Successfully Guiding an 8" Celestron SCT - Any ideas?

[red dwalf]

Here's a pic - hardly any processing done yet (as you can see!), but the star shape is evident:

M81-01-Crop-Levels.png

Here's a 100% crop:

M81-01-Crop-Levels-cropped.png

these look like exactly the same as the star shapes as in my M51 which i took with my 8" sct with a 6.3 reducer fitted and a canon 1100d, i thoiught it might be a collimation problem as i havent done a very good star test as yet. PHD graph was almost flat when guiding for 5 minutes, one of the best graphs i`ve had, but still the funny star shapes.

i was guiding with a atik titan and a st80 

[PhotoGav]

Oh OK. Well good luck when you get the chance to give it all a try and I very much look forward to hearing how you get on.

The results of my experiments yesterday evening were fairly conclusive. I managed a maximum of 120s unguided and that was increased to 420s guided. The next guided sub length I tried was 600s and that wasn't successful. I am about to order the focal reducer, so will see how that improves things.

I failed to get the 2x Barlow to work with the guidecam on the 9x50 finderscope. I just couldn't find focus at all. I must have been doing something wrong, but I don't know what... I simply put the QHY5-II into the barlow (an old Meade 2x barlow that I had in the box of bits) and put that into the finderscope adapter. I moved the guidecam as far forward as it would go and then slowly moved it backwards, but I never got close to seeing anything vaguely star like. I also tried moving the front objective focus, but that gave no result either.

When you add a barlow does the focus point move nearer to the objective lens or further away?

Hopefully I will solve all this soon and can get back to actually making some images!

[PhotoGav]

these look like exactly the same as the star shapes as in my M51 which i took with my 8" sct with a 6.3 reducer fitted and a canon 1100d, i thoiught it might be a collimation problem as i havent done a very good star test as yet. PHD graph was almost flat when guiding for 5 minutes, one of the best graphs i`ve had, but still the funny star shapes.

i was guiding with a atik titan and a st80 

Mmm, interesting and rather worrying too! I am about to get the focal reducer, but what you say suggests that this won't make any difference.... Argh!

Would poor collimation give that star shape? I have checked collimation by defocusing a bright star and to my very untrained eye it seemed that the black blob was central in the doughnut of light, so I left the front mirror well alone.

[red dwalf]

what you really need to do is get on a bright star and really barlow the lens , to maybe even 4x, then try a star test,  webcams are good for this as you can see the image on the pc screen,

[Owmuchonomy]

I'm following this with interest as I am in a similar position.  I can only quote my experience in the field not provide expert analysis.  So here are some observations.

I have just set up an Altair Astro finder and SLX guide cam as a guide scope (FL 320mm, F/4.1).  I used it once on my refractor (FL 750mm) and once on my 9.25 XLT SCT (FL 2350mm) this month.  I discussed with the retailer and we both agreed it may be pushing it on the SCT but I agreed to try it.  I use PHD2 to guide.  At my first attempt I imaged M66 with the SCT using 160 second subs.  Stars were fine.  When I pushed it to 300 seconds I got trailing but at the edges so it may be a hint of field curvature.  I have got a 0.63 reducer/flattener but haven't used that with this set up yet.

I don't see how using a Barlow will help in a prime focus guiding situation (no back focus).  Also a 2x would reduce the light by a factor of 4.  I'm happy to try it though as an experiment.  I assume PHD2 can take care of the image transition with a barlow?

All loaded up my guided SCT set up is about 11kg which is a bit much on an HEQ5 Pro.  For imaging I should be keeping it around 9kg so this doesn't help.  This is critical having researched on this forum.  50% of the mount capacity is the limit for good imaging.

Having researched PHD2 then with my slightly longer FL guiding set up I use between 750 and 1200ms for calibration steps.  Somebody mentioned 4000ms which I've not seen quoted before.  That would suggest that the star may not be focussed very well?  (Very slightly off focus is fine).

I'm just waiting for more clear skies and then I will have a play around.

[Christopher Davenport]

Note: 

The barlow is a standard 2x Skywatcher 1.25, but using the extension that came with the cam I get a effective x1.6 increase.

This was calculated through planetary imaging of Jupiter.

Note:

My finder is a 50mm achromatic refractor with 200mm focal length,

With a 1.6x barlow I alter the focal length to 320mm  (F6.4)

The finder has a extremely tight focus with the my QHY5L II (3.75um) in the back of the finder.

To achieve focus without the barlow, the camera is about 1/2 way in.

To achieve focus with the barlow, the camera is all the way out and only the Barlow is in.

A word of warning, finding a guide star with the Barlow in is tricky and frustrating as it has reduced the FStop.

I tend to push the gain right up and only use it when I need precise long exposure guiding with bright stars about.

Otherwise I tend to guide fine without the Barlow on most subjects with both my 750mm ED and my 1500mm Mak.

(But I am still a relative noob with cheap kit and am currently happy with the images I get.)

PS: I focus my camera and my Finderguider on a bright star like  Vega. so that I can use the onion ring to move the scope onto the target.

[Owmuchonomy]

QHY5L-II-x1.6.jpg

Note: 

The barlow is a standard 2x Skywatcher 1.25, but using the extension that came with the cam I get a effective x1.6 increase.

This was calculated through planetary imaging of Jupiter.

F-Guider.jpg

Note:

My finder is a 50mm achromatic refractor with 200mm focal length,

With a 1.6x barlow I alter the focal length to 320mm  (F6.4)

The finder has a extremely tight focus with the my QHY5L II (3.75um) in the back of the finder.

To achieve focus without the barlow, the camera is about 1/2 way in.

To achieve focus with the barlow, the camera is all the way out and only the Barlow is in.

A word of warning, finding a guide star with the Barlow in is tricky and frustrating as it has reduced the FStop.

I tend to push the gain right up and only use it when I need precise long exposure guiding with bright stars about.

Otherwise I tend to guide fine without the Barlow on most subjects with both my 750mm ED and my 1500mm Mak.

(But I am still a relative noob with cheap kit and am currently happy with the images I get.)

PS: I focus my camera and my Finderguider on a bright star like  Vega. so that I can use the onion ring to move the scope onto the target.

All makes sense.  I've got a similar 2x barlow and will have a play around.

[Christopher Davenport]

Here's a pic - hardly any processing done yet (as you can see!), but the star shape is evident:

So in my humble opinion there are a couple of things that can help here. (this is not definite answer and the issue may lie elsewhere.) 

Just looking at these elongations again and they all point in the same direction, so it looks like a alignment issue on a single axis.

This is either due polar misalign, unbalanced scope or a periodic error on this axis. Where all the exposures doing this or only some of them?

If all exposures then I would suggest check your scope balance and spending more time on drift aligning both axis.

(Star near horizon in east or west and one near the meridian.)

I drift align for 2 x longest exposure on each axis with corrections and I use 5x zoom on my live view. (5 x focal length)

The 30min spent on alignment is worth it when you are doing 3hrs of data capture.

If it is only some of the exposures, then I would suggest that you enable dithering in your capture program to take out the slack in the drive and minimise period errors.

With dithering on and over about 15 captures, Cappa sigma stacking is possible which takes out random noise and some movement in the stars.

In processing with layers a star mask can also be used to clip your stars and bring out colour. (all depends on how much this bugs you.)

[PhotoGav]

Mmm, lots to digest there - thank you folks.

All the exposures suffer from the elongated stars, so that would point to a polar alignment issue or balance, or both! I thought that I had balanced things well, but will be more careful with that step next time. I use Alignmaster to improve Polar Alignment, but will try several iterations next time rather than just the one I normally use. Yes, Christopher, you are right, time spent drift aligning would be time well spent. This is something that I haven't had to do so far, but perhaps it's time to start!

As for the Barlow. Thank you for posting those pics and the details - that's wonderfully helpful. I notice that you can screw your barlow into the end of the QHY5. I've just tried with my barlow and I can't do that... I have to slide the camera into the holder bit of the barlow - should work? Interesting about the focus point. I have a feeling that I was doing it completely wrong the other night. I was moving the camera in the barlow tube rather than the barlow & camera in the finder adapter... Will try it properly next time!

I have just taken delivery of the reducer. Hooray!!

So, thank you for your help. I have plenty to try out next time I can set up. This is likely to be in about three weeks time as I have a busy busy time at work over the next couple of weeks. Can't wait to get set up in the garden again and try and make some more progress.

Clear skies all!

[Thalestris24]

Mmm, lots to digest there - thank you folks.

All the exposures suffer from the elongated stars, so that would point to a polar alignment issue or balance, or both! I thought that I had balanced things well, but will be more careful with that step next time. I use Alignmaster to improve Polar Alignment, but will try several iterations next time rather than just the one I normally use. Yes, Christopher, you are right, time spent drift aligning would be time well spent. This is something that I haven't had to do so far, but perhaps it's time to start!

As for the Barlow. Thank you for posting those pics and the details - that's wonderfully helpful. I notice that you can screw your barlow into the end of the QHY5. I've just tried with my barlow and I can't do that... I have to slide the camera into the holder bit of the barlow - should work? Interesting about the focus point. I have a feeling that I was doing it completely wrong the other night. I was moving the camera in the barlow tube rather than the barlow & camera in the finder adapter... Will try it properly next time!

I have just taken delivery of the reducer. Hooray!!

So, thank you for your help. I have plenty to try out next time I can set up. This is likely to be in about three weeks time as I have a busy busy time at work over the next couple of weeks. Can't wait to get set up in the garden again and try and make some more progress.

Clear skies all!

Hi

An ST80 http://www.firstlightoptics.com/startravel/skywatcher-startravel-80-ota.html might be better as a guidescope. It's F5 with 450mm focal length and quite light. You could also try Astrotortilla for precise polar alignment via plate solving. It also lets you precisely slew to a target via Stellarium/Stellarium scope or CdC. AT takes a bit of setting up but can be really useful. However, I think if you had a balance or alignment issue it would show up in the PHD graph. Having said that, any slight perturbations detected via a finder would be magnified by the long focal length of the scope. A good reason for having a compatible guide scope. It's also really worth reading Craig Stark's PHD troubleshooting guide: http://www.cloudynights.com/item.php?item_id=2755

Hth

Louise

[PhotoGav]

Hi Louise,

Thanks for the thoughts. The problem with a larger 'proper' guidescope is weight... I am approaching the limits of the HEQ5 with the 8" SCT as it is. Unfortunately I'm sure that you are right though!

I have just started using AT and it is absolutely superb. I didn't realise that you could do polar alignment with it though.... Please tell me more...

I will read and digest the troubleshooting guide - thanks for the link.

[Thalestris24]

Hi Louise,

Thanks for the thoughts. The problem with a larger 'proper' guidescope is weight... I am approaching the limits of the HEQ5 with the 8" SCT as it is. Unfortunately I'm sure that you are right though!

I have just started using AT and it is absolutely superb. I didn't realise that you could do polar alignment with it though.... Please tell me more...

I will read and digest the troubleshooting guide - thanks for the link.

Hi

Well the ST-80 is pretty light - only about 1.5kg, I'd say. The HEQ5 should take up to about 11kg ok for imaging. The EdgeHD OTA comes in at about 6kg? That's similar to my 150pds which is well within the heq5 imaging limits. So, add a kg for camera and bits, that's 6 + 1.5 + 1 = still only 8.5kgs. I'm assuming you don't have the evostar 80ed mounted at the same time?? If you use the ST-80 for guiding you can get away without the finder and just use AT to put you in exactly the right place coupled with the guide cam output + Liveview/ APT/Byeos. I think it's really your scope's long focal length that's making the guiding challenging though flex is also often a hidden enemy.

I'm sure there must be others here that have a similar setup to you.

The AT polar alignment is in the 'tools' options. I must admit I do find actually adjusting the HEQ5 a pain even with upgraded alt/az bolts.

Louise

[PhotoGav]

Interesting Louise.

Weight wise: OTA + Reducer @ 6.45kg + 60D @ 0.75kg = 7.2kg. St-80 = 1.3kg. Total so far = 8.5kg. This leaves 2.5kg for guide cam and other bits and pieces. So, indeed, it could work! I'll give the finder guider one more try now I have the reducer, but I have a sneaking suspicion that eventually FLO will be taking even more of my hard earned!

I will be investigating AT polar align.

Thanks.

[Tinker1947]

Interesting Louise.

Weight wise: OTA + Reducer @ 6.45kg + 60D @ 0.75kg = 7.2kg. St-80 = 1.3kg. Total so far = 8.5kg. This leaves 2.5kg for guide cam and other bits and pieces. So, indeed, it could work! I'll give the finder guider one more try now I have the reducer, but I have a sneaking suspicion that eventually FLO will be taking even more of my hard earned!

I will be investigating AT polar align.

Thanks.

The celestron OAG is very sturdy, have a read of the manual first.....

http://www.celestron.com/c3/support3/index.php?_m=downloads&_a=viewdownload&downloaditemid=850&nav=0,1

[PhotoGav]

Actually, on researching dovetails, scope etc. I remembered the OAG route and think that while this is probably a lot more learning, it would be the perfect guiding solution... It is, of course, slightly more expensive too!

[PhotoGav]

Tinker1947, I see in your sig that you have the C11, reducer & OAG. Can you use the OAG with the reducer on the scope? I'm presuming so, but just want to check...

Also, is the QHY5-II guide cam going to work with the OAG - is it sensitive enough? Again, hoping so!

Cheers!

[Tinker1947]

Tinker1947, I see in your sig that you have the C11, reducer & OAG. Can you use the OAG with the reducer on the scope? I'm presuming so, but just want to check...

Also, is the QHY5-II guide cam going to work with the OAG - is it sensitive enough? Again, hoping so!

Cheers!

The quick answer is no as it come out of the box, adding up the distances quoted it comes out over the 105mm recommended for the f6.3 FR, i sent the female 48mm adapter away and had it converted to a SCT thread this now works at the required distance, testing has been good, but due to 2 things its been delayed, the first is the clouds and the second i brought a Hotech laser collimator, the expensive one and set about collimating this is nothing like doing a reflector, so  getting the collimation back to where it should be is testing my patience to the limit......But the OAG is a very sturdy bit of kit and if it suits what you want to do spacing wise it will be money well spent...

[PhotoGav]

Thank goodness I asked! I will do my research properly on this to make sure that it can work. Thank you!

Sorry to hear of your collimating woes... I will heed your warning and leave that until I absolutely have to. Mine seems to be fine as is.

Looks like we should get some clear skies over the next period. As full moon approaches and I'm flat out at work, it is guaranteed to be clear!

Sent from my iPhone using Tapatalk

[Tinker1947]

Check your collimating screws mine were would you believe philips headed so to stop the cam out required a hand on the back of the scope and press hard on the front, there now changed to 3mm Knurled knob type, the Hotech does require a solid floor with no vibration, the best way i have seen in other threads is a jig to hold the scope and collimator on a bench so there tied together i think it would still need tweaking when put back on a mount with a out of focus star....as for the AOG it will work i think with CCD/Filter Wheel/ f6.3 FR but the 55mm required by DSLR's is around 10mm to much, another test with the  space at 115mm might show it doesn't matter but thats at the back of a list of other stuff thats needs doing first.....