advice needed

[andyo]

Hi I am presently imaging the crescent nebula and decided to go for longer subs for narrowband increasing from 600secs to 900secs. I have noticed that with pushing the sub exposures longer I am now getting some elongation on the stars. My polar alignment is good  and I am using a reducer/flattener. The guiding seemed to be pretty good also not peaking over 1" and most of the time half that. My guiding resolution is 2.68arcsecs/pixel and my imaging resolution is 1.84 arcsec/pixel giving me a 1.45 difference in scale therefore the imaging would not have peaked above 1.45" and was around 0.75 most of the time. I would have thought this would give me good enough guiding for these exposures so if anyone can help with this it would be most appreciated thanks

 

HERE IS A STRETCHED SUB

[astro mick]

No expert but have you got the distance between the reducer and the ccd sensor correct.

Usually around 55mm.

Mick.

[andyo]

2 hours ago, astro mick said:

No expert but have you got the distance between the reducer and the ccd sensor correct.

Usually around 55mm.

Mick.

Hi Mick thanks for the reply, yes the distance is correct within a mm.I think if it was the reducer then it would also show on the five or ten minute subs and be mainly limited to the outer edges .I have just double checked tihis and it is 55mm

[astro mick]

Andy what is your set-up, it could well be flexture creeping in.How are you guiding.

[andyo]

5 minutes ago, astro mick said:

Andy what is your set-up, it could well be flexture creeping in.How are you guiding.

ah yes the dreaded flexture that may be it. The scope was pointing nearly straight up at the zenith.Mick I have attached a photo of my set up the guide scope is attached with a dovetail on top of the imaging scope(ed80) all connections are tightened down.

[astro mick]

Hi Andy.That looks fairly secure and whats more the cables are well tied down.

Imaging directly overhead will show any weakness in the set_up, but mainly polar alignment.

Did you drift align? 

Can you get 900 sec subs when imaging at the celestrial equator?

If yes, i would say its polar alighment or Flex.

How many seconds do you use in PHD.

Experiment between 1 and 4 secs.

Maybe someone with more knowledge could chip in.

Mick.

 

[peter shah]

There a number of things that this could be....I wouldn't say it was polar alignment as it would normally give rotation in the field when guiding. Differential flexure is much more common. However if you were imaging across the meridian at the time then poor balance would also cause this. Backlash in the Dec as well as over aggressive guide settings  could also be a possibility.  If you have you camera orientated x and y axis to correspond with North and East, then you image would suggest an error on both axis as the star shape is diagonal across the fov... its going to be a process of elimination... if I was to put money on it I would go with flex, but you still need to pay close attention to these other issues.

 

[andyo]

30 minutes ago, astro mick said:

Hi Andy.That looks fairly secure and whats more the cables are well tied down.

Imaging directly overhead will show any weakness in the set_up, but mainly polar alignment.

Did you drift align? 

Can you get 900 sec subs when imaging at the celestrial equator?

If yes, i would say its polar alighment or Flex.

How many seconds do you use in PHD.

Experiment between 1 and 4 secs.

Maybe someone with more knowledge could chip in.

Mick.

 

Hi Mick 

I didn't drift align I collimated the polarscope and then set the home position by getting the pole star in the crosshairs then adjusting the alt to place the pole star at the 6 oclock position then rotated the mounts RA to get the polar circle over the star then set this as the home position in eqmod. Then using eqmod aligned the polar scope. When using PHD2 it showed the polar alignment as fluctuating between zero and 1 arcminute. I have yet to attempt a 900sec near the celestial equator. I adjusted the guiding time to 3 secs as that gave the best results (after trying 2.5 and 3.5)

Andy  

[ollypenrice]

A great reply from Peter. If shorter subs give good stellar shapes then it has nothing to do with optics - ie spacing, etc.

You say your PA is good. Of course it may be but what is your evidence? The acid test, PA software-free, is to take an early and late sub separated by a good long time - ideally hours - and stack one onto the other by means of star alignment. Then zoom out and look at the relationship between the edge frame of the first image and the edge frame of the last. That will give you a 'no messing' indication of your polar misalignment.

Having seen numerous guests and robotic shed clients get different PA values from different software on the same mount I am a firm advocate of drift alignment and the test I describe above. Neither can lie.

I'm only guessing but I do suspect PA.

Olly

[andyo]

10 minutes ago, peter shah said:

There a number of things that this could be....I wouldn't say it was polar alignment as it would normally give rotation in the field when guiding. Differential flexure is much more common. However if you were imaging across the meridian at the time then poor balance would also cause this. Backlash in the Dec as well as over aggressive guide settings  could also be a possibility.  If you have you camera orientated x and y axis to correspond with North and East, then you image would suggest an error on both axis as the star shape is diagonal across the fov... its going to be a process of elimination... if I was to put money on it I would go with flex, but you still need to pay close attention to these other issues.

 

Hi Peter 

That gives me a few things to work on. The camera is not perfectly aligned x and y to correspond with north and east I think I will set this right for the next session.I used the guide assistant in PHD2 and went with the recommended settings.Will check the balance of the scope again but I did balance it on the three axis when first setting it up. Backlash could well be an issue

[andyo]

6 minutes ago, ollypenrice said:

You say your PA is good. Of course it may be but what is your evidence

Hi Olly,

Thanks for the advice I will definitely try this out as my only evidence was the guide assistant in PHD2 and eqmods own alignment routine 

[michael8554]

People swear by the 80mm refractor for guiding, but all I see is typified by your image.

The scope is well mounted, but the guide cam is hanging free 12" from the rings, albeit on decent screwed connectors.

Is this good enough folks?

Michael

[andyo]

1 hour ago, michael8554 said:

People swear by the 80mm refractor for guiding, but all I see is typified by your image.

The scope is well mounted, but the guide cam is hanging free 12" from the rings, albeit on decent screwed connectors.

Is this good enough folks?

Michael

Good question but what is the alternative, a short FL guide camera could mean you are guiding at as much as  3 or 4 times less than your imaging scale. An off axis guider is not always possible for a particular rig and also has its problems. I have considered getting a second hand ED80 for guiiding as it has a far better focuser and primary lens. Is this just overkill for guiding though.

[astro mick]

Hi Andy.

I,m glad you have received better answers.

One other trick is to not to have perfect balance, as this can cause backlash in the gears.

I always have the Dec axis slightly bottom heavy, and the RA axis weighted on the East side of the mount.(think thats right).This then ensures the gears are better engaged, and therefore hoping to avoid slip.

Something else to try.

Mick.

 

 

[andyo]

Thanks a lot for your help Mick,Pete and Olly all the answers I have received have been a great help and have given me some things to work on. I think I will be spending some time attempting a drift alignment next clear night.Mick I have also heard about this slightly off balance trick (weighting the RA axis slightly on the east side ) will give that a go also. I don't believe there is much more I can do if it turns out to be flex. I am starting to think PA as this would cause field rotation around the guide star. Thanks again everyone your help is much appreciated 

[PatrickGilliland]

Following up on Olly's comment - using polar scope is only as good as your eyes, head position and a number of other factors!  I used alignmaster when i set up on my old Eq6 and improved PA by a factor of 5.  I would be disappointed to be over .2.  I would also try loosing the bits above the guidescope, whether securely fixed or not could be causing issues.

Have you balanced out all 3 dimensions if not try that and then weigh east a fraction to keep the gear teeth engagaed.

Paddy

[andyo]

1 hour ago, PatrickGilliland said:

Following up on Olly's comment - using polar scope is only as good as your eyes, head position and a number of other factors!  I used alignmaster when i set up on my old Eq6 and improved PA by a factor of 5.  I would be disappointed to be over .2.  I would also try loosing the bits above the guidescope, whether securely fixed or not could be causing issues.

Have you balanced out all 3 dimensions if not try that and then weigh east a fraction to keep the gear teeth engagaed.

Paddy

Thanks for the advice Paddy. Yes I did balance out on all 3 dimensions. Will be doing a drift align next clear sky

Andy

[michael8554]

21 hours ago, andyo said:

Good question but what is the alternative, a short FL guide camera could mean you are guiding at as much as  3 or 4 times less than your imaging scale. An off axis guider is not always possible for a particular rig and also has its problems. I have considered getting a second hand ED80 for guiiding as it has a far better focuser and primary lens. Is this just overkill for guiding though.

Agree entirely Andy, I just ask if the overhang is a problem or not, people bang on about flex in guidescope setups.

Michael

[andyo]

1 hour ago, michael8554 said:

Agree entirely Andy, I just ask if the overhang is a problem or not, people bang on about flex in guidescope setups.

Michael

I cant say its ever been a problem for me with the shorter 5 min subs I normally take, even the 10 minute subs I took last imaging session turned out ok. However as Mick said when imaging directly overhead that is when any weaknesses in the set up are likely to show such as flex or PA. This may well turn out to be PA rather than flex I will do a drift align next clear night to at least rule this out,I will post back to say if this cures it

[andyo]

Did  a drift align last night and found while the azimuth was almost spot on the altitude adjustment was drifting after a minute or so. When I had finished the drift alignment the star was holding steady for 20mins. I decided to go and check where this placed the pole star with the eqmod method and found the star was now near on exactly the same for azimuth but was half way between the circle and centre for altitude. The polar scope is collimated so surely it shouldnt be this far out