe get a tutorial on how to set it up etc. I have a shop near me that sells DSLR's (which is next on my list) and I have a laptop. I've been recommended MEPC a number of times and shall be ordering it next week

[DaveS]

Another thought.

Going away from 'fracs, how about one of these:

http://www.firstlightoptics.com/reflectors/skywatcher-explorer-130p-ds-ota.html

Add the coma corrector and T-ring and you'd have budget left to consider a mount upgrade to a HEQ5 which is in a whole different league to the EQ5, and is designed for guiding. I had a look on FLO to see if there was a bundle but couldn't see one.

[Jay T]

I've just sold a newt, I fancied trying something different I've had a look at complete Celestron SCT set ups but they don't really appeal to me. The William Optics GT 81 triplet is sparking my interest though...

[DaveS]

Fair enough.

If you're hankering after a 'frac then the Imaging Star 71 is still worth a look. I think there are a few of us here who use one, either the Imaging Star or the WO version (Which is a fair bit more expensive). If you go for a conventional ED doublet or triplet bare in mind that you'll have to budget for a flattener or flattener / reducer.

[ollypenrice]

I must say that this is the one which catches my eye, recommended earlier by Dave S. The fact that it is designed for imaging with an inherently flat field (and a very large one claimed) means freedom from hassles over chip spacing. http://www.iankingim...uct.php?id=1605

As ever with the cheaper apos, the big question is How well does it control the blue channel?  This is one of the main reasons behind people forking out over £2K for the Takahashi FSQ85. I'll certainly be looking out for results from the TS scope. When you do trawl the net looking for results from scope 'x' do beware of narrowband images. There's nothing wrong with NB images, of course, but the filters used are almost monochromatic so they they don't test the colour correction of the lens at all. If you're going to do broadand (natural colour) imaging you need to look at results from one shot colour or LRGB imagers. Badly corrected optics can still work well in narrowband.

Olly