Which flattener will give me round stars?

[AndyKeogh]

Hi all,

After a few months of trying to photograph without guiding, I bought a Megrez 90 to use as a guide scope for my C8 but its turning out to be my main imaging scope for the moment. I finally got PHD working with my QHY6 last week and I have attached a jpg produced from one of the 5min raw images. Camera is an unmodified EOS 50D with an Astronomics CLS-CCD clip filter.

The problem is that the stars become elongated as you move away from the centre . I have looked on Craig Stark's site where he reviews some flatteners and has lovely round stars in the corners.

But he was not using a Megrez 90!. I have also read a number of threads in here but although there is at least one member with a Megrez 90 and a flattener I do not recall any pictures of the stars in the corner.

So will a WO Flattener III or IV or some other make solve my problem or am I expecting too much from the Megrez?

TIA

Andy

[RobH]

I'm interested in the replies you get here Andy, as I have a WO ZS80FD.

I am now using a large format CCD with it and get the same problems, and may well be moving to a Megrez 90 before too long too.

Cheers

Rob

[AndyKeogh]

I have attached a better view of the corner as the detail got lost in the full image.

Andy

[Beyond_Vision]

If you can try to pick up a Televue TRF2008 focal reducer/field flattner. The WO reducer III is ok but from what I have heard does not correct the field totally. The WO IV actually lengthens the focal length slightly by 1.1 so is not a reducer. I'm not sure how well it fares in correcting the field. What you ideally want is something that reduces the focal length to give you a faster scope for imaging also giving you a wider field of view. The Televue FF/FR should do both.

Regards

Kevin

[roundycat]

I wish I could help. I have a ZS80FD and a TV reducer but the one intended for the longer refractors, ie, 600-800mm. I use it with my TMB which is 1200mm and it works just fine. Strangely I have never used it with my ZS.

Dennis

[AndyKeogh]

Thanks, I believe SCS Astro are the Televue importers so I will; give them a call tomorrow.

I have been browsing the net since I posted this question and there have been a number of reviews, many with pictures but not using the Megrez 90 I was realy hoping to see some photographs taken with a Megrez 90 and a flattener before I talked to the dealers.

So far the only positive pictures I have seen have been with the Borg .85 reducer flattener. There have been quite a few mention of the TRF-2008 but I will need to check further and see what pictures are available.

Meanwhile I am still hoping that someone here will have something positive to say.

Andy

[AndyKeogh]

A quick update.

In the end after a lot of trawling the net, the choice came down to the TRS-2008 or the Borg DG-L. I could not find any technical specs on TRS other than 0.8 reduction whereas the Borg had much more information available and is supposed to flatten the field for the full 35mm frame size. It also comes with adjusting spacers so that the distance between the front and back elements can be tuned for different focal lengths. This was impoprtant to me as measuremts so far suggest my Megrez 90 has a focal length of 561 and is F6.2 rather than the specified F6.9.

In the end I was heavily influenced by the following reports though neither featured the TRS-2008

Borg DG-L 0.85 reducer (#7887) vs WO 0.8 Flattener III - IceInSpace

William Optics Flattener 4 - IceInSpace

Last night ( actually this morning ) I managed some test pictures of the coathanger. They were unguided and I did not have the CLD-CCD clip filter fitted and the moon was pretty bright but you can see the effect of the flattener on the star shapes

The results show a considerable improvement but are not perfect. I do not know if this is down to being unguided, the clouds, Rubbish seeing or just my inexperience.

To check how well it flattens I have compared this mornings photograph with one taken a few weeks ago. This has allowed me to calculate that the DG-L gives a measured focal reduction of 0.89 with the thin spacer fitted.

I have then cut the bottom left hand corner and top right hand corner from each photograph and as can be seen from the attachments the stars are considerably rounder despite no guiding.

The APS-C chip in the Canon EOD 50D has a diagonal of 26.8mm

Andy

Flattened BLHC versus unflattened BLHC

Flattened THRC versus unflattened TRHC

[buddyarrows]

Hi Andy,

The effect you describe is known as Field Curvature and most APOs have this effect due to optical physics.

I had a similar problem with my Meade Series 5000 80mm APO and got the use of a Skywatcher Field Flattener designed to suit a F-6 scope. The SW FF drastically reduced coma and with out breaking the bank at £69.99 Although not perfect on a large blow-ups (crops) and maybe, but more probably, not as good as a WO FF could do, but I am pleased with the results I have obtained so far. The nice thing about the SW FF is there is no power factor increasing the FOV. (When using a Canon 40D and Meade 80mm F-6 APO)

Hope this will help

Regards.

William

[Dangerous-Dave]

I use the Flattener III with the Megrez 90 and as others have said it does not flatten outlying stars perfectly when using a DSLR. They are an improvement, but rather than being elongated radially, are elongated at 90 degrees to the centre, suggesting overcorrection.

The advertised spacing of 55mm is too long in my opinion. My experiments with the SXC-H9 and Megrez 90/Megrez 72 have shown it to be far shorter (about 40-45 IIRC). Unfortunately, it is nigh on impossible to achieve this spacing with a DSLR.

[DaveS]

This is interesting as I have been contemplating a Megrez 90 / HEQ5, primarily for visual but moving into AP.

Currently I have an EOS 5D, with no immediate plans for either an APS-C or CCD cam (The M 90 /HEQ5 will eat all my budget).

From what's been posted I may have to rethink my plans.

Comments?

Dave

[Dangerous-Dave]

This is interesting as I have been contemplating a Megrez 90 / HEQ5, primarily for visual but moving into AP.

Currently I have an EOS 5D, with no immediate plans for either an APS-C or CCD cam (The M 90 /HEQ5 will eat all my budget).

From what's been posted I may have to rethink my plans.

Comments?

Dave

The Megrez 90 is an excellent telescope for the money and suffers no more coma than any other doublet of the same size Dave. In fact, if you use a 2/3" CCD you see none as they all fall outside the sensor! The 5D, on the other hand, will show up more bad stars than most on account of the full frame sensor.

If that's likely to bother you, then you might want to save up for a triplet and dedicated flattener, or better still a Petzval.

[DaveS]

Sorry for not getting back earlier.

Yes, I think a Megrez 90 may be my best bet (Reminder to self... check bank balance!)

And dream about a FLT 132 and triplet flattener.

Dave