Celestron 8" edge on HEQ5 pro

[sinbad40]

Hi all,

I am wondering if the Celestron 8" edge HD would be feasible on the HEQ5 pro.  With the weight of the camera, it is still within the recommended weight for AP, but reading what other people have done, they have used an OAG to help with guiding instead of a mounted guide scope to keep the weight down and to help with the star tracking.

Do you think this can be done without major headaches, or is this a "better upgrade the mount first" type of issue.  Real world experience with similar setups would be good

I don't want to be upgrading the mount if i can get away with it just yet, and i don't mind getting the OAG, save me taking the guide scope off the refractor, make things easy for swapping them round..

Thanks

 

G 

[ollypenrice]

The key thing is to set weight aside for a moment and think in terms of what matters, which is resolution. Inevitably, with modern amateur cameras, you will be imaging at a very high level of resolution in arcseconds per pixel. What camera do you intend to use and do you aim to use a reducer? Run it through this calculator: https://www.12dstring.me.uk/fovcalc.php  What you need to know is the resolution of the proposed system in arcseconds per pixel. Once you know that, consider the following:

- your autoguided mount must deliver a tracking accuracy of no more than half the final image scale. A very good HEQ5 or HEQ6 can deliver about 0.5" RMS so it will support an image scale of about 1 arcsec per pixel. Your mount might not be as good as that and you might have to accept twice that error, meaning a best image scale of 2"P.P.

- then again, the seeing might limit you to a best possible resolution of 1.5"PP anyway (or worse) so that means a tracking accuracy of 0.75 arcsecs is all you need.

- in reality, with the Edge HD, you are quite likely to be imaging at a completely unrealistic image scale and will need to bin your data or resize it, which isn't a problem, really.

If we are going to think this through properly, we must start with 'What camera?' and 'what image scale does this give you?'

Olly

[wimvb]

48 minutes ago, ollypenrice said:

in reality, with the Edge HD, you are quite likely to be imaging at a completely unrealistic image scale and will need to bin your data or resize it, which isn't a problem, really.

If we are going to think this through properly, we must start with 'What camera?' and 'what image scale does this give you?'

This reminds me of a story that is told about a swedish cook who lived a few centuries ago, and is supposed to have started her recipes with "you take whatever you have" (man tager hvad man haver). In the op's case that is probable the Edge 8" and an osc camera based on the Sony 533 sensor. Binning 2x2 is easiest done by deBayering in super pixel mode. Ie, 2×2 RGGB colour pixels are combined into one RGB pixel. The 3.76 um pixels would give a pixel scale equivalent to a 7.5 um pixel sensor.

[sinbad40]

Yes, i will be using the 533 that i already have, and thank you for the calculator.  I would like to use it as is, but a reducer would also come into play at some point..

[ollypenrice]

10 hours ago, sinbad40 said:

Yes, i will be using the 533 that i already have, and thank you for the calculator.  I would like to use it as is, but a reducer would also come into play at some point..

A reducer would help because, as it is, you are still heavily oversampled even when binning 2x2.

The fundametal problem is that modern amateur cameras are easily oversampled by more than a metre of FL. or even less than that. Binning and resizing will work, though.

Olly

[sinbad40]

Thanks for the advice again Olly, I will keep looking round  and may re-think my options, but good to know that it could be done if i went that route.  Looking at the calculator, it could be done at Resolution 0.76"/pixel using 2x2 binning, that would go to 1.09 with a 0.7 reducer.  So as long as my mount is set the best i can get it, it should be alright to do what i am thinking, if i am right in what i have put.

 

Edited April 18, 2023 by sinbad40

[WolfieGlos]

On 17/04/2023 at 20:45, ollypenrice said:

The key thing is to set weight aside for a moment and think in terms of what matters, which is resolution. Inevitably, with modern amateur cameras, you will be imaging at a very high level of resolution in arcseconds per pixel. What camera do you intend to use and do you aim to use a reducer? Run it through this calculator: https://www.12dstring.me.uk/fovcalc.php  What you need to know is the resolution of the proposed system in arcseconds per pixel. Once you know that, consider the following:

- your autoguided mount must deliver a tracking accuracy of no more than half the final image scale. A very good HEQ5 or HEQ6 can deliver about 0.5" RMS so it will support an image scale of about 1 arcsec per pixel. Your mount might not be as good as that and you might have to accept twice that error, meaning a best image scale of 2"P.P.

- then again, the seeing might limit you to a best possible resolution of 1.5"PP anyway (or worse) so that means a tracking accuracy of 0.75 arcsecs is all you need.

- in reality, with the Edge HD, you are quite likely to be imaging at a completely unrealistic image scale and will need to bin your data or resize it, which isn't a problem, really.

If we are going to think this through properly, we must start with 'What camera?' and 'what image scale does this give you?'

Olly

Hi Olly. This is very interesting, and thanks also for the detailed explanation, as I’m on the verge of purchasing a 6” Stellalyra RC, to be paired with my canons (77D and 800D), also on a HEQ5. No reducer as the diagonal size is smaller than the cameras sensor.
Using that link, this puts me at 0.56” / pixel. So as I read it, this means I would need to achieve a maximum RMS of 0.28? My HEQ5 doesn’t get near that (I think I get 0.7, but it’s been a while due to clouds…). What are the issues with being over or under sampled?

Sorry to hijack your thread sinbad, but seems like I’m in a similar situation! Happy to delete and repost separately if necessary 

[wimvb]

10 minutes ago, WolfieGlos said:

as I’m on the verge of purchasing a 6” Stellalyra RC, to be paired with my canons (77D and 800D), also on a HEQ5.

Why an RC, if I may ask? It's a slow scope which will give oversampled images. Oversampling means that light that should fall onto one pixel, is spread over several. Less light per pixel, means longer exposure time to clear the read noise floor. Unless your sky conditions can support the pixel scale, there's not much point in having such a long fl, in my opinion. In comparison, the Stellalyra 8" f/4 newtonian has much more light gathering power, and a more favourable pixel scale, at about the same price.

[WolfieGlos]

28 minutes ago, wimvb said:

Why an RC, if I may ask? It's a slow scope which will give oversampled images. Oversampling means that light that should fall onto one pixel, is spread over several. Less light per pixel, means longer exposure time to clear the read noise floor. Unless your sky conditions can support the pixel scale, there's not much point in having such a long fl, in my opinion. In comparison, the Stellalyra 8" f/4 newtonian has much more light gathering power, and a more favourable pixel scale, at about the same price.

Hi Wim, essentially it's a decision I've been around on quite a lot in a separate thread; https://stargazerslounge.com/topic/408459-stellamira-110ed-or-sw-150pds/ , based on wanting to pull in some galaxies, price, focal length and what the mount can realistically handle. Next payday I was going to purchase it, but you're right about the slow scope and it is something that is concerning me, but resolution is not something I had considered until I saw this topic.

My skies are bortle 4, based on a website and the clearskies app.

Would the 8" f/4 newt be suitable with a HEQ5 though? The OTA is 8.4kg alone.

Edited April 18, 2023 by WolfieGlos

[AndrewRrrrrr]

interested to know what people's opinions are on what is an ideal modern camera that would be suited to the edge8HD? 

[wimvb]

46 minutes ago, WolfieGlos said:

Would the 8" f/4 newt be suitable with a HEQ5 though? The OTA is 8.4kg alone.

That would be my main concern. The heq5 is supposed to be able to carry 11 kg, but an 8" Newtonian is large, and can act as a sail in the wind. Otoh at f/4 or f/5 you can use much shorter exposures than at f/9. And a long fl telescope, even a lighter one, can be sensitive to wind gusts.

[wimvb]

31 minutes ago, AndrewRrrrrr said:

interested to know what people's opinions are on what is an ideal modern camera that would be suited to the edge8HD? 

What if you rephrase that question as: what is the best scope for your camera? Scopes come in all shapes and sizes, but pixel size is much more restricted. Most ZWO models seem to have either 3.76 um or 2.4 um pixels. The size is most likely determined by the manufacturing process and the desired sensor characteristics.

When I bought my first astrocamera, the ASI174MM-Cool, it was mainly because of its pixel size (5.86 um) and associated large full well depth. When I replaced it for a camera whit a larger sensor, I opted again for a sensor with large pixels (the ASI294MM has 4.63 um pixels). The ASI174MM gave me a pixel scale of 1.2 "/pixel, the ASI294MM gives me 0.96"/pixel.

If you want to match a camera to a non-reduced edge 8, you'd need 10 um pixels (or 5 um binned 2×2) to get 1"/pixel. The ASI342MM has 9 um pixels. Close enough. Unfortunately, there doesn't seem to be a cooled version of this camera.

Edited April 18, 2023 by wimvb

[ollypenrice]

9 hours ago, WolfieGlos said:

Hi Olly. This is very interesting, and thanks also for the detailed explanation, as I’m on the verge of purchasing a 6” Stellalyra RC, to be paired with my canons (77D and 800D), also on a HEQ5. No reducer as the diagonal size is smaller than the cameras sensor.
Using that link, this puts me at 0.56” / pixel. So as I read it, this means I would need to achieve a maximum RMS of 0.28? My HEQ5 doesn’t get near that (I think I get 0.7, but it’s been a while due to clouds…). What are the issues with being over or under sampled?

Sorry to hijack your thread sinbad, but seems like I’m in a similar situation! Happy to delete and repost separately if necessary 

0.56"PP is flatly impossible to achieve with any mount or guider simply because the seeing won't allow it. The seeing will blur our any detail below a certain value in arcsecs. What that value is depends on your site's seeing but it will not be less than 1" and almost certainly won't be less than 1.5." You mount will support 1.5 or so as well. If I were you I'd forget these long FL scopes and go for something shorter, with as much aperture as possible.

If you go for an over-sampled system (long FL) you will starve each pixel of light, as Wim said, and you will also restrict your FOV for absolutely no gain whatever in fine detail resolution.  To my mind, the modern camera works with FLs up to a metre or so, after which they are pretty pointless except in places with very exceptional seeing.

Our RASA 8 is working at about 1.9"PP and can - just - be out resolved by a TEC140 working at 1.4.  Given the RASA has a FL of just 400mm it shows how the long FL is a thing of the past - in my view.

Olly

[sinbad40]

18 hours ago, WolfieGlos said:

Hi Olly. This is very interesting, and thanks also for the detailed explanation, as I’m on the verge of purchasing a 6” Stellalyra RC, to be paired with my canons (77D and 800D), also on a HEQ5. No reducer as the diagonal size is smaller than the cameras sensor.
Using that link, this puts me at 0.56” / pixel. So as I read it, this means I would need to achieve a maximum RMS of 0.28? My HEQ5 doesn’t get near that (I think I get 0.7, but it’s been a while due to clouds…). What are the issues with being over or under sampled?

Sorry to hijack your thread sinbad, but seems like I’m in a similar situation! Happy to delete and repost separately if necessary 

Not at all, the more info i can gain the better.

[wimvb]

20 hours ago, WolfieGlos said:

Hi Wim, essentially it's a decision I've been around on quite a lot in a separate thread; https://stargazerslounge.com/topic/408459-stellamira-110ed-or-sw-150pds/ , based on wanting to pull in some galaxies, price, focal length and what the mount can realistically handle. Next payday I was going to purchase it, but you're right about the slow scope and it is something that is concerning me, but resolution is not something I had considered until I saw this topic.

My skies are bortle 4, based on a website and the clearskies app.

Would the 8" f/4 newt be suitable with a HEQ5 though? The OTA is 8.4kg alone.

I started out with the 150 PDS and "upgraded" to a 190MN Maksutov Newton. Focal length wise I went from 750 mm to 1 000 mm, not much of an increase. If you take a look at my Astrobin page, you can see what these scopes achieve in a Northern European climate (link is in my signature). I also went from 1.6 "/pixel with the 150PDS / ASI174 to 0.96 "/pixel with the 190MN / ASI294. Not that I wanted a better pixel scale, it just happened to turn out that way. Any difference in the  images is mainly due to me getting better at guiding, focusing, and processing. As Olly said, there's absolutely no point in getting to smaller pixel scales unless you put that scope on a cold mountain top in a desert.

The decision between a refractor or a reflector is mainly about three things. 1. Do you mind tinkering with collimation and mirror cleaning now and then? 2. Where do you put the scope? A reflector is always more of a sail in the wind than a refractor, and a smaller mount (heq5) may struggle with that. 3. What is your budget? A newtonian is cheaper than a refractor.

[WolfieGlos]

15 hours ago, ollypenrice said:

0.56"PP is flatly impossible to achieve with any mount or guider simply because the seeing won't allow it. The seeing will blur our any detail below a certain value in arcsecs. What that value is depends on your site's seeing but it will not be less than 1" and almost certainly won't be less than 1.5." You mount will support 1.5 or so as well. If I were you I'd forget these long FL scopes and go for something shorter, with as much aperture as possible.

If you go for an over-sampled system (long FL) you will starve each pixel of light, as Wim said, and you will also restrict your FOV for absolutely no gain whatever in fine detail resolution.  To my mind, the modern camera works with FLs up to a metre or so, after which they are pretty pointless except in places with very exceptional seeing.

Our RASA 8 is working at about 1.9"PP and can - just - be out resolved by a TEC140 working at 1.4.  Given the RASA has a FL of just 400mm it shows how the long FL is a thing of the past - in my view.

Olly

Thanks Olly. Your images with the Rasa are amazing, and I did briefly look at a Rasa… but both the price and weight of it (on a HEQ5) are beyond me at this stage sadly!

Im imaging on the HEQ5 with the 72 ED as I type, and my guiding error from phd2 is at 0.82” RMS, but is hitting 1” to 1.2” possibly influenced by some light wind tonight (and dithering?). 5 minute subs showing no trailing, similar with 10minutes using an lenhance filter on previous occasions. 

This is interesting regarding the longer focal lengths, this subject has opened my eyes somewhat, because initially I had aspirations of imaging some of the smaller targets such as The Sombrero and Sunflower galaxies, globular clusters, a close up of the bubble nebula and pillars of creation as examples (I already knew the f/9 RC would struggle with those especially if I used a triband filter…). I had been trying different scopes on those targets with Stellarium and AstronomyTools, and that’s how I eventually ended up with the RC to also suit both budget and mount payload. Back to the drawing board it seems! 

To over simplify and to put it in layman’s terms, the way I’m reading this is that it’s better to have a shorter FL, with a larger aperture that’s correctly sampled…and simply crop the image to “zoom in”. 

3 hours ago, wimvb said:

I started out with the 150 PDS and "upgraded" to a 190MN Maksutov Newton. Focal length wise I went from 750 mm to 1 000 mm, not much of an increase. If you take a look at my Astrobin page, you can see what these scopes achieve in a Northern European climate (link is in my signature). I also went from 1.6 "/pixel with the 150PDS / ASI174 to 0.96 "/pixel with the 190MN / ASI294. Not that I wanted a better pixel scale, it just happened to turn out that way. Any difference in the  images is mainly due to me getting better at guiding, focusing, and processing. As Olly said, there's absolutely no point in getting to smaller pixel scales unless you put that scope on a cold mountain top in a desert.

The decision between a refractor or a reflector is mainly about three things. 1. Do you mind tinkering with collimation and mirror cleaning now and then? 2. Where do you put the scope? A reflector is always more of a sail in the wind than a refractor, and a smaller mount (heq5) may struggle with that. 3. What is your budget? A newtonian is cheaper than a refractor.

Thanks Wim. That’s a great collection of images on astrobin.

1) It’s not something I’ve done before, but was open to an RC (which I’ve read are the some of the hardest to collimate!), so happy to learn it.

2) Always in the back garden. Generally with the the wind direction we get here it’s fine, but to image targets in the south, i set up in the side passage which can be a bit of a wind tunnel between the wall and fence that follow a road all the way through the village. 

3) Budget is no more than £1000, I wasn’t aiming to go that high but even the RC with accessories was coming in at £700. 150PDS was a little less, and I was put off from skywatcher refractors after tilt/collimating/aberration issues with the 72ed (different topic) so i ruled out the 100ed… One of the reasons I initially went along the lines of the stellamira 110 f6. I was recommended the Starfield 102 f7, which with a flattener is over budget, but if it’s the right scope then happy to hold off. 

My intention is to stick with the dslrs for the time being, and I was considering a 533mc next year as I know others who have paired it with the 72ed, and I’ve just realised it has the same pixel size as my dslrs, so whatever longer scope I get, in theory all the cameras should work with both scopes. I think I might make a list of what scopes match the cameras and what the mount can deliver. I’ve no intention of changing the mount, I’ve only had it a few months after spending a year with the star adventurer! 

[sinbad40]

Sorry to hear you had issues with the 72ED, I have the 80ED and i find it an excellent scope bang for the buck wise.  I had an LX90 10" many years ago for visual and planet/moon AP as it was fork mounted, but don't want to go to that size again.  I would love to keep with refractors, but size and weight soon go up with aperture (in my price range).  Maybe the 120ED could be a better coupling for my needs at 900mm fl, but will need to keep the plan open for now.

No major rush, and thanks for all the info to all.

Just checked my log from last night using the phd2 log viewer, RA 0.40 RMS, Dec 0.40 RMS Total 0.57, not to much wind around, but it did have the odd jump to 1.

Edited April 20, 2023 by sinbad40

[WolfieGlos]

Hi Sinbad, that's some great guiding then on the HEQ5 🙂 . Out of interest, how do you polar align your HEQ5? I'm currently using the polar scope and lining it up with the SW app, just like I used to with the star-adventurer. I think this could be influencing my poorer RMS, but for my current setup it works.

I've run a check on my current setup (HEQ5 + Evostar 72ED + 0.85 reducer + Canon 77/800D), and my resolution is 2.14 "/pixel. Using the tracking accuracy of no more than half the final image scale, suggests my RMS should be no more than 1.07". Which I achieve 🙂 

Taking that a step further using https://astronomy.tools/calculators/ccd_suitability , I've found that I am slightly undersampling in OK Seeing, and being widefield I guess losing that slight detail by undersampling isn't a big problem. In better seeing, it actually significantly undersamples. Quite how you measure Seeing I'm not sure though, obviously it varies all the time ! 

 

Researching scope combinations for some more FL, it appears as though there are a few options according to astronomy tools with regards to Seeing:

Stellalyra 6" f/4 Newtonian + 1.1x coma corrector works out as 660 FL, 1.16"/p resolution and samples correctly in OK Seeing, and slightly undersamples in Good. Over samples in Poor.

Stellamira 110 f/6 ED, non-reduced is identical to the Newt, but with a 0.8x reducer is 1.45"/p, and also samples correctly in Poor Seeing.

Stellalyra 6" f/9 RC, + 0.7x reducer works out at 1028 FL, 0.74"/p, and samples correctly in Good and OK Seeing. *Reducer only has a 15mm imaging circle, so image would need to be cropped for a APS-C, but reduces to f/6.72.

Starfield 102 f/7, +0.8x reducer works out at 572 FL, 1.34"/p, and samples correctly in OK and Poor Seeing. 

All weigh between 4-6kg's, OTA only.

Looking at this now, I agree with @wimvb to look at the 6" f/4 Newt, giving both good FL and fast optics. But the RMS requirement of 0.58 is pushing my guiding a bit. Would an OAG help with this?

Although I would like to get more FL, it seems beyond what I can realistically achieve without other investment. Ironically, I've just seen over on CN someone using a HEQ5 + 6" RC for this image of M63: https://www.cloudynights.com/topic/872973-bortle-8-m63-sunflower-rc6-photoshop-vs-pixinsight/#entry12645094 , although with an ASI-071 (which has a larger pixel size) and 15 hours of data!

[wimvb]

1 hour ago, WolfieGlos said:

how do you polar align your HEQ5?

The hand control of SW mounts has an "all star" polar alignment routine that is better than the polar scope alone (which may not sit straight in its housing). If you use a computer connection (not ST4), you can use PHD for polar alignment. Try to get the misalignment within a few arc minutes.

1 hour ago, WolfieGlos said:

it appears as though there are a few options according to astronomy tools with regards to Seeing

Don't sweat the small stuff. Nebulae seldom have fine detail, so they can look really good at >2 "/p. Galaxies need better sampling, but there's really no point in going finer than 1 "/p or so. As Olly wrote earlier, in practice you won't see much of an improvement in detail if you go beyond the lower limit. But, start by choosing a scope and a camera that you like and will operate with ease. This is more important than fine tuning pixel scale. I have a SkyWatcher Maksutov Newton for all my imaging. It sits permanently on a heavy SkyWatcher mount in an observatory, where it gives great results. A guy I know who lives in northern Sweden, used the same model scope on an EQ6-R in the field (ie he drove to a very dark site and needed to shovel snow before he could set up his gear). He sold the scope after one season, probably because it's too cumbersome to haul around as part of a mobile setup in deep snow.

1 hour ago, WolfieGlos said:

Would an OAG help with this

Yes. If you have a reflector, the mirror is probably not fixed in place (it shouldn't be or you will get pinched optics in cold weather). OAG will compensate for mirror movement, a guide scope won't. With an OAG, use a sensitive guide camera (eg the ASI290 mini) or a guide camera with larger sensor (ASI174). But make sure the OAG stalk and prism can cover the sensor of the guide camera.

At f/4 you will also need to look into the mechanical quality of the scope, especially the focuser. High grade optics are useless if the focuser can't carry the camera.

[sinbad40]

I used to use the polar scope, but when I last tested the alignment with it, it was off, and found that one of the screws had threaded (probably me being a little heavy with it the previous time).  So before I went to get another, I had started to use NINA and installed the polar align.  I spent a good hour getting it to 00.00.23 out, it showed a little square in the circle and thought with cloud coming in, that will do for now, as each little movement seems to put it back to 00.01.33 or something similar.  I have made sure that the pier is fully even as well after a settle period.

One thing I did with my phd was play with the aggressiveness, that is around 80 on both, and when testing the time, I have kept it at 1-1.5 seconds, some say do it shorter, others I have seen say try upto 2-2.5 seconds, but I will stay with what my system seems happy with.  Put a shot in to show what i have from the log, I hope i am looking at it right.

Still looking myself, even looking at the 130/150/200 line up of skywatcher reflectors, just don’t want to blast into something and then wish I had gone for something else.

I will be having a look at the ones you have listed now

[WolfieGlos]

On 21/04/2023 at 16:40, wimvb said:

Don't sweat the small stuff. Nebulae seldom have fine detail, so they can look really good at >2 "/p. Galaxies need better sampling, but there's really no point in going finer than 1 "/p or so. As Olly wrote earlier, in practice you won't see much of an improvement in detail if you go beyond the lower limit. But, start by choosing a scope and a camera that you like and will operate with ease. This is more important than fine tuning pixel scale. I have a SkyWatcher Maksutov Newton for all my imaging. It sits permanently on a heavy SkyWatcher mount in an observatory, where it gives great results. A guy I know who lives in northern Sweden, used the same model scope on an EQ6-R in the field (ie he drove to a very dark site and needed to shovel snow before he could set up his gear). He sold the scope after one season, probably because it's too cumbersome to haul around as part of a mobile setup in deep snow.

Yes. If you have a reflector, the mirror is probably not fixed in place (it shouldn't be or you will get pinched optics in cold weather). OAG will compensate for mirror movement, a guide scope won't. With an OAG, use a sensitive guide camera (eg the ASI290 mini) or a guide camera with larger sensor (ASI174). But make sure the OAG stalk and prism can cover the sensor of the guide camera.

At f/4 you will also need to look into the mechanical quality of the scope, especially the focuser. High grade optics are useless if the focuser can't carry the camera.

Thanks Wim. Fair play to that person carrying that setup to the field, I doubt I'd be doing that with a setup half that size...never mind the snow! In fact, I found it awkward taking my SA on holiday last year .....

I take your point on the operating with ease part, that's a good point. You'll crucify me for this, but I did start to re-consider the 6" RC (!) , but using a 0.75x reducer to f/7...yes the edges would be cropped as it only has a 15mm imaging circle, but if used for smaller targets (i.e. M63), it would be cropped in anyway as they won't fill the frame. This brings the resolution to 0.74"/p, still unrealistic for guiding it seems.

However I think I'm actually leaning back towards a refractor again, more for ease but also the max RMS Olly mentioned. However they are coming in at twice the price. Looking at a few, the Stellalyra f/4 Newt + coma correcter + accessories £525 - £600, Stellamira 110ED+Reducer = £898, Starfield 102+Reducer = £1078. The 6" RC? £800. Quite a difference, but hopefully the refractors would need little to no adjustment or maintenance, and be a grab-and-go scope.

The f/stops with the coma corrector / reducers work out as 4.3, 4.8 and 5.6 respectively (ignoring the RC). In reality, is there much difference here? I'm currently imaging at f/4.9 with a reducer, it used to be f/5.8 with a non-reducer and I've noticed little difference in the light gathered in the subs on the histogram (just better corner stars).

[Clarkey]

On 23/04/2023 at 00:34, WolfieGlos said:

This brings the resolution to 0.74"/p, still unrealistic for guiding it seems

Yes, but if you bin x2 it will give you 1.5"/px and make the whole system 'faster' and be within your guiding limitations.

Personally, I am yet to be convinced of the F4 newts (cheaper ones anyway). I had to add a new focuser and use an aplanatic coma corrector so the set up ended up costing around £1000. Not only that, but they need pretty constant fettling.