Opinions on an ED80 or GSO RC 8" for imaging

[Russ59]

Without sounding like I'm jumping in too deep at the beginning, I am looking at an imaging OTA for my NEQ6 in a few months time. My budget will be around the 1K mark and I had my mind set on an Orion ED80 (seems to be a natural choice for many), however I see Bintel are selling a GSO RC 8" (f8) for just over the 1K also.

http://www.bintel.com.au/Telescopes/...oductview.aspx

vs.

http://www.bintel.com.au/Telescopes/...oductview.aspx


I know this is an open question and there may not be a specific answer to it, but I would like some opinions on which would be more practical/suitable for imaging. I plan to use both a DSLR and/or a QHY for imaging with the usual bits and bobs for guiding. My first thought was 'aperture rules' and the 8" RC would be a night light bucket for visual also.

Any pros' or cons on either? Any members have any feedback on the GSO RC's?

Cheers,
Russ

[jjjnettie (Jeanette)]

I own both.
The ed80 is awesome. It has a good wide FOV that fits so many objects up there. You can keep adding data to any images you take through it, whether it's the next night, or next week, as there are no diffraction spikes in the images. This gives it a versatility that the RC8 doesn't have.
The RC8 is pretty good too. Personally, I find it a hassle that I can't keep adding data to any photos I've taken through it. If I had a permanent set up, and don't move the camera between different images, it wouldn't be a problem.

[RickS (Rick)]

The ED80 will be much easier to get started with. The longer focal length and weight of the RC8 would make it a lot more challenging for a first imaging scope. Not saying it can't be done but be ready for more of a struggle if you start with the RC8...

Cheers,
Rick.

[Astro_Bot]

Permit me to jump in with a similar question:

Cost aside, is an ED120, f/7.5, FL=900mm a better choice for a "rich field telescope" mostly for imaging but maybe some visual work as well? Is a 3 degree TFOV realistically achievable with an ED120? Or, is the ED80 quite sufficient for that purpose and greater 'bang for the buck'?

I note that the light-gathering power of an RC 8" is much greater that an ED80, so I'm wandering why the ED80 is so popular.

[mithrandir (Andrew)]

Originally Posted by Astro_Bot

Permit me to jump in with a similar question:

Cost aside, is an ED120, f/7.5, FL=900mm a better choice for a "rich field telescope" mostly for imaging but maybe some visual work as well? Is a 3 degree TFOV realistically achievable with an ED120? Or, is the ED80 quite sufficient for that purpose and greater 'bang for the buck'?

I note that the light-gathering power of an RC 8" is much greater that an ED80, so I'm wandering why the ED80 is so popular.

Assuming a 0.8 flattener, APS-C chip and Andrews' prices:

BD-ED80 600mm f/7.5 gives you 158x106 arcmin for $899

GSO RC8 CF 1600mm f/8 gives you 58x39 arcmin for $1099

BD-ED120 900mm f/7.5 gives you 105x80 arcmin for $1999

[Russ59]

Thanks Rick and jjj,

I was purely looking at the aperture advantage but I will most definitely take your advice and stick to the original plan of an ED80.

jjj, I didn't even know that the process of taking subs on different nights was possible? or even practical?

Can you also elaborate on the reason why a longer focal length would make it more challenging? Sorry but I'm still trying to grasp even the pros and cons of short vs. long focal lengths in scopes. Obviously in the examples Andrew has just provided, the RC-8 is obviously a smaller FOV. Is that the main factor?

Russ

[Poita (Peter)]

It is just harder to get perfect tracking with a longer focal length scope, and you typically get a smaller FOV.

The disadvantage with the ED80 to me is that it is only good for really big targets, its focal length is way too small for tiny, faint galaxies etc.

[Astro_Bot]

Originally Posted by mithrandir

Assuming a 0.8 flattener, APS-C chip and Andrews' prices: ...

Thanks. What I know about astrophotography would fit on a post-it note ... but then, that's kinda why I'm here.

[Mighty_oz (Marcus)]

Russ, JJJ - Food for thought, with the RC8 u could do this adaption to allow for shooting at diff angles/times http://astroanarchy.blogspot.com.au/...y9-camera.html to allow for getting back to the same camera angle.
If this guy can do these type of images with this i'm sure the rest of us can also.
What u have to ask is what do u want to image now vs what u may like down the track and how big a learning curve do u want as a challenge ?

Marcus.

Originally Posted by jjjnettie

I own both.
The ed80 is awesome. It has a good wide FOV that fits so many objects up there. You can keep adding data to any images you take through it, whether it's the next night, or next week, as there are no diffraction spikes in the images. This gives it a versatility that the RC8 doesn't have.
The RC8 is pretty good too. Personally, I find it a hassle that I can't keep adding data to any photos I've taken through it. If I had a permanent set up, and don't move the camera between different images, it wouldn't be a problem.

[Marke (Mark)]

I take my RC10 images at different angles at times - I just use CCDIS in CCDstack and it scales, shifts and rotates all your subs to match .

[RickS (Rick)]

I use plate solving and manual rotation of the camera to get back to approximately the same angle as subs from previous nights. Image registration takes care of any small variations...

Cheers,
Rick.

[jjjnettie (Jeanette)]

If you're starting out with Astro Imaging, you'll find it frustrating enough with just a straight forward rig, learning drift aligning, guiding, the inevitable problem solving of hardware and software, and then the stacking and processing.
Do yourself a favour and make the learning curve an enjoyable experience. The ED80 can produce spectacular images. You won't regret the purchase.
And when the time comes that the whole process becomes second nature, then it's time to challenge yourself with something with a bit more focal length.
The ED80 is a keeper. You don't see very many of them on offer in the IceTrades do you.

[cometcatcher (Kevin)]

The popular term "light bucket" might be apt for visual use with the eye being the sensor, but it doesn't really apply to astrophotography. The camera doesn't see it that way. What the camera does care about is how fast the scope is (F ratio) and focal length.

Most ED80 scopes will be faster - have a lower F ratios than the RC's. This means shorter exposures even though the objective diameter is much less.

Focal length equals magnification. In a visual telescope, we simply change eyepieces to go from either wide field to close up views. However when we shoot deep sky at prime focus, the magnification of the scope is more or less fixed. For the ED80 it's fixed at wide field (Planetary imaging excluded). The RC8 is fixed at narrow field.

This makes the ED80 suitable for large objects like nebula, open clusters and comets, not so good planetary nebula and galaxies which are smaller. The RC8 being narrow field is better for small objects like galaxies, small nebula and globular clusters.

So you see, to cover more objects in the sky, you need both, and we haven't even touched on imaging planets!

Starting with the ED80 will be easier. As mentioned in the above posts, it's more forgiving of guiding errors.

[CoolhandJo (Paul)]

ED80 is about 50% easier all round in my opinion. but you dont get 50% less quality images (user dependant ofcourse!)

[Astro_Bot]

Thanks for the advice. I have recently acquired an ED80 ... the Skywatcher "pro series" with (as I understand) FPL-53 glass.

and we haven't even touched on imaging planets!

I've been reading about "image scale", amongst other things. Other than that and trying to achieve good contrast, is there anything particularly different about planetary imaging?

[cometcatcher (Kevin)]

Originally Posted by Astro_Bot

... is there anything particularly different about planetary imaging?

Just about everything. The small diameter lens and relatively short focal length of an ED80 make getting a large and bright enough image difficult and the resolving power isn't there. Also a large imaging sensor used for deep sky is not optimal for planets. Not that it can't be done, but there's better ways to do it.

[Astro_Bot]

Sorry, I should have explained ... my existing scope is an 8" Mak-Cass, f/12.3, FL 2500mm. But I'm thinking of selling and getting something more versatile ... maybe an 8"/9.25" SCT that'll take a focal reducer, or maybe an RC8". I'm not going to rush into that decision, though ... I hope.

Originally Posted by cometcatcher

Also a large imaging sensor used for deep sky is not optimal for planets. Not that it can't be done, but there's better ways to do it.

I didn't know that. I thought more pixels would produce a better image? Or is it that the pixel density that works on DSOs isn't high enough to get good resolution on planets? Or am I way off?

Why is that, if you don't mind me asking?

[RickS (Rick)]

Originally Posted by Astro_Bot

I didn't know that. I thought more pixels would produce a better image? Or is it that the pixel density that works on DSOs isn't high enough to get good resolution on planets? Or am I way off?

Why is that, if you don't mind me asking?

DSOs are dim and need long exposures. They are often quite large as well. Planets are bright and quite small at the focal plane, even at long focal lengths. For planetary you want a small sensor that you can read quickly to get lots of short exposures. Then you pick out the best ones from the short, transitory periods of good seeing and process them to produce your image. It's a very different technique to imaging DSOs.

Cheers,
Rick.

[Astro_Bot]

Originally Posted by RickS

... you want a small sensor that you can read quickly to get lots of short exposures. Then you pick out the best ones from the short, transitory periods of good seeing and process them to produce your image. ...

Thanks for that.

OK, so a sensor fast enough to read sequential images quickly, take many and reject the fuzzy ones, stack and process. I guess that's why webcams are popular.

But don't DSLRs have a sequential mode, say, 3 to 5 frames/sec, which might as well be used at higher ISO since inherent noise would be less of a problem with such a bright target? Or maybe movie mode (1080p)? Is either of those a good option for planetary imaging?

I don't presently own a DSLR .... so maybe that's an issue in that I don't fully understand their limitations.

[Astro_Bot]

I found this, which suggests that a DSLR can work OK when used in 640x480 crop mode at 60fps with 1:1 resolution.

Otherwise, video mode used the full pixel array then downsamples to 1080p, probably destroying any hope of good resolution.

Then there's the Bayer filter interfering with colour rendition and a low-pass filter.

It's beginning to sound like DSLRs are a compromise in every respect.

Anyhow, this is getting a long way off the topic of ED80s vs RC8 vs whatever as AP OTAs.