8" Edge vs C 9.25

[Anthonyexmouth]

ok, permanent pier half way complete and now im thinking what next, last year i enjoyed using my little mak90 for some planetary video imaging but the shakes really annoyed me. now with nearly a ton of concrete holding my heq5 in place im looking at something in a longer focal length to play with. will mostly be imaging but some visual would be nice. is it worth sacrificing aperture with the 8" edge over the C 9.25 as they are both similar price. 

[vlaiv]

What sort of imaging are you aiming for?

Planetary imaging - go with C9.25

Regular imaging - go with 8" Edge

 

 

[Craney]

Good question....   Since planetary imaging will involve only a very small field, then having the 8" HD flat field will I guess, have marginal benefits.

Whereas, the 9.25 has greater F.L and more light gather....   

For DSO's and other larger targets, hmmmm ?? tricky one that.....

 

 

[Anthonyexmouth]

Just now, vlaiv said:

What sort of imaging are you aiming for?

Planetary imaging - go with C9.25

Regular imaging - go with 8" Edge

 

 

i think i'm happy with my ed80 and canon for regular dso but really enjoyed the mak90 between shakes. the mak and an asi290 colour were a bit of an impulse buy. maybe the mak will be ample for now on a better pier but well all love new toys. 

maybe an investment into a dual saddle clamp so i can have the ed80 and mak90 set up might be a cheaper option

[vlaiv]

If you already have ED80 and you are happy with that one for DSO AP, but have cravings for visual and planetary ap work - it's easy - C9.25.

I would even think of C11 if funds allow. It's a bit of stretch for HEQ5 but not much. We are not talking long exposure ap stability here, and that scope is around 12Kg, while Heq5 can handle up to 15Kg max.

[Anthonyexmouth]

the FOV calculator doesn't show much in it, am i reading it wrong? 

[Anthonyexmouth]

6 minutes ago, vlaiv said:

If you already have ED80 and you are happy with that one for DSO AP, but have cravings for visual and planetary ap work - it's easy - C9.25.

I would even think of C11 if funds allow. It's a bit of stretch for HEQ5 but not much. We are not talking long exposure ap stability here, and that scope is around 12Kg, while Heq5 can handle up to 15Kg max.

I'm pretty confident that the heq5 will not be on the pier too long, in the summer i like heading out to dartmoor so the pier will probably be getting an EQ6-R at some point. 

[vlaiv]

With planetary it's not about FOV - its about sampling rate and maximum resolution. When you image with ASI290, you won't be using full FOV anyway unless you are going after the Moon.

Full frame download will slow down rate that you can achieve, and you want good FPS rate to capture as much frames as possible.

Aperture size is also very important in planetary - larger the scope, more detailed the image.

With C9.25 vs 8" edge, its not that much aperture as cost. This additional cost goes into special design and rear element that enables you to have tight stars and flat field - which is good for imaging. Any SCT will have sharp central region (when properly collimated) but you need correctors for outer region which Edge has built in and matched.

[Anthonyexmouth]

2 minutes ago, vlaiv said:

With planetary it's not about FOV - its about sampling rate and maximum resolution. When you image with ASI290, you won't be using full FOV anyway unless you are going after the Moon.

Full frame download will slow down rate that you can achieve, and you want good FPS rate to capture as much frames as possible.

Aperture size is also very important in planetary - larger the scope, more detailed the image.

With C9.25 vs 8" edge, its not that much aperture as cost. This additional cost goes into special design and rear element that enables you to have tight stars and flat field - which is good for imaging. Any SCT will have sharp central region (when properly collimated) but you need correctors for outer region which Edge has built in and matched.

so with planetary i'm only concerned with the center of the frame so a flat field isn't as important? 

[vlaiv]

Just now, Anthonyexmouth said:

so with planetary i'm only concerned with the center of the frame so a flat field isn't as important? 

Correct.

You'll also probably add barlow - that reduces FOV even further.

You will use ROI of camera to get smaller frames - higher FPS. Take a look at specs for ASI290

You want to go for 640x480 to have at least 380fps available. This does not mean that you will "use it up". This will depend on your exposure, but sometimes coherence time is such that you need to use 3-6ms subs. If you want to use 5ms subs to full potential you need 200fps for that.

With planetary imaging, here are key points:

1. Aperture

2. Matching resolution depending on pixel size (by use of barlow)

3. Targeting proper exposure length depending on coherence time. Coherence time is time needed for light bending in atmosphere to change. If you image in time shorter than that - you will get distorted image but you will not get motion blur of atmosphere. Distorted image is something that stacking programs can deal with. Blur can't be removed and will end up in stack and you will need to try to remove it with wavelets or other sharpening methods like deconvolution. On the other hand longer subs give you better SNR.

Planetary imaging is really balance of these last two points - go with longer exposure and you get less usable subs to stack - go with shorter exposure and your individual subs will have lower SNR but you will have more frames to choose from (and less motion blur if you keep at or under coherence time).

[Anthonyexmouth]

i was pleased with my first attempt last year at planetary with the mak90 and the asi290.