Upgrading from 130p for imaging

[LouisJB]

Hi,

I have a modest HEQ5 pro synscan, 130P, ST80/QHY5L-II/PHD guiding setup. I still have much to learn in terms of imaging/guiding etc.

But assuming I do get the hang of it, I'd naturally at some stage want to upgrade the tube.

Since I'm in no rush, as I can use what I have and learn from it, if I can identify a good upgrade path I can keep an eye our for second hand bargains in the background. So given I'd like to image (galaxies, nebula mainly, probably dabble on planets but that's much more secondary). Oh, at the moment I use a Pentax K5, later hopefully the new K3. Maybe one day even a cooled CCD, but that's not really something I'm that concerned with right now.

So, from what I read bigger isn't always better, especially for imaging. There's problems of wind, weight, focal lengths, fields of view being reduced etc. However, at 5.1" the 130p is naturally a little on the small side and having to work around the back-focus issue isn't ideal, so I will replace at some stage. I'll probably keep the 130 as it's nice and small and I can easily take it out to a dark site any time.

I think I've not the patience to go much bellow F5 for problems of colmination. Carbon fibre is an option if it helps stability (thermally) and keeps the HEQ5 not too loaded. Also, I probably would rather stay within the realms of the HEQ5 as it's 'portable' rather than upping to an NEQ6. I don't mind taking longer exposures to compensate for aperture, to some degree (with guiding limits etc). In any case I'm an amateur and this is a hobby and I don't have perfect skies (or a permanent setup in an observatory), so something appropriate to that is what I'm looking for.

Given all that, it seems like some naturally choices are: 

Skywatcher 150PDS. No back-focus issues, better focuser, more light capture, slightly longer focal length, still lightweight, easy to colminate and so on. I'm pretty sure this would work and be a worth while upgrade.

Then there are 8" tubes. I think I rule out the Quattro F4s. A carbon fibre 8" RC would be an option.

It would be nice if the visual views are not abysmal, but given the skies around here I'm likely only to see impressive things once I've captured 5mins or more of photons

Any recommendations? What would you do, or what have you done perhaps in a similar situation?

[nmoushon]

I think I've not the patience to go much bellow F5 for problems of colmination. Carbon fibre is an option if it helps stability (thermally) and keeps the HEQ5 not too loaded. Also, I probably would rather stay within the realms of the HEQ5 as it's 'portable' rather than upping to an NEQ6. I don't mind taking longer exposures to compensate for aperture, to some degree (with guiding limits etc). In any case I'm an amateur and this is a hobby and I don't have perfect skies (or a permanent setup in an observatory), so something appropriate to that is what I'm looking for.

Given all that, it seems like some naturally choices are: 

Skywatcher 150PDS. No back-focus issues, better focuser, more light capture, slightly longer focal length, still lightweight, easy to colminate and so on. I'm pretty sure this would work and be a worth while upgrade.

Then there are 8" tubes. I think I rule out the Quattro F4s. A carbon fibre 8" RC would be an option.

Firstly when you say, "I don't mind taking longer exposures to compensate for aperture" I'm hoping you been you dont want to increase aperture to reduce f/ratio?

Secondly, imaging galaxies and nebula have two different needs. Nebulas need a wide fov other wise you'll have to be doing mosaics a lot. You can still capture galaxies with the same wide fov but they will be small and not show as much detail. But this also depends on what your trying to achieve when imaging galaxies. If you don't mind the wide fov the shows them floating in a field of stars then your fine but if you want the "up close and personal" type shots where you can see lots of detail in the arms and fine the smaller galaxies then you'll need a larger FL scope.

Thirdly, the 150PDS is a good scope and a decent upgrade with a modestly wide fov. The 8" RC is a HUGE upgrade (in comparison). Not because of its aperture size but because of its long FL and this will also require a reducer because of its native F/8. This will also provide a challenge on guiding at these long FL and collimating. Both can be over come but its learning curve so just a heads up.

Forthly, What I would do: If I was wanting a wide fov for some serious nebula imaging I would be tempted to either keep the scope you have and upgrade to a CCD or actually switch over to a refractor. If I was more interested in galaxies (which I am personally) I would start saving up for that 8" RC and the extra bits needed. My next upgrade from my ED80 will be to an 8-10"RC, depending on my future budget and if I'm in the dog house or not with the accountant.   aka my wife. One other thing would be to double check if the HEQ5 can handle the size and FL of an 8"RC. I think it can but will need some more expertly advise on that one.

[LouisJB]

By 'taking longer exposures', I mean I'd be happy with a 6" over an 8" (or larger) if guiding/weight and all the other factors benefited the smaller scope. Sure I'd need longer exposures to capture the same number of photons (which I don't mind, within reason). In either case I think I'm constrained to >= F5 and between 6" - 8". I don't think a big/long 8" or a 10" is going to work. I suppose a compact tube (SCT) might work, but weight would be a problem for sure, as might guiding.

So, what sort of focal lengths are appropriate for galaxies and nebulae?

Having two tubes is an option over mosaics, I'd prefer that. I can add magnification for a short focal length tube and maybe get a reducer for a long focal length tube, so I guess there's some leeway either way, to some degree.

The 8" RC carbon was my first choice and it was recommended to go with the HEQ5 pro for an astro setup, it has a focal length of 1600mm. Would that mean there's no way to image a nebula like M42 using some reducers, I'd always need mosaics and/or another tube for that? What about or a small planetary nebula? At 650mm focal length the 130P would have been ideal as a second tube for wide-view stuff, but I had not accounted for back-focus. It's effectively 1300mm for me at the moment. I could look to modify it.

Say I want to image the Ring Nebula, what sort of focal length would be ideal? The Dumbbell nebula? Say now I want to image M42 in all it's glory, what would I need for that?

I don't mind some black/star field around galaxies, but it would be nice to see some detail at the same time. I appreciate some targets will be out of my reach so if I can image the easier targets well I think that would be a good start!

Maybe I need something like a 150PDS and also an 8" RC

[nmoushon]

Combining a scope for nebula and galaxies can be a bit difficult. It depends on the size of galaxies you are wanting to image. The largest of the galaxies are the most common ones. M31,M33,M51,M101,ect. There other out there that are close to those sizes that aren't in to "popular" catelogs but most tend to be small. If you really don't want mosaics then an ED80 with reducer is what you will want. With my ED80 and reducer I have a FL of 380mm, which fits M42 very nicely on my DSLR chip with a decent amount of room to spare. I wouldn't add magnification to to scope to image, you will increase the F/Ratio very fast and make your scope very slow. My ED80 has a native 500mm FL for a f/ratio of f/6.25. To add just a 1.2x extender will only increase FL to 600mm but just the f/ratio to f/7.5! Making it go from a decently fast scope to slow scope. Two scopes, one for wide fov imaging and one for galaxies is an option if you have the budget.

With long FL scope you will need mosaics for large objects like M42. But the plus side on the long FL scopes is, besides grabbing the small galaxies, is you can get better detail on individual parts of the larger nebulas. i.e. A detailed shot of the elephents trunk which is a small part of a much larger nebula or the heart of the Heart Nebula. So nebula is not out of the picture with long FL scopes its just taking a different side of the nebulas. Its just if you want the entirety of a nebula then you need a short FL scope. But even then there are some that need mosaics even at my 380mm.

The 130P is a decent wide fov but not really wide. You can fit plenty of nebula at the 650mm but there are plenty that would need mosaics even at that.

If you are wanting to image Planetary Nebula thats a whole different boat lol. Those require a lot of FL to get a really detailed shot. There are a couple recent images (this last week) posted of a couple PN at very long FL of near or over 2000mm! There are also some very good images of them at 1000mm also so it can be done a shorter FL too. These are just typically very small objects. Theres a couple larger ones though.

If you were to run two scopes I would get an ED80 (or something similar) and then the 8"RC (or something similar). And just a note you'll have to switch out the scopes as the mount can't handle both those and the ST80 at the same time.

To get a better sense of the scale an object at different FL do a search in the imaging-deep sky portion of the forum and search for an object. Say M81 and M82 or M51 and look to see what scope and FL they use to get what they got. Can search nebulas and PNs as well. This will be the best way to help you decide on what FL you are wanting and to help you understand the scale of an object. I'm still trying to wrap my head around that and do this often. Also keep in mind the size of the camera chip also come into play here.

[LouisJB]

Thanks, excellent advice. Definitely many things to consider.

I guess if I could have a go at imaging many of the well known galaxies, M33, M81, M82, etc and some globular clusters and some planetary nebula in their entirety I'd be pretty busy and happy for the foreseeable. In terms of nebula rather than planetary nebula, I guess if I can extract some interesting parts of the whole thing and have the option to mosaic (ok maybe not so bad for special cases) that could work. I'll check out some images and focal lengths. I have camera lenses to 300mm F4 so with guiding that's always an option for super wide field also, I guess.

So silly q but between a 150P and 8" RC there's much more than just focal length, aperture and construction changes - the RC design will also give better images and IQ? I read somewhere the large central obstruction loses contrast, I assume that's visual only. Would it still be good for some visual work (good = better than my little 130p)?

If so, are there any other contenders in that sort of 8" 1000-1600mm focal length 'fast' reflectors I could consider also?

As for the ED80, it's a nice idea. I think I'll spend more time imaging smallish galaxies like M82 than large nebula, but I suppose I could image through my ST80 and guide with 130p to get a sense of what that sort of field is like (ST80 is 400mm). I think I'm coming to terms with the fact when i say nebula I mean planetary nebula and bits of bigger nebula I can't always get into a single image. At least for now, I mean it seems like these are opposite ends of the spectrum and I'll end up with two tubes at some stage. Initially the bigger astrograph with longer focal length sounds like what I need to get started. Will it be reasonable for occasional visual / casual observations?

What about the 9.25 SCT, too heavy? too hard to guide?

[nmoushon]

RCs were design for imaging so they are superior than just the typical Newt. How much so? I'm not sure as I have no personal experience with either. I've only imaged with a refractor. You can use a RC for visual but its not the best. The SCTs are much better for visual. The difference between them is 1- the central obstruction and 2-the native focal lengths. RCs are typically built at F/8 where SCT are at F/10. And yes, you can image with a 9.25. Whether your mount can image at that FL is something else but, there are a lot of people that do that on this forum. You will need the reducer though. Thats a must. I'm not sure on the weight comparison but I think they are close because of similar builds. Guiding a SCT is the same as a RC of a similar size. Yes weight plays into it and wether its balanced correctly and your mount can handle it but the main factor is the focal length. They longer the FL the more precise your guiding has to be to an equal length exposure at a shorter FL. Thats why long FL imaging is so hard and such an achievement and requires expensive equipement. Its good you have some experience with guiding already other wise that would make the big learning curve of guiding at long FL even bigger.

I think there is an 8" Newt @1000mm to put it at F/5...or maybe its a 10" @1000mm to put it at F/4....I forget which. You'll have to do some searching of different manufacturers.

You can image with your ST80 to help you get a sense of scale but it wont produce the best images. It wasn't designed to image with so you'll get a lot of CA with. An APO refractor is what you will want to image with.

To have a scope that great for visual and great for imagin doesn't exist. They are two different beast that need two different things. All imaging scopes can be used visually but they are not as good a scope made for visual only and how good they are visually varies. i.e. My ED80 provides very good visual views but has a very wide fov and a small aperture so it can't pick out the fainter objects except in the darkest of skies and if you compare it to a Dob of a similar price the Dob would win hands down because you can have a bigger aperture.

[LouisJB]

Certainly giving me a lot to think about. I'm going to practice more imaging with what I have to get to understand the difficulties better. I only got guiding working recently.

In fact I see you have a similar setup. I nearly went 9x50 guider but got the ST80 instead as I thought it would be more versatile. Might end up going to 50mm guider if weight gets to be an issue. I'm using QHY5L-II

That's the other thing, in terms of focal lengths it depends on sensor size right, and really it's the image scale that matters - I've not worked out how to work that out yet but I should as I'm using APS-C DSLR so guess I should see how that translates on different focal length tubes into field of view.

One thing for sure, my 130P and having to use a barlow is suboptimal. From what you said that's probably making my F5 quite slow. So anything will be an improvement. Question might come down to making a cheap improvement with something I can keep when I spend more on a longer focal length, higher end astrograph or just get the latter now.

as I say, no rush. More research to do. I'll check out some tube weights, maybe put some candidates here for consideration

[nmoushon]

I'm still working on guiding as well. It can be very tricky at times. My 9x50mm works great but sometimes I have to move it a bit to find a decent size star to guide on. I can always find one though. The ST80 will be a minimum if you go for long FL scopes. 9x50 just wont cut it for that. Or get an OAG. I'm currently shooting at 5min subs but with finally getting by CCD up and running and mount perfectly balanced I'm now going to try and push it to 10min and eventually want to get to 30min subs. That might be a while though.

If you're sticking with your DSLR just have to note that different camera give a different size image with the same scope. The scale of the object is the same the FOV is cut down....I think I got that right. I just switched to CCD so I'm still trying to get through that learning curve so might not have that information quite right. Sorry if its a bit off. If your using a 2x barlow with your F/5 scope then you are imaging at F/10. Which is an extremely slow scope for imaging. If your having trouble focusing get and extension not a barlow. Its basically a spacer.

[LouisJB]

the one and only time I got to try it I tried a 4 minute picture of M82 and it showed up, stars are round too. Stars picked up in the image are down to around mag 16.5 by the looks of it. The galaxy image isn't great, a little colour but naturally I'd need many shots like that, probably 5 mins each to get to see something producing a good image. However, was still very chuffed to see it. I could barely see it in the eyepiece but actually the QHY5L-II picked it up on the screen. Think the guiding might work well, probably I'm over-guided for my current tube but the idea was to dabble and get something that might work for longer focal length tubes.

So the barlow thing isn't great, however I think the size of say M82 at 650 with x2 barlow = 1300mm was still quite small so until I get a tube of longer focal length I might need to continue using it even if I solve the back-focus somehow (by modification of the tube).

The 8" RC tubes are 1600mm F8, from what's been said that sounds actually quite slow now. Still maybe that is fast enough for most astro imaging?

[nmoushon]

The reason it was very dim after a 4min exposure was because you shot it at F/10 with the barlow. Give it a try without the barlow. Yes M81/82 will be smaller but you can crop the image in PS to help make it look larger. I think you will be very surprised at what you get. Plus there is no such thing as over-guided lol. That just mean you need to push the subs longer. Try taking an exposure for as long as you can. If you could get 5min at 1300mm you should easily get at least 10min or more at 650mm. Which will really make them pop in brightness and color. Make sure to grab lots of exposures and stack them. 

The RCs are usually paired with a reducer to bring down around F/6ish. I've seen them used at F/8 but most imagers pair them with the reducer, which is an added cost. Also a lot of RC dont come with a focuser so make sure to check and see if they do or not. Some do and some don't. Note sure why but its how it is. 

[LouisJB]

The problem with the stock 130p is that it can't achieve sufficient back-focus for DSLR prime focus photography, if I'd realised before I got it I'd have probably started with a 130 or 150 pds with are designed to support it. So given what I have it's about the best I can do. I was thinking about looking for a much lower power Barlow, a 1x would be ideal but say a 1.2x, but then it's still a stop-gap and might be better spent on the new OTA. The other thing is to modify the 130 for the time being so it can act as a better stop-gap before getting a new tube, thing is that makes me nervous as I'm no expert on these things, alignment and colmination etc - I'd rather not render it broken until I have something else to use.

I don't know how well guided I am yet, I only got one session and 2 pictures, but the 4 minute one of M82 (which was low in the sky in the direction over some light pollution) came out like this http://www.flickr.com/photos/31369139@N06/10544625485/ it's naturally not outstanding but for a single underexposed shot at least you can see it! It's really the first picture I've taken through a telescope of a DSO, so naturally I was pretty pleased to see anything at all!

I will definitely try longer exposures on the next outing and try to tune PHD and see if I can push the exposures out to a sweet spot, then as you say take many and stack. That's the other thing, got no idea how to tune PHD either yet, I just do polar alignment best I can then let PHD calibrate, focus DSLR on a bright star then slew to target and shoot. Pretty crude but you have to start somewhere...

Seems like the 8" RC CF will be a good start for me. I won't splash out on that until I've mastered all the things I have (guiding, imaging, stacking etc) as I've found going out with too many new things leaves you struggling to master any of it properly. Still got plenty to learn, just a shame I've got such an effectively slow scope to work with in the mean time.

[RikM]

I'd be inclined to dump the 130P for a 130PDS + SkyWatcher 0.9x coma corrector. It has everything you need for a great imaging telescope. Fast optics, free from false colour, shortish focal length for a wide field and easy guiding, holds collimation well. With your Pentax K5 the reduced focal length gives you an ideal 1.7 acrsec/pixel sampling rate for deep sky.

Changing to a 150PDS (good on a HEQ5), or 200PDS (possible on a HEQ5) isn't an 'upgrade'. They are exactly the same as the 130PDS just with a different field of view (and are harder to guide for long subs due to the longer focal length, so may even be a downgrade!)

Even changing to an ED80 you would have more or less the same field of view. You would gain some ease of use (no collimation) but at the expense of..expense 3x the price before you think about dew bands etc. With a CCD camera, the ED80 makes sense, but for a DSLR the potential f/4.5 of the 130PDS is very attractive.

[LouisJB]

ok that's very interesting, as 130pds can be found for relatively cheap. If I modify my 130 I would have effectively the same (except the focuser isn't as good) but maybe you're right on that, I should just try out a small scope of similar characteristics, for now. Would a 150pds be much worse, not compensated by bigger aperture? It's 750 vs 650mm so not that different.

[RikM]

I love the 150P for imaging on my NEQ6. I am sure it would be fine on an HEQ5. The 200P, at 1 metre focal length, is where it starts to get difficult.

'Aperture' can be a bit of a red herring when thinking about imaging. Since both the 130P and 150P are f/5, they will give the same image brightness on extended objects, it will just fill a bit more of the field in the 150P compared to the 130P. Do you want a very slightly more 'zoomed in' look? Imagine taking a picture with a 50mm lens and one of the same subject with the 100mm lens, both at f/5. They will be the same brightness right? Think of the 130PDS as a 650mm lens and the 150PDS as a 750mm lens, the 200P as a 1,000mm lens, they are all at f/5, so they will all give the same brightness for the same exposure time, you would just 'see' different areas of sky in your images.

For visual use aperture rules (because the eye doesn't build up exposure I guess), the 150P is a nice step up from the 130P, so that is a clear advantage, if you want look through the thing as well as take pictures.

I use a 150P for imaging and a 250PX for looking through.

[LouisJB]

When you say 150P I presume you mean PDS or does the 150P not have a back-focus problem?

I must say, I'm starting to think I should get a 130 or 150 PDS so I can take 650/750mm wide field images at F5 and maybe then later on get the 1600mm 8 RC. As has been pointed out they are going to do different jobs but jumping in with the more expensive and hard to guide RC at the start might be too much, whereas for a very modest outlay I can start getting images much better than I can right now using effectively an F10 scope. Also I'm still a newbie, I worry about damaging an expensive tube. It's nice to play around on something inexpensive to start with. The images I've seen from 130/150 can look amazing.

So both being F5 really the only consideration between them is 650 or 750 mm focal length, or will aperture make a small improvement (I guess it would for visual observing if not imaging).

[nmoushon]

I might be miss understanding but the way to fix your back focus problem is not a fix to your tube but a fix to you draw tub in the focuser. And its not really a fix but an added piece. There are extension tubes you can but for the focuser so you can get farther focus. This is an exmaple of what im talking about. http://www.telescope.com/Astrophotography/Astrophotography-Accessories/Orion-Extension-Tubes-for-Telescope-Focusers/c/4/sc/61/e/11.uts I could be wrong since I don't use a reflector but this is what I thought fixed the problem.

[LouisJB]

the issue is the 130P won't focus close enough in, not out.

The modifications are along the lines of;

shorter focuser (probably as expensive as the tube!)

different colmination bolts moving primary forward

some form of barlow to project the image forward to the DSLR sensor

cutting the tube down!

(or moving secondary but that's probably not very feasible)

by the way, RC aside, for normal reflectors I'll need a coma corrector of some sort?

[nmoushon]

Oh my bad. I knew I'd probably have that wrong. How about this then: http://www.firstlightoptics.com/skywatcher-focusers/skywatcher-dual-speed-low-profile-1252-inch-crayford-focuser.html You might find one second hand to make it worth getting.

And yes you will need a corrector. Some are at x1.0 and some do x.90 so with that you might be able to achieve focus. Not sure though.

[RikM]

I use a standard single-speed 150P, but it has a different focuser from the 130P. It's a 2" crayford and actually I find it better than the PDS dual speed focuser, but that could be just me. No focus issues with either Canon EOS cameras or a Starlight Xpress mono CCD and filter wheel. I use a 1x Baader MPCC coma corrector. The 0.9x Skywatcher will mean you need even more in focus travel.

If you click on my deep sky gallery you can see some of my images. It's not up to date though, sorry. Some of them were taken with an NEQ6 mount but the older ones were unguided with an EQ3-2. The notes should say which.

[LouisJB]

Why do you find the single speed better than the dual, out of interest.

I think the PDS has more back-focus, maybe because of the different focuser only? Not sure if I'd need it then by the sounds of it.

I only have 1.25 eyepieces etc, it comes with a 2" to 1.25" adapter for normal visual? The other thing is I'd need a 2" to t-thread adapter I guess, but that's no big deal to get.

[RikM]

The PDS has more back focus but the ones I've seen (I've never owned one) don't seem to hold focus as readily as the single speeds. There are threads and threads of people griping about the poor SkyWatcher DS focusers slipping. I have had three single speed SkyWatcher Crayfords with medium weight camera stuff hung off them and I have the focus slip...once, when I forgot to tighten the lock screw. That has been my experience, as with all things, your mileage may vary. I don't have big heavy camera gear, but an SXV-H9 + 5x1.25" filterwheel+MPCC is a good start.

I would assume it comes with the 2" - 1.25" eyepiece adaptor. Mine all did but as I said all single speed versions.

As for 2" t-thread adaptor, I would strongly recommend a coma corrector instead, otherwise you will have distorted stars in the corners of the field. What's the point of a wide field of view if you have to crop the edges to get rid of eggy stars?

[Uranium235]

RikM, on 31 Oct 2013 - 07:52 AM, said:

I'd be inclined to dump the 130P for a 130PDS + SkyWatcher 0.9x coma corrector. It has everything you need for a great imaging telescope. Fast optics, free from false colour, shortish focal length for a wide field and easy guiding, holds collimation well. With your Pentax K5 the reduced focal length gives you an ideal 1.7 acrsec/pixel sampling rate for deep sky.

Already done that here mate

The 80ED reduced gives me 510mm @ f6.38, but the 130pds with the fr/cc gives me 585mm @ f4.5 (1.9" p/p) - only a slight loss in FOV but double the speed. It means that flats will be mandatory, but I dont mind if its cutting my project times in half.

Then if you mod it up with a compression ring, flocking and a fan - then it might become a half decent imaging tool. Im even thinking of shortening the focuser drawtube, but I have to get familiar with the focuser mechanism first (need to find out what part governs the distance of out focus).

Oh, about the focuser. If its anything like the one on the 80ED, all it takes is a few tweaks of the focuser tension screws to tighten it up. I was having a terrible time trying to hang a 383 + reducer and filterwheel off mine, but after a bit of trial and error I've tweaked it to the point where i can focus and lock the focuser on stars that are nearer the zenith (dont get any drift on focuser lock either).

[RikM]

Nice one Rob. I've toyed with it but with my dinky Sony chip it still doesn't give me enough extra sky over the 150P to be worth it right now. Look forward to seeing the results

[Uranium235]

Ive got first light scheduled for Monday or Tuesday night, ive already set up the coma corrector for a 56mm spacing (55 + 1 for filters and CCD window). All I need now is a telescope to hang it off

Hopefully the collimation wont get too messed up en route.

[LouisJB]

ok, in that case I might try modifying my 130p to get focus without a Barlow. I can do that non-destructively. 

Where does the coma corrector go in the optical chain, it's a 1.25 filter on the end of the T to 1.25 adapter?

So just to check, the PDS dual speed uses a 2" focuser but the P uses something smaller, it's definitely not 2". If I get a coma corrector I need the right one for the tube size or they are always the standard 1.25 filter thread?

And should I even consider a PDS at all (130 or 150) if the focuser might slip, that sounds like a nightmare - is there no way to tighten it up?