Advice on Guidescope upgrade

Using SharpCap's Polar Alignment feature
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JLP
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Advice on Guidescope upgrade

Post by JLP » Tue Nov 28, 2017 9:40 pm

Just been using Sharpcap for setting up polar alignment (PA), what a gem SC is thanks Robin, it’s a game changer for me. I am in my second year of trying astrophotography imaging using a DSLR and lenses up to 500mm on a Star Adventurer (SA). It’s not the best mount for this stuff I know but it is cheap and portable and gets you into producing ok images of DSOs. It also teaches you about the importance of rigid mountings and balancing. PA until recently was a hit and miss for me relying on setting the reticule in the SA polar finder with a right angle x2 magnifier attached and when imaging I had no idea if PA was being maintained. I have tried PHD drift align but its soooo long winded.

My current setup uses a StarTravel 80T (ST), 80mm aperture fl 400mm giving an f5 ratio and a Altair GPCAMAR0130C. With this sensor it gives me an angle of view 30’ x 41’ somewhat outside the recommended range, but it seems to work and I can keep the PA to under 30 arc sec showing “Excellent” most of the time. However I get a fair number of can’t resolves, but don’t know if that’s down to the angle of view or the poor view of stars or focusing. I am using 4sec exposures with the gain at 1000 and can only see a few stars. I am also using dark frame subtraction. Sharpcap seems to see more stars when plate resolving. What’s really helpful to me is making small adjustments to PA during the gaps between exposures during an imaging session. With my 300mm f4 lens I have been able to get 4-5min exposures.

The ST 80T is a bit too big for my setup so I was thinking of getting the QHY Miniguidescope a 30mm aperture fl 130mm giving f4.3 ratio. With my sensor this would give an angle of view 1o 35’ x 2o 6’. Is anyone using this combination; do you need an extension piece and how well does it focus and what sort of exposure time can you use? Would I be better off getting the 0.5 focal reducer or another scope circa 200mm?
Portability is important to me as my back garden is in a dip and I am surround by skyline obstructions, bushes and trees roof tops (including my own) and insecurity lighting by some of my neighbours. I try and get away a bit and try and pick places with low light pollution and times with low moonlight. So portability is what I am after so that I can put it in the car without it taking over.

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Re: Advice on Guidescope upgrade

Post by admin » Tue Nov 28, 2017 9:52 pm

Hi,

good to hear that the polar alignment is working well for you, even if you are pushing the limits a little ;)

I've certainly had reports of people using the mini-guidescope with 30mm aperture with success with the polar alignment. It looks like the QHY (and clones) have a C/CS thread which will match the thread on your Altair camera and should bring you to focus with at the very most the addition of a 5mm C thread spacer (you may have this already as I think some of the Altair cameras came with this). C has a backfocus of 17.5mm and CS 12.5mm, so the scope is probably set up with a 17.5mm back focus to work with a C mount camera and it will also work with a CS mount camera with the inclusion of the 5mm spacer.

If you are getting unsolved frames intermittently then it is probably a combination of poor star detection and the small field of view. You can try tinkering with some of the star detection settings in the first stage of PA to see if you can find a combination that works better. If 1000 is the max gain on the GPCAM (I think it is - newer models like the GP224 have more gain range) then it might be worth trying 2x digital gain to see if that helps - note that doing this does not brighten the image but does affect the star detection.

cheers,

Robin

JLP
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Re: Advice on Guidescope upgrade

Post by JLP » Wed Nov 29, 2017 11:24 am

Thank for the helpful reply Robin.

I assume the 2 x digital gain does not affect the angle of view.

Is there a way to log or trend the PA once you have finished making adjustments?

John

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Re: Advice on Guidescope upgrade

Post by admin » Wed Nov 29, 2017 6:52 pm

Hi John,

the digital gain doesn't affect the FOV.

Not sure what you mean by log the PA - you could re-run the alignment to check you get the same result, but the only way to track it would be to keep the scope pointing at the pole, which rather stops you from doing anything useful with it! Sureyly once you are aligned you are aligned - unless you are on soft ground and the tripod starts to sink!

cheers

Robin

JLP
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Re: Advice on Guidescope upgrade

Post by JLP » Wed Nov 29, 2017 10:01 pm

Thanks again.
By log I mean a trend graph of dec and alt verses time much as PHD does.
I have noticed that on my cheap mount the alignment does drift and I think that is primary because I am operating at the limits of the mount. I am sure the wedge flexes depending on the position of the ra axis Its also difficult to balance my DSLR with the ST80T guide scope and the balance changes depending on the position of rotation on the mount. I keep the guide scope pointing at Polaris and my imaging DSLR pointing at M31 or M33 currently.

I am leaving the Sharpcap polar window live when doing an imaging session. I could switch to PHD but I really prefer the way SC shows PA and when my PA drifts a small correction puts it right. I have now ordered the 30mm Miniguidescope as that will significantly reduce the weight on the little Star Adventurer and I am hoping to mount my 2.5kg worth of DSLR and lenses towards the centre of the SA. One day I will upgrade my mount to a more rigid goto mount its more likely sooner now that I have an excellent way to PA thanks to SC.

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Re: Advice on Guidescope upgrade

Post by admin » Thu Nov 30, 2017 7:01 am

To be honest, I'm still rather confused here - Once you start imaging, the guide scope will be pointing to some other random part of the sky and the pattern of stars will mean nothing to SharpCap's polar alignment code. I'm afraid I'm not familiar with the PHD functionality you are describing either, unless you are using PHD to monitor the motion of the mount without guiding?

cheers,

Robin

JLP
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Re: Advice on Guidescope upgrade

Post by JLP » Thu Nov 30, 2017 3:04 pm

When I am imaging my guide camera is still pointing towards Polaris now that I am using SharpCap. Maybe I am doing something wrong perhaps a picture will help. My imaging camera the DSLR is pointing to M33 in the picture.
PB300031.jpg
PB300031.jpg (988.44 KiB) Viewed 60 times

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Re: Advice on Guidescope upgrade

Post by admin » Thu Nov 30, 2017 6:39 pm

Well, I've never seen a setup quite like that before!

The normal approach would be to set up the DSLR parallel to the main scope, then to slew the whole mount to whatever taget you are interested in. That way you can also use PHD to guide (you can't guide in the config you are using because the stars near the pole will basically not move at all!)

Any reason you have gone for that setup? Am I missing something?

Robin

JLP
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Re: Advice on Guidescope upgrade

Post by JLP » Thu Nov 30, 2017 10:41 pm

I could be wrong with the way i set things up I am still on a steep learning curve. The reason I set up this way is because of the flexure in the mount. If I were to set up the DSLR parallel with the scope and slew round having got PA the whole PA will be out because of the shift in weight even if balanced. By setting my DSLR to the DSO (M33) when in the final position of SharpCaps PA I can be more confident that PA is maintained. I have real problems finding the DSO's with the various heads I have used, the tightening and undoing of locking nuts on the heads often ends up upsetting the balance of the mount and hence the PA. Because I am not piggybacking my imaging OTA with my guide scope I cannot see the need to set it up pointing at Polaris to start with. When I want to guide I rotate my guide scope round to the something on the celestial equator.
The result is something like this. These are 20 x 2 min exposures (Olympus EM1 MKII 300mm f4) stacked in DSS and processed in Star tools.
M31 2017-11-25a.jpg
M31 2017-11-25a.jpg (877.92 KiB) Viewed 51 times

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Re: Advice on Guidescope upgrade

Post by admin » Sat Dec 02, 2017 9:48 am

Hi,

Ok, I see - it obviously works and helps with the balance, but as you say it must make finding the target tricky as you have to do everything with a manual camera mount! It's not the normal way of doing things though!

cheers,

Robin

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