Tak FSQ-85 Issue

[Jonesdee]

In October I broke the piggy bank and bought an FSQ-85ED, which was supplied with the 1.01x Flattener.   Given our rubbish weather, I have only had a couple of opportunities to use it, the first being with my ASI1600 mono.  I had the flattener fitted for that session and was pleased with the results. 

In December I purchased an ASI2600 OSC  and fitted that to the Tak without the flattener,  first to save the aggravation of setting accurate back focus and secondly because it should not be needed for that sensor size (the flattener is supposedly only required to correct astigmatism with full frame sensors).  I imaged M52 and IC1805 in the first session and noticed that the centre of the stacked image appeared to be very bright., despite being calibrated with flats, dark flats and dark frames.  Using DBE in Pixinsight I was able to reduce the flare, but it returned quite significantly when noise reduction was applied to the image . 

Next clear night I imaged close to M81/82  with  dark sky in the centre of the frame.  Sure enough, there was a bright circle in the centre.  Suspecting the ASI2600 to be faulty, last night I repeated the test with my ASI294, expecting it to be fine. Unfortunately the same bright patch  was present, pointing the blame at the scope instead of the camera.  Here is a (slightly cropped) screen grab of the ASI294 image.  Subs were calibrated with darks, flats and dark flats, stacked and for quickness, gradients removed with ABE in Pixinsight.  No other processing performed .  The issue is very clear even in a low-res snip:

 

 

Has anyone experienced anything similar to this? I wonder if use of the 1.01x flattener is mandatory with the latest FSQ85s, i.e. have the main optics been changed to better work with the flattener which is now supplied free of charge with the scope? 

Next test will obviously be to try one of the OSCs on the scope with the flattener fitted. 

Cheers

Dave

[Stuart1971]

8 minutes ago, Jonesdee said:

In October I broke the piggy bank and bought an FSQ-85ED, which was supplied with the 1.01x Flattener.   Given our rubbish weather, I have only had a couple of opportunities to use it, the first being with my ASI1600 mono.  I had the flattener fitted for that session and was pleased with the results. 

In December I purchased an ASI2600 OSC  and fitted that to the Tak without the flattener,  first to save the aggravation of setting accurate back focus and secondly because it should not be needed for that sensor size (the flattener is supposedly only required to correct astigmatism with full frame sensors).  I imaged M52 and IC1805 in the first session and noticed that the centre of the stacked image appeared to be very bright., despite being calibrated with flats, dark flats and dark frames.  Using DBE in Pixinsight I was able to reduce the flare, but it returned quite significantly when noise reduction was applied to the image . 

Next clear night I imaged close to M81/82  with  dark sky in the centre of the frame.  Sure enough, there was a bright circle in the centre.  Suspecting the ASI2600 to be faulty, last night I repeated the test with my ASI294, expecting it to be fine. Unfortunately the same bright patch  was present, pointing the blame at the scope instead of the camera.  Here is a (slightly cropped) screen grab of the ASI294 image.  Subs were calibrated with darks, flats and dark flats, stacked and for quickness, gradients removed with ABE in Pixinsight.  No other processing performed .  The issue is very clear even in a low-res snip:

 

 

Has anyone experienced anything similar to this? I wonder if use of the 1.01x flattener is mandatory with the latest FSQ85s, i.e. have the main optics been changed to better work with the flattener which is now supplied free of charge with the scope? 

Next test will obviously be to try one of the OSCs on the scope with the flattener fitted. 

Cheers

Dave

Hi,

I Have this same scope, and the flattener is only for full frame sensor cameras, it was included in the package as now sensors have got much bigger, and the scope was designed when DSLR sensors were the largest used, it saved a complete re design for Takahashi.... it’s not needed with the size sensors you are using, I have used DSLR, SXVR M25c and KAF8300, sensors with mine and all good, never used the flattener...

Are the optics nice and clear, have you shone a torch in the end to check the glass that’s up inside the tube about 2/3s the way, and make sure there are no marks on the glass....?

[Stuart1971]

Or it could be a calibration issue...what are the individual subs like...?

[Jonesdee]

Hi Stuart, thanks for your reply.  Interestingly I cannot see the issue in either individual subs or the master flat.  Good to know that you do not use the flattener as this points to an issue with the main optics of the scope.  I will check with a torch as you suggested, however looking through the OTA at the sky seems clear.   

[newbie alert]

What's a full uncropprd image look like?

[Stuart1971]

2 hours ago, Jonesdee said:

Hi Stuart, thanks for your reply.  Interestingly I cannot see the issue in either individual subs or the master flat.  Good to know that you do not use the flattener as this points to an issue with the main optics of the scope.  I will check with a torch as you suggested, however looking through the OTA at the sky seems clear.   

Sounds to me like it’s a Calibration artifact......👍🏼

Edited January 24, 2021 by Stuart1971

[discardedastro]

4 minutes ago, Stuart1971 said:

Sounds to me like it’s a Calibration artifact......👍🏼

Agree - that looks like an issue with your flats. Can't imagine any physical issue would produce such an effect (it'd be pretty extreme vignetting, and that wouldn't be consistent across cameras with different sensors!). How are you generating flats, physically and in terms of processing?

[Stuart1971]

There looks to be some sort of strange pattern over the whole image....which again points to calibration issues...which TBH, is the best outcome, as it means the scope is good....and you can sort it.... 👍🏼

[Jonesdee]

All

Thanks for your input - I will start again from scratch and reprocess the data.

Will post the results . . .

Dave