Barlow lens plate solving failing

Dspital

Can ASIAIR plate solve with a Barlow lens in the light path? I’ve had no luck trying this with either 2x or 2.5x Barlow.

stevesp

It depends on the FOV you have
There is a min and max FOV where it works

PMTeam@ZWO

stevesp ASIAIR could complete the solve plate with a minimum FOV of 0.2º. May I ask what is the Main scope focal length after adding the Barlow mirror?

Dspital

PMTeam@ZWO adding 2x Barlow to Celestron C6 resulted in FL=5808. It solved once and Asiair returned the FL 5808, failed every time after that. The camera is a dslr eos T3i modified.

w7ay

Dspital adding 2x Barlow to Celestron C6 resulted in FL=5808.

Something is not right. A C6A has a native focal length of 1500 mm. With a 2x Barlow lens, the focal length should be closer to 3000 mm instead of 5800 mm.

Be sure to check the backfocus (how far the camera sensor is away from the Barlow) since that will change the Barlow's magnification factor.

Telecentric "Barlows" (like the Tele-Vue PowerMates) allow you to change magnification by changing the spacing of the camera from the Barlow. But cheap Barlow lenses will distort significantly when your magnification is different from the designed magnification. So, that is the first thing that you need to fix.

That being said, even at 2x magnification, the Barlow would put Plate Solving out of reach with your C6 and the ASIAIR, since it will produce a FOV of 0.17º x 0.13º. At 5800 mm focal length, it would be waaay out of reach.

You can get the plate scale (and thus derive the true focal length) by using astrometry.net to plate solve for you, since the ASIAIR is too crude to plate solve the small FOV.

Chen

PMTeam@ZWO

Dspital If the focal length is filled in incorrectly, it will fail to be resolved. You can fill in 0 in the focal length, ASIAIR will automatically calculate the focal length after taking pictures

Dspital

Yes I suspected that the back distance between the camera and the Barlow was the reason that it wasn’t working. Thank you for the input.

David

PhotomanTom

Would it work to switch a guide scope and camera (240fl at f4 and asi120 mm-s) as main camera, do the plate solving to center your object and then switch back to the narrower fov of the barlowed scope and camera as the main camera?

I do that to focus both the main and guide cameras and temporarily mislabeling each camera works.

SigurdG

Same problem: have a 130mm, f6,6. No problem plate solving, but putting on a 3x Barlow (focal length 2574), plate solve failed. Why?

SigurdG

Why is Asiair unable to plate solve this?

w7ay

SigurdG Why is Asiair unable to plate solve this?

Your image is out of focus. Once in focus, the dimmer stars will start to appear, too. Use a Bahtinov mask.

Your image also has very distorted stars. Plate solving requires sharp stars. Check your flattener/reducer's backfocus and tilt. It is unusal to have coma in the entire field, which your imaging train exhibits. The stars are so poor that even if you can plate solve, you will not be happy with the resultant stacked image -- you will just have a bunch of tadpoles.

Chen

SigurdG

Ran it through autofocus first (ZWO EAF), but it evidently didn't do the job as it should have.

--Sigurd

w7ay

SigurdG Ran it through autofocus first (ZWO EAF), but it evidently didn't do the job as it should have.

ASIAIR autofocus is a joke. That is why I suggested using a Bahtinov mask.

ASIAIR "autofocus" works OK with medium focal length scopes, and well flattened images. But if the field is not flat, ASIAIR runs amok since it will be choosing stars of different shapes from different parts of the frame that it takes.

Even so, its autofocus is never very consistent. Two autofocus performed just seconds apart on ASIAIR could produce very different results.

Chen

SigurdG

Focussed to the best of my ability, ut it's still not solving.

SigurdG

Seems like ASIAIR does not like Barlows.

w7ay

SigurdG Seems like ASIAIR does not like Barlows.

Works fine with my Barlows. Of all the problems with ASIAIR (so numerous that I don't recommend the ASIAIR), the use of Barlows is not one of them.

What is your plate scale?

ZWO's documentation says ASIAIR requires an FOV of larger than 0.2º x 0.2º to plate solve. If you have the original ASIAIR, then the FOV needs to be 0.4º x 0.4º or larger.

Remember that Barlows have a defined backfocus -- if you use a longer backfocus, a 2x Barlow will generate greater than 2x magnification. And that could really increase your plate scale if you are not careful with the backfocus distance.

Some telecentric "Barlows" can have moderately constant magnification factor (I have a Televue PowerMate that does that, and a couple of other Televue PowerMates that don't ). Most Barlows, especially cheap onces, will not have constant magnification however. Check the spec sheets of your Barlow.

The shape of your stars seems to point to a cheap Barlow, and you may not even have a spec sheet.

Chen

johnpd

SigurdG

According to the information I was able to piece together from what you posted, you have a 130mm telescope with a focal length of 858mm. The camera you are using is an ASI2600 set at Bin2. With the 3x Barlow in place that gives you a focal length of 2574mm which would give you a FOV of 0.52° x 0.35°. This should be sufficient. However as Chen said, your stars look somewhat distorted. Have you tried the Bahtinov mask on a bright star? Try PhotomanTom's suggestion of using a guide scope as the main camera to center on a bright star, then switch to the 2600 as the main camera and use a Bahtinov mask to focus. If that does not work, then Chen's suggestion of the Barlow causing the distortion may be what's happening.

JohnD