Guiding at longer FL

[kirkster501]

..boy is it hard! My first proper session with my RC8 scope tonight at over 1600mm FL. I am getting trailing at 5 mins subs and I've had to drop it right down to two minutes to get pinpoint stars. On my ED80 I can do 10 min subs. I think the FL is almost an exponential increase in difficulty. I am using guidescope and not a OAG.

I need a Focal Reducer for the RC8 to slow it down a bit. I am capturing M57 right now and the subs are pinpoint at 120 secs.

People who guide at 2800mm and longer I salute you!

Steve

[JamesF]

What you need is a Mesu 200

James

[kirkster501]

What you need is a Mesu 200

James

Indeed! And a pay rise to pay for it......

[jgs001]

What are you guiding with Steve? I know the RC6 is shorter (1390mm) than the RC8 but I've not had problems guiding with an st80, for 10 minutes.

[kirkster501]

Hey John, I am guiding with an ST80 and PHD. Wondering if an ST80 cuts it due to being too short FL wrt to the main imaging scope...... But as you say, the RC6 is not that much shorter FL wise.... Could be something else. Need to check everything is bolted down tight. I think it is though. Will post a sub later.

EDIT: meant to say the collimation of the scope is creak on too.

[Merlin66]

Hmmmm

I guide with a beamsplitter, Lodestar, PHD a C11 at f10 on a NEQ6pro mount and can keep a target star on a 25 micron slit gap for at least 10 min at a time......

It is possible.....

[kirkster501]

Yes something was wrong somewhere on my rig last night. I shall have to investigate further.

Thanks, Steve

Sent from my iPhone using Tapatalk

[Bizibilder]

Please don't shout if this is silly - but can you use a Barlow with the ST80 to get a longer focal length guidescope? (Just an idea, probably not a very good one either).

[kirkster501]

Please don't shout if this is silly - but can you use a Barlow with the ST80 to get a longer focal length guidescope? (Just an idea, probably not a very good one either).

I have been considering doing exactly that Roger. I am not sure if it is feasible...

[jgs001]

It depends on what crop factor camera you're using, but if you convert the focal lengths to 35 mm equiv, using a qhy5v (about the same as a webcam) on an st80 gives you am eq focal length of 3400mm, and the rc6 with a 1.6x aps-c slr is 2224mm. I know that's being very simplistic and there are a lot of other factors involved, but it's enough to help me get t head aroun it . The rc8 will be about 2560mm eq, therefore shorter than the eq guide scope length. I think a Barlow on the st80 would possibly reduce the fov to far and give a much dinner target forthright guidecam. If the guidecam is sensitive enough that may not be a problem of course.

[Jessun]

I wouldn't think the ST is to blame, and there shouldn't be a need to barlow it at all. (Less stars to chose from, and PHD does sub pixel anyway).

Good luck with finding the fault. Flexure perhaps?

/Jesper

[kirkster501]

Good luck with finding the fault. Flexure perhaps?

All part of the fun and the learning process! You learn most when things dont work, not when they do

I have a Celestron guide scope as well which has a longer FL than the ST80. It has a hard mounting connection too, none of this rings business. I will try this as well. I am convinced there is no flexure in my system - althogh how many times have flk said that on these boards! ??

Pictured below (end of a night in Jan with lots of dew) is my rig with the Celestrion guidescope before I swapped back to the ST80. Looks like I shall be swapping back to the Celestron guider again (was hoping to sell it to contribute funds towards a 460EX). Neh bother, getting the guiding right is the number 1 thing here

[jackrussell0232]

I know I'm a bit behind the curve with this bit of advice, but looking at the photo above, I've no idea how you managed to avoid flex with that setup. The USB cable hanging out of the guidecam should really be secured to something near the scope. If you left a cable dangling off the back of the guidecam attached to an ST80 focuser, it will almost certainly create flex in that part of the rig.

I know you're probably taking the egg out of you mouth as you read this, but I'm constantly stunned by the number of imagers - some of whom occasionally get really good shots - who seem to think that flex only applies to scope mountings. I've seen it in focusers, extensions, barlows and even in a QHY5 PCB, which was loose inside the camera housing.

Doubt this will help, but wanted to try at least.

Jack

[kirkster501]

.....If you left a cable dangling off the back of the guidecam attached to an ST80 focuser, it will almost certainly create flex in that part of the rig.

Great piece of advice - many thanks! Yes, that could cause some issues, never thought of that. I will place a cable tie at a strategic place and ensure the cable is routed through it to take the weight off the USB connector next time.

Thanks for your commentary, gratefully received

[sloz1664]

Hi Steve,

Only just noticed this thread. It doesn't matter a jot about differing focal lengths when guiding with digital cameras.

The "like for like" focal length criteria was borne from old film cameras.

I have a Tal 200K 1700mm focal length I have adapted a binocular lens as a makeshift focal reducer

It's now imaging at f5.2 in lieu of it's original f8.5. That makes a hell of a difference regards tracking and more importantly imaging time.

With the SXV H9 (SX equivalent of the Atik 314L) I can capture all the smaller DSO's nicely. I use the skywatcher 9x50 Guide scope with either

the SXV guider or if I use the QHY8Pro I guide with the QHY5L II. Lots of pinsharp guide stars, with no problems at all.

+1 For securing all your cables.

cheers

Steve

[r3i]

Hi Steve,

I've also just come across this thread. I'd heartily recommend that you get an OAG for imaging with this scope in order to reduce the potential for flexure. Also I've recently found that when using an OAG the guiding performance with PHD significantly improves when I switch the Dec guiding from Auto to the appropriate North or South option.

If you want to make the scope faster, I can recommend the Astro Physics CCDT67 focal reducer as it works well with the 8" RC.

[Zakalwe]

I know I'm a bit behind the curve with this bit of advice, but looking at the photo above, I've no idea how you managed to avoid flex with that setup. The USB cable hanging out of the guidecam should really be secured to something near the scope. If you left a cable dangling off the back of the guidecam attached to an ST80 focuser, it will almost certainly create flex in that part of the rig.

I know you're probably taking the egg out of you mouth as you read this, but I'm constantly stunned by the number of imagers - some of whom occasionally get really good shots - who seem to think that flex only applies to scope mountings. I've seen it in focusers, extensions, barlows and even in a QHY5 PCB, which was loose inside the camera housing.

Doubt this will help, but wanted to try at least.

Jack

I'd echo all of that. The focuser on those ST80s isn't the best thing going. Perhaps it'd be worth junking the guidescope and moving to an OAG? It'd take a far bit of weight out of the imaging train and remove a heck of a lot of possible flexure sources?

I've moved to an OAG, and there was an initial bit of faffing about getting spacing and orientation correct. The first time I used it though I was running 15 minute subs with no problems.

[sloz1664]

I am still trying to get my TS OAG to focus on my Tal 200K. I've tried all manner of adaptors to reduce the "in travel" so I can

get the correct spacing between the Fr, the ccd and still achieve focus with the guide cam on the oag.Last resort

modify the oag. :Envy:

cheers

Steve

[kirkster501]

Thank you guys. Yes, OAG is something I am considering. It's a jump I am going to make when I get into CCD I think. I'll see. It will self-fund itself probably because I can then sell my two very nice guidescopes. I have been a bit put of though with OAG by the faffing with finding stars and all the spacers etc required and as such I have given it a wide birth. Clearly this needs to be reconsidered.... I will see how I get on with the CCDT67.

R3i - I have just got my CCDT67 to slow the RC8 down a bit. Great minds think alike

[r3i]

R3i - I have just got my CCDT67 to slow the RC8 down a bit. Great minds think alike

Great Steve - just to be pedantic with the terminology, a focal reducer speeds up a scope , i.e. in this case you'll be going from F8 to around F5.4. As well making guiding easier, the CCDT67 will give you more options in getting larger targets into the FOV.

[kirkster501]

Great Steve - just to be pedantic with the terminology, a focal reducer speeds up a scope , i.e. in this case you'll be going from F8 to around F5.4. As well making guiding easier, the CCDT67 will give you more options in getting larger targets into the FOV.

Yes indeed Mike - I realized that after the edit window had closed I thought not to put a comment on correcting myself and drawing attention to my numptyness!

[ollypenrice]

I'm constantly banging on about FL and not weight as being the key thing in choosing a mount. In the wealthier parts of the imaging world everybody knows this already, of course.

To guide on budget mounts at long FLs is going to be difficult but propbably possible. Polar alignment. Balance. Off axis guider. Cable routes. Guiding parameters. (You can't just ask PHD to auto calibrate. It is more complex than that.) People like Tim and others have guided at very long FLs with EQ sixes. I'll say this though; despite their impressive and genuine success they tend to head for 10 Microns, Mesu Mounts etc!!

Olly

[Mark72]

I was guiding RC 10'' (2m) with a finder. But I used Lacerta MGEN. I feel it can guide everything and anything as long as it has a guide star.

Mark

[astrosathya]

Hi Kirk,

If you are serious about imaging, then it pays dividends to drift align before you start guiding. This eases the job on the guider and it gives longer subs as a result of that. The new firmware release by sky-watcher has a polar alignment routine that works very well, specially for me because i cannot see the pole star from my rooftop. 20 minutes spent on drift alignment is well worth it.

Regards,