Flattener

[Big Bang!]

Good evening.

I started AP two years ago with a Ritchey Chretien 250 TT.   Needing more flexibility in terms of F/L, I recently added an Esprit 120ED Pro refractor to my armory, along with its purpose-made flattener. I didn't understand anything about flatteners or whether I would need one but I felt I ought to get it just in case.  I am just about to swap over the OTAs so I now need to know what I don't know.  If anybody is able to give me a brief outline of the issues making flatteners necessary and what I ought to expect whilst using one I would be grateful.  Treat me like I know nothing please - words of one syllable etc!

The critical piece of information seems to be that this combination of Esprit and flattener gives an imaging circle of 44mm.  I am using an Atik One 9.0 camera, which is 3380x2704; pixel size is 3.69.  Is the flattener only necessary with really large sensors?

Many thanks,

Gus

[ollypenrice]

Your camera has a small chip and might not need the flattener. You give an image circle spec with the flattener but not without it so there is no way of anticipating the non-flattened field size unless SW publish it somewhere, but this is not their strong point. So I'd be surprized if you needed it but who knows? A user, so let's hope someone pops up!

If you do use the flattener these things have a chip distance to respect. This should be given with the flattener and it describes the necessary distance between the back of the flattener and the chip. If using mono with filters you should add 1/3 of the filter thickness (usually meaning about about 1mm) to the specified distance. There is endless debate about whether you should add 1mm or subtract it. I feel it has to be 'add' and Don Goldman of Astrodon Filters says the same. Good enough for me.

Olly

[Big Bang!]

Hi Olly,

Thanks.  

Yes, I cannot find a figure for the image circle without a flattener but the review I read on the 120 EDpro seems to suggest that one is necessary for imaging with a DSLR so I thought I ought to buy one just in case. The stated spacing is 75mm in my case and FLO have been helping with an adjustable adapter.  They also say 'add' and when I think about it, seems logical.  I assume that any error in the spacing will show up as out-of-focus stars in the corners of the frame or will they be out across the whole field?

Can you confirm that in essence the purpose of a flattener is simply to ensure that the flat imaging circle is at least as big as the chip?  Does it do this by increasing the cone size or does it create some other clever optical effect to achieve this? I am looking into getting a larger format camera, possibly an ATIK 383L+, to take full advantage of the larger flat field. Is there anything else I need to consider, like, say, pixel size?  My main OTA is an RC250TT "Astrograph" - does that necessarily mean I would not need a flattener for it with a larger format camera?

Gus

[Pompey Monkey]

16 hours ago, ollypenrice said:

Your camera has a small chip and might not need the flattener. You give an image circle spec with the flattener but not without it so there is no way of anticipating the non-flattened field size unless SW publish it somewhere, but this is not their strong point. So I'd be surprized if you needed it but who knows? A user, so let's hope someone pops up!

If you do use the flattener these things have a chip distance to respect. This should be given with the flattener and it describes the necessary distance between the back of the flattener and the chip. If using mono with filters you should add 1/3 of the filter thickness (usually meaning about about 1mm) to the specified distance. There is endless debate about whether you should add 1mm or subtract it. I feel it has to be 'add' and Don Goldman of Astrodon Filters says the same. Good enough for me.

Olly

It is most certainly "add"....

Unless you happen to be in possession of a rather exotic filter set made from a material with a negative refractive index

15 hours ago, Big Bang! said:

Hi Olly,

Thanks.  

Yes, I cannot find a figure for the image circle without a flattener but the review I read on the 120 EDpro seems to suggest that one is necessary for imaging with a DSLR so I thought I ought to buy one just in case. The stated spacing is 75mm in my case and FLO have been helping with an adjustable adapter.  They also say 'add' and when I think about it, seems logical.  I assume that any error in the spacing will show up as out-of-focus stars in the corners of the frame or will they be out across the whole field?

Can you confirm that in essence the purpose of a flattener is simply to ensure that the flat imaging circle is at least as big as the chip?  Does it do this by increasing the cone size or does it create some other clever optical effect to achieve this? I am looking into getting a larger format camera, possibly an ATIK 383L+, to take full advantage of the larger flat field. Is there anything else I need to consider, like, say, pixel size?  My main OTA is an RC250TT "Astrograph" - does that necessarily mean I would not need a flattener for it with a larger format camera?

Gus

I have an 80 ED pro with the flattener and that covers my 8300 chip comfortably with a stated imaging circle of 33 mm. I have another slight problem with orthogonality in one corner, but have it on very good authority that this is due to something else in the imaging chain. I don't exactly know how flatteners work, but I believe that it is slightly more than just increasing the cone size as that would imply a drastic change in effective focal ratio.

You RC is designed specifically for AP and should, as far as I understand it, not need any kind of flattener.

[swag72]

If your spacing is not correct when you use the flattener then you will get distorted stars in the corners. There's a couple of excellent images on the forum that help should you need it, with regards to trying to get the correct spacing. If it's right you will get nice round stars across the frame. The two images below have been posted before (although I can't remember by who) and they are most useful in trying to ascertain whether you may need more or less spacing when trying the flattener.

I hope whoever it is doesn't mind me posting these

If your camera sensor is too close to the flattener then you will get the 'whooshing effect' in the corners and if you need to reduce the spacing then you will get the radial corner effect.

Hope that helps.

 

[Big Bang!]

O wow, Sara!  Yes, does that ever help; I had visions of not knowing which way to go.  Thank you.  I understand the positional tolerance is only about 1 mm.

Paul, how do you find the 8300 chip? (Being a Pompey lad you can't be too far from me)

 

Gus

 

[Pompey Monkey]

19 hours ago, Big Bang! said:

Paul, how do you find the 8300 chip? (Being a Pompey lad you can't be too far from me)

 

Gus

 

It seems ok to me, but it is the first ccd that I've owned so I don't have much experience to compare with. It is, however, leaps and bounds ahead of the DSLR that I started with. I took a set of darks a year ago and I only used these to create a bad pixel map. I'm not even sure how much use that is because dithering with pixel rejection in the stacking process seems to remove the bad pixels anyway.

BTW, I run it at -20C

You're in Fareham? I'm just down the A27 in Portchester, deep in the  Portsmouth skyglow

PS MODS - The way that the cursor jumps around the text while I'm typing a post is getting a tad tiresome. It seems n I use an emoticon.worse whe