Skywatcher Star Adventurer - Unguided Focal length & Exposure times

Jasp05 (Aaron) · #1 23-10-2017, 02:20 PM

Hi guys,

I considered buying a star adventurer for quite a while but in the end decided to build a home made tracker. (Not a barn door type, but more like a traditional EQ mount).

I'm curious to know what kind of performance people can get from the star adventurer unguided as a comparison to my homemade tracker.

After my first night out with very little in the way of polar alignment performed. (I can't see the pole, therefore I guess in the direction of south and use an app on my phone to set latitude) was able to get what I thought was pretty decent results for an unguided platform.

tested initially at 18mm on an aps-c sensor and was able to do 2.5 min subs.

When i tried longer focal length of 50mm i noticed alignment was a bit out so made a small adjustment and was then able to go up in focal length again to 135mm (200mm equivalent on crop sensor) and was getting acceptable results at 60-90 sec subs.

With a proper PA and some more careful setup (My latitude adjustment was slipping throughout the night) I'm hoping to get to 2-3 min at 200mm unguided.

Can anyone comment on the performance of the Star Adventure (without guiding) and what focal lengths your using?

omegacrux (David) · #2 23-10-2017, 05:50 PM

Hi last night I got with a Olympus OMD 40-150 @150 plus a 2x teleconvertor which equals 600mm with the M4/3 , I had sharp stars after 60 seconds
David

Atmos (Colin) · #3 23-10-2017, 06:03 PM

I’ve got a basic old HEQ5 (just stepped motors) that I can get 2-3 minutes at 10”/pixel. The issue here is the periodic error, at 3 minutes it is about half a worm cycle. My EQ6 has lower PE than my HEQ5 as it’s had numerous upgrades so it could probably do near indefinitely at 10”/pixel; 85mm lens with a 24MP APS-C with no anti-aliasing filter.

Of course if I used my D700 with a 300-500mm lens it would probably get the same results because it’s pixles are huge and it has blurring filters that make it very user friendly

Jasp05 (Aaron) · #4 23-10-2017, 10:21 PM

Just to confirm. These are unguided results yeah? 600mm at 60 secs exposure seems really good unguided regardless of what mount you have?

Atmos (Colin) · #5 23-10-2017, 11:23 PM

Unguided yes.
Consider what Kevin and Raymo have done. 8” F/5 reflectors with 30-90s exposures both with HEQ5s.

Jasp05 (Aaron) · #6 24-10-2017, 10:31 AM

Well thanks for the feedback. looks like i have some work ahead of me to weed out some things to get longer exposures.

AFter some more tinkering last night was able to approach 10 min exposure with short focal length. but once I start approaching 100-200mm FL things go wobbly after a min or so.

How can you determine if these errors are from bad polar alignment or an issue with my tracking mechanism??

currently its driven by a stepper motor with a 27:1 ratio gearbox which then feeds a 5:1 belt drive. After microstepping (1/16th step) is accounted for I have over 400,000 steps of resolution in one rotation of my RA axis. So i'm pretty certain my drive speed is good. But I haven't devised an accurate way to test it other than in the field use..

Atmos (Colin) · #7 24-10-2017, 10:45 AM

Field use is the only way I’m afraid.
What you need is the longest focal length you’ve got and use a program like PHD2 to track a bright star over time.

Taking 1s exposures and having software track it over a long period (2 hours?) will give you multiple cycles. One you have the star movement you open up the log file in something like PemPro.

I’ve never done any of this personally (never measured my PE this way) so others can help more on this topic.

xelasnave · #8 24-10-2017, 01:22 PM

Quote:

Originally Posted by Jasp05

How can you determine if these errors are from bad polar alignment or an issue with my tracking mechanism??

..

I think you could assume its polar alignment and be correct. If you are sure its spot on then look for other problems but with out polar near perfect ...well first make sure it is perfect.

Good luck with your photos.

alex

raymo · #9 24-10-2017, 01:30 PM

With my HEQ5 accurately polar aligned I was able to get almost all my
subs acceptable @60 secs, and about 50% acceptable @90 secs, and
about 20% @ 120 secs, and around 5-10% @ 150 secs in tests I did
at my scope's prime focus {1000mm].
How are you polar aligning? You need to be within a few arc seconds accuracy.
Your belt drive will pretty much eliminate PE at that point in the drive system,
but the gearbox will probably introduce it's own.

Jasp05 (Aaron) · #10 24-10-2017, 02:00 PM

Raymo, my polar alignment is very crude at this point. My only real telescope I've owned is my 8 inch dob. So never had to align till now.

I basically point the axis as close to SCP as i can and use an app on my phone to set the latitude.

Then i do a few test exposures and move the mount around the azimuth axis to get as close as I can. (I can't see SCP as there are trees in the way).

I do have a dark location where I can see the SCP but haven't made the trip out yet. Weather has been a bit iffy up here.

I'm assuming a drift alignment could be made to work on my setup? I think i may be doing that in a way already tbh. I note the direction of movement in my subs then adjust the mount to combat it. If its vertical movement I adjust the latitude. and horizontal i adjust the azimuth. and it works for my short FL subs. but anything with longer FL is suffering still.

raymo · #11 24-10-2017, 03:09 PM

You can get accurate PA without being able to see the SCP.
Google DARV. It is a PA routine by Robert Vice. After a few
iterations to familiarise yourself with it, you can get accurate PA
in 15-20 mins. All you need is a telescope and a DSLR.
raymo

Jasp05 (Aaron) · #12 24-10-2017, 04:38 PM

Raymo I should be able to use that DARV method if just using my tracker and Camera correct?

I've attached a quick drawing of my setup so you can see.

I've got a base plate hinged to a top plate to act as latitude adjustment. there is a fine tune adjustment but its not included in the drawing.

and at this stage I'm just turning the base plate on the tripod for adjustment in azimuth.

I should be able to turn the camera and focus on stars in the areas designated by the DARV guide and follow the instructions to get alignment?

Camera is attached to a ball head mount. the big gear on the back is a belt drive attached to a geared stepper which mounts on the top plate also.

DarkKnight (Kev) · #13 24-10-2017, 06:17 PM

You mentioned your latitude adjustment was slipping during the session and it might be a good idea to check that your ballhead isn't doing the same.

Jasp05 (Aaron) · #14 24-10-2017, 07:41 PM

Pretty sure ive fixed the latitude from slipping. Could be flex or slippage in the ball head. I guess a bit more trial and error wont hurt. Will focus on proper polar alignment the next few nights and post back.

Thanks for the responses guys.

raymo · #15 24-10-2017, 08:26 PM

I don't see a motor in your sketch, so I don't know if you are tracking
with a motor, or with a manual slow motion control. Either way you should
be right to do DARV, just more convenient with a motor.
raymo

jenchris (Jennifer) · #16 25-10-2017, 01:21 AM

First light tonight with star adventurer.
Canon70dw at135mm
1x30 seconds at 10,000iso.
Jpg only processed in mobile phone

Jasp05 (Aaron) · #17 25-10-2017, 11:30 AM

This is my the result of my efforts the other night.

1min exposures at 135mm approx F5 ISO 3200. about 10 subs stacked with a few darks.

Started to get a hint of the horsehead nebula in the subs, but going to need alot more time to bring that out properly. And don't mind the big black dot in the middle of my image. My kids got hold of my camera/ lenses without me knowing... Didn't realise till I was looking at the images on the PC.

Just added another image that had a bit more data and cropped slightly. For some reason the stack in DSS made the edges very blurry. But more detail in the nebula and hints at the horsehead.

Jasp05 (Aaron) · #18 25-10-2017, 03:51 PM

Quote:

Originally Posted by raymo

I don't see a motor in your sketch, so I don't know if you are tracking
with a motor, or with a manual slow motion control. Either way you should
be right to do DARV, just more convenient with a motor.
raymo

Raymo with the DARV method of alignment. I've got some newbie questions.

Step 2: Point your telescope Due south and at 0 degrees dec. where is that pointing exactly

. 0 declination is on the celestial equator at 90 degrees to the SCP? which for me is basically directly overhead. My latitude is 23.4 degrees. Can you reference a star that I can go find on a star chart that may help orientate me.

In step 7 they say to set the drive speed to something like guide rate. Shouldn't it be faster or slower than sidereal rate? to create the streak? If you move at sidereal your only going to get the usual small streak from bad alignment...?

EDIT: Have just remembered I can use stellarium for reference of a Star at 0 degrees Dec. which going by it is basically directly overhead if not a little bit further back. And I assume that this process will work even you aimed at a star at 10-15degrees dec or something?

Instructions are pasted below for reference.

1. Setup and align your telescope normally.
2. Set your telescope to point due south and at 0 degrees DEC.
3. Find a semi-bright star. A 6th magnitude star works perfectly, but a dimmer star can be used.
4. Insert your CCD or DSLR camera into the eyepiece holder or attach via the t-adapter.
5. Focus the star for the CCD or DSLR.
6. Once focused, move the star to the right hand side of the camera sensor.
7. Set your telescope to its lowest drive speed. Typically a guide rate mode.
8. Set your camera software to take an exposure of 125 seconds. The first 5 seconds is used to create a point of reference on the image.
9. As soon as the first five seconds have elapsed, then press the W on the telescope keypad to cause the star to move to the opposite side of the sensor.
10. For the first minute continue to move the telescope West. As soon as the first minute has elapsed, immediately reverse the telescope direction.
11. When the second minute has finished, stop moving the telescope.

raymo · #19 25-10-2017, 04:25 PM

Obviously 0 Dec is 90 degrees from the Southern Celestial Pole, and the
scope mount[saddle] is perpendicular to it's Right Ascension axis, so if the mount is roughly aligned with the SCP when you put your scope in a
upright position it will be near enough 0 Dec for your purposes. You can
swing the scope in an East/West direction until you find a usable star
fairly close to 0 Dec.
raymo

Jasp05 (Aaron) · #20 25-10-2017, 05:46 PM

Thanks Raymo.

And as far as the tracking speed whilst using the alignment method. Should it just be left at sidereal rate?

or slower than that?