Guiding with an 8" SCT

[Galaxyfaraway]

I wanted to check whether it would work to guide with my Celestron 8" EdgeHD (I would be using the WO GT81 as my imaging telescope). Or would it be better to purchase a smaller guiding camera specifically for that?

Guiding would be done with Lodestar Xpress.

Thanks, gfa

[wuthton]

I think I'm confirming your suspicions but your Celestron is everything you don't want in a guidescope, long FL, big and heavy.

Get a cheap ST80, the exact opposite.

[nmoushon]

You can guide with it but it will be very difficult. For the FL of your 81mm you could even get away with modding a 9x50 to fit your guide cam and use that. Thats what I did and could easily get 10min subs with my 80mm + reducer. Probably could've pushed it to 15min if I ever bothered to do more than a 2min PA. Or can get another 60-80mm guide scope that would work fine too. Though I would suggest having them mounted in tandem and not with rings. Rings tend to have flexture issues, especially if you are doing very long exposures. If you are only wanting to plan 5-10min exposures max then rings are fine. 

[Galaxyfaraway]

"you could even get away with modding a 9x50 to fit your guide cam and use that. "

Thanks. Any idea on how to do that? (and how difficult it would be?) I presume I need to fit something on the pack in order to screw on the Lodestar camera?

If difficult, is one of these any good: https://www.widescreen-centre.co.uk/Products/William_Optics_50mm_guiding_.html

[salas]

I have a GT81 and use an ST80 for guiding, advice I've heard / read is that its probably best to use a scope with a similar field of view as you imaging scope for guiding.

[Davey-T]

Well I have an OAG and Loadstar on my 10"SCT and it guides fine when imaging with a piggy backed 100mm refractor.

Dave

This may be of use

http://www.modernastronomy.com/shop/accessories/adapters/qhy5-ii-series-lodestar-adapter-for-celestron-1mm-straight-through-finders/

[salas]

Am sure guiding can be achieved using a large scope even with longer FL (haven't tried personally) but I guess the real question is do you want to be rigging up a mahoosive scope for guiding when a budget 80mm or smaller scope will do the job. Judging by the mount it could probably handle it though.

[nmoushon]

"you could even get away with modding a 9x50 to fit your guide cam and use that. "

Thanks. Any idea on how to do that? (and how difficult it would be?) I presume I need to fit something on the pack in order to screw on the Lodestar camera?

If difficult, is one of these any good: https://www.widescreen-centre.co.uk/Products/William_Optics_50mm_guiding_.html

Here is a link to mine : http://stargazerslounge.com/topic/175734-diy-9x50-finderguider/

The one you linked hsould work just fine as well. You can usually find them on secondhand sites as well that people have modded themselves. 

[Davey-T]

Am sure guiding can be achieved using a large scope even with longer FL (haven't tried personally) but I guess the real question is do you want to be rigging up a mahoosive scope for guiding when a budget 80mm or smaller scope will do the job. Judging by the mount it could probably handle it though.

Wasn't suggesting it as an ideal but it works for me as I'm permanently setup in the obs'y.

Dave

[salas]

No Worries Dave I know you weren't - ahhh a permanent obsy.....one day....one day. Life would be so much easier.  

[Galaxyfaraway]

At the (ever present) danger of appearing daft, what is the actual difficulty in guiding with a long scope? (such as a SCT 8"). Isn't it a matter of finding a star (ANY star) and go with it, so to speak? What is the major difficulty? (I haven't tried yet as I just got the lodestar the other day).

I do have a side by side dovetail and it's not a problem mounting both of them next to each other.

I am inclined getting a smaller scope specifically for guiding, just need a good reason (and excuse...) and to understand the potential pitfalls.

Thanks for great replies. gfa

[Owmuchonomy]

Ok, well lets take the advice of Craig Stark, the original designer of PhD guiding.  "A ratio of 4:1 [in arc seconds per pixel] guiding:imaging should not be exceeded."  As an example, my guiding rig which comprises a 328mm FL guider/finder and a Lodestar guides at 5.15 arc seconds per pixel.  My 4" frac images at 1.24 arc seconds per pixel.  That's coincidentally a ratio close to 4:1.  So if you calculate the resolutions of your set ups then you'll get a feel for how successful your guiding is going to be.

[Galaxyfaraway]

Is there a formula or a link that explains how to calculate the arc seconds per pixels of my WO GT81mm refractor and to work out the ideal ratio?

The scope is f4.72 (with reducer) with a focal length of 382mm and I am really not certain how to work out its arc seconds per pixel.

Unfortunately an offaxis guider would not work with my Celestron 8" and modding a 9x50 finder scope is a little too uncertain for me that it would work well enough.

[Scorpius]

Is there a formula or a link that explains how to calculate the arc seconds per pixels of my WO GT81mm refractor and to work out the ideal ratio?

The scope is f4.72 (with reducer) with a focal length of 382mm and I am really not certain how to work out its arc seconds per pixel.

Unfortunately an offaxis guider would not work with my Celestron 8" and modding a 9x50 finder scope is a little too uncertain for me that it would work well enough.

Camera’s photosite dimensions in microns / focal length of OTA in mm * 206.3. Ideal ratio depends on what you're imaging -  planets or DSO's?

[Galaxyfaraway]

"Camera’s photosite dimensions in microns / focal length of OTA in mm * 206.3. Ideal ratio depends on what you're imaging -  planets or DSO's?"

DSOs (or trying to).

My Nikon DSLR d750 sensor size is 35mm (35.9 x 24). Pixel size is 5.97u. 

So 5.97 / 382mm * 206.3 = 3.22

(Where does the number 206.3 come from?)

Lodestar pixel size is given as 8.4um x 8.2um.

What do I do with this information? 

[MartinB]

You can use your 8" SCT to guide with that set up, in fact it is a fairly common permanent set up, long focal length and short focal length scopes which gives you good imaging options. I have used my 10" SCT with OAG a few times focally reduced to around F7. You could reduce further if you wanted using a Meade 3.3 reducer.

The three main problems are i)the narrow field of view which can make it a right royal pain finding a decent guidestar, this is where hefty focal reduction can help. ii) mirror flop, you need to have the SCT mirror firmly in place especially if you go anywhere near the meridian. iii) last but no least is flex. The SCT is quite a weighty tube so everything needs to be pretty solid. The impact of flex will be the same regardless of how much you focally reduce

Give it a try!

[Scorpius]

"Camera’s photosite dimensions in microns / focal length of OTA in mm * 206.3. Ideal ratio depends on what you're imaging - planets or DSO's?"

DSOs (or trying to).

My Nikon DSLR d750 sensor size is 35mm (35.9 x 24). Pixel size is 5.97u.

So 5.97 / 382mm * 206.3 = 3.22

(Where does the number 206.3 come from?)

Lodestar pixel size is given as 8.4um x 8.2um.

What do I do with this information?

Don’t want to get in over my head so will defer to the more experienced members to explain the intricacies of calculating optimal resolution for imaging. Also forgot to mention that sky conditions are a factor as well and I believe there are even ways to calculate the maximum resolution current sky conditions will support but this is beyond my current understanding.

But in general, I believe a higher sampling rate (lower number) is preferable for planetary imaging while a lower rate (higher number) is better for DSO’s. But again, it depends on what current conditions will support.

This chart from Sky & Telescope is probably somewhat dated, but seems to indicate 0.7 or less pixels per arc second is best for planets and 1.5 - 2.5 is preferable for DSO’s which means you're in the ballpark for DSO's at 3.22 per pixel. I’d like to learn more about this subject so hoping someone with a deeper understanding will enlighten us with the details.

As far as guide cameras, not sure if it’s much of an issue but this may not be the case. Another thing that occurs to me is – unless you have a large assortment of OTA’s, cameras, reducers & barlows to choose from – you more or less have to “take what you can get” based on the equipment at your disposal.

Regarding the multiplier 206.3 - who knows but someone much smarter than me apparently figured out that's the correct number to use...

[ollypenrice]

In the early stages of my astrophotgraphic life I used a 10 inch SCT to guide a piggybacked WO ZS66. It worked but, as Martin said, finding a guide star wasn't easy and such were the lurchings of the autoguider at that FL that learning how to interpret them was difficult. When I started guiding with a short FL guidescope everything became more coherent and I could begin to learn about what was happening. And it can never be a good idea to guide through anything with a mirror, or to guide a reflector other than through its mirror, because mirrors move. You might be lucky, and folks will jump up and say, 'I do it' and they are not wrong - but they are probably lucky.

Sure, give it a try, but go into it open minded and be prepared to say 'it does' or 'it doesn't' work for me. Nobody on here can tell you whether or not it will - me included, of course.

Olly

[Psychobilly]

I used to use my CPC800 reduced to f6.3 to guide when imaging through a piggybacked Megrez 72... 

Mirror flop could  be a problem on the CPC  but IIRC  the Edge has mirror locks so that should help...

In the end I added a 500/f8 T mount telephoto lens as a guide scope and was able to image through both the Megrez72 and CPC800 ant the same time to give widefield and close up images of the target at the same time...

Peter

[Galaxyfaraway]

Ok, I just tried guiding through the EdgehD 8" and....I am no certain what happened but all exposures had some star trailing. I tried 2 minutes guided and unguided and the unguided exposures were much cleaner. Any ideas what went wrong?

I used the Lodestar for the first time so perhaps set it up wrongly? I used PHD guiding software (I haven't installed Eqmod yet so under "mount" I selected on-camera (is this correct?).

Any looping exposures under 10 secs produced too much noise.

I will get a dedicated guiding camera next week, just want to understand what I am doing wrong.

[Scorpius]

Regarding the multiplier 206.3 - who knows but someone much smarter than me apparently figured out that's the correct number to use...

Maybe it was Olly - how bout it 'ole wise one. What's the low-down on image resolution? 

[MartinB]

The optimal sampling rate is a theoretical starting point.  Next thing to think about is seeing conditions.  Anyone who has ever looked at a guiding graph from one night to the next knows that the seeing is going to be a factor in resolution.  So if you have a scope of f/l 1000mm and a pixel size of 6microns your sampling rate is 1.24 arc secs per pixel.  You will need  pristine seeing to achieve the potential resolution of your system.  If you have a small refractor of f/l 500mm and a large pixelled camera of 9microns you are at 3.7 arc secs per pixel are unlikely to be seeing limited.  If you are at 2000mm f/l then bin!  

All this is purely theoretical because in reality most of us look at our images on a computer screen and rarely actually view our images at their native resolution.  How many images on SGL are presented at 100%?  The venerable KAF 11000 has a pixel size of 9 microns and is often used with small refractors.  The under sampling is only an issue if they are presented in the form of a very large poster.  

If you are working a long focal lengths and oversampled you need to work out when it's best to bin since this can give you advantages of reduced read noise.

IMHO people get far too hung up about sampling rates.

[ollypenrice]

Ok, I just tried guiding through the EdgehD 8" and....I am no certain what happened but all exposures had some star trailing. I tried 2 minutes guided and unguided and the unguided exposures were much cleaner. Any ideas what went wrong?

I used the Lodestar for the first time so perhaps set it up wrongly? I used PHD guiding software (I haven't installed Eqmod yet so under "mount" I selected on-camera (is this correct?).

Any looping exposures under 10 secs produced too much noise.

I will get a dedicated guiding camera next week, just want to understand what I am doing wrong.

I'm lost. (I'm easily lost!) The Lodestar is a dedicated guide camera. I don't know how to guide with EQmod, though.

However a Lodestar should have no trouble at all working in unbinned subs of just a couple of seconds. I used one in an OAG at 2.4 metres FL unbinned at it was fine with 3 seconds or so.

On sampling rate for imaging I agree with Martin that folks do get too hung up on it. I've imaged recently at 0.66 arcsecs PP, 1.8 and 3.5. I think 0.66 was slower than necessary because I think we'd have done as well at about 1.0. But they all work. Even at 3.5 with the dual Tak rig Tom and I try to make every image presentable at 100%. I downsize them a little for the web - about 10% when I link to a full size, but I think they look OK. (Well, we never posted the 33 panel Orion at full size... )

This should let you see 3.5"PP at close to full size. https://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-SgKgDDN/0/O/Kemble%27s%20Cascade.jpg

Olly

[Galaxyfaraway]

"However a Lodestar should have no trouble at all working in unbinned subs of just a couple of seconds. I used one in an OAG at 2.4 metres FL unbinned at it was fine with 3 seconds or so."

Well, stupid me is all I can say! I was latching onto hot pixels, thinking those were stars! 

I eventually found a star (it was so huge, it was taking up all my screen, till I got it in focus).

It seemed to guide ok after that. Posting a photo of the graph: does it look ok to you or is it crappy guiding? I did manage 10 minutes exposures and the stars look ok to me.

The reason I got trails in my stars previously was because I was taking exposures while PHD was still calibrating...Reminds me of that time when I pressed gas in full confidence that my car would go backwards. (It turned out it was in first gear...)