Worth considering this scope for dso imaging?

[sbj]

Hi there,

I'm considering getting something shorter and faster than my current evostar 120 for imaging.  I am interested in imaging galaxies, clusters and nebulae.

I've seen this offering from RVO:

http://www.rothervalleyoptics.co.uk/rvo-90mm-f500-achromat-refractor-ota.html

Just wondered if someone with a little more technical expertise than me could offer an opinion.  I realise that being an achromat there will be CA present, but is this likely to be an issue given the type of use I intend it for?

Would I be significantly better of with an ed offering if my budget was flexible?

Thanks

Steve

[GuillermoBarrancos]

If Your Budget is a little flexible, like say another 130, then this I think is the cheapest 80mm ED Scope you can get:

http://www.firstlightoptics.com/pro-series/skywatcher-evostar-80ed-ds-pro-ota.html

But if your budget is tight (I see you already used to imaging with an achromat no?) then it seems like very decent quality little achro. But you will get some false color on bright stars.

[ronin]

A f/5.5 achro will have CA present on brighter objects, most of what you give is not exactly bright so may not impact a great deal. One other aspect is that an achro will have a greater depth of focus, meaning a less sharp image is created so what you image will not be quite as sharp as with an ED.

The problem of an ED is simply they cost a fair bit more, so your outlay is greater.

You could wait patiently, or impatiently, for a used item to appear, occasionally a WO 72 appears at £200 price area. But it means waiting and then you have to be the one that gets the scope.

It is always easy to say wait, but the RV one given should be better then the Evostar for what you want, shorter. lighter and faster. I grabbed a couple of ED/apo's some time back for myself.

Will say with an NEQ6 do you intend to get an ED/apo at some stage?

Just a mount that substantial seems slightly oddly mated with an Evostar 120.

[GuillermoBarrancos]

PS. If Your Budget is really tight and you really want to start out with a little widefield APO, this is also an excellent value for around 290 pounds:

https://www.teleskop-express.de/shop/product_info.php/info/p1151_TS-Optics-70mm-f-6-ED-Refractor-APO-Telescope---Carbon-Tube---2--Crayford.html

Tho the problem is you will need a flattener tho (with an Apo), adding up 100-150 pounds of aditional costs. :-/

So sadly you have to take that into consideration as well.

[Alien 13]

Hi Steve

The problem with achromats even well corrected ones is that the CA will be a problem and show up as star bloat on long exposures even if it seems fine when used visually.

An ED doublet refractor will be better but even they have difficulty controlling the blue part of the spectrum but in most cases the effect is not too bad unfortunately there are not that many real choices and they are not that cheap.

The use of camera lenses in the 200-400 mm can work reasonably well for the larger wide field objects but again are not cheap unless you go down the used old SLR type lenses with adapters.

Alan

[sbj]

Thanks for the input so far.  I'm guessing I really would be better off being patient and adding to the budget.  No doubt I would soon be hankering after an upgrade.....

I bought the Neq6 really with a thought to future kit, and the evostar was really just a first dabble with refractors.

Looks like an ed is the way to go then, and the sw certainly gets good reviews.  I also like the look of the opticstar ed80s gold.

[Alien 13]

The one problem with the ED80 range of refractors is that they are not very fast so are a bit marginal if imaging with a DSLR in terms of exposure time and camera noise they are however very well CA corrected though.

Faster scopes like the small aperature WO range dont have quite as good CA performance in the blue spectrum as the ED80 types so its a trade off as to what is more important to you.

Alan

[sbj]

right, more complications!

I would be guiding and using an unmodded dslr initially.

[DaveS]

I'm going to stick my neck out a bit here and say that your target list is looking for two (Or three) distinct 'scopes.

Galaxies (Apart from the very nearest) are small targets needing a good FL, I would be thinking of a metre or so, while still keeping a fast focal ratio, which means a big aperture. The SW 190 mak-newt comes to mind. It will go well on your NEQ6, but needs fettling.

Nebulae and clusters are larger, so can benefit from a shorter FL. I'm also thinking that for clusters of stars, which behave as point sources, aperture might be more important than focal ratio. Nebulae, of course, being extended objects need speed rather than aperture or FL, which is why the ubiquitous fast 80mm apo (Of various manufacturers) is so widely recommended.

[lensman57]

Hi there,

I'm considering getting something shorter and faster than my current evostar 120 for imaging.  I am interested in imaging galaxies, clusters and nebulae.

I've seen this offering from RVO:

http://www.rothervalleyoptics.co.uk/rvo-90mm-f500-achromat-refractor-ota.html

Just wondered if someone with a little more technical expertise than me could offer an opinion.  I realise that being an achromat there will be CA present, but is this likely to be an issue given the type of use I intend it for?

Would I be significantly better of with an ed offering if my budget was flexible?

Thanks

Steve

I had the Opticstar version of this scope for a while. It makes a good widefield observing scope but as an imager it is hopeless. At F5.5 it is pretty fast and that is its downfall as the CA is unacceptably high for imaging,  best to save a bit more and go for something like WO ZS 71 ED APO, @ f5.9 it is almost as fast but with much better correction for CA.

Regards,

A.G

[IanL]

Using a large chip like a DSLR you'd almost certainly want the flattener/reducer with the SW 80ED, which pushes the cost up over £500, but does speed it up to f6.37.  Without you will get significant elongation of stars in the corners of the chip.  It is perfectly possible to crop the worst of it out of course as there is still a fairly large area that will be reasonably flat.  

f6.37 isn't that fast, but with a DSLR and the NEQ6 you should easily manage 10 or 20 minute (guided) exposures after a bit of practice so you don't have to grab it all in three minute exposures.  Noise is an issue with those long exposures but you can process most of it out with a bit of skill and practice.  The star bloat you would have to live with, but you would have to spend a lot more on either a mono camera and filters or a triplet to overcome it fully whatever you do.

As others have said, you will have to decide what your realistic budget is, how long you are prepared to wait and which set of issues you can live with unless money is not object.  Bear in mind that if you did look at the 80ED or a similar model, there is nothing to stop you getting the OTA first and adding a flattener/reducer later if that is the route you choose.

There are only going to be a few galaxies that are frame fillers in these type of scopes, but don't let that put you off them. I have done a fair few mid/widefields with multiple galaxies using my 80ED. They come out small but perfectly formed and having a bunch of galaxies in context make a nice change from the long focal length spectaculars that we see so often in the glossy magazines!

[laser_jock99]

Even a £500 budget is a bit tight for good apo optics. If you want a 'true' apo at that sort of cost then possibly consider a Newtonian?

Knowing what I know now I'd buy this:

http://www.telescopehouse.com/acatalog/info-BC185.html

and this:

http://www.telescopehouse.com/acatalog/Revelation-2--Coma-Corrector-for-Newtonian-Telescopes.html (or maybe a better coma corrector for a bit more (Baader MPCC III?)

Your EQ6 will handle it fine, 800mm FL will bring in galaxies a bit closer and F4 is fast enough for DSLR work.

Whilst the ED80 is a great little scope (I have two of them) even with the 0.85x Focal Reducer/Flattener- at f6.37 they are a bit slow.

[sbj]

Thanks again,

In terms of imaging galaxies, If "Galaxies (Apart from the very nearest) are small targets needing a good FL", would something like a mak be a better idea than a short fl refractor?

[DaveS]

Well, as I said, the Skywatcher 190mm f/5.5 Maksutov-Newtonian is well regarded for imaging, though as ollypenrice has said it needs fettling. With a FL of a metre it sounds good for galaxies.

http://www.firstlightoptics.com/maksutov-newtonian/skywatcher-explorer-190mn-ds-pro-mak-newt-astrograph.html

[lensman57]

Thanks again,

In terms of imaging galaxies, If "Galaxies (Apart from the very nearest) are small targets needing a good FL", would something like a mak be a better idea than a short fl refractor?

Maks are too slow, Mak Newtonians are designed for imaging and they are fine but with a few  caveats. Collimation, dewing problems on the corrector and the weight. The focuser on the SW is adequate but just and the last but not least they are not exactly cheap. You also have to understand that guiding @ 400mm FL is rather different from guiding @1000mm, so do not expect tight round stars unless the PA and the balance are spot on. You could try a plain old Newt such as the SW 200P and see how you go, it is a lot cheaper, lighter and if things don't work out you can always sell it with only a small loss, Mak Newts are specialised and selling them off is not an easy job. I am presently experimenting with a 150 PDS and a couple of cooled Atiks and the scale is just right for most of the popular galaxies, that is if this horrible weather allows any imaging soon.

Regards,

A.G

[sbj]

aah, the weather.....

I will look into imaging Newtonians.  I think the mak-newt is not the way I want to go. 

[RikM]

I also use a 150P f/5 Newtonian with a coma corrector. At f/5 it is okay with a DSLR and is rock steady on the NEQ6.

[sbj]

Lovely Rik.   What exposures did you use for that shot?

[RikM]

Lovely Rik.   What exposures did you use for that shot?

I can't remember exactly. Probably something like 35 x 7min with an unmodded 1000D.

[davefrance]

A f/5.5 achro will have CA present on brighter objects, most of what you give is not exactly bright so may not impact a great deal. One other aspect is that an achro will have a greater depth of focus, meaning a less sharp image is created so what you image will not be quite as sharp as with an ED.

The problem of an ED is simply they cost a fair bit more, so your outlay is greater.

You could wait patiently, or impatiently, for a used item to appear, occasionally a WO 72 appears at £200 price area. But it means waiting and then you have to be the one that gets the scope.

It is always easy to say wait, but the RV one given should be better then the Evostar for what you want, shorter. lighter and faster. I grabbed a couple of ED/apo's some time back for myself.

Will say with an NEQ6 do you intend to get an ED/apo at some stage?

Just a mount that substantial seems slightly oddly mated with an Evostar 120.

"an achro will have a greater depth of focus"..............an F5.5 achro has a greater depth of focus to say a F6.3 Apo? Can you explain please, as a photographer this doesn't make sense to me. Thanks in advance.

[IanL]

Whereas this is what you get with an 80ED, 0.85FR and an unmodified 500D.  Now this is 30 x 10min, which is probably a fair comparison with the one above given it is f6.7 vs f5.

[DaveS]

No it won't. Depth of focus is proportional to focal ratio. Longer FR will give  a more forgiving depth of focus.

[lensman57]

aah, the weather.....

I will look into imaging Newtonians.  I think the mak-newt is not the way I want to go. 

Just to add context to the discussion I attach a crop of M82 with its now famous super-nova, it was taken using the new 150PDS,  an Atik 428EX and an IDAS D1 filter, it is a stack of 11 X 240s subs, the focus drifted a bit as the scope cooled down but it was not critical. I think that if you are comfortable with a scope that is not plug and play and don't mind tinkering with collimation, the focuser and the clumsy nature of a NEWT then they represent good value and they are fast.

Regards,

A.G

[ollypenrice]

'An achro will have a greater depth of focus' leaves me wondering what the point in question was. Achromats have a greater depth of mis-focus, for sure! That's why they are not apochromats. The colours don't come to the same focus. As stated above, depth of field depends on the steepness of the light cone, ie the F ratio.

Don't use an achromat for imaging. Over a hundred years ago achromats were corrected for visual or for film photography but not for both.

Olly