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Celestron Edge and HyperStar over RASA

Cassegrain
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#26 herpmcderpington

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Posted 29 August 2021 - 09:43 AM

 

First there is mechanical difference you didn't mention between the R8 and R11.  That is sensor size compatibility and ability to use a filter wheel.  The 8 is limited in maximum sensor area and single filter; the 11 can use larger sensors and a filter wheel especially helpful if shooting Mono. 

   Per Celestron and the 11... "Large 43.3mm optimized image circle maintains pinpoint stars to the far corners of even the largest astroimaging sensors, while the usable field extends even further to 52mm for larger format sensors"

   Per Celestron and the 8... "Flat field free of optical aberrations like field curvature, coma, astigmatism and chromatic aberration across an entire APS-C sensor. Unlike the larger RASAs that work with DSLR and large CCD cameras, the 8” model was designed with color astronomical CMOS cameras, smaller CCD cameras, and mirrorless cameras in mind. The telescope does not work with standard DSLR cameras.

Next concerning aperture effects on imaging the same target, the 11 has a longer focal length meaning the maximum field of view is a bit smaller 620 vs 400mm but has about 2/10 arc second greater resolution which doesn't sound significant, but if you're imaging for detail with significant hours of data that is detail that will sift out as signal.  The larger aperture means more signal per unit of time to fill the pixel well given near identical focal ratios. 

Looking at images on Astrobin is in someways useful but in other ways useless because the data collected more often has gone through levels of processing rather than being a single unaltered image.  That processing has the tendency to blend detail to the aesthetic pleasure of the individual.  A more helpful resource would be raw single image data of the same target through the 8 & 11 using comparable imaging system to match pixel scale.  This would identify better the reality of the difference between 8 & 11.  

In general...
If I were to state the reasons for choosing the 11 over the 8 they would be as follows:

  • greater signal per unit of time
  • versatile options for sensor and filter

If I were to state the reasons for choosing the 8 over the 11 they would be as follows:

  • budget friendly
  • can be used on wider array of mounts

 

What sort of filter wheel can you use on the RASA 11? I have looked everywhere online and could not find a filter wheel setup which does not substantially increase CO or causes weird asymmetric diffraction spikes.



#27 Rasfahan

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Posted 29 August 2021 - 11:40 AM

What sort of filter wheel can you use on the RASA 11? I have looked everywhere online and could not find a filter wheel setup which does not substantially increase CO or causes weird asymmetric diffraction spikes.

I use the ZWO 5x1.25" mini filterwheel - it is still quite large in front and it does vignette a M4/3 sensor but this is surprisingly well-corrected with flats.. It does not cause asymmetric spikes, but the star shapes when out of focus are asymmetric which makes judging collimation difficult. Due to mount problems I only have a few images with my RASA in my astrobin. Please note that diffraction spikes/star shapes are asymmetric in these images. That is not caused by the filterwheel but by the cables leading to the camera, as I did not have an assembly yet that fixed those correctly, and by my collimation problems. Still, if you want perfect stars, the RASA probably is not the way to go (or remove the stars in post and replace them with refractor stars).

https://www.astrobin.com/jnak9q/B/

https://www.astrobin.com/ktrtu3/

https://www.astrobin.com/ir706p/

 

Due to the increased central obstruction, the RASA 11 with the mini filterwheel is T/2.5 instead of T/2.4.


Edited by Rasfahan, 29 August 2021 - 11:41 AM.


#28 Rasfahan

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Posted 29 August 2021 - 12:00 PM

Thank you!  Indeed, I've been thinking about this wrong, which is why I'm so appreciative of folks like yourself taking the time to share and help me understand.  Thinking about it wrong comes from not knowing what I didn't know.  Grateful for you and others helping to fill in those gaps.

 

Just a point of correction on the image I shared above.  That's actually the newer, and larger, M-Due.  It has a higher payload capacity than the M-Uno, which topped out at 44 lbs, while the M-Due is 55 lbs for a single scope and 70 lbs for dual scopes.  At 43 lbs for the RASA 11 plus camera, guide scope, guide cam, I should be within that single scope load limit.  I could even put the guide scope and guide cam on the other saddle to distribute load better.

What I've learned is helping me net out the price calculation in favor of the RASA 11, since another key factor I have to deal with is that clear skies are rare where I live (Pacific Northwest, famous for rain, and well below the national average for cloudless days).

 

I was trying to decide if I would remain an 8" astro guy, with an EdgeHD 8" and adding a RASA 8". or if I would make the jump to 11" for both.  This discussion really helped with my understanding.  I'm ready to join team 11.  Now if only I can find them!

Glad to be able to help.

 

I know where you are coming from. Germany is a wet place, too (this year we only had on season: rainy season). It's why I got my RASA. But there's no free lunch. Be prepared to invest a significant amount of (clear-sky) time in getting it setup. I would not recommend it for travel or if you set up nightly. Backfocus and tilt adjustment need to be very exact, and removing and reattaching the camera or even moving the scope will mess with that. Currently, if I had used a refractor, I would probably have got more images done than with my mirrored scopes. Now that the teething pains are over, I hope to catch up to that in the next months - tough chance as it's turning autumn already.

 

With 55 lb on the M-Due I'ld e-mail Avalon, see what they say. They were very responsive and knowledgeable when I contacted them, and even dissuaded me of their M-Uno mount for my use case (a 10" RC or 11" EdgeHD). I like the Avalon mounts, and have heard good things about the recent samples. I'm seriously looking at the M-Zero for mobile use.

 

The guide cam/scope really is not needed for the RASA, especially as you are planning on a mount with absolute encoders. You'll stay well below 30s on most exposures anyway, and the thing that kills the most subs for me is the mirror flop (not bad, < 10%). I'ld just do good PA with unguided + dithering and recenter every half an hour or so to account for remaining drift.


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#29 herpmcderpington

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Posted 29 August 2021 - 01:22 PM

I use the ZWO 5x1.25" mini filterwheel - it is still quite large in front and it does vignette a M4/3 sensor but this is surprisingly well-corrected with flats.. It does not cause asymmetric spikes, but the star shapes when out of focus are asymmetric which makes judging collimation difficult. Due to mount problems I only have a few images with my RASA in my astrobin. Please note that diffraction spikes/star shapes are asymmetric in these images. That is not caused by the filterwheel but by the cables leading to the camera, as I did not have an assembly yet that fixed those correctly, and by my collimation problems. Still, if you want perfect stars, the RASA probably is not the way to go (or remove the stars in post and replace them with refractor stars).

https://www.astrobin.com/jnak9q/B/

https://www.astrobin.com/ktrtu3/

https://www.astrobin.com/ir706p/

 

Due to the increased central obstruction, the RASA 11 with the mini filterwheel is T/2.5 instead of T/2.4.

Thanks for the images. Yeah these stars are brutal. I still cannot find a fast astrograph to replace my f/7 slow refractor, other than the 12k dollar RH200. I get 2-3 nights/month of clear skies at most, so I need something that allows me to complete a full SHO project in a single night.



#30 cougarone

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Posted 29 August 2021 - 01:49 PM

Glad to be able to help.

 

I know where you are coming from. Germany is a wet place, too (this year we only had on season: rainy season). It's why I got my RASA. But there's no free lunch. Be prepared to invest a significant amount of (clear-sky) time in getting it setup. I would not recommend it for travel or if you set up nightly. Backfocus and tilt adjustment need to be very exact, and removing and reattaching the camera or even moving the scope will mess with that. Currently, if I had used a refractor, I would probably have got more images done than with my mirrored scopes. 

Can you comment on set up pain if nightly. I was thinking of purchasing a RASA 11 to put on the AP 1100 (no encoders yet, waiting 9 months so far) that sits in the garage on a rolling pier that I move in and out of my garage as one unit (about 35 yards) onto to my driveway with an SV130/SV80 attached.  Would moving this in and out of my driveway cause half the night to be spent on set up. Just learning astrophotography (crawling), and I can currently be up and running in 30 minutes . 



#31 Rasfahan

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Posted 29 August 2021 - 03:33 PM

Can you comment on set up pain if nightly. I was thinking of purchasing a RASA 11 to put on the AP 1100 (no encoders yet, waiting 9 months so far) that sits in the garage on a rolling pier that I move in and out of my garage as one unit (about 35 yards) onto to my driveway with an SV130/SV80 attached.  Would moving this in and out of my driveway cause half the night to be spent on set up. Just learning astrophotography (crawling), and I can currently be up and running in 30 minutes . 

No, I do not think just moving the whole setup will be a problem. But as soon as you remove or change some part of the imaging train you need to do adjustments.



#32 Rasfahan

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Posted 29 August 2021 - 03:40 PM

Thanks for the images. Yeah these stars are brutal. I still cannot find a fast astrograph to replace my f/7 slow refractor, other than the 12k dollar RH200. I get 2-3 nights/month of clear skies at most, so I need something that allows me to complete a full SHO project in a single night.

Yes, my last adjustments made them quite a bit better, but I could finish no image yet with them. Some of them are with the 2.3 micron unbinned pixels of the QHY294M. The Officina Stellare RH seems to have its own problems, too. The AP Riccardi-Honders is not only expensive, but not produced anymore and very rare. For the RASA I think binning to 7micron pixels is a good way to hide some aberrations and not go crazy trying to get adjustments perfect. Going for that perfection has become part of the hobby for me, though, and as I seem to have solved my mount problems, that is the next step to my „final“ setup (10“ RC and RASA 11 as a double imaging setup).



#33 555aaa

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Posted 29 August 2021 - 05:59 PM

The filters on the RASA are a real problem IMHO. The design as is already vignettes about 25% in the corners because the flattener/corrector lens group is too far away from the focal plane and too small (this is I suspect a concession made so that a DSLR will focus with it). The RASA36 in comparison has a huge flattener group and doesn't have this issue.  When you use a small camera and small filters you're throwing away most of the image. There are some filter drawer type holders that are available but it is very hard to get a 50mm round filter close enough to the sensor, since the angle of the cone of light is so steep.  I also have a micrometer tilt/spacing adjustment doo-dad that makes collimation easy, but it doesn't work with filters. I personally think the individual filter holders are the way to go (for now) since the 11 will cover 6 square degrees of sky with good correction to the corners, so you can get pretty good illumination in the corners for individual filters. The RASA probably needs rectangular filters but no one makes those yet. Smaller filters work if you're willing to discard most of the field of view in terms of square degrees, and as I pointed out above, the design sacrifices resolution for field of view.

 

The other thing about it is that it's heavy. The RASA11 is similar as a payload to a C14.



#34 Rasfahan

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Posted 29 August 2021 - 06:43 PM

The filters on the RASA are a real problem IMHO. The design as is already vignettes about 25% in the corners because the flattener/corrector lens group is too far away from the focal plane and too small (this is I suspect a concession made so that a DSLR will focus with it). The RASA36 in comparison has a huge flattener group and doesn't have this issue.  When you use a small camera and small filters you're throwing away most of the image. There are some filter drawer type holders that are available but it is very hard to get a 50mm round filter close enough to the sensor, since the angle of the cone of light is so steep.  I also have a micrometer tilt/spacing adjustment doo-dad that makes collimation easy, but it doesn't work with filters. I personally think the individual filter holders are the way to go (for now) since the 11 will cover 6 square degrees of sky with good correction to the corners, so you can get pretty good illumination in the corners for individual filters. The RASA probably needs rectangular filters but no one makes those yet. Smaller filters work if you're willing to discard most of the field of view in terms of square degrees, and as I pointed out above, the design sacrifices resolution for field of view.

 

The other thing about it is that it's heavy. The RASA11 is similar as a payload to a C14.

Correct on all accounts, I think. Full-frame narrowband imaging is not well possible with the RASA 11. But its light grasp is good enough that I can image many targets even without NB-filters and an OSC camera, in Bortle 6, and with a good resolution, too. And even when you reduce the FOV to m4/3 for narrowband (about 40% vignetting in the corners) there is no other system that can give the same (pixel) etendue and comparable resolution that‘s even close in price.



#35 herpmcderpington

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Posted 29 August 2021 - 06:49 PM

What about smaller form factor cameras with integrated filter wheels? I was looking at the SBIG STC-7 or even QSI cameras. I haven't found any images on Astrobin. Would they cause weird star shapes/diffraction spikes or severe vignetting?



#36 Rasfahan

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Posted 30 August 2021 - 05:23 AM

Those cameras have quite a bit larger diameter than the ZWO Mini Filterwheel, causing a huge CO. I believe the mini is really the only option that is at least somehow practicable for the RASA 11. The scope is designed for OSC, though.



#37 JethroXP

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Posted 30 August 2021 - 01:26 PM

Glad to be able to help.

 

I know where you are coming from. Germany is a wet place, too (this year we only had on season: rainy season). It's why I got my RASA. But there's no free lunch. Be prepared to invest a significant amount of (clear-sky) time in getting it setup. I would not recommend it for travel or if you set up nightly. Backfocus and tilt adjustment need to be very exact, and removing and reattaching the camera or even moving the scope will mess with that. Currently, if I had used a refractor, I would probably have got more images done than with my mirrored scopes. Now that the teething pains are over, I hope to catch up to that in the next months - tough chance as it's turning autumn already.

 

With 55 lb on the M-Due I'ld e-mail Avalon, see what they say. They were very responsive and knowledgeable when I contacted them, and even dissuaded me of their M-Uno mount for my use case (a 10" RC or 11" EdgeHD). I like the Avalon mounts, and have heard good things about the recent samples. I'm seriously looking at the M-Zero for mobile use.

 

The guide cam/scope really is not needed for the RASA, especially as you are planning on a mount with absolute encoders. You'll stay well below 30s on most exposures anyway, and the thing that kills the most subs for me is the mirror flop (not bad, < 10%). I'ld just do good PA with unguided + dithering and recenter every half an hour or so to account for remaining drift.

Thanks again for the advice!  I did email Avalon and Stefano got back to me very quickly.  This is a direct quote:  "The M-due can manage the Celestron RASA 11 telescope weight."

My mind is made up, I'm getting the RASA 11, the EdgeHD 11, and the ASI6200MC Pro.  Probably in that order, but not all at once, that's going to take some time lol.gif I can see my ASI2600MC Pro being the default camera for the EdgeHD 11 while the ASI6200MC Pro will live on the RASA 11, though there may be some targets that would benefit from the wider FOV at f/10 or f/7 on the EdgeHD.


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#38 555aaa

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Posted 30 August 2021 - 02:48 PM

Thanks again for the advice!  I did email Avalon and Stefano got back to me very quickly.  This is a direct quote:  "The M-due can manage the Celestron RASA 11 telescope weight."

My mind is made up, I'm getting the RASA 11, the EdgeHD 11, and the ASI6200MC Pro.  Probably in that order, but not all at once, that's going to take some time lol.gif I can see my ASI2600MC Pro being the default camera for the EdgeHD 11 while the ASI6200MC Pro will live on the RASA 11, though there may be some targets that would benefit from the wider FOV at f/10 or f/7 on the EdgeHD.

Good choice. FYI you can get a M68 tilt plate to replace the M54 tilt plate on the ASI6200MC camera. It improves the corners a little if you have a big enough adapter.


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#39 Alfredo Beltran

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Posted 07 September 2021 - 10:17 PM

Just found this thread.

 

I use a C9.25 EdgeHD with hyperstar since 2013 and I’ve found it to be a very versatile combination. Not only gives a wide FOV for an APS-C sensor at 540 mm (f2.3), which allows imaging a lot of big objects, but also gives a very flat field at f10 for imaging small planetary nebulae, clusters and galaxies. Lately I’ve also used it for planetary at f20 with very satisfactory results.

 

And of course, once in a while, used for visual its performance is also remarkable.

 

For me, versatility and optical quality were the main reasons to get this OTA. Sure, a combination of other telescopes could be also an option, but in the end is a matter of each person’s preferences.
 

You can see the images I’ve taken with it on the galleries I have at Flickr and Instagram on my signature.

 

Regards,

 

Alfredo




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