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Help needed choosing a new telescope for imaging


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Hi SGL, I’m hoping to pick your brains…

I’ve been using an Askar FRA400 (f/5.6, 400mm) astrograph for 18 months now, and am having a great time. It’s easy to use, very forgiving, produces good images, and is basically a lot of fun. I’m vacuuming up wide-angle targets and am starting to want something with a longer focal length, say between 750 and 1000mm, basically to get “closer up” to nebulae in particular, which are my favourite targets. I reckon I’ve got another 6 months or so left before I want to make a jump, so am thinking about it now…

I’ve got some criteria for a new telescope:

  • Ideally I’d like to keep my existing mount, which is an Orion Sirius EQ-G (very similar to an HEQ5)
  • I’d also like to keep and use other existing kit: ASIAIR Plus; ZWO EAF; William Optics 30mm guidescope & ZWO ASI 120MM Mini guidecam; ZWO ASI2600MC-Pro camera with ZWO Filter Drawer, plus Optolong L-eXtreme filter (although I’ll probably buy one of the 3nm dualband filters when they come out)
  • The mount is rated for 14kg of imaging kit, but let’s say 10kg to be safe. All those accessories come to just under 2kg. That leaves about 8kg for a telescope plus tube rings etc.
  • Budget is up for negotiation, but under £3k would be good

There’s one telescope that jumps out as ticking a lot of my boxes: the Askar 107PHQ. It’s basically a scaled-up version of my beloved Askar FRA400. Good points of the Askar 107PHQ:

  • Petzval-like design, meaning no backfocus issues
  • 749mm focal length
  • 6.9kg including tube rings and dovetail
  • Comes with a Vixen dovetail (instead of Losmandy, which my mount would need an adapter for)
  • Apparently easy to fit a ZWO EAF
  • By my measurements looking at the schematics, an ASIAIR Plus should fit neatly below the OTA, between the tube rings
  • £2499

I’m slightly hesitant for one reason: at f/7, it’s a fair bit slower than my f5.6 Askar FRA400. 1.6 times slower, if I’ve understood correctly. I like to rack up long integration times on my targets, currently aiming for a minimum of 20 hours. With an Askar 107PHQ, that would become 32 hours. Ouch. (And that’s good quality data, I normally collect 1/3 more that doesn’t make the cut). There is an optional 0.7x reducer, but that would bring the focal length down to 524mm, as well as requiring a specific backfocus thereby negating a plus point of the Askar 107PHQ’s design.

This brings me to the questions:

  • Are there any benefits of f/7 over f/5.6 that might soften the blow of needing longer integration times?
  • Should I be considering a different telescope altogether? I’ve always been a refractor guy, but am open to other suggestions. Telescopes like the Sky-Watcher Explorer 190MN DS-PRO, TS-PHOTON 8" f/4, and Vixen R200SS F/4 all have appealing specifications but are beyond the scope of my experience so I'm naturally wary. I worry that they might be a bit fiddly in some way or another, especially coming from the Askar FRA400 that is basically idiot-proof!

Thanks for reading this far. Any help / advice / suggestions / words of wisdom would be appreciated!

-Lee

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A 130pds will be good though your mount could probably handle the larger ones, larger aperture slightly longer focal length and F5. Collimation is also easier, there are other Newtonians to consider, the Orion astrograph being one.

I don't think you'd want to go for a similar f ratio to the one you've already got, you'll get benefit from the aperture which will give you some speed benefit. Having shot with a samyang at f2.8 compared to a vintage lens at f4 and f5.6 you do notice the difference, this is amplified by a scope due to the aperture increase.

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Sounds like an 8" newtonian is what you need, although i am not sure if i would recommend one. Depends on how much you want to tinker really.

Dont worry about F/ratio in terms of imaging speed, it only affects the length of your subexposure and in your light pollution they will remain short with higher F/ratios. The thing that matters is aperture and working resolution, and if you keep the resolution as same (binning) and increase aperture you get a faster scope. So in reality the F/7 107mm scope will be much faster than your current one (if binned).

If i were you i would go for this premade quality newtonian and add a good focuser and coma corrector on top: https://www.teleskop-express.de/shop/product_info.php/info/p6119_TS-Optics-8--f-4-ONTC-Carbon-Tube-Newtonian-telescope---fully-customizable.html

The price is high but compare it to the 107mm APO and its much more palatable now. I think youll be surprised how much DIY you need to bring a cheap newtonian up to astrograph standards so up to you to figure out whether elbow grease or more spending is the way to go.

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58 minutes ago, Lee_P said:

Ideally I’d like to keep my existing mount, which is an Orion Sirius EQ-G (very similar to an HEQ5)

First decide the focal lenght, based on the desired field-of-view. Also bear in mind that you most likely will continue to use your current scope, unless you sell it. Therefore, it makes little sense to increase the focal lenght just a bit, with a 400mm I'd at least want 600mm, maybe 800mm.

I have a HEQ5 with a SW 200PDS newt witch I use with a DSLR and the finderscope as guidescope. It is heavy for this mount, and as I recently got my hands on a Canon 5D (which adds additional weight) I'm considering a downgrade of the scope. What about a 6" newt, either f/4 or f/5?? SW has their 150PDS (750mm focal lenght) at a reasonable price, and the more pricey 150P f/4 Quattro (with 600mm and a coma corrector). StellaLyra has a 6" f/4 without coma corrector which is priced something between the two SW-scopes.

The SW 130PDS is somewhat hyped, in my opinion, and a bit wee for the HEQ5. The smaller the scope, the more prominent the obstruction caused by the secondary mirror.

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2 hours ago, Elp said:

A 130pds will be good though your mount could probably handle the larger ones, larger aperture slightly longer focal length and F5. Collimation is also easier, there are other Newtonians to consider, the Orion astrograph being one.

I don't think you'd want to go for a similar f ratio to the one you've already got, you'll get benefit from the aperture which will give you some speed benefit. Having shot with a samyang at f2.8 compared to a vintage lens at f4 and f5.6 you do notice the difference, this is amplified by a scope due to the aperture increase.

Thanks, I hadn't considered Orion astrographs. The AG8 looks very good, although it'd be pushing the budget somewhat 🥴

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2 hours ago, ONIKKINEN said:

Sounds like an 8" newtonian is what you need, although i am not sure if i would recommend one. Depends on how much you want to tinker really.

Ah, this is it, I don't like to tinker at all if I can help it! Hence being drawn to the 107PHQ.

2 hours ago, ONIKKINEN said:

Dont worry about F/ratio in terms of imaging speed, it only affects the length of your subexposure and in your light pollution they will remain short with higher F/ratios. The thing that matters is aperture and working resolution, and if you keep the resolution as same (binning) and increase aperture you get a faster scope. So in reality the F/7 107mm scope will be much faster than your current one (if binned).

I'm not fully clued up on binning... could you explain a bit more? If it only affects the length of my subexposures, then that's no problem. I currently shoot 2-minute subs, it would be easy to switch to 3-minutes. You're saying that if I boost those subframes then I could still aim for 20-hour integrations, rather than the 32-hours I calculated? I'm not sure what's meant by "working resolution". If I bin 2x2, that'll get me SNR faster, but will lower the final resolution? 

2 hours ago, ONIKKINEN said:

If i were you i would go for this premade quality newtonian and add a good focuser and coma corrector on top: https://www.teleskop-express.de/shop/product_info.php/info/p6119_TS-Optics-8--f-4-ONTC-Carbon-Tube-Newtonian-telescope---fully-customizable.html

The price is high but compare it to the 107mm APO and its much more palatable now. I think youll be surprised how much DIY you need to bring a cheap newtonian up to astrograph standards so up to you to figure out whether elbow grease or more spending is the way to go.

This is a possibility, and one I hadn't considered. I'm definitely rather spend a bit more to get something good rather than go down the DIY route.

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The OP states that his specific aim was to get “closer in” to his targets…So this means imaging at a more higher image scale. Currently he’s at approx1.9 arcsec per pixel and with good guiding on a HEQ5he may be able to get to somewhere around 1.2 arcsec per pixel (this will require a guiding accuracy of around 0.6 arcsec RMS). 1.2arcsec per pixel requires a focal length of around 650mm with the 2600camera- so the 107PHQ is a bit longer than you need, but if you regularly get guiding errors lower than 0.6 RMS then you may be able to make use of the extra FL. I’d expect an 8inch newt on the HEQ5 might be pushing it a bit on all but the very best (and calmest) of nights. A 6inch F5 with coma corrector may be the sweet spot.

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1 hour ago, Rallemikken said:

First decide the focal lenght, based on the desired field-of-view. Also bear in mind that you most likely will continue to use your current scope, unless you sell it. Therefore, it makes little sense to increase the focal lenght just a bit, with a 400mm I'd at least want 600mm, maybe 800mm.

I have a HEQ5 with a SW 200PDS newt witch I use with a DSLR and the finderscope as guidescope. It is heavy for this mount, and as I recently got my hands on a Canon 5D (which adds additional weight) I'm considering a downgrade of the scope. What about a 6" newt, either f/4 or f/5?? SW has their 150PDS (750mm focal lenght) at a reasonable price, and the more pricey 150P f/4 Quattro (with 600mm and a coma corrector). StellaLyra has a 6" f/4 without coma corrector which is priced something between the two SW-scopes.

The SW 130PDS is somewhat hyped, in my opinion, and a bit wee for the HEQ5. The smaller the scope, the more prominent the obstruction caused by the secondary mirror.

Thanks! In my post I said I'm aiming for something in the 750 - 1000mm range, which matches with your advice. The Skywatchers and StellaLyras are possibilities, although I'm a bit wary of the quality given their low price. My current telescope is £1000, the ones you've suggested are half that. I could spend £3000 or thereabouts, so I'd like to get something higher-end if possible. 

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2 minutes ago, Lee_P said:

I'm not fully clued up on binning... could you explain a bit more? If it only affects the length of my subexposures, then that's no problem. I currently shoot 2-minute subs, it would be easy to switch to 3-minutes.

You can work out a measure of the “speed” of your current system- it’s aperture squared multiplied by image scale squared. In your case 71^2x1.9^2=18,200…if you do the same calculation for your new options then you’ll get an estimate of how much faster or slower it’s going to be…So a 6inch newt at approx 1.2arcsec 150^2x1.2^2=32,400…So about 1.8x faster than your current set up…if you bin pixels that will change your working resolution, but provided you use the correct resolution for your bin factor I think this calculation should still give you a good guide.

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17 minutes ago, catburglar said:

The OP states that his specific aim was to get “closer in” to his targets…So this means imaging at a more higher image scale. Currently he’s at approx1.9 arcsec per pixel and with good guiding on a HEQ5he may be able to get to somewhere around 1.2 arcsec per pixel (this will require a guiding accuracy of around 0.6 arcsec RMS). 1.2arcsec per pixel requires a focal length of around 650mm with the 2600camera- so the 107PHQ is a bit longer than you need, but if you regularly get guiding errors lower than 0.6 RMS then you may be able to make use of the extra FL. I’d expect an 8inch newt on the HEQ5 might be pushing it a bit on all but the very best (and calmest) of nights. A 6inch F5 with coma corrector may be the sweet spot.

Thanks for the input! My mount is on a pier and I do often have an RMS around 0.6. 

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I guess the point I was making was that If you want to image at higher resolution and not be any ‘slower’ than you currently are you’ll need both aperture and focal length and good guiding. It may be difficult to get all three because of your mount. An 8inch newt is a pretty big thing to put on a HEQ5class mount and expect 0.5”-0.6” rms guiding. What’s your current guiding performance like?

If you can achieve the required guiding accuracy then one of the Orion AG scopes might be a good bet- a step up from the Skywatcher/Stellalyra. I think the MN190 might be a step too far- it’s both heavy and long…But you might fancy the explore scientific MN152- FL=750,  a bit easier on the mount in both weight and length might give your guiding system less aggravation.

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3 minutes ago, Lee_P said:

Thanks for the input! My mount is on a pier and I do often have an RMS around 0.6.

But that’s with a 70mm refractor, not a sail disguised as an 8inch Newtonian…I think the 6inch mak newt should be nice quality and no issues with spacing/back focus. 

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Image scale as mentioned above is a good guide, and if your skies are bortle high then go down in scale from your 1.9 "/px  to roughly double your average guiding rms so that you get all the resolution your scope gives you for given skies.

With 2600mc, 650 mm is about 1.2 "/px and 760mm hits about 1" /px. If you look at your average star fwhm (maybe asi studio fits viewer has a star size button now), divide that value by 1.4-1.6 as a guide for your best seeing, and that might help decide the focal length and image scale. 

I'm guessing around 700mm will be your limit, any longer and you may not see the benefits compared to a crop ay 700mm. 

Then, I would decide aperture for brightness and some resolution benefit, and then the size and weight and use all these criteria to narrow down. 

And the there are spikes. If you don't like them, the Mn190 is great if you have the mount payload capacity and the right pixel size, or a long, expensive heavy apo. The TS 130/910 is very good, and with 0.79x reducer is about 715mm or so at f/5.5. Perfect in my view. 

A 6 inch imaging newt, decent one is a good option, such as the ontc range from ts which are outstanding. 

Or an epsilon 130d with the 1.5x extender to give 650 mm at f/5 ( there is one available, hint hint, about to put mine on the block). The f ratio for objects that will now span across more pixels compared to the FRA5. 6 is partially moot once you are above the noise and plan do long longish integrations. Its longer, but does t scale with ratio of squares of f ratios unless the focal length and pixel scale are the same. So slower f ratios are no thin to be feared unless its f/12 or something! 

TS also have fcd 100 106mm apos with beefy focusers at f/ 6.something. 

Lastly, you are osc so binning in software is an option for ~1200mm mm at bin 2, giving you the same pixel scale as 600 mm with bin 1 on the same camera. 

 

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42 minutes ago, catburglar said:

But that’s with a 70mm refractor, not a sail disguised as an 8inch Newtonian…I think the 6inch mak newt should be nice quality and no issues with spacing/back focus. 

Yes ok, that's a good point. The Explore Scientific MN152 does seem like it could be a good option. And no backfocus issues you say..?

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@Lee_P

What is it that you are hoping to achieve?

400mm FL with 72 giving you somewhere about 2"/px is very good combination for nebulae. If that is your interest - I don't really see what you intend to improve upon.

There is certain relationship between quantities that you need to consider:

- Size of FOV

- Sampling rate

- Image pixel count.

You don't really have too much wiggle room as far as sampling rate goes. You are already at 2"/px - and realistically - you can go down to 1.4" - 1.5".  You can certainly sample at higher resolution - like 1"/px or below that - but there is really no point in doing so as aperture sizes, seeing and mount performance won't allow you to go lower.

This puts some constraints at above dependence - you can't shoot large detailed FOV if you have certain amount of pixels at your disposal. Longer focal lengths will reduce FOV and at some point you will have to start to bin your pixels in order to maintain sampling rate (and ultimately speed - defined as total imaging time to reach certain SNR).

That will in turn result in image with less total pixels.

For example - you can happily image with 8" aperture on HEQ5 class mount if it is in form of compact scope - like EdgeHD 8" or 8" RC

Both of these scopes will have very large focal lengths - but you can still image at around 1.5"/px with them. EdgeHD will have 2000mm of FL which you can reduce using their reducer to something like 1400mm - RC has 1600mm. Bin your pixels x3 and you have effective focal length of about 500mm (which is increase over your current 400m).

Cost is reduction in FOV and your images will have something like 1500x1200 px if you don't do mosaics (btw doing mosaics negates aperture advantage as you need to spend time on each panel - no free lunch).

In any case - question is - what you hope to change compared to your current setup?

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1 hour ago, catburglar said:

You can work out a measure of the “speed” of your current system- it’s aperture squared multiplied by image scale squared. In your case 71^2x1.9^2=18,200…if you do the same calculation for your new options then you’ll get an estimate of how much faster or slower it’s going to be…So a 6inch newt at approx 1.2arcsec 150^2x1.2^2=32,400…So about 1.8x faster than your current set up…if you bin pixels that will change your working resolution, but provided you use the correct resolution for your bin factor I think this calculation should still give you a good guide.

This is so so helpful, I can't thank you enough!

So, going through those calculations I come up with the speed of three options:

Askar FRA400 (as a baseline): 19,510
Askar PHQ107: 12,264 (0.63x the speed of the Askar FRA400)
Explore Scientific MN152: 26,008 (1.3x the speed of the Askar FRA400)

That's a point in the MN152's favour. 

I thought that the focal ratio was an important factor in the speed of a system though, have I just had the wrong end of the stick all this time?

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1 minute ago, Lee_P said:

I thought that the focal ratio was an important factor in the speed of a system though, have I just had the wrong end of the stick all this time?

It is if you keep pixel size the same,

As you increase aperture - focal length also increases, and if you keep pixel size the same - sampling rate increases - which negates effects of aperture increase.

EdgeHD with 0.7 reducer and x3 bin will have

203^2 * 1.68^2 = 116308 which is x5.96 baseline

If you can accept 1500x1200 px image and narrow FOV ...

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11 minutes ago, vlaiv said:

@Lee_P

What is it that you are hoping to achieve?

400mm FL with 72 giving you somewhere about 2"/px is very good combination for nebulae. If that is your interest - I don't really see what you intend to improve upon.

There is certain relationship between quantities that you need to consider:

- Size of FOV

- Sampling rate

- Image pixel count.

You don't really have too much wiggle room as far as sampling rate goes. You are already at 2"/px - and realistically - you can go down to 1.4" - 1.5".  You can certainly sample at higher resolution - like 1"/px or below that - but there is really no point in doing so as aperture sizes, seeing and mount performance won't allow you to go lower.

This puts some constraints at above dependence - you can't shoot large detailed FOV if you have certain amount of pixels at your disposal. Longer focal lengths will reduce FOV and at some point you will have to start to bin your pixels in order to maintain sampling rate (and ultimately speed - defined as total imaging time to reach certain SNR).

That will in turn result in image with less total pixels.

For example - you can happily image with 8" aperture on HEQ5 class mount if it is in form of compact scope - like EdgeHD 8" or 8" RC

Both of these scopes will have very large focal lengths - but you can still image at around 1.5"/px with them. EdgeHD will have 2000mm of FL which you can reduce using their reducer to something like 1400mm - RC has 1600mm. Bin your pixels x3 and you have effective focal length of about 500mm (which is increase over your current 400m).

Cost is reduction in FOV and your images will have something like 1500x1200 px if you don't do mosaics (btw doing mosaics negates aperture advantage as you need to spend time on each panel - no free lunch).

In any case - question is - what you hope to change compared to your current setup?

Hi vlaiv, thanks for your input -- invaluable as always! I guess in non-technical terms, I want to take "closer up" images of DSOs, primarily but not exclusively nebulae. With my current wide-field set-up I can get a large nebula in a single field of view, which is good, but I find I'm wanting to zoom into specific areas more. I can crop in, but then the image resolution and quality go down. If that makes sense? Perhaps I've been thinking about it too simply, but my logic was "get a telescope with a longer focal length to get closer in, but also a larger aperture so it doesn't take months to gather enough light to make a decent image."

It would also be good to get sufficient signal-to-noise ratio faster than I'm able to currently, but that's secondary. I'm quite patient 😁

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55 minutes ago, Clarkey said:

How about a 115mm triplet such as a TS photoline? There are a number of 'clones' too. Probably the limit for an HEQ5 but could be used at native FL or with a reducer.

That could work, similar to the Askar 107PHQ I'm pondering.

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22 minutes ago, Lee_P said:

I can crop in, but then the image resolution and quality go down. If that makes sense? Perhaps I've been thinking about it too simply, but my logic was "get a telescope with a longer focal length to get closer in, but also a larger aperture so it doesn't take months to gather enough light to make a decent image."

There is really little difference between the two - I mean cropping current setup and using longer focal length telescope.

Difference is in ability to actually achieve certain resolution - which depends on guide performance, seeing and aperture size (in non trivial way).

First thing you should do is to measure what you've been able to achieve in terms of resolution / star FWHM with your current setup.

My guess is that you'll find your stars to be in 3.5" to 4" FWHM range.

In order to say properly sample at something like 1.5"/px - you'll need about 2.4" - 2.5" FWHM stars. This requires decent seeing and at least 6" of aperture if not more.

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1 hour ago, Lee_P said:

I'm a bit wary of the quality given their low price

They are not high-end, far from that, but the mirrors and focusers on those I mentioned is good. If you want to throw in some extra bucks, choose one with a carbon tube. Other than that, it's much more satisfactory to tune and enhance a midrange scope to top performance, than just buy the one on the top shelf. On this endeavour, you learn a lot, and you hone your skills. This is typical for many of us who use newtonians, we love to tinker. And our scopes lasts forever, or at least as long as the coating on the mirrors. Sooner or later all refractors fills up with god-knows-what, and you will have a hard time dismantling it and cleaning it up. Once in a while people with problems posts flats here, pretty obvious which ones uses refractors..........

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