FSQ 85 elongated stars

[AstroBruiser]

Hi everybody,

Firstly as i am new here i would like to say a BIG thank you for the warm welcome received since joining. I am truly overwhelmed!!

Now to the problem at hand  

I currently have a Moravian 16200 paired with a Tak 85 and i seem to be getting elongated stars only in the corners.

I have rotated my camera 180 deg and taken an image with no change ,polar alignment is as close as it gets with the Polemaster so i can rule that out . In the top corners the stars seem to be streaking inwards and the bottom corners outwards.

i have attached a file so you can a better idea of what i am on about.

Any input on this matter would be greatly appreciated.

Regards,

George

 

 

 

acrux ha.fit

[Martin-Devon]

George, when I put your image through CCD Inspector it shows some tilt and curvature, there's are fairly big differences also between FWMH values from the centre compared to the edges. I'm not sure if you are using the reducer on your Tak, but if so, then the spacing is obviously critical and if this is not right you'll get issues with the corners. If this is not the cause then look carefully at the focuser, and especially how you tighten up after rotating with the CAA, some Tak owners have found that they can get different tilt/curvature readouts depending on how this is done. 

Martin

[AstroBruiser]

10 minutes ago, Martin-Devon said:

George, when I put your image through CCD Inspector it shows some tilt and curvature, there's are fairly big differences also between FWMH values from the centre compared to the edges. I'm not sure if you are using the reducer on your Tak, but if so, then the spacing is obviously critical and if this is not right you'll get issues with the corners. If this is not the cause then look carefully at the focuser, and especially how you tighten up after rotating with the CAA, some Tak owners have found that they can get different tilt/curvature readouts depending on how this is done. 

Martin

Hi Martin,

Thank you for looking into this. 

I am not using a reducer although i just did a bit of research and found the spacing from  flattener to ccd  is greater than Takahashi recommend. I have no other way to shorten this distance to achieve focus.

Does this mean the 85 will never work with this combination as the flattener is inbuilt?

George

[Martin-Devon]

In the absence of a reducer you should have less issues - it's possible that one of the causes of your problem though is that the focus is not spot-on. If there are aspects of field curvature present as well you may want to experiment with the position of the star that you use for focusing. Some folk use a star right in the middle of the frame, but it can be better to try using a star 1/3rd the way in from the bottom and side of the frame, this can be effective in some cases of curvature. 

Spacing is only critical if you have a reducer in place. For a non-reducer set-up, which is what you are running, you just need to ensure that you get perfect focus and have the spacers and focusing compatible to get this. I don't think the in-built flattener is at fault here.

Another thing to check is that the imaging circle of your Tak85 covers the 16200 chip properly. I'm not familiar with this CCD (I have the bigger G4-16000), but it looks like the chip size is 27x22mm for the 16200, just double-check that that is OK for the imaging circle in your case. It also looks like the 16200 needs 2" filters, and again this flags up the need to check if this is compatible with the Tak85. It may be, but check it out. 

If you eliminate the question of coverage across the imaging circle, then my bets would be on either imperfect focusing and/or the way the focuser is re-tightened once you rotate it.

Martin 

[AstroBruiser]

4 minutes ago, Martin-Devon said:

In the absence of a reducer you should have less issues - it's possible that one of the causes of your problem though is that the focus is not spot-on. If there are aspects of field curvature present as well you may want to experiment with the position of the star that you use for focusing. Some folk use a star right in the middle of the frame, but it can be better to try using a star 1/3rd the way in from the bottom and side of the frame, this can be effective in some cases of curvature. 

Spacing is only critical if you have a reducer in place. For a non-reducer set-up, which is what you are running, you just need to ensure that you get perfect focus and have the spacers and focusing compatible to get this. I don't think the in-built flattener is at fault here.

Another thing to check is that the imaging circle of your Tak85 covers the 16200 chip properly. I'm not familiar with this CCD (I have the bigger G4-16000), but it looks like the chip size is 27x22mm for the 16200, just double-check that that is OK for the imaging circle in your case. It also looks like the 16200 needs 2" filters, and again this flags up the need to check if this is compatible with the Tak85. It may be, but check it out. 

If you eliminate the question of coverage across the imaging circle, then my bets would be on either imperfect focusing and/or the way the focuser is re-tightened once you rotate it.

Martin 

I will give this a go, hopefully next week if clear skies allow me to do so and let you know the outcome.

Many thanks

George

[ollypenrice]

If you are not using the Tak reducer then there is no chip distance to respect. You put the camera in and focus, as Martin says. That is that. Now Tak claim that this scope will cover a full frame chip. It won't, if you consider the Kodak 11 meg to be full frame, and that is also that. A friend and I have tried two examples of the scope with this chip. (The 11 meg chip might be a little bigger than the old 35mm film format, depending on where you look on the net.) However, if your chip is only 27x22 then the claimed image circle should cover it easily. Alas a significant number of posts are appearing along these lines and I think Takahashi have a problem that they need to address. I have been a great advocate of this scope having had great results from it with a smaller (15x15mm) chip but you should, without doubt, be able to plug in a 27x22mm chip and image away straight from the box. I really do think it is as simple as that.

Olly

[Ken82]

Actually I get very good results with a full frame canon 6d with this scope . The star elongation is only very slight and at the very corners of the image . 

[PatrickGilliland]

When you say polemaster is this the QHY polemaster camera?

If so at best this gives an approximate alignment only, i would still complete a secondary process such as alignmaster or tpoint model - polemaster get with 10-15 arcseconds if you are lucky - not really good enough got imagimg IMHO.

Paddy

[Martin-Devon]

George - take a look at this thread also, may give you some direction as well

 

 

[AstroBruiser]

On 8/21/2016 at 05:16, ollypenrice said:

If you are not using the Tak reducer then there is no chip distance to respect. You put the camera in and focus, as Martin says. That is that. Now Tak claim that this scope will cover a full frame chip. It won't, if you consider the Kodak 11 meg to be full frame, and that is also that. A friend and I have tried two examples of the scope with this chip. (The 11 meg chip might be a little bigger than the old 35mm film format, depending on where you look on the net.) However, if your chip is only 27x22 then the claimed image circle should cover it easily. Alas a significant number of posts are appearing along these lines and I think Takahashi have a problem that they need to address. I have been a great advocate of this scope having had great results from it with a smaller (15x15mm) chip but you should, without doubt, be able to plug in a 27x22mm chip and image away straight from the box. I really do think it is as simple as that.

Olly

Fingers crossed it is focuser related so i can resolve this issue  quick and easy otherwise i'ts off to Tak. My biggest concern is Tak may not accept responsibility for its performance on the 16200 shifting the blame on the camera itself. Visually the Baby q is fine ,this may be their argument. Interesting times ahead :S

George

[AstroBruiser]

12 hours ago, PatrickGilliland said:

When you say polemaster is this the QHY polemaster camera?

If so at best this gives an approximate alignment only, i would still complete a secondary process such as alignmaster or tpoint model - polemaster get with 10-15 arcseconds if you are lucky - not really good enough got imagimg IMHO.

Paddy

Yes it is the QHY polemaster. I always do a drift align in PHD2 after to monitor performance and it seems to be very good,or at least i haven't had a problem with it yet. 

By the way your Astrobin gallery is quite impressive!!

George

[AstroBruiser]

I had it originally on a Pentax 645z and had the same problem with the corners too,given the size of the MF chip thats understandable.

[Whirlwind]

6 hours ago, AstroBruiser said:

Yes it is the QHY polemaster. I always do a drift align in PHD2 after to monitor performance and it seems to be very good,or at least i haven't had a problem with it yet. 

By the way your Astrobin gallery is quite impressive!!

George

You should be able to check whether it's Polar Alignment by taking a much shorter exposure (say 15 seconds) and then checking the brighter stars in the corners.  If you are getting the same effect you can rule out Polar Alignment issues.  On the other hand if the stars are all round in such a short exposure then the issue is probably in your mechanical setup.  If it is and you think that PA is good then have you checked you are not getting any movement in your imaging train because of dragging cables, adapters that aren't secure etc that might be causing your camera to rotate or slip in the focuser? 

[AstroBruiser]

15 hours ago, Whirlwind said:

You should be able to check whether it's Polar Alignment by taking a much shorter exposure (say 15 seconds) and then checking the brighter stars in the corners.  If you are getting the same effect you can rule out Polar Alignment issues.  On the other hand if the stars are all round in such a short exposure then the issue is probably in your mechanical setup.  If it is and you think that PA is good then have you checked you are not getting any movement in your imaging train because of dragging cables, adapters that aren't secure etc that might be causing your camera to rotate or slip in the focuser? 

I have noticed that my CAA can still rotated  with minimal pressure  even when tightly locked so it may be this. I  will have to further investigate this issue and let you know for sure. 

George 

[PatrickGilliland]

On 22/08/2016 at 02:39, AstroBruiser said:

Yes it is the QHY polemaster. I always do a drift align in PHD2 after to monitor performance and it seems to be very good,or at least i haven't had a problem with it yet. 

By the way your Astrobin gallery is quite impressive!!

George

Thanks George I do try  

I found it is adequate - for portable rigs it's excellent but for perm set ups more accuracy can be found.  That said having now looked at the image i am better informed (on mobile phone previously) it does look like field is not flat, centre seems OK and four corners all out.  Check your spacing again if not follow the advice of those here who are better equipped than me to answer  

Paddy