Imaging Issues with CPC925

[1CM69]

Hi all,

I was out with my scope on Saturday night and it was the first time I had setup using my Celestron Pro HD Wedge.

I followed very detailed guidance to Polar align my scope and was really blown away at how straight forward the whole procedure was and the accuracy that I had achieved straight away. The longest & most taxing part of the alignment process was initially locating and making sure that is was indeed Polaris in my finder and not another nearby star.

The scope was set to track mode EQ North & slewing from star to star was very accurate.

I attached my f6.3FR to gain a slightly wider field in order to image Pleiades in all it's glory.

I took a number of images and all except one had star trailing, even on relatively short exposures, shorter even than when I use my scope without the wedge in AltAz configuration.

Here are two images, the first is taken at ISO1600 for 5 seconds and you can see the stars are round except at the extremities which show slight trailing. The second image is again at ISO1600 but one second shorter exposure than the first, at 4 seconds and the trailing is visible, albeit slight across the whole of the image.

 

 

 

This was extremely frustrating, how can the second image show the starting of trailing when the exposure time is 1 second less than the first image?

I tried many other exposure/ISO settings but all showed trailing, I never got better than the result in the first image here.

I decided to start over, switching the scope off and restarting the whole Polar alignment procedure once again but this didn't help, I was getting disappointing results for very, very short exposure times.

I thought that maybe it was to do with my counterweights on my scope, it was far more trickier balancing the scope when mounted on the wedge than it is when just in AltAz configuration. Right from the get go I was concerned how the scope could possible be balanced correctly when normally in AltAz setup the counterweights are along a vertical plane but wedge mounted they could end up near to a horizontal plane.

Surely this could add to strain of the Az motor trying to rotate the scope in this format.

I assumed that maybe this was causing the trailing as the Az motor was trying to keep up with field rotation but having to lift the counterweight up on one side whilst maintaining a braking action to stop the downward action of the other counterweight directly opposite, (hard to describe, hope you get the gist).

I removed the counterweights altogether but to my embarrassment I didn't capture any images at this stage because something else crossed my mind, namely the GPS time.

I had noticed when setting up that the GPS time was off by one hour, it was still showing British Summertime & not GMT. No matter how long I left the handset hoping it would resync to the correct time it didn't & stayed this one hour ahead.

Now I hadn't experienced this before because I only got my scope in August when BST was in full swing.

Thinking that perhaps this time discrepancy could cause the trailing I was seeing I manually altered the time, once again by switching the scope off then on again to reset it & causing the need to realign.

With the scope realigned, time running out and clouds rolling in I decided to target some other stars for quick images and I seemed to get some pleasing results with much longer exposure times, although still hit and miss.

 

I am just trying to get to the bottom of what was going wrong here, could it really have been the hour difference in GPS time or the counterweights causing premature trailing?

 

I took two images of M42 at 15sec exposure, one shows the stars completely round while the other shows them as elongated and this is after manually altering the GPS time etc...

 

I would have thought that I could achieve exposures in the minutes at least.

 

What is going wrong?

As an aside I did notice that when attempting to centre an object for imaging using the right arrow button on the handset resulted in the object slowly continuing to move in that direct even after releasing the button, I had to put a couple of blips on the left arrow button to counteract this and keep the target still. Is this normal?

 

Any help would be great.

 

Kirk

 

 

[cfpendock]

I had a CPC on a wedge, and from what I remember, I always used the main scope rather than the finder for polar alignment.

Chris

[ronin]

Any difference between the finder and the main will add to inaccuracy. Also pointing at Polaris is nice but that is not the earths rotational axis, it is about 1 degree off of the rotational axis. With a poalr scope Polaris is not in the centre it is off to one side to account for this offset.

A 9.25 has a long focal length even with a reducer, so any inaccuracies will be shown up fairly prominently. For scopes like yours I have only really seen them on a permanent pier where the pier is very accurately aligned to the earths axis. It is not the scope that is aligned in EQ mode but the pier/mount.

[SkyBound]

Hi with such a long focal length scope the PA would have to be very very good Indeed, even at f6.3 it would still need to be pretty close, lining up with Polaris is not good enough, as Polaris is not the true pole, you should be polar aligning on the NCP, which is the true pole, this will then help quite a bit., especially at your focal length.

secondly the GPS will pick up UTC (coordinated universal time)  time which is correct, the fact that your handset was an hour out does not matter, that is just a setting in the handset, you can ignore that, all it means is that the handset will display either BST or not, I don't bother altering mine which means the handset is always correct for 6 months of the year......and an hour out the other 6 months.

thirdly when balancing, do it in the Alt AZ position, first slide weights back or fourth to counter whatever you have on the back of the scope, (camera etc) then point the tube straight up and make sure it stays there, and does not fall one way or the other, you may need to add weight to the top of the tube to get balance, when you have this is fine for wedge mode (equatorial) use.

i have used an 8" Meade SCT and got subs of around 3 mins with round stars using the above method of balancing at f6.3 and about 1.5-2 mins at f10, also without the f6.3 reducer the stars will be out of shape at the edges due to field curvature, this is normal, it will be a lot better with the reducer but still not perfect.

balance is the key with fork mounted SCT scopes used on an equatorial wedge.

[michael.h.f.wilkinson]

As others have said, aligning on Polaris is inadequate. I have used the true NCP alignment available in my Great Polaris EQ mount, and whilst very accurate, longer exposures with FR still required guiding, I found, even at the slightly shorter focal length of the C8. Drift aligning might be able to improve the result

[1CM69]

54 minutes ago, SkyBound said:

Hi with such a long focal length scope the PA would have to be very very good Indeed, even at f6.3 it would still need to be pretty close, lining up with Polaris is not good enough, as Polaris is not the true pole, you should be polar aligning on the NCP, which is the true pole, this will then help quite a bit., especially at your focal length.

secondly the GPS will pick up UTC (coordinated universal time)  time which is correct, the fact that your handset was an hour out does not matter, that is just a setting in the handset, you can ignore that, all it means is that the handset will display either BST or not, I don't bother altering mine which means the handset is always correct for 6 months of the year......and an hour out the other 6 months.

thirdly when balancing, do it in the Alt AZ position, first slide weights back or fourth to counter whatever you have on the back of the scope, (camera etc) then point the tube straight up and make sure it stays there, and does not fall one way or the other, you may need to add weight to the top of the tube to get balance, when you have this is fine for wedge mode (equatorial) use.

i have used an 8" Meade SCT and got subs of around 3 mins with round stars using the above method of balancing at f6.3 and about 1.5-2 mins at f10, also without the f6.3 reducer the stars will be out of shape at the edges due to field curvature, this is normal, it will be a lot better with the reducer but still not perfect.

balance is the key with fork mounted SCT scopes used on an equatorial wedge.

So my theories about counterweights or incorrect time flew out the window.

Balancing is no problem, done many times when not wedge mounted, just thought it could be an issue with the Az motor now having to lift the weights when wedge mounted instead of only rotating them through a vertical plane.

55 minutes ago, ronin said:

Any difference between the finder and the main will add to inaccuracy. Also pointing at Polaris is nice but that is not the earths rotational axis, it is about 1 degree off of the rotational axis. With a poalr scope Polaris is not in the centre it is off to one side to account for this offset.

A 9.25 has a long focal length even with a reducer, so any inaccuracies will be shown up fairly prominently. For scopes like yours I have only really seen them on a permanent pier where the pier is very accurately aligned to the earths axis. It is not the scope that is aligned in EQ mode but the pier/mount.

I did already know that Polaris is slightly off exact centre and thought that even if Polaris is located & set as bang in the middle of the crosshairs, the brain of the scope allowed for this using the set position & GPS data etc...

1 hour ago, cfpendock said:

I had a CPC on a wedge, and from what I remember, I always used the main scope rather than the finder for polar alignment.

Chris

That does sound more sensible, I'm currently reading through this: http://www.shadycrypt.com/pages/Polar/Polar.htm and it seems to lay things out in a more understandable form.

[1CM69]

14 minutes ago, michael.h.f.wilkinson said:

As others have said, aligning on Polaris is inadequate. I have used the true NCP alignment available in my Great Polaris EQ mount, and whilst very accurate, longer exposures with FR still required guiding, I found, even at the slightly shorter focal length of the C8. Drift aligning might be able to improve the result

I think that guiding is ultimately going to be the way that I will need to go.

Just find it odd that I was getting better & longer results, still sub 30sec though, using the scope in AltAz & not on the wedge.

[SkyBound]

Are you allowing the scope to track in all directions, as you mention  "EQ track mode north"....not sure what that means..?

[1CM69]

15 minutes ago, SkyBound said:

Are you allowing the scope to track in all directions, as you mention  "EQ track mode north"....not sure what that means..?

The scope has 3 tracking modes; AltAz, EQ North & EQ South. 

I had it set as it should have been, EQ North

[Jupiter1]

Just to pick up on the last point of the lead query, when I set up my GO TO for visual use on my 9.25 ALT/Azimuth Evolution scope , after I have it nicely aligned with a Planet dead centre , it this starts to slide away ,as if the clutches have been left loose . I do , of course make sure that is not the case.

Also , After many attempts to get my centring  as accurate as possible the object will still be in the Bottom of the 40mm wide Field lens, just about in.

 

Anybody else having a similar problem or any Suggestions for which I would be very Grateful , Many thanks , Jupiter1

[kirkster501]

Even if you are guiding during your exposures you will still get field rotation if you are a degree off the NCP - which you will be if you are aligning the mount on Polaris.You need to be much more accurate than that.

Even for visual, with the F10 FL of the C9.25 objects will not be in the FoV in your Goto's if you Polar Aligned the scope on Polaris.  I have a C9.25 myself and can tell you this for a fact.

Edited November 10, 2016 by kirkster501

[beka]

On 07/11/2016 at 17:32, 1CM69 said:

Hi all,

I was out with my scope on Saturday night and it was the first time I had setup using my Celestron Pro HD Wedge.

I followed very detailed guidance to Polar align my scope and was really blown away at how straight forward the whole procedure was and the accuracy that I had achieved straight away. The longest & most taxing part of the alignment process was initially locating and making sure that is was indeed Polaris in my finder and not another nearby star.

The scope was set to track mode EQ North & slewing from star to star was very accurate.

I attached my f6.3FR to gain a slightly wider field in order to image Pleiades in all it's glory.

I took a number of images and all except one had star trailing, even on relatively short exposures, shorter even than when I use my scope without the wedge in AltAz configuration.

Here are two images, the first is taken at ISO1600 for 5 seconds and you can see the stars are round except at the extremities which show slight trailing. The second image is again at ISO1600 but one second shorter exposure than the first, at 4 seconds and the trailing is visible, albeit slight across the whole of the image.

 

 

 

This was extremely frustrating, how can the second image show the starting of trailing when the exposure time is 1 second less than the first image?

I tried many other exposure/ISO settings but all showed trailing, I never got better than the result in the first image here.

I decided to start over, switching the scope off and restarting the whole Polar alignment procedure once again but this didn't help, I was getting disappointing results for very, very short exposure times.

I thought that maybe it was to do with my counterweights on my scope, it was far more trickier balancing the scope when mounted on the wedge than it is when just in AltAz configuration. Right from the get go I was concerned how the scope could possible be balanced correctly when normally in AltAz setup the counterweights are along a vertical plane but wedge mounted they could end up near to a horizontal plane.

Surely this could add to strain of the Az motor trying to rotate the scope in this format.

I assumed that maybe this was causing the trailing as the Az motor was trying to keep up with field rotation but having to lift the counterweight up on one side whilst maintaining a braking action to stop the downward action of the other counterweight directly opposite, (hard to describe, hope you get the gist).

I removed the counterweights altogether but to my embarrassment I didn't capture any images at this stage because something else crossed my mind, namely the GPS time.

I had noticed when setting up that the GPS time was off by one hour, it was still showing British Summertime & not GMT. No matter how long I left the handset hoping it would resync to the correct time it didn't & stayed this one hour ahead.

Now I hadn't experienced this before because I only got my scope in August when BST was in full swing.

Thinking that perhaps this time discrepancy could cause the trailing I was seeing I manually altered the time, once again by switching the scope off then on again to reset it & causing the need to realign.

With the scope realigned, time running out and clouds rolling in I decided to target some other stars for quick images and I seemed to get some pleasing results with much longer exposure times, although still hit and miss.

 

I am just trying to get to the bottom of what was going wrong here, could it really have been the hour difference in GPS time or the counterweights causing premature trailing?

 

I took two images of M42 at 15sec exposure, one shows the stars completely round while the other shows them as elongated and this is after manually altering the GPS time etc...

 

I would have thought that I could achieve exposures in the minutes at least.

 

What is going wrong?

As an aside I did notice that when attempting to centre an object for imaging using the right arrow button on the handset resulted in the object slowly continuing to move in that direct even after releasing the button, I had to put a couple of blips on the left arrow button to counteract this and keep the target still. Is this normal?

 

Any help would be great.

 

Kirk

 

 

Hi 1CM69,

I am considering purchasing the Celestron HD Wedge and saw your post when doing a search. I would like to know if you solved your problem and if you are happy with your wedge. I am also thinking - had you considered backlash as a cause for your issues, as your last statement about the view drifting seems to suggest this. I image sometimes with my CPC 1100 in altaz mode and I have found that I have to wait a while after slewing for it to settle before starting to image. There are settings for backlash compensation which I have not tried so far.

Best

Edited February 10, 2022 by beka
Typo