EAA - small apo vs 10" dobson

[chrisv]

I've started EAA with Skywatcher ST120/600, mounted on an EQ5 (with a DIY goto) and with a ZWO 120MM-S camera. I was very pleased with it initially, since it opened a whole new world. Unfortunately nowadays I can only allocate 1-2 hours for my observing sessions and I find myself taking all the equipment outside less and less frequently.

My main issues are,

• complicated setup - lots of parts, polar alignment, Onstep settings (used for my DIY goto), camera drivers, lots of cables
• the 600mm focal length is a bit too narrow for the ZWO 120MM-S camera; I wasn't able to observe some targets due to not enough stars, especially when using narrowband filters; I've tried a cheap focal reducer, but the results were very bad
• EQ GOTO - EQ5 is enough for my needs, but even if the setup only takes me 10 minutes, it's still a lot, if I only have 1 hour available
• chromatic aberrations - they are not that bad when using narrow band filters, but they are noticeable on bright stars, even in mono (bloated stars)

Lately I've read more and more posts from people using small APOs on the AZ GIT mount - the mount is small enough (I could even try to leave everything assembled) and the setup doesn't seem to be that bad. If I can get it to work with Astroberry or Stellarmate, I could have something very light and portable, which could also be used remotely during the winter months.

I was considering getting the TS Optics 72ED F6 APO together with the AZ GTI month, but after summing all costs, it's almost the same as getting a 10" Dobson together with an equatorial platform. Of course, it is a different style, but there are some success stories for this approach as well.

The 10" Dobson would be a pleasure to use for visual, but it could also work for EAA (maybe with a focal reducer). I enjoy star hopping, but I have some challenges with targets from south, where there are some strong street lights. My observations will be from my backyard (I just need to take everything outside) or from the balcony. I'm concerned more about the setup time, rather than the weight, if I only have 1-2 hours available, I want to spend them observing and not setting things up.

What do you think? Would a 10" Dobson on an equatorial platform take longer to carry / setup than a small APO on AZ GTI? Which option would be more versatile, easier to expand? Are there other options I should take into account in this budget?

Thank you!

[RobertI]

I would have thought if you are really pushed for time, then the 72mm plus AZ GTI would get the most use, and because of the short focal length (especially if reducer used) then less strain in the tracking. Also have the option of starsense for auto-alignment and possibly even an ASIAir for full automation?
 

Funnily enough I have the same issues and am experimenting with a similar setup. Previously I have used an RC6 on an CG5 (same as EQ5) - great for galaxies and smaller DSOs, but setup takes time. I am now experimenting with my 66mm APO with a reducer on my SkyProdigy mount for auto alignment - in theory super quick setup, but don’t know yet whether the tracking is up to it and whether my Lodestar camera with its big pixels will work at this short focal length. Results are promising so far and the fun factor is high. 
 

I can’t really comment from practical experience on the Dob option though, I’ll leave others to do that.  

Edited July 4, 2022 by RobertI

[PeterC65]

I have the TS-Optics Photoline 72 that you're considering and can confirm that it's a great piece of kit. Very well made with a great focuser and small / light enough to be very convenient to use. There is no CA to my eyes. I use it on a Sky-Watcher SynScan AZ GOTO mount (similar spec to the AZ GTI but without the WiFi) and find that on this mount targets are easy to find and track and are almost always right in the centre of the field of view. The scope is very small when the dew shield is stowed and it could easily be left on the mount.

My mount is very easy to align, taking only a couple of minutes, and if you can put the mount back exactly where it was last time (I drilled holes in the patio for the tripod legs) then the alignment can be maintained across multiple sessions.

So far I've used this setup for visual only but I've recently acquired an Altair GPCAM2 327C camera for EEVA once the nights get darker. In daylight testing I could achieve focus more easily with the Photoline 72 than with my Explorer 150, mainly because the refractor has twice as much focus travel as the Newtonian. I expect a 10" Dob would be the same, as most Newtonians seem to have limited focus travel.

 

[chrisv]

Thank you for your responses. I'm more inclined to go with the 72ED on AZ GTI, but there still is something holding me back.

With the small APO I'm concerned that it will be very disappointing for visual use. My current ST 120/600 already provides good wide field views visually, but it's terrible at planets or high magnification, the same areas where 72ED doesn't shine.

How much affect does the aperture have for EAA, especially under light pollution? From my understanding, for imaging the aperture will affect the resolution - how much detail you can get, e.g. if the aperture increases and the f-stop remains the same, the focal length will increase - which translates to higher magnification or resolution, depending on the camera sensor size.

Is a 72ED APO enough as a main scope for EAA? Would there be a major difference between a 72ED and a 80ED, provided they have the same f-stop?

Can you see more with a 72ED APO using EAA than with a 10" F5 Dobsonian visually?

Edited July 6, 2022 by chrisv

[RobertI]

5 hours ago, chrisv said:

Is a 72ED APO enough as a main scope for EAA? Would there be a major difference between a 72ED and a 80ED, provided they have the same f-stop?

It would work best for nebulae, large galaxies and wide field in my opinion. Also very good for emission nebulae with Ha filter. If that’s your area of interest then it might work for you. If you are more interested in globulars, planetary nebulae and smaller galaxies, then not so good.  Personally I think you would eventually run out of things to look at with the wide field set up, so probably not ideal as a main EAA scope, there are so many more interesting small DSOs to look at in my opinion, especially in galaxy season. 
 

5 hours ago, chrisv said:

Can you see more with a 72ED APO using EAA than with a 10" F5 Dobsonian visually?

I would say definitely yes for larger nebulae and galaxies as described above. The visual dob would be more satisfying for smaller DSOs I would think (based on my experience with my 8” SCT). 

Edited July 6, 2022 by RobertI

[chrisv]

That was my concern as well. When I started EAA with my ST120/600 on EQ5, I had very good results, but also very bad ones. The small sensor ASI 120MM is very narrow, even for a 600mm focal length, and for a lot of targets, especially when using narrow band filters, there weren't enough stars for stacking.

With a small APO on the AZ GTI, I was hoping to address this issue and to have an easier to use setup. Do you have any other scope suggestions, to complement a 72ED and get more resolution for smaller DSOs, without overloading the GTI mount (max 5kgs)? Would a different camera, maybe one with smaller pixels, help?

Thank you.

[RobertI]

I’m not really up with the latest EAA cameras, so wouldn’t like to advise what might work, but it does sound like it is worth investigating a different EAA camera which would better complement the 72ED. But having re-read your your post, I am thinking there are two alternative solutions to your problem:

1. Low cost option - build a semi-permanent pier and leave the mount and scope set up under a weather proof cover or box. A ‘Todmorden Pier’ is virtually zero cost and I have seriously been thinking about one of these.
2. Higher cost option - invest in a good quality AZ mount like a Skywatcher AZ-EQ5 or an i-Optron AZ Pro to reduce setup time. The former also allows the the possibility of Starsense to reduce setup time even more. 

With both above options you can then look to upgrade the scope and camera over time to remove the problem of starbloating, lack of stars and field of view. I think Skywatcher 130PDS or 150PDS would make a great EAA scope.

 

[RobertI]

If you're interested I did a session with my 66mm APO, Skyprodigy mount (basically a Celestron SLT, a similar class of mount to the Skywatcher GTi I would say) and lodestar camera - might give you an idea of what's possible? You could probably do better than me with a better camera and mount!!

 

 

Edited July 8, 2022 by RobertI

[chrisv]

Thanks Rob, those are some great images, way better than anything that I've taken with my previous setup ST120/600.

I've decided to go with the small apo option, TS Optics 72 F6 together with AZ GTI. It was a hard decision, since a 10" inch dobsonian would have been great for visual as well, but at least it seems an approach that's more tried and tested.

[Cosmic Geoff]

On 06/07/2022 at 23:10, chrisv said:

The small sensor ASI 120MM is very narrow, even for a 600mm focal length,

I tried an ASI120MC against a ASI224MC as a trouble-shooting excercise recently, and concluded (among other things) that while the ASI120MC worked well enough as a planetary camera (its intended role) it is too noisy to be very useful for long exposure deep space imaging.  Yes, the field with these planetary cameras is small, but it might conceal optical aberrations in some scopes that would be more obvious with a larger sensor chip.

[Steve in Boulder]

Low read noise cameras are very good for EAA, and for alt-az mounts, because you can take shorter subs and more of them without read noise becoming a significant factor.  From ZWO, the 290MM, 533MM/MC, and 294MM/MC are all worth consideration.  

[chrisv]

I've had the chance to try the setup (AZ GTI, TS Optics 72/432 and ZWO ASI 120mm) for a night and the results were acceptable. Compared to the 120mm F5 achromat, the stars are sharper, but the overall image quality isn't that much better. The noise level is quite high and the resolution a bit too small. I've switched from Sharpcap to Astroberry (EKOS & ASTAP), hence it's a bit hard to compare the results, but the setup is very portable - I can keep everything assembled and just place it outside.

I know that everyone recommends ZWO 290MM, but what about 178MM? It has higher resolution and sensor size, while the pixel size is in the same range. It does have slightly higher read noise, but it's also a bit cheaper at my local astro shop. Is it worth it?

ZWO announced some new cameras which will be available later this year and 662MC got my attention - it's in the same price range, has lower read noise (at least on paper), but it's only available as color.

[Steve in Boulder]

1 hour ago, chrisv said:

I've had the chance to try the setup (AZ GTI, TS Optics 72/432 and ZWO ASI 120mm) for a night and the results were acceptable. Compared to the 120mm F5 achromat, the stars are sharper, but the overall image quality isn't that much better. The noise level is quite high and the resolution a bit too small. I've switched from Sharpcap to Astroberry (EKOS & ASTAP), hence it's a bit hard to compare the results, but the setup is very portable - I can keep everything assembled and just place it outside.

I know that everyone recommends ZWO 290MM, but what about 178MM? It has higher resolution and sensor size, while the pixel size is in the same range. It does have slightly higher read noise, but it's also a bit cheaper at my local astro shop. Is it worth it?

ZWO announced some new cameras which will be available later this year and 662MC got my attention - it's in the same price range, has lower read noise (at least on paper), but it's only available as color.

I have a 178MM.  It has a lot of amp glow so you’ll want darks but otherwise it’s pretty good and definitely a big improvement over the 120MM.  I had it out the other night on my C6 Hyperstar, here’s a portion of NGC 7000.  From what I read, a good planetary camera too though I haven’t tried it in that capacity yet.  
 

Are you using ASTAP for live stacking?

[chrisv]

Nice image, the extra resolution pays off.

I've tried using ASTAP for EAA, since that was the only option available in Astroberry by default. I was put off at first, because it doesn't do capture, but in the end, the workflow is not that bad. I've used EKOS to capture the images to a separate folder and configured ASTAP live stacking to read the images from the same folder. Whenever I changed targets, I had to manually change the folder in both places.

It was my first try with ASTAP and I probably need to practice a bit more.

Two nights ago I've also tried capturing using my Canon 80D, but 24 MP was too much too handle for the Raspberry PI - it crashed due to out of memory in the end. This reminded me that higher resolution, will require more processing power, hence a different setup is needed.

[Steve in Boulder]

On 18/07/2022 at 03:18, chrisv said:

Nice image, the extra resolution pays off.

I've tried using ASTAP for EAA, since that was the only option available in Astroberry by default. I was put off at first, because it doesn't do capture, but in the end, the workflow is not that bad. I've used EKOS to capture the images to a separate folder and configured ASTAP live stacking to read the images from the same folder. Whenever I changed targets, I had to manually change the folder in both places.

It was my first try with ASTAP and I probably need to practice a bit more.

Two nights ago I've also tried capturing using my Canon 80D, but 24 MP was too much too handle for the Raspberry PI - it crashed due to out of memory in the end. This reminded me that higher resolution, will require more processing power, hence a different setup is needed.

If you have a suitable laptop, you could consider continuing to use Astroberry on the RPi to find targets, take images, and so on, but using software like Jocular or SharpCap on the laptop to stack the images via a watched folder and a WiFi connection to the RPi.