Long Exposure - all the gear and no idea!

[Johns22]

I've recently purchased a guidecam (ASI 120mm) from the helpful guys at FLO in the hope of increasing my exposure time. My first night of trying to guide was a nightmare last Sunday night with calibration taking ages and then the correction graph looked like a portrait of the Alps...

Monday night I discovered that you really need to nail your polar alignment for guiding to be effective, so I invested in Sharpcap Pro. Wednesday night I got all of this working, SharpCap polar align said it was Excellent, PHD2 caliberated quicker (had an advisory on DEC backlash) and showed far less adjustments when guiding, I set focus with a bahtinov mask and then run my first few 60 sec exposures to find significant star trails. I ended up doing 40 sec exposures which were the absolute maximum before trailing became an issue. Rather disappointed having shelled out a few hundred quid!

I'm running a Skywatcher 200P on an EQ5 Goto Mount, guided with a ZWO ASI 120mm fitted to the standard Skywatcher 50mm finderscope. 

Am I pushing the mount too far with a DSLR and guidecam fitted?
Could the guidescope need realigning with the OTA?
Have I missed something really fundamental?

I plan to image again tonight and am keen to understand the errors of my ways!

Any help very much appreciated.

[jif001]

The EQ5 has a payload limit of about 9kg for visual or 6.5 kg for imaging (some would say 50% of the visual payload - so 4.5kg for imaging). That ‘scope must be about 9kg before you add anything to it, so as you have suggested yourself, payload may be your issue. 

[almcl]

I used an EQ5 with a SW200p and Canon DSLR for a while, and although results weren't always brilliant and I agree it's not an ideal combination, it could usually manage more than 40 seconds.

A good PA helps, a good balance (slightly east heavy) is also useful.  Getting the right figures into PHD2 is essential, what figure did you enter for the focal length of you guide scope? 

Can you post a PHD2 guide log file plus details of your imaging camera?

Edited March 27, 2020 by almcl

[Johns22]

Thanks, I will try the east heavy balance tonight. I have entered 190mm for the focal length of the guidescope (following advice on others using the standard 50mm finder on here with the camera). If I get chance before setting up tonight, I'll post the guide log file. The imaging camera is a Canon 600D unmodified.

Cheers

John.

[Anthonyexmouth]

2 minutes ago, Johns22 said:

Thanks, I will try the east heavy balance tonight. I have entered 190mm for the focal length of the guidescope (following advice on others using the standard 50mm finder on here with the camera). If I get chance before setting up tonight, I'll post the guide log file. The imaging camera is a Canon 600D unmodified.

Cheers

John.

Wind or even a light breeze may be your problem too rather than a technical issue. That thing is gonna be a sail on that mount. 

[almcl]

190 sounds about right.

Without a coma corrector you're imaging at  0.89 "/px so, by the time you have debayered this will be  equivalent to 0.88"/px, so you'll ultimately want to guide at about 1 arc sec.  Won't be possible every night (tonight is looking a bit windy here) but some nights that should be doable according to my logs from 4 years ago.

[almcl]

whoops! double post. Sorry!

Edited March 27, 2020 by almcl

[Tommohawk]

Also bear in mind that finderscope mounts can be pretty bendy - any kind of drag on the cable and you'll be all over the place. Maybe at least tape the cable to the scope 

[discardedastro]

5 hours ago, Tommohawk said:

Also bear in mind that finderscope mounts can be pretty bendy - any kind of drag on the cable and you'll be all over the place. Maybe at least tape the cable to the scope 

Definitely - OAG is preferable for guiding a Newtonian (as it lets you account for mirror movement) but is a lot more fiddly, not always possible with backfocus available, and requires a sensitive guide camera. Flexure in guide rings can be a huge issue - I went to the extreme of having a friend CNC-mill a friction fit set of rings for mine, which more or less fixed it:

But you can still be at the mercy of flex elsewhere - e.g. the helical focuser on the Primaluce Lab scope will move laterally if you let it, so cable anchoring is a good move regardless. I use velcro and a bit of gaffer tape.

I'd echo the wind comment above, too - I see huge difference in guiding between calm and windy nights. Calm nights I can achieve <0.8" RMS, windy nights can push that up to <2" RMS (and imaging at 0.5"/px, that can be a bit problematic)

Good PA helps but to be honest I can get <0.8" RMS with just the polar drift align in PHD2. There's no perfect alignment between guidescope and telescope (not that I can adjust it!). This is also just using as 120MC (in mono, so much like a MM). Rigidity and stiffness is king, basically.

Edited March 29, 2020 by discardedastro

[Johns22]

Thank you guys, I have little option but to persevere with it and I will be trying the tips outlined above. I have been letting the cables trail from the scope to the laptop (the guide cam 2m cable makes it difficult) but I have recently acquired a small powered USB hub which I aim to mount on the scope and then have some cable management. 

Appreciate all the comments, thanks.

 

[Pompey Monkey]

On 29/03/2020 at 18:03, discardedastro said:

You can still be at the mercy of flex elsewhere - e.g. the helical focuser on the Primaluce Lab scope will move laterally if you let it, so cable anchoring is a good move regardless. I use velcro and a bit of gaffer tape.

I

This happens with the generic guide scopes too. The fix i used was to add two more locking screws to secure the helical part of the focuser. The holes were already there, drilled and tapped!

Just takes a bit of nerve to tweak them up tight enough without stripping the threads.....

[malc-c]

To be honest IMO you are pushing the equipment beyond its limits.  I've been off the forum for a while and a few years ago the recommended mount for use with a 200P when imaging was an HEQ5.  I started with an EQ5 and soon realised that the mount wasn't geared for imaging.  The HEQ5 has finer stepping stepper motors, and higher load capability making it the more suited platform, especially when using DSLRs as the main camera.

However, as you have already purchased the equipment there isn't much more you can do other than change the way you guid the scope.  Using a finder as a guide scope has the advantage of being lightweight,  but being a short focal length not a lot of movement would be detected, hence why your PHD traces were good but the resulting image had trails.  Longer focal lengths will improve guide accuracy, to a point in that each guide step will only be as good as the resolution of the mount, so even if you opted for an off axis guider it may well be that the stars appear more rounded.  Having said that the results may be perfectly acceptable, but most OAG, especially the narrow ones for Canon cameras to ensure there is enough travel on the focuser to focus the image on the sensor and guide camera are quite expensive, between £120 and £180.

Also, an element of luck comes into play... sometimes the image you are capturing is in a position where balance of the scope works in your favor and it all works...

[Pompey Monkey]

29 minutes ago, malc-c said:

Using a finder as a guide scope has the advantage of being lightweight,  but being a short focal length not a lot of movement would be detected, hence why your PHD traces were good but the resulting image had trails. 

I don't agree with this diagnosis: Although the SW finder has a fairly short FL (about 180 mm), This, coupled with the 3.75 um pixels in the ASI 120m, should guide the 200P adequately. Smooth guide graphs with star trails on the main image is a classic indicator of differential flexure.

I do agree, however, that a 200P on an EQ5 is pushing it for reliable guiding.

[malc-c]

Maybe I worded that wrong.   I was suggesting that guiding through a longer focal length would give better precision.