Equipment for Autoguiding with EdgeHD 8" SCT

[souls33k3r]

Hi Guys,

Yes yes me again with another one of those stupid questions

Right so i'm in the market looking for more to make more purchases and hopefully not un-necessary ones too.

Since I own an EdgeHD 8" SCT which is on a NEQ6 Pro mount, I know that imaging DSO's will be a tough task with a few DSO's out there.

So basically I was told that I need a .7x focal reducer, an autoguider (was suggested an Orion Star shoot) and a off-axis (Celestron) guider which I know is a perfect combo.

Now here comes my questions.

1) Is a focal reducer really really really necessary? I know it will bring the f/10 down to a one full f-stop and make it faster but I mean I'm no expert in these f-stops, how does it boil down to number in minutes/hours?

2) What other autoguider options do I have? I do not want to break my bank with all this but if Orion star shoot is the best for that price bracket then I will go for it otherwise is there a cheaper and better version with whatever company? I've seen people use ASI and ZWO

3) I know off-axis will give me precision, again is this an absolute necessity to have?

4) Can I not hook up the autoguider with a finderscope and use that? My scope came with a 9x50 finderscope by default. Will this work or do I need to buy something bigger and better? If so then what would you recommend?

I will be in the market back again for a ED80 refractor too, so will all this equipment still be valid? If I'm not mistaken, the only thing valid will be an autoguider correct? Or will I still need a better finderscope?

I want to weigh in the complete thing, what would be more cost effective.

Thank you

[souls33k3r]

Wow no replies ... Is this really a tough question?

[MattJenko]

Few things here.

Focal redcuer on an SCT is more to get a wider field of view than necesarily make it "faster". A reducer doesn't gather more light, it just makes whatever light that comes in fit on a smaller part of the sensor, thus filling the pixels up quicker, hence the faster link.

As for guiding, the long focal length again kind of dictates an OAG is best as the image scale on a short fast ST80 is going to not be accurate enough I am assuming. The edge has mirror locks, so the argument of using an OAG because the mirror shifts when tracking is not so valid. I have seen SCTs guided by finderscopes, but nearly all use OAGs for good reason.

I personally have used an ASI120M for guiding and any likeminded camera will work. I have used an old Meade DSI Pro etc and people have luck with the QHY5, which is a bargain price these days. You need sensitive cameras for OAG guiding, which is why the lodestar is so popular, but this is some big $$$ for a guidecam.

Hope this helps.

Cheers

Matt

[steppenwolf]

1) Is a focal reducer really really really necessary? I know it will bring the f/10 down to a one full f-stop and make it faster but I mean I'm no expert in these f-stops, how does it boil down to number in minutes/hours?

Long focal lengths are more difficult to track accurately than shorter focal lengths and many deep sky objects are quite wide making a shorter focal length (and thus a wider field of view) more attractive for this use, The effectively shorter focal length  achieved by the use of a focal reducer is a win-win here.

2) What other autoguider options do I have? I do not want to break my bank with all this but if Orion star shoot is the best for that price bracket then I will go for it otherwise is there a cheaper and better version with whatever company? I've seen people use ASI and ZWO

The Orion Starshoot is fine but another option to consider if price is important would be the QHY5 which, as Matt says is a bargain.

3) I know off-axis will give me precision, again is this an absolute necessity to have?

An off axis guider is almost a requisite for use on an SCT in particular but generally on longer focal length telescopes and even with the reducer, the effective focal length of your telescope will still be around 1400mm which is fairly long.

4) Can I not hook up the autoguider with a finderscope and use that? My scope came with a 9x50 finderscope by default. Will this work or do I need to buy something bigger and better? If so then what would you recommend?

A modified finderscope is fine for use as a guide 'scope for short focal length imaging 'scopes but, again, not for focal lengths greater than 1000mm as an absolute maximum of 1000mm.

In an ideal world, a much shorter focal length imaging instrument would be a better solution but you could use your existing 8" SCT provided you are aware of the potential issues for tracking and object size.

[swag72]

I used an OAG on my C9.25 a couple of years ago. The first thing that became VERY apparent is that there are precious few guide stars around in that small field of view, so you'd benefit massively from a more sensitive guide camera. I got the Lodestar anticipating this very issue and it's not let me down...... but even with that, it is quite surprising how few stars there are to choose!

[ollypenrice]

I think there's a bit more to the focal reducer question.

In bald numbers the exposure time goes as the square of the F ratio, so 7x7 = 49 (say minutes) while 10x10 = 100 minutes. I'd call that a lot of difference!

Now Matt is right that a reducer brings in no new photons so, for objects that will fit on the unreduced chip, you don't gain anything with a reducer. You could just as well downsize your F10 image of the object to the F7 screen size of the object and get much the same result. The 'F ratio myth' points this out.

However, how many objects will fit on your chip at 2 metres? A lot of classic targets won't. In this case the reducer will do something useful. It will widen your field, as Matt says, but it will also make that wider field image reach an acceptable S/N ratio in half the time.

And then there's your sampling rate. (How many arcsecnds per pixel are you covering with your setup?) Here's a calculator. Plug in scope and camera, with and without the reducer. http://www.12dstring.me.uk/fov.htm To be honest, anything less than an arcsecond per pixel is probably going to be difficult to retain, given the effects of guiding and seeing. Many UK imagers like to stick to something like 2 arcsecs per pixel. Using the calculator you can see where this puts you. If your sampling rate is too fine each pixel gets insuffient light, slowing you down badly, and you don't get the extra resolution anyway. This is lose-lose.

As for the guiding, you have to be better off with an OAG because of the elimination of flexure, mirror movement and for the better match in focal lengths. However, people do report success with guidescopes and even finder guiders. If you go for an OAG I would recommend a top quality guide cam like the Lodestar. For faster FR guidescopes lesser cameras are fine.

As I think you know, the small and unbinnable pixels of the DSLR are not a good match for longish FL SCTs and nothing is going to change that.

Olly

Edit, crossed with Sara and Steve. It's funny that Steve looked at the reducer from a 'win win' point of view and I looked at it from the other end I said 'lose lose.' Maybe I'm a 'glass half empty' person (but I don't think I am really!) Anyway we all seem to agree.

[steppenwolf]

Edit, crossed with Sara and Steve. It's funny that Steve looked at the reducer from a 'win win' point of view and I looked at it from the other end I said 'lose lose.' Maybe I'm a 'glass half empty' person (but I don't think I am really!) Anyway we all seem to agree.

Tee hee, Olly - the bottom line is that we all basically seem to agree on the main issues, we are just considering slightly different attributes! I didn't mention mirror flop which is certainly a big issue with a standard SCT but I believe that the Edge HD version have a fairly good mirror lock system?

[PhotoGav]

All great advice so far and interesting about reducers and what they actually do. I have an EdgeHD 800, the dedicated reducer and OAG. I find that the only good thing about this set up is the focal length - wow, you can get in 'close'. However it is a complete pig of a system to use and to get acceptable results from! I shall expand...

This is a sloooow optical system, so to image successfully you need looooong sub length exposures. That means the best guiding possible. I first used my set up with a finder guider and a QHY5 guidecam. I failed miserably. I couldn't get decent star shapes at anything longer than about 120s sub length. I wanted 1800s subs!

I realised the only way forward was to use the Celestron OAG. This is a fiddle to use, though certainly not impossible. As Sara said, finding a guide star can be a challenge, especially when galaxy hunting and stars are less abundant. There was also a spacing issue when trying to use the reducer with the OAG - it required a particular adapter that at that time, 18 months ago I think, didn't exist! It might do now. Anyway, without the reducer and with a great deal of patience I did have some success.

I then upgraded my DSLR to a CCD with integrated OAG and a Lodestar X2 guidecam. This combination works better - the guidecam is amazing and can find stars in darker patches of sky than the Hubble Deep Field area! Guiding works well enough and, while still a frustrating system to use mainly down to the slow optics, if sky conditions would allow many many hours of imaging, I can make half decent images.

All in all, this is not an easy scope to image with and you mention that you are considering an ED80. If you have not yet done much imaging, I would seriously consider purchasing the ED80 first, using a finder guider adapter with a 'cheaper' guidecam such as a QHY or ASI, and get to grips with the whole imaging challenge. Then, a little further down the line, when you are more confident, you can consider trying to get the Edge up and imaging!

[knobby]

On a positive note ... Things like M57 are small and pretty bright !

[PhotoGav]

Tee hee, Olly - the bottom line is that we all basically seem to agree on the main issues, we are just considering slightly different attributes! I didn't mention mirror flop which is certainly a big issue with a standard SCT but I believe that the Edge HD version have a fairly good mirror lock system?

Yes, the EdgeHD has an excellent mirror lock system, so long as you remember to tighten it up....!

[steppenwolf]

Gavin's story pretty much sums up what you will be up against and I would endorse his last paragraph fully.

[ollypenrice]

Gavin's story pretty much sums up what you will be up against and I would endorse his last paragraph fully.

So would I.

Olly

[knobby]

Yes, the EdgeHD has an excellent mirror lock system, so long as you remember to tighten it up....!

And more importantly ... Loosen off afterwards before using the stock focuser !