Exposure Query and Bumps.

[Spacehead]

Hi All,

I am just ordering a QHY5l-IIc and an Orion 50mm guidescope for my SW200P to start my journey in guiding with a laptop.
So currently tracking, I get quite sharp 1 min subs.

I have two questions if its ok.

1.  Is a 5 min sub, guided, going to produce more detail than 5 x 1 min subs stacked?  If so is there some kind of scale - say a 5 min sub will be 30% "better" than the 5 x 1 min subs stacked?  

2.  I put the motors on my eq5 myself, I am "vaguely" aware of this issue of a bump every so often whereby the cogs dont fit 100% so there is a jerk every x minutes whilst they are running.  Does PHD get round this?  Can anyone expand on this topic for me?

Cheers all

 

[daz]

There are huge numbers of discussions on the merits of lots of shorts vs fewer longs - see 

 as an example

 

PHD won't get around a mechanical issue with your motors, not that I'm aware of anyway...

[ronin]

Could well be limited by the scope in a way. The 200P has reasonable cross section and so any wind or breeze could cause a lost exposure. The longer the exposure then in real terms the greater the chance of something occuring. Just thinking that you could lose a whole 5 minute exposure if you were unlucky.

I believe that a single longer exposure is better, S/N ratio. But there is more then just that involved. Guiding is to make a good setup better. If the setup is poor then the guiding is constantly compensating, and that compensation is movement.

You appear to have a 200P on an EQ5 and adding guiding in the form ofcamera and sope and attachements is starting to push at the mechanical limits of the whole lot, you may simply start to not drive as easily or smoothly.

The other is that a DSLR heats up so the sensor will have increeaded thermal noise, although I suspect that this plateaus after a fairly short period. Would 5x1 minute exposures create less thermal noise on the images the a single 5 minute exposure - I would suspect there is less in this scenario, but I suspect that 2x150 second exposures creates as much thermal noise as a single 5 minute one.

My preference would be to keep it simple and set it all up as normal, get the guiding operational then say try 2 minutes, then if all good increase a bit more. It is not a competition to see who can get the single longest exposure after all. Think the HST likely holds that.

Will say I am quite happy at collecting multiple short exposures. So maybe not the best advice.

 

[Spacehead]

The thing is I am quite happy with tracking in that I dont mind doing lots of 1 min jobbies.  But I keep getting told I need to guide to get better results.

Last night I got 130 x 1min subs of the Pleiades - but there is no nebulosity showing.  That made me think I need to guide for longer subs - but then again - the posts above would indicate it wont make much diff.

So where's the dam nebulosity?  Odd - because the images are pretty good and at 800 ISO id have thought id get some.

 

[newbie alert]

Its a trade off on collect a good signal without the noise...if you do a min sub on say iso 800 then try to do a 5 min sub on the same iso you're going to get far more noise..

The difference you get from a 1-2 min is significant...same as a 3 min sub..get get a far better signal..someone describes it as the signal photons filling the signal pots up..sp a 1 min signal might only fill the pot up part way but a 3 min might fill it up more..  try 2 mins...then 3 mins and more ..the limit will be what length you can guide at and the noise level of the camera..in cold conditions the noise level will be lower..

[ollypenrice]

Modern CMOS cameras are changing the rules about sub length because, with their low read noise, they do well with multiple short subs. My own experience with CCD is unequivocal: lots of very short subs do not, on faint nebulosity, match fewer longer ones. The fainter the data, the longer the subs you need.

With a DSLR a minute stikes me as very short and I'd expect you to catch more nebulosity with longer subs. Give it a try.

PHD will not take out sudden mechanically induced jumps but what it might do is put the camera back on target quickly enough for an insignifcant amount of 'jumping signal' to land on the chip.

Olly

[Spacehead]

Cheers all - lots for me to think about..........

[bobro]

On 01/09/2017 at 11:17, Spacehead said:

I put the motors on my eq5 myself, I am "vaguely" aware of this issue of a bump every so often whereby the cogs dont fit 100% so there is a jerk every x minutes whilst they are running.  Does PHD get round this?  Can anyone expand on this topic for me?

Having spent a bit of time recently attempting to adjust backlash out of a CG5 (same as EQ5) after fitting motors, some thoughts : is the 'bump' every 10 minutes? If so this represents 1 revolution of the RA worm axis. The bump could be due to a defect on the motor cogs (assuming a stepper motor with 2 gear wheels), with the worm drive or with the backlash adjustment of the RA axis.

Suggestions (in order):

1) see if the RA axis is binding. Check by manually rotating the axis using the worm drive through a full revolution - it should be quite easy to rotate the worm drive. If it isn't readjust the worm backlash so that it freely turns without binding.

2) visually examine the 2 RA gears for defects. A small defect can be gently filed down.

3) remove the worm drive and check the worm for damage.

A bit of backlash in the RA axis isn't a problem as the axis is permanently driven in one direction.

HTH

 

[dph1nm]

On 01/09/2017 at 13:51, Spacehead said:

Last night I got 130 x 1min subs of the Pleiades - but there is no nebulosity showing.

Which is odd, because with a Canon 1000D I can pick it up with about 10 mins worth of 30sec subs on a 8.5" scope at 1290mm FL from a light polluted site. If you really mean you had 130mins of exposure then it should be booming out, so I suspect this is some problem other than exposure length.

NIgelM

[Spacehead]

dph1nm & bobro - I have a couple of issues atm - will get back to these.